The Detection and Intensification of X-Ray Images

THE DETECTION AND INTENSIFICATION OF X-RAY IMAGES

CYRIL WILLIAM SMITH ABSTRACT

"THE DETECTION AND INTENSIFICATION OF X—RAY IMAGES".

An historical introduction is followed by a survey of the physics of X—rays, and the production of X—rays for radiology. The possibility of detecting X—ray images is related to measurable quantities, and existing techniques are shown to be close to the quantum limit. Improvements in the diagnostic value of the X—ray image, and the reduction in the patient dose require: the reduction of scattered radiation; increased X—ray absorption in the primary detector; the development of contrast media; the control of image contrast; the presentation of the final image at a high brightness level. The methods of X—ray image intensification are surveyed. Work towards these ends is described in respect of an X—ray sensitive photo— conductive camera tube for converting an X—ray image into a television signal. Its time lag was reduced to 1/25th second by reducing the sulphur content of the amorphous selenium used for the photoconductor, but the tube was shown to be of little use for dose rates of less than 1 r/Min at the tube. This was on account of the small photocurrents which were due to the difficulty of extracting charge from a thick layer by an applied electric field, 2. without also producing a background of white spots. The intensification of the weak light images from X—ray fluorescent screens becomes practicable with reduction in the background glow in intensifier tubes. A tube design in which this had been reduced by excluding caesium from the high voltage part of the tube was improved by using a spiral accelerating electrode to give a more linear electric field, and the remaining background glow was shown to depend on the presence of the solenoidal magnetic focusing field in the region of the tube cathode. 3.

CONTENTS

Abstract. Page 1. Section 1: Introduction. Page 4.- Section 2: X—ray Absorption. Page 14. Section 3: X—ray Images. Page 26. Section 4: An X—ray Sensitive Photoconductive Camera Tube for X—ray Image Intensification. Page 63. Section 5: The Intensification of the Faint Light Images from an X—ray Fluorescent Screen. Page 130. 1G5 Conclusions. Page i-&t. 16 Acknowledgements. Page -1-frd-. 170 References. Page 4-69.. 4. The Dection and Intensification of X-ray Images.

Section 1: Introductj.on.

Historical Survey.

While repeating Hertz"and Lenard's experiments with electrons, RUntgen made an observation on November 8th, 1895, which resulted in the discovery of X-rays. In his "Preliminary Communication", Rtntgen (1896) described the fluorescence which the rays produced in various substances and their power of penetrating solid bodies. Of the X-ray image of his hand he wrote: "... if the hand be held between the discharge tube and the screen, the darker shadow of the bones is seen within the slightly dark shadow image of the hand itself ...". He published photographs from X-ray images and observed that the rays were subject to rectilinear propagation with the intensity decreasing as the square of the distance from their origin at the bright spot on the wall of the discharge tube where the cathode-rays impinged. Discharge tubes and high voltage induction coils were a part of the equipment of most physics laboratories at the time and Rbntgen's experiments were immediately confirmed and extended. There were at least 1044 publications concerned with X-rays during 1896. (Glasser, 1933). The subsequent development 5. of radiology is described in a series of articles entitled "Sixty Years of Radiology" (Stead et al., 1956). Rlintgen's work was followed by a development of the physics of X-rays. Stokes and J.J. Thomson (1898) put forward the theory that X-rays were irregular electromagnetic pulses produced when charged particles were decelerated on striking a target. Rdntgenis failure to obtain refraction of X-rays with a prism was explained by Malt.ezos (1896) on the Helmholtz dispersion formula which gives a refractive index tending to unity for very short wavelengths. Haga and Wind (1899) and Walter and Pohl (1909) confirmed that the wavelength of X-rays was of the order of 10-8 cm, by diffraction experiments. Imbert and Bertin-Sans (1896) found that X-rays underwent scattering rather than specular reflection and J.J. Thompson applied the classical pulse theory of X-rays to this scattering. Barkia (1906) went far towards confirming the predicted angular distribution and polarization of the scattered X-rays and deduced the number of electrons in the carbon atom. He recognised series of line spectra in the scattered X-rays which were characteristic of the scattering substance, and used these X-rays (Barkia and Sadler, 1909) as a source of homogeneous radiation to show that the general law for the absorption of radiation could be applied to X-rays. 6.

On the basis of the electromagnetic theory, Laue predicted that X—rays would be diffracted by a crystal lattice acting as a three—dimensional grating, this was confirmed (Friedrich, Knipping and Laue, 1913). W.I. Bragg (1912) introduced the concept of reflection from a set of equidistant planes in the crystal and deduced the spatial position of the diffraction pattern for a given wavelength of X—rays. W.H. Bragg and W.L. Bragg (1913) developed the X—ray spectrometer and observed the characteristic X—ray emission spectra for the target material superimposed on the continuous X—ray spectrum. Moseley (1913, 1914) made a systematic study of the L-ray emission spectra of various elements from which he derived a relation between the frequency of the X—ray spectral lines and the atomic number of the radiating element. Bohr (1913) used the Rutherford (1911) model of the atom to calculate the energies of the electrons in their orbits and the frequencies of the radiation they would emit in making transitions between different energy levels. These were found to correspond to the observed absorption limits. Besides the discontinuous energy states which give rise to the characteristic spectra, there are states forming a continuous range which correspond to the hyper— bolic orbits of Bohr's theory. Transitions involving 7. these give rise to the continuous X-ray spectrum (Born, 1952). The continuous spectrum has a well defined lower wavelength (maximum energy) limit corresponding to the kinetic energy of the incident electrons and is given by Einstein's (1905) equation for the photoelectric effect. With very thin targets the maximum of the X-ray energy spectrum occurs at this lower-wavelength-limit. For thicker targets the energy spectrum is modified by the loss of energy by some incident electrons and by the absorption of the emitted X-rays in the target material. The wavelength of maximum energy occurs at about 12 times the wavelength of the lower-wavelength-limit. KUlenkampff (1926) suggested that the energy distribution in the continuous X-ray spectrum can be given by a system of parallel straight lines for values of the tube voltage V and the frequency of the radiation ;v. Corresponding to each value of V there is a maximum frequency vo given by Einstein's (1905) photoelectric equation: Ve hvo where e is the electronic charge and h is Plank's constant. Then the equation is expressed in terms of the minimum wavelength X0 and the values of the constants are substituted it becomes x _ 12,354 0 - V 8.

where X o is in Angstrom units if V is expressed in volts. The energy E, of X—radiation of frequency v emitted per second is proportional to (/ 0 v) and to the tube current is Fiv = Ki(v•0 — v• ) where IC is the constant of proportionality. The energy

emitted per second in the frequency interval v 2 —v 1 is given by 2 E d.v = Ki( vo v )cv . v-1 Expressing the energy emitted per second in terms of the wavelength: E =Li ,1) ^ with a new constant L. This expression must be further adjusted to take account of X—ray absorption in the target, the tube window and any filtration used (de Waard, 1947).

X—ray Tubes.

The early discharge tubes were improved by the use of a concave cathode and adjustable electrodes to enable the electron beam to be focused on the target. A potash—bulb enabled oxygen to be generated as required to adjust the gas pressure and hence the break—down potential of the tube which determined the effective wavelength of the 9.

X-radiation. Coolidge (1913) introduced a high-vacuum tube with a thermionic cathode in which the tube current was independent of the applied voltage. Greatorex (1960b) points out that no fundamental change in the operation of the tube has taken place since that time. There have been considerable refinements in the tube technology and electron optics and the high-voltage generators and control units now offer precise and fully automatic selection of the appropriate parameters by means of a single push-button. The X-radiation is generated by a beam of high energy electrons which are decelerated by striking a target. The electron source is a cathode of tungsten wire heated by an electric current and mounted in a metal focusing cup. The anode is usually a tungsten block about 0.25 cm thick around which is vacuum cast a copper block to conduct the heat away from the tungsten. The efficiency of X-ray generation n is proportional to the atomic number z of the target and to the tube voltage V volts (usually 40 to 120 kVp). It is given empirically by n = zV x 10-7%. A tmEsten target can be operated at temperatures up to 2700°07 and at a power density of 250 watts per square mm. The glass walls of the tube will tend to charge up during operation and this must not affect the tube performance. The anode is usually mounted with the normal to the surface 10.

making an angle between 15° - 20° with the axis of the tube. The area effective in heat dissipation is then three times the area of the focal spot as viewed from directly below the centre of the anode. However this makes the apparent size of the focal spot vary over the area of the film which affects the resolution obtained. Typical values for the projected area of the focal spot are in the range 1 to 5 mm . The tube rating for short exposures is determined by the melting point of the material in the focal area and for long exposures by the thermal capacity and heat loss of the anode block. The tube geometry determines the voltage rating. The limit to the performance of the tube set by the melting point of the tungsten is raised by the use of a rotating—anode in which the target is a disc of tungsten cast into the copper cylinder. The latter forms the rotor of an induction motor, which has its stator outside the vacuum envelope of the tube. The effect of increasing the area of the target is to increase the current rating by a factor of about six over that for the stationary anode tube having the same size of focal spot.

X—ray Apparatus:

Small and Mobile Sets.

The electrical dangers from the high voltages associated with the X-ray tube are reduced by mounting the X-ray tube and its transformer as a single unit in an oil-filled container which is also the radiation shield. The high voltage leads are completely enclosed and the low voltage leads which are connected to the control unit do not interfere with the free manipulation of the unit. If the gray tube is of the self-rectifying type, a tube voltage of 85 kVp will result in a peak-inverse voltage of the order of 120 kV because of the voltage drop across the tube during the conducting half cycle. Thermionic emission from an over-heated focal spot under conditions of inverse voltage will damage the filament. A technique known as "inverse reduction" prevents the peak-inverse voltage from rising more than a few kilovolts above the useful voltage. An argon filled diode capable of passing a current of 10 amps with a voltage drop of less than 10 volts is connected in series with the primary circuit of the high voltage transformer so that it only conducts during the useful half cycle. This distorts the secondary voltage waveform reducing the peak-inverse voltage across the X-ray tube.

Larger Installations.

For X-ray sets capable of current of more than 100 mA, the transformer is separate from the X-ray tube so that 12.

only the weight of the tube and its casing needs support. Semiconductor rectifiers are now used in preference to vacuum tube recifiers giving a reduction in size and weight, an absence of heater current requirements and an almost indefinite life, together with the practicability of 6—phase high—voltage circuits. The output voltage may be stabilized using specially developed high voltage valves as series stabilizers. A stable X—ray tube voltage is important because it deter— mines the tube current, the exposure, and the effective wavelength of the X—rays, which determine the density and contrast of the radiograph. Exposure times as short as 1 millisecond can be obtained by pulsing the grid of the series control valve, and the bias on this valve determines the output voltage. To keep the controls at a potential near to earth, a radiofrequency oscillator is amplitude modulated by the bias voltage. The oscillations are transformer coupled to the grid of the series control valve where the carrier is rectified to recover the bias voltage. While an X—ray exposure is being made there may be a load of 70 amps on the power supply line, the voltage of which may fall by as much as 10%. Because of the thermal time lag in the filaments of the X—ray tube and any vacuum 13 ' tube rectifiers, the resulting drop in emission will not take place immediately, but will occur during the exposure time. A booster transformer is used to compensate for this by increasing the filament voltage in proportion to the current load imposed by the equipment. The X—ray tube filament current needs checking after 100 exposures for changes due to evaporation of the filament unless the supply is adequately stabilized. With a stable filament supply, the tube current passing through the low—potential centre— tap of the secondary winding on the high—voltage trans— former may be integrated to determine the X—ray exposure. The X—ray set can be switched off automatically as soon as the appropriate exposure has been made and the average film density can be kept constant to within plus or minus 10%. An auxiliary fluorescent screen with a photomultiplier; or an ionization chamber may also be used to monitor the radiation received by the film (Fransen, 1959). 14.

Section 2: X-ray Absorption.

The intensity of a narrow beam of X-radiation is reduced on passing through matter in such a way that the change of intensity AI is proportional to the thickness Ax of the absorber and to the intensity I of the incident radiation: Al = µ I Ax where µ is a constant of proportionality known as the absorption coefficient or absorption cross-section. If the radiation is homogeneous then µ is constant and the integral of the above equation is:

I - x x = exp 10 where Io is the intensity at x = 0, and Ix is the intensity after the beam has passed through a thickness x of the absorber. The thickness x may be expressed in any of the forms given in table 1, and the product p x must be dimensionless. 15. Table 1.

Thickness x cm x gm/cm' lx atoms/cm' x electrons/cm` of absorber Absorption µ cm-1 -U cm2/gm a µcm.2,/atom e µ cm2/electro: coefficient p

Absorption , , tZ coefficient p N(1-: TI)e µ Ze g e g in terms of eµ Z is the atomic number N is Avogardo's number A is the atomic weight p is the density (gm/cm3)

The absorption coefficient shows the least variation from element to element when expressed in terms of cm2/electron or cm2/gm (mass absorption coefficient), since except for hydrogen and helium changes slowly as Z increases.

X—ray absorption coefficients.

Available information on X—ray absorption coefficients is summarized by Grodstein (1957) for the energy range 10 kV to 100 MeV. Other references are: Davisson and Evans (1952), Nelms (1953), Victoreen (1948, 1949), Richtmeyer and Kennard (1950). An empirical formula for the absorption coefficient is µ c X 3 .1. a where c and a are constants. The term C X 3 represents 16.

absorption due to the photoelectric effect and a represents absorption due to scattering. The equation is valid so long as the photoelectric effect contributes appreciably to the absorption. The constant C has a different value in each of the wavelength ranges bounded by the absorption edges and also depends upon the atomic number of the absorber. For heavy elements the equation is useful throughout the range of clinical X—rays. For water and aluminium, the values given by the equation are 10% too large at an X—ray energy of 80 kV. If the absorber is a mixture or compound of different elements in fractional amounts byweight: al , a2 an ...; if M is the total density of the mixture, and if the absorption coefficients of the constituents are

, .21 -0. then: p1 P2 p3 Aa bal + (la)a + (IQ)a,10+) IO = exp p2 2 P3 Pa

The coefficients of absorption in pyrex glass (Whitcher and Todd, 1953) have been calculated on this basis.

Half—Value—Layer.

The intensity Ix of X—rays after passing through a distance x in the absorber is:

Ix = exp J-0 where Io is the intensity at x = 0; and hence for a layer 17, of thickness VII the intensity is reduced to 1/exp of the intensity at x = O. It is often convenient to consider the thickness of the absorber which will halve the intensity. This is called- the half-value-layer, (H.V.L.) and is given by: (H.V.L.) - 1112 _ 0.693

For radiation which is not monochromatic, it is possible to measure an effective absorption coefficient and then from the tables of absorption coefficients to determine an equivalent monochromatic beam energy and wavelength. Oaks (1958) determines the half-value-layer as follows. First, half of a film is exposed to give a developed density of 2.0. Then with the first half shielded, the unexposed part is exposed through a step wedge and is given twice the exposure which the first part received. From a plot, of density against step thickness, the thickness of the wedge material can be found which corresponds to the photographic density of the half of the film first exposed. This thickness is the half-value-layer for the material of the step wedge and the radiation used. As a practical rule, Oaks states that the optimum voltage for X-rays is that at which the half-value-layer of the object is one-fifth of the total thickness of the object. The half-value-layers of a number of materials are given in the following .* 18.

table 2 based on values of mass absorption coefficient from Grodstein (1957).

Table 2.

Thickness in cm which reduces the X-ray intensity to half the initial value.

Energy Wavelength Air 'Water Calcium Aluminium Copper Lead keV 1 Angstroms N.T.P. Phosphate

10 1.24 846 0.13 0.005 0.009 0.0003r0.0007 20 0.62 1205 0.88 0.50 ' 0.074 0.002 0.0009 40 0.31 .1750 2.62 0.36 0.46 0.016 0.006 80 y 0.16 2440 3.78 1.37 1.30 0.10 0.037 150 0.08 3020 4.58 2.31 1.86 0.35 0.32 300 0.04 3400 5.82 , 3.18 2.47 0.69 0.16m k Absorption edges in lead. absorption edge 15.89 keV J-02 " 13.07 keV a K 11 11 88.23 keV

The Processes of X-ray absorption (Richtmeyer and Kennard, 1950; Fermi, 1953). Robinson (1914, 1923, 1925, 1927, 1930) showed that the absorption of X-rays at low energies is due to photoelectric absorption and classical scattering. A.H. Compton (1923) described a scattering process in which an X-ray photon interacts with a "free" electron to give a scattered 19.

X-ray photon of longer wavelength and a recoil electron carrying the balance of the energy. The production of an electron-positron pair can occur when the energy of the X-ray quantum is at least two electron-rest-mass-units, an energy of 1.02 MeV (Anderson, 1933). The main processes in the clinical X-ray region are photoelectric absorption and the Compton scattering, with an additional contribution from coherent scattering. The processes are independent and the absorption coeffient µ can be separated into the scatter and photoelectric

coefficients a and T respectively:

Q + T where o = kncoherent + Coherent'

Photoelectric Effect (Davisson and Evans, 1952; Nelms, 1953; Grodstein, 1957).

If an electron is ejected from the K shell of an atom, an electron from the Z or M shell can replace it and a quantum of radiation corresponding to the energy difference between the two levels is emitted. Moseley (1913) deduced the following law for the frequency v of the (n,m) transition line for an element of atomic number Z:

v = Ro(Z - 20.

where Ro is the Rydberg constant, and a is a constant representing the screening effect of the other electrons. For the K shell, n = 1, m = 2, 3, 4, ... etc. For the L shell, n = 2, m = 3, 4, 5, ... etc. The probability of absorption of an X-quantum by photoelectric effect in the K-shell decreases as the 7/2 power of the energy of the incident X-quantum and is proportional to the 5th power of the atomic number of the absorber. The maximum energy of the ejected electrons is given by Einstein's (1905) photoelectric equation.For electrons from the K-shell: 2 2mvmax = h( v where v is the frequency of the incident radiation, 4vx the frequency of the K-absorption edge and where the work function is negligible. The emission of the characteristic (fluorescent) radiation from an atom is also a consequence of photoelectric absorption, although an atom in a state of single ionization can also undergo spontaneously an Auger transition to a state of double ionization without the emission of a photon, providing that the initial energy of the singly ionized atom is greater than the final energy of the doubly ionized atom. 21.

The kinetic energy of the electron released from the atom removes the excess energy. The range of an electron is the total distance travelled MI; C El 1 before its kinetic energy becomes zero, and istprcportional to the density of the material. The range of electrons in material of density 1 gm/cm3 is given in Table 3, as a function of their energy.

Table 3. (Lea, 1946).

Electron energy Range in microns for material in keV of density 1 gm/cm3

0.1 0.0030 0.3 0.0101 1.0 0.0534 3.0 0.3124 10.0 2.517 30 17.33 100 141.2 300 832.0

Following photoelectric absorption in the K-shell, the percentage yield of fluorescent radiation is very small for the light atoms up to 80; 1% for loNe; 11% for 1701; 34% for 26Fe; 60% for 34Se; 85% for 50Sn (Broyles et al, 1953). Most of the products of photoelectric absorption in the human body will be Auger electrons. 22.

In the output of an X—ray tube at 70 kV the intensity of radiation in the ccntinuous spectrum is so high that it is difficult to isolate the characteristic radiation from the tungsten target using elements with absorption edges at the appropriate wavelengths as filters. A molybdenum target will generate very intense Ka radiation at 20 kV where the continuous spectrum is relatively weak. Clark (1947) has in7estigated the use of monochromatic radiation and has found a markedly greater change in density per step of a wedge with the MoKa radiation than with the continuous spectrum,. The following table 4 of suitable target materials and filters is quoted:

Table 4.

Target K series Predominating Isolating excitation Ka doublet filter potential wavelength kilovolts

69.3 0.21 Won Mo 20.0 0.71 Zr Cu 8.86 1.54 Ni Co 7.71 1.79 Fe Fe 7.10 1.93 Mn Cr 5.98 2.286 V

Mackay (1962) discusses the use of the spectral information in an X—ray image for 'chemical analysis in the living body by the isolation of fluorescent radiation 23, wavelengths for a three-colour television display.

Rayleigh. Scatter (coherent scatter).

When a quantum of radiation is absorbed by an atom, the latter may pass to an excited energy state and then return to the ground state with the emission of radiation of the same wavelength but not in the original direction. The probability of this process is large only for X-quanta with low energy. This is also the region where the photoelectric effect predominates, so that an accurate knowledge of the Rayleigh. scattering is not usually required for acceptable accuracy in the absorption coefficients. However it should be considered in connection with the degradation of an X-ray image by scattered radiation. Additional scatter may arise from interference among X-rays scattered coherently by electrons in different atoms. The scatter becomes extremely large for crystalline solids under conditions of Bragg reflection. An approximate value for Vcoherent) p is given by c/coh= 0 542115 (Glocker and E1. 7 Messner, 1957) where Z is the atomic number of the scatterer and E the X-photon energy in keV. For the absorption of X-rays in air, Rayleigh scattering gives a significant contribution to the absorption coefficients in the energy range 50 - 100 keV. 24.

Compton Scatterim (Compton, A.H. 1923)

Compton scattering (incoherent scattering) is a process in which radiation of frequency v and wavelength X acts

like a particle, having a well defined energy E = hv , and momentum P = 11./X . In the collision between an X—ray quantum and an electron, energy and momentum are conserved so the energy of recoil of the electron equals the difference between the energies of the incident quantum and the scattered quantum, and the increase in wavelength & of the scattered radiation is given by (Born, 1952, p.328) 2h = sing

where m—c = 0.0242 1 and is called the Compton wavelength, m is the rest mass-of the electron, T is the angle of the scattered quantum relative to the direction of the incident quantum. The increase in wavelength is independent of the wavelength of the incident radiation and depends only on T the angle of scatter. Graphs of the Compton Energy—Angle Relationship are given by Nelms (1953).

Anihilation Radiation.

When an X—ray of energy greater than 2mc2 (1.02 MeV) is absorbed in matter, loss of energy by pair production 25.

can take place. In this process the X—photon vanishes and is replaced by a positive and a negative electron. The threshold energy is 2mc2 because for a free electron there are no energy states between — mc2 and + mc2. In order to conserve momentum, the negative energy electron must be near a nucleus, or another electron, or else there must be an interaction between two X—photons coming from different directions. If a positron unites with a free or loosely bound electron both disappear and their entire energy is dissipated as two photons of equal energy.

Bremsstrahlung.

Energy is lost as radiation from electrons with a velocity close to the velocity of light when they undergo an acceleration as for example in the field of the nucleus. The resulting energy loss per cm. path is proportional to the energy of the electrons and becomes equal to the energy loss by ionization when the electron energy in MeV is approximately equal to

800 MeV where Z is the atomic number of the nucleus (Fermi, 1953, p.47). The effect is not significant in the production of clinical X—rays but Bremsstrahlungen may occur in the emission spectra of radioactive isotopes. 26.

Section 3: X—ray Images.

The Detection of X—ray Images.

Photographic Emulsion.

The detection of X—ray images by their effect on a photographic emulsion was shown by Wintgen in his original paper. For parts of the body which are no thicker than the shoulder and where maximum detail is required a film coated on each side with thick emulsions (non—screen film) is used. It has a wide latitude to variations in the exposure and the quality of the X—rays. A change of 6 — 7 kVp in the tube voltage is needed to produce a visible change of density in this film. A reduction in the X—ray exposure is obtained by placing intensifying screens of phosphor materials in contact with each face of a double—coated film. The light from the screens produces agceetter- density in the film than the X—rays. The latter only contribute to about 2% of the final density by direct action in the emulsions. Approx— imately five developable grains can be produced by each absorbed X—ray quantum but the X—ray exposure must be greater than this in order to obtain a useful density for direct viewing of the film. The light emitted by the screen increases with the size of the phosphor crystals but there is a corresponding loss in detail due to optical scattering. 27. As a compromise between speed and definition three grades of screen are produced for radiography: "high-speed", "par-speed" and "detail". With "par-speed" screens the exposure is approximately proportional to the photographic density which it produces. For radiography, the "high- speed" screens resolve between 3* and 42 line pairs/mm the "par-speed" screens between 4 and 5 line pairs/mm and the "detail" screens up to 7i line pairs/mm. The photofluoros- copic and fluoroscopic screens (including the Patterson-B2) resolve from 2-3 line pairs/mm (Morgan and Corrigan, 1955). Ardran (1962) has concluded that for the best definition most film screen combinations are already too fast since graininess due to quantum effect is usually a limiting factor. Reciprocity law failure is shown by film exposed to X-radiation with intensifying screens, but films exposed to X-radiation without screens do not Show this effect. The light output from a screen decreases by 0.1% to 1% per degree centigrade rise in ambient temperature.

Fluoroscopic Screens.

The energy yielded by an inorganic phosphor in the form of light may be as much as 30% of the X-ray energy 28.

absorbed. About half the total light output is emitted into the solid angle 2n steradians on one side of the screen. The light is emitted according to Lambert's Cosine Law. The optimum thickness for an X—ray fluoroscopic screen of 40% zinc sulphide and 60% cadmium sulphide, (silver activated) is given by Leverenz (1950) as 100 mg/cm2 and by Kallmann (1949) as 80 mg/cm2. Taking the density of the phosphor as 4.5 gm/cm3; 100 mg/cm2 gives a layer 0.022 cm thick, but due to stacking of the crystals the thickness for purposes of light scatter is several times greater than this. Figures quoted from various sources for the quantum efficiency of fluol'oscopic screens are given in table 5.

Table 5.

Screen X—ray No. of useful Source energy light quanta per keV absorbed X—ray quantum

ZnS.CdS:Ag 100 3000 Leverenz (1950) Patterson—B2 70 5000 Sturm & Morgan (1949) Fluoroscopic Screen 90 1125 Teves & Tol (1955) Philips Image Intensifier 90 500 Teves & Tol (1955) Calcium Tungstate 110 500 Coltman (1947) 29.

The quantum yields given in table 5 are consistent with those which can be deduced from the energy yields given in table 6.

Table 6.

Properties of Luminescent Materials.

Liminescent Wavelength - Energy Yield Transparency Material of Maximum .thickness for (Activator Emission. 'Gamma rays Soft X—rays 10% of light shown after Angstrom . to penetrate colon) Units % % I to surface. mg/cm?

ZnS : Ag 4500 14 20 80 ZnS : Cu 5200 22 30 200 ZnS.CdS :Cu 5900 18 10 80 Zn2S104 :Mn 5250 11 8 100 ZnO 5500 10 4 CdB407 6200 10 4 CaW04 4300 8 4 100 KBr : Tl 3600 7 completely transparent Diphenyl 3850 7.5 2.5 very good i: crystallized Thmanth- 4500 11 1.5 from melt racene Napthalene 3850 5 0.8 1 KI (Tl) blue probably completely CsI (Tl) about 5% transparent

Kallmann (1949) Albrecht et al (1959)

30.

Pieces of single crystals of CsI(T1) and KI(T1) mounted to form a transparent screen, give almost 100% X—ray absorption to high energies, and less sensitivity to scattered X— radiation. (Albrecht, Oosterkamp and von Osenbruggen, 1959). Values up to 80% were found quoted for the X—ray absorption in X—ray fluorescent screens whereas Keller and Ploke (1955) stated that in practice the X—radiation absorbed by the fluorescent screen of an image converter was only 2% of the incident radiation (table 7).

Table 7.

Reference X—ray Absorption (Clinical X—rays)

Sturm & Morgan (1949) 51% to 68% Fluoroscopic screen Teves & Tol (1952) 65% Teves & Tol (1955) 80% 24% Philips Intensifier Keller & Ploke 2% Image Converter Measured by Dr. Evans and writer 27% Levy & West "H..W Screen Measured by Dr. Evans and writer 39% Levy & West "H.S" Screen Keller & Ploke 35% Amorphous selenium 15 microns thick

Since this made their X—ray television system compare very favourably with image converters, further confirmation of 31. the higher values for the X—ray absorption in a fluoroscopic screen was obtained is private communications from Dr. J.F. Fowler (King's College Hospital) and Mr. G.A. Hay (Leeds General Infirmary). In addition, the X—ray absorption of two samples of fluoroscopic screen material were measured with the assis— tance and apparatus of Dr. H.D. Evans, Health Physicist at Imperial College. The Levy—West "high definition" screen had an absorption of 27% and the "high sensitivity" screen, 39%. The actual values for the absorption may have been higher owing to the possibility of scatter in the measuring apparatus.

Diagnostic Radiology.

X—rays are selectively absorbed by various substances in proportion to their densities and the normal human body contains a sufficiently wide range of densities from the air filled lungs to the skeleton;to permit an accurate anatomic study of the living body using X—rays. However, the appearance of the "normal" is complicated by the wide range of variations encountered in so called "normal" tissues. Diagnostic procedures often require radiographs of normal tissues for comparison with suspected abnormalities, in addition to the special techniques such 32. as the introduction of contrast-media whereby the differ- ences in contrast between organs and their surrounding tissues can be enhanced. Because of complicating shadow from overlying structures many views may be required for a complete diagnosis. Most radiographs are based on an "average" technique for the X-ray opacity of a 5' 8" male weighing 150 lbs or a 5' 2" female weighing 120 lbs. The effect of the variation of the thickness of the body in the X-ray beam is such as to require a change of 2 to 3 kVp per cm thickness when screens are used. Individuals engaged upon active work requiring muscular exertion develop an increased tissue opacity which may require as much as 10 kVp over the average, while an individual whose work and habits are sedentary may require 6 kVp less than the average. Tissue changes resulting from disease and atrophy affect X-ray density and contrast unpredictably. Table 8 shows the wide range of photographic density encountered in radiographs which were made with par-speed intensifying screens. 33. I—ray Dosaz2.

Early workers with X—rays found that they were receiving superficial burns on their hands and faces due to the radiation. The original purpose of protective measures in diagnostic radiology was to prevent immediate signs of radiation damage and when it was found that the radiation effects were cumulative the dose allowed to the operator was further reduced. Until recently it was taken for granted that the dose to the patient was rel— atively unimportant as it was only received once. The effects of radiation on man are difficult to assess (Roberts, 1949). A whole body dose of 300 rad (see p.34) has a 50% chance of being fatal to man. After large doses of radiation, various acute symptoms appear in a few days. The reproductive organs and blood forming cells in the bone marrow are the most sensitive to radi— ation damage. There may be a long—term effect which appears after many years, often as a cancer or as abnormalities in subsequent generations. Modern work on the biological effects of radiation tends to be biochemical, using as its basis the initial distribution of ions produced by the radiation. 34.

The units for measuring X-radiation.

The roentgen was defined in 1925-1928 as the time integral of the radiation field at a point, in terms of the ionization produced in a unit mass of air. The RAD (an abbreviation of Radiation-Absorbed-Dose) is the specific energy absorption at a point in an irradiated medium. The unit 1 rad = 100 ergs/gm; for clinical X-rays 1 roentgen is equivalent to an absorption of energy by soft tissues of between 88 and 97 ergs/gm. The REM (RAD-Equivalent-Man) is for ionizing radiations in general, and takes into account the Relative-Biological-Effectiveness (RBE) of the radiation. The above are related by the eqaation REM = RAD x RBE. In most places the dose received from natural sources of radiation is of the order of 0.1 rad/year; the recommended whole body dose for those exposed to radiation in the course of their occupation is 0.1 rad/week (Ministry of Health, 1957). The approximate dose which is received by a patient is given in table 8 for a number of types of examination. The range of gonad doses indicated covers children and adults, both male and female, and is derived from data in the Adrian Committee Report (1960) and Recommendations of the International Commission (1954). The measurement 35. Table 8,

Type of Dose from Dose to Approximate, Range of Examination Radiography gonads only additional photographic due to dose from density in radiography fluoroscopy radiographs exposed with r mr r Parspeed screens

Fingers and hands (non-screen film) 0.1 3.4-10 - - Shoulder 1.4 0.3-2.4 2.4-8 2.2-0.38 Skull 1.8-3.1 94-145 - 2.0-0.26 Dentistry (non-screen film) 9-15 3-66 - - Spine 3.6-7 12-132 - 3.5-0.94 Pelvis 4.6-5.8 420-930 9-28 3.0-0.40 Pregnancy 5.1-10.2 Mother.. - - 780-1560 Foetus 240-3800 Lungs 0.05-0,24 1.5-15 1.5-12 1.19-0.4 Lungs, Mass Radiography (1 exposure) 0.04-0.50 0.1-0.72 - - Stomach 8-16 230-1830 8-30 - Kidney 4-15 150-4800 2-40 - Cardiac Catheteris- ation 1.5-3 8-30 30-150 - 36.

of the intogrul absorbed dose during each diagnostic investigation is discussed by Reinsma (1959). The Adrian Committee Report notes that the wide spread in. the gonad doses was due to high values measured in a small number of hospitals, particularly where the gonads were . unnecessarily included in the main beam, and if the standards shown by 25% of the hospitals were achieved by the remainder, then the annual genetic dose could be reduced from 19.3 mr to about 2 mr per person.

Factors giving a reduction of X—ray dosage.

The Adrian Committee (1960) reported that, of the various parameters affecting the gonad dose, the diameter of the primary beam can have the greatest single effect. It should be large enough to cover the part being radio— graphed, but efficient light beam collimators and rec— tangular fields should be used so that the edges of the beam of radiation are shown on the film. A fast film and the use of intensifying screens result in some loss of resolution but the number of retakes needed due to movement may be reduced. Fluoroscopy should only be used when the information cannot be obtained from films. Methods of intensifying the image are reported to be desirable and their use is 37.

recommended as they become available. Obsolete erratic equipment leading to unnecessary repeat examinations should be replaced and timing switches are recommended for mobile equipment. The use of a Potter—Bucky grid increases the dose to the patient in proportion to the grid ratio but it may be justified by the great improvement in the film quality. The dose may be reduced by the use of filters to remove the low energy radiation from the primary beam. of X—rays (Reinsma, 1959i Ardran and Crooks, 1962).

The of

Table 9. Factors which determine the quality of an X—ray Image. (Tuddenham, 1957.) Quality of Image 1 Physiology of Vision Spatial X.-re) Intensity Gradient

X—ray intensity Distance across difference across cbntour an image contour Position of X—ray X—ray intensity X—ray energy Scatter source in relation or exposure 1 to Detector I Object and Filtration Grids Cones 1 background Object Focal absorption Motion spot size

The perception of detail in the final presentation of an X—ray image is a visual process. Each part of the 38. image in the visual field limits the perception of every other part of the image. The likelihood of detecting significant detail is increased by suppressing non- contributory images but so also is the likelihood of the observer only seeing what he expectsto see. The memory contributes towards perception in a creative way and a distinction must be made between the subjective and objective information u'ed in perception (Bouwers, 1960). The retina of the eye is a packed mosaic of receptor cells which respond to the stimulus of the incident light by transmitting discharges along the optic nerves. Exposing the cells to a constant level of illimination results in an initial burst of activity followed by a decrease in the rate at which the discharges occur. Exposure. to a fluctuating level of illumination results in sustained acitivity. The repetition frequency of the discharges varies as a logar- ithmic function of the change in illimination level. The retina is in constant motion relative to any image pattern presented to it, so that in areas of the image where there is an illumination gradient, the cells are subject to a fluctuating level of illumination. These cells determine the image contours and hence the perception of the image. The degree of activity depends on the rate of change of illumination AI with respect to distance across the 39. retina Ax. There is a minimum value of - --x which will result in the perception of an image. The retinal illumination gradient can be increased or decreased by increasing or decreasing the viewing distance or by the use of concave or convex lenses respectively. Electronic devices are also a-vs.:Liable to adjust the illumination gradient of images converted to a television signal waveform. In fluoroscopy, the screen brightness is limited by the permitted X-ray dosage. For a chest examination a typical screen brightness is 0.02 ft-lambert and for an abdominal examination it may be from 0.005 ft-lambert to 0 0003 ft-lambert. The fluoroscopic screen can resolve 2 - 3 line pairs per mm. In bright light the eye can resolve about 14 line pairs per mm for high contrast objects at 25 cm, but at brightness levels in fluoroscopy the eye can only resolve 1.2 to 0.17 line pairs per mm respectively. The visual cells of the retina are of two kinds: cones and rods. The latter are concerned with vision in brightness levels below about 0.01 ft-lambert. They are easily fatigued by bright light and do not begin to function properly until after at least 20 minutes in the dark; their efficiency improves materially for up to 40 minutes and the maximum sensitivity is only attained after 10 hours

40.

in darkness, The Adrian Committee (1960) recommend at least 10 minutes dark adaptation time before commenoing-a fluoroscopic examination. The rods are most sensitive to green light, and are not affected by red light so the radiologist can wear glasses which only transmit red light ana 3o some work in normal surroundings whilst acquiring dark adaptation (Sheard, 1944).

Factors Affecting the Spatial X—ray Intensity Gradient. * X—ray Intensity or Exposure.

That a given system can resolve the X—ray image of a high contrast grid does not give an indication of the resolution for a low contrast object with the same system. A given area of an X—ray image contains a certain flux of X—ray quanta which fluctuates in a random way during any finite period of observation. The quanta represent a number of isolated events in a continuum of time, so that a Poisson distribution is appropriate to represent them. In this case the variance is equal to the

ONO mean, and the standard deviation of the mean number n is 2 / 4r; quanta/cm/ /sec. For an image composed of random events and free from aria6 -9

added background, where na is number of quanta/cm/ 2/sec/

corresponding to a final image brightness B, is is the To be published in Brit.J.Radiol. 41. storage time of the system, and A is the area in cm2 of a part of the image of brightness B AB, the number of events in the area A during the storage time is Anats with a standard deviation of JiI7;. For area A to be distinguishable from the background: AB > kAv where k is the threshold signal-to-noise ratio. Since B = Anats, the contrast B -> k(Anats)2 (Hay, 1960) The change of intensity AI of an X-ray beam of intensity Iy, at a depth y in a medium, due to the absorption in a further thickness Ay is given by AI , µIy Ay where µ is the linear absorption coefficient for a narrow beam of radiation. The difference in the intensities of beams emerging from two adjacent small objects at depth y, having linear absorption coefficients 111 and 1'2 respectively, is - AI2 = ( 112 - µ.1 )Iy Ay. The radiation contrast between the two is Ail - AI2 = ( 122 - ) AY. Iy 1

If the process of X-ray detection alters the contrast by a factor G, then 42.

B G. Ail — AI2 + I Y .4 Hence G( 42 -- 41) Ay = k(Anats)

and nats (µ2 — 41)2(A "2) 2 Now tS is the total number of events/cm at the first object, and is proportional to the integrated radiation dose for a given energy. The quantum equivalent of the reontgen is given in table 10 from Mulvey and Ballinger (1959) as a function of the energy of the quanta. The term (A. Ay2) can be evaluated for certain shapes of object as follows: Cube of side a.

In this case A = a2 and. Ay2 = a2, whence (A Ay2 % ) = a4. Hence in order to halve the size of a cubical object at the quantum limit of resolution, the X—ray dose must be increased sixteen times. Redangular Solid of sides a, b, c.

Let A = ab and Ay2 = c2, whence (A Ay2) = a c2 . Solid cylinder of diameter a and length 1.

Let the cross—section of the object be bounded by the circles x2 + y2 = r2 (where r = ), whose centres lie 43.

Table 10. (Mulvey and Ballinger, 1959).

Photon energy E ergs/cm2/roentgen N quanta/cm2/roentgen keV

10 18.1 1.13 x 109 15 63.0 2.62 x 109 20 155 x 109 30 560 1.17 x 1010 40 1250 1.96 x 1010 50 2240 2.80 x 1010 60 2930 3.06 x 1010 80 3640 2.85 x 1010 100 3800 2.38 x 1010 150 3400 1.42 x 1010 200 3200 1.00 x 1010 300 2950 6.16 x 109 400 2900 4.53 x 109 44. on the axis of the cylinder. Let the radiation be iteident from the positive y direction; then summing all the elements:1 dx of A and 2 (r2 - x2)2 of Ay between the limits + r and - r:

(Aii y2) = 4(r2 x2)1 dx -r

_ 16 r31 or 72 a2 1. - 3

Sphere of diameter a.

Summing for all elements of A such as 27c x dx, and of Ay such as 2 (r2 - x2)2 between the limits + r and 0 : fr (AA Y2) = 8n (r2x - x3) dx 0

=2 TC r4

= z 4 8

X-ray Scatter.

The processes which remove X-radiation from the primary beam to form the X-ray image result in secondary radiation which can be considered as "noise" added to the X-ray image.

45.

Let Is be the intensity of scattered radiation assumed uniform over the image area. The radiation con- il I2 trast was defined as ; but with the addition of scattered radiation, it becomes

A - A I2 All 1I2 I I + I + I y s Y s

All _ AI2 1 1 + 9

An equation can be written: T 2 121") + ?) kG/ nats uti )2(A Ay2) (µ2

Seeman and Splettstosser (1955) give the results of measurements from which the values of (1 + -=.4) given in the following table 11 have been derived. The measurements involved a series of radiographs of step wedges which were separated from the film by various thicknesses of water. 46.

Table 11.

Values of 1 -3,- s derived from Seeman and Splettstosser Iy ( (1955).

Thickness of ,ter 5.7 cm 5.7 cm 194 cm 11Acm 17 cm 17 cm lay-el.

Thickness and 0.9 cm 0.3 cm 1.9 cm 0.6 cm 2.8 cm 0.9 cm material lucite bone-mix lucite bone-mix lucite bone-mix of wedge , . Tube voltage kVp 40 2.25 :3.3 4.5 ,6.5 7.0 8.4 60 2.5 3.3 5.2 6.8 8.8 8.5 80 2.8 3.9 5.7 7.7 10.2 9.5 100 3.0 4.2 6.3 7.2 12.3 11.7 120 3.0 4.1 7.4 .6.6 12.2 17.0

The effect of scattered radiation on the size of a cube which can just be resolved is proportional to + L)* while the reduction in the dose required to I (1 Y Is 2 resolve a given cube is proportional to (1 + - . Iy. The linear d ions of a cube shaped object which can just be resolved from quantum noise by a dose of 1 47.

roentgen at 80 kVp, taking G = 1, k = 2, and including the effect of scattered radiation, are given by the above equations as follows: a cube of calcium phosphate immersed in 10 cm of water, a = 0.03 cm a cube of air immersed in 10 cm of water, a = 0.10 cm. Under the conditions of fluoroscopy given by Sturm and Morgan (1949), the above equation gives the dimensions of a cube of calcium phosphate immersed in water, which could just be resolved: 0.3 cm for the anterior—posterior chest examination, 1.0 cm for the lateral abdominal examination. The dimensions are of the same order of magnitude as the resolution of the eye at the brightness levels concerned. An estimated value is included for' the effect of scattered radiation which is based on table 11.

5. Improvement of the Diagnostic Value of the X—ray Image.

The purpose of a radiograph is to provide the clinician with information for use in arriving at a judgement of the patient's internal condition and the correct course of action or inaction to be adopted. He is concerned with the visibility of objects as they stand out from their surrounding regions. For "correct technique", 48.

the exposure and X-ray tube voltage are adjusted to give a degree of density and contrast which is considered to be "satisfactory" (Files, 1946). Considerable information is potentially available from a radiograph although not directly visible, and while a re-take under different oc:;?,15.tions might render it visible, this would involve additional patient dosage. Both density and contrast can be controlled by :Logetronography (St. John and Craig, 1957). A scanning light source provided by a cathode-ray tube exposes printing paper ar aim which is placed in contact with the radiograph; Light penetrating this is detected by' two photocells. One collects light from the entire field and produces a signal corresponding to the density variation in the radiograph. This is amplified and used to modulate the brightness of the cathode-ray tube. The amount of feedback may be varied to give results ranging from a facsimile reproduction to an overall grey having high detail contrast. The second photocell covers a 2" diameter portion of the radiograph which has been selected for reproduction at the mean density. Fisher and Gershon-Cohen (1958, 1959) describe a system which combines the advantages of immediate viewing with the facilityfor simultaneous adjustment of contrast. By removing the gradations in brightness outside the range 49. under analysis and expanding the remaining narrowed portion of the brightness bard to the full range of contrast of the display monitor, areas of less than 2% contrast difference can be made perceptible. The radiologist can view a single film with many contrast conversions and the pat .ant is spared the multiple exposures which would other— wise be needed. Fisher and Gershon—Cohen (1958) have extended this technique by the use of colour translation. Near black signals are displayed as blue, 25% signals as cyan, 50% signals as green, 75% signals as yellow and 100% signals as red. This is of value in locating isolated regions of the radiograph having the same density.

Image Intensification.

The diagnostic value of the fluoroscopic image is limited by the low intensity of light produced in the screen by any allowed dose rate, and before any of the above techniques would become of value in fluorosocopy the available X—ray information must be efficiently used to produce a visual image at a high brightness level.

Image Intensification Techniques.

1. Image Converter Tubes. 50. 2. Television with an image converter tube. 3. Television with a fluoroscopic screen. 4, solid state intensifiers. 5. Flying spot X-ray scanning systems. 6 7-ray sensitive television camera tubes.

Ima Converter Tubes.

The X-ray image converter tubes for X-ray image have been described by Chamberlain (1942), Coltman (1948), Teves and Tol (1952 and 1955), Dosse (1953), Van der Tuuk and Kuhl (1959), and Botden (1959). An X-ray fluoroscopic screen (5", 7" or 9" in diameter) is situated inside a vacuum envelope. It is supported on a thin aluminium base behind the front window. The light emitted from the screen by X-radiation produces electrons from a photocathode which is in optical contact with the screen. These photoelectrons are accelerated in an electric field produced by a potential difference of about 25 kV between cathode and anode; they are focused by an electrostatic electron lens on to a fluorescent screen where they give a visible image one ninth (one eleventh in the 9" tube) of the original linear dimensions. This is viewed through a magnifying eye-piece so that the angle subtended at the eye is the same as that subtended by the original X-ray image. The brightness of 5'!

the final screen is about 3 ft-lamberts but ion spot and background glow may be present, which limit the contrast and -r-ie perception of detail in the image. In recent tubes, the ion spot can be removed by operating an incor- peTated ion-getter pump. The gain in brightness is of the order of 1000, which io sufficient for fluoroscopy in normal lighting without increase in the X-ray intensity beyond that used for the fluoroscopic screen when viewed in darkness (Stevenson and Fergusson, 1961). In practice the radiologist should be able to palpate the patient, manipulate the instruments whilst viewing the image, and also consult with other practitioners. An instrument which is viewed through an eyepiece is inconvenient, a television display is prefer- able.

Television * in con4unction with an Image Converter Tube.

In 1955 an image intensifier used in conjunction with an industrial television channel was demonstrated at the Royal College of Surgeons (Mayneord, 1955). By 1959 the industrial television channels were well enough developed

* A.A. Campbell-Swinton pioneered both X-rays in Great Britain (Campbell-Swinton, 1896, 1912) and the basic ideas of television (Campbell-Swinton, 1908, 1926). 52. and sufficiently inexpensive for hospital installation. They were found by Stevenson (1961) to be reasonably reliable, needing only occasional servicing and were used for over 1000 abdominal examinations. Photoconductive camera tubes (Vidicon) were favoured for reasons of economy, simplicity and small size. The tubes were espec— ially selected for this application. The time lag in the response of these tubes particularly at low light levels was reported to be unimportant in most cases; details of the waves of contraction in the stomach and small bowel were not affected. The rapid passage of barium through the pharynx and cervial oesophagus could be demonstrated on account of the higher brightness levels. The outline of the heart was reasonably sharp, but there was a con— siderable difference in detail of the vascular pattern at the lung base due to transmitted cardiac pulsation on the left side. Disturbing loss of detail only occurred when the operator moved the screen carriage very rapidly. The optical coupling from the output phosphor of the intens— ifier to the photoconductive target of the Vidicon employed two lenses. With the 5" intensifier, the collimator lens fitted inside the recessed portion of the intensifier. With the 7" and 9" intensifiers, Stevenson (1961) dispensed 53.

with the semi-transmitting mirror of the image distributor and m- unted the two lenses close together. :ter 18 months use of X-ray television, Stevenson (1961) finds the ability to carry out examinations in room lighting the most outstanding virtue of the system. Radiol- ogical details are shown on a separate monitor in the operating theatre when the patient is sent to the X-ray department for part of an operation. The surgeons can approach any part of the body which can be fluoroscopically defined with an exploratory needle under the guidance of the television screen. In cardiac catheterisation the tip of the catheter is clearly visible on the monitor and with room lighting the. readings on all instruments can be easily observed. In Sweden, Lindblom (1960) has been using an Orthicon or Vidicon camera coupled to an image intensifier since 1958. Two X-ray tubes are energized alternatively at a frequency of 25 cycles, and the two images on the monitor are examined through perforated discs rotating synchronously with the 25 cycles to give a stereo-effect.

Television of a Fluoroscopic Screen.

Image intensification by the television of the light imagBfrom a fluoroscopic screen was demonstrated in 1949 54. at the Johns Hopkins Hospital (Sturm and Morgan, 1949). Brief tests were made by Hay in 1954 and 1956 from which measurements and subjective estimates enabled the perfor— mance of a final system to be predicted. In 1957 an image orthicon camera and channel were installed at the Leeds General Infirmary (Hay, 1958). A Levy—West type H.S. visual screen was used in conjunction with a lens of focal length 3i" and relative aperture f(1.5. The image orthicon had a target—mesh spacing three times that used for the standard tubes. This reduced the charge storage capacity giving higher sensitivity at the cost of a reduced range of signal levels. There was also a pre.-amplifier designed for use with low light level signals. The principle defect in the final image was noise originating in the image orthicon when used at low light levels. To keep the magnification of the optical system as large as possible, an image—orthicon was developed in which the effective diameter of the photocathode was 31" and the target diameter 2e, the reduction being achieved by a magnetic lens at the image section. The target—mesh spacing was made less than that used in the standard image orthicon to increase the target storage capacity (Banks, 1958). 55.

In 1959, Marconi Instruments Ltd. (Garthwaite, 1960) demonstrated a complete X-ray television set for use with a standard diagnostic X-ray table; this is now in hospital use (Medical Press, 1960). The fluoroscopic screen is used to convert the X-ray image to a light image which is then converted to a television signal with the modified image orthicon camera tube (Banks, 1958) used with the wide aperture optical system (f/0.68). The equipment can be used for thorax radiology with a dose rate to the patient of 0.12 roentgen/minute, and cine-recording can be carried out simultaneously.

Solid State Intensifiers.

The application of solid state devices to M-ray image intensification takes the form of the Panel X-ray Amplifier (Kazan, 1957, 1958; Diemer, Klasens, and van Stanten, 1955; Fowler, 1959). This consists of a photoconductive layer in contact with an electroluminescent layer. An alternating potential is applied between opposite faces of the layers, and the brightness of the electroluminescent layer depends upon the alternating potential developed across it. X-rays absorbed in the photoconductive layer lower the resistance so that a larger fraction of the applied potential is developed 56. across the electroluminescent layer increasing the brightness at that point. The image persists for half ante after the X-rays have been turned off but some image is still visible after ten minutes. This renders the panel inefficient for the intensification of moving objects. The panel can be used for short term storage with X-ray intensities up to 0.25 roentgen/minute at which dose rate the screen brightness is 1 ft-lambert and the image contrast is high.

Flying Spot X-r4z Scannin& Systems.

An X-ray television system using a scanning beam of X-rays can be made very efficient but it is not practical for clinical radiology. In such systems (Moon, 1950; Greatorex, 1960a), a narrow pencil of X-rays is scanned over the area to be examined in the manner of a television raster. The Xo.rays penetrating the ob4ect are absorbed by a scintillator which is coupled optically to a photomultiplier. The output signal from the photomultiplier is in the form of a television signal with the amplitude proportional to the intensity at each point in the X-ray image. To obtain adequate resolution, the pinhole colli- mating the X-rays must be small, thus a high current density is needed at the X-ray tube target to obtain an 57. adequate X-ray intensity after collimation. This has not been obtained in practice and Greatorex suggests that the X-ray tube should have a target capable of dissipating 10 kW, for example 100 mA at 100 kV to give a continuous viewing of the image.

X-ray Sensitive Camera Tubes.

It was shown in 1954 that a photoconductive camera tube (Heijne, Schagen and Bruining, 1954; Cope and Rose, 1954) could give a television signal corresponding to an X-ray image absorbed in the photoconductor. The thin targets of the light sensitive vidicon tubes abeorbed- only about 10% of the radiation and intensities of a few roentgen per minute at the tube were needed to give adequate signals. The targets were of the order of one inch in diameter and hence too small to be of practical use. Keller and Ploke (1955) reported work carried out during 1952-53 on an X-ray sensitive camera tube. This tube was demountable and continuously pumped. It had a sensitive area 30 cm square, the photoconductor was a layer of amorphous selenium prepared by evaporation in high vacuum, which absorbed 35% of the incident X-rays, and the 0.8 cm thick aluminium end window absorbed a further 20%. The target was scanned by a beam of electrons of 1,500 eV energy 58.

which stabilized the free surface by secondary emission to a potential near that of a high voltage collector electrode. This technique which is termed "anode potential stabilization" results in the inefficient discharge of the target by the electron beam. It was thought that the operation and sensitivity of this tube could have been improved by stabilizing the target at the potential of the cathode (C.P.S) using a low energy beam of electrons (Blumlein and McGee, 1934). This gives storage of charge on the target between scans. It now seems likely that lag in the photoconductor gave the effect of charge storage to Keller and Plokers anode-potential-stabilized tube. Photographs from the television monitor are given by Keller and Ploke (1955) showing a thorax taken with an exposure of 1/10 second at an X-ray tube current of 4 mA, with 110 kV X-ray tube potential and the X-ray tube target 75 cm from the camera tube target; under these conditions the dose rate would have been about 0.15 roentgen/minute at the camera tube, and 1.5 roentgen/minute at the patient. After some success had been obtained by the writer with X-ray sensitive photoconductive camera tubes using amorphous selenium as the target material, Jacobs and Berger (1956) and Berger and Pace (1957) described an X-ray camera tube in which the photoconductor was lead monoxide. 59.

The target was 18 cm diameter, and was prepared and tested before being sealed into the tube. The deflection of the scanning beam of electrons was electrostatic. It was reported that a mesh with 2 apertures per mm could be clearly resolved at an X-ray intensity of 0.01 r/min. The sensitivity extended to X-rays of energy greater than 1 MeV. The speed of response was that imposed by the scanning system.

Image Storage.

In addition to image intensification, the radiologist carrying out fluoroscopy with television may wish to study an image more closely or consult with his colleagues. Image storage permits him to retain this image or sequence of images without further irradiation of the patient. Systems ,which permit the storage of an intensified image are: 1. Magnetic Drums. 2. Video Magnetic Tape. 3. Image Storage Tube. 4. Cine-radiography. 5. Solid State Intensifiers.

Magnetic Drums.

The magnetic drum is designed to record the image 60.

points of a single picture on each track which occupies a complete circumference of the drum. The read-out time is unlimited and the memory is permanent (Schutt and Oosterkamp, 1959).

Video Magnetic Tape.

For video-recording on magnetic tape four magnetic recording heads are situated at 90° intervals around the rim of a disc. The tape is curved to fit the edge of the disc and the heads rotate past the tape at a speed greater than 2 km/sec. Jutras (1959) discusses the economics of the system for radiology. Apart from the capital cost of the equipment, the tapes may be run past the heads only 100 times before signs of wear appear; by then the heads will also need renovation. He estimates that with less than 10 minutes of actual recording time, the running cost would be about $3.00 for each patient, which is a fifth of the cost when radiographs are used. The images can be trans- ferred to 16 mm film for demonstration purposes, but the main applications seem to be to cases where immediate replay is required. 61.

Image Storage Tubes.

Direct View Tubes.

The image is stored as a pattern of electric charge on a dielectric surface which is formed on a metal mesh which, together with a collector mesh, is situated behind the viewing screen. A beam of low energy electrons from a flood—gun cathode charges the surface of the storage mesh to its cathode potential. Electrons are unable to land on the storage mesh and pass through the holes to produce fluorescence on the viewing screen. By taking the storage mesh to a more negative potential the electrons from the flood gun are unable to pass through apertures in the storage mesh and are repelled on to the collector mesh. The screen then becomes dark and the tube is ready for the writing operation. The writing gun scans the storage mesh with a modulated beam of high velocity electrons, so that secondary electrons are liberated from the dielectric on the storage mesh which takes up a positive charge propor— tional to the signal applied to the writing gun. This allows electrons from the flood gun to pass through the mesh so that a visual image is formed on the screen. There is very little loss of quality for the first minute and the image can be viewed for periods of up to 10 minutes. In 62.

the absence of tac flood beam the image will be stored for up to a week. The charge images can be removed and the tube prepared to store a new image in a time between sec and 1/20 sec. The number of distinguishable grey tones in the image is only five and the resolution is not so good as that of a television monitor. Other types of storage tube use a scanning gun for reading—out, which generates a television signal that can be displayed on a separate mon— itor. Sons tubes have the facility for reading and writing simultaneously.

Cine—Radiography.

Although a series of radiographs of a frog's leg were. made for tine.-projection in 1897, it was not until 1936 that a 16 mm cine—film could be made without danger to the patient, and not until 1951 for a 35 mm tine—film at 50 frames/second; even so the dose rates required for this would be of the order of hundreds of roentgens per minute. With image intensification the cinephotography of the screen of a television monitor can take place simultaneously and at the dose rate required for the examination.

Solid State Intensifiers.

Henderson (1962) describes a luminescent panel which is sensitive to X—rays and retains an image that can be excited by 60 to 120 volts do at a current of 1 mA/sq.in. 63. Section 4. The X-ray Sensitive Photoconductive Camera

Tube for Image Intensification.

Introduction.

While X-ray images permit accurate anatomic examinations of the body, their use in medical diagnosis is limited by the amount of radiation to which the patient and the population as a whole may be exposed. Ultimately the limit to the imge quality is set by the quantum nature of the radiatj.on and an expression has been derived giving the size of certain shapes of object which can be resolved, with a given dose of radiation, by a detector operating at the quantum limit. The techniquesof radiology are already close to this limit, although in fluoroscopy the poor resolution of the eye at the low light levels conceals this. Image intensification can improve the diagnostic value of the X-ray image by raising the brightness level so that the radiologist can work in conditions of bright ambient illumination, without the need for dark adaptation, and reduce the X-ray dose by an increased speed of working. The conversion of the X-ray image to a television signal waveform has the advantages that established techniques are available for the display of the final image at a 64.

high brightness level; that there are few restrictions on the number and locations of the picture monitors; that techniques of image storage are available; that techniques of contrast control permit the adjustment of the spatial intensity gradient, and the improvement of the resolution near the quantum limit by increasing the contrast factor G (of page'4'Let seq.). Each„,technique. available for intensifying the X-ray image to a level at which a television camera tube could be used had some disadvantage: image converter tubes gave a bright background which limited the contrast and amplification obtainable; solid state intensifiers had inadequate gain and showed too great time-lag to be of use for moving objects; the fluoroscopic screen had such a low light output that a costly large aperture optical system was required to over come the quantum limitations. In addition the flying-spot scanning X-ray tubes did not give a useful intensity of X-ray output, and the photoconductive camera tubes had targets of too small an area and too low an X-ray absorption to be of practical use. Of these systems the photoconductive camera tube offered the most promise. A technique was available which would enable a tube with a large area target to be made and some success with targets having a 35% X-ray absorption had 65. been reported (Keller and Ploke, 1955). An increase in sensitivity was expected from the use of cathode-potential- stabilization (Smith, 1960).

Design. Considerations.

The design of the X-ray sensitive photoconductive camera tube sets the following problems of technique. Certain of the small photoconductive camera tubes were known to function as X-ray sensitive camera tubes but the direct scaling of these tubes to an acceptable size would have involved large vacuum envelopes and magnetic focusing fields throughout a large volume with consequent high power consumption and dissipation. It was thought that a continuously pumped tube would not be acceptable as a part of a diagnostic X-ray television system so only a processed and sealed-off camera tube was considered. The use of anode potential stabilization of the target results in the shading of the image and the inefficient discharge of the target by the scanning beam of electrons. Shading is avoided and the target is discharged more efficiently if it is scanned with electrons of low energy so that the secondary emission ratio of the electrons at the target is less than unity. The target then stabilizes close to the potential of the cathode from which the electrons originate (Bltm1ein and McGee, 66. 1934). For amorphous selenium which was used as the photo— conductive material the secondary emission ratio was found to be less than unity for electrons with an energy less than 40 eV. If the electron beam makes an angle (1) with the normal to the target surface and if the energy of the electrons is Ve electron volts then the component of energy normal to the surface is Ve cos24) and the component parallel to the surface is Ve sin24 . Thus for an obliquely incident electron beam, the energy component normal to the surface is less than in the case of normal incidence by an amount Ve sin21) thus the photo— conductive layer will stabilize at a potential Ve more positive than the cathode from which the electrons originate (McGee, 1950). It was pointed out by Zubszynski and Rodda (1934) that the variation of potential across a large target could be overcome by making the target of spherical form with the centre of curvature at the centre of deflection of the scanning system. The electron beam would thus land orth— ogonally to the surface over the whole area scanned. The objection of complicating the optical, system does not apply to an X—ray image, which has no plane of optimum definition. This technique offered the possibility of stabilizing a 67.

large area target at cathode potential using a relatively large angle of deflection and of focusing the beam with a short magnetic lens. It was found by the use of a specially constructed mono— scope that the best arrangement for the decelerating elec— trodes was to keep the maximum potential as close to the target as possible whilst allowing for one earthed ring electrode to isolate the signal plate from the high voltage electrode. The decelerating field acts as a diverging lens which increases the target curvature required. A layer of photoconductive material a few hundred microns thick is to be evaporated on to a surface inside an already outgassed tube under high vacuum before it is sealed off the pump. This is the only stage at which the layer of photoconductor could be formed, since photoconductive materials will not withstand the high temperature bake which must be carried out if the tube is to retain a high vacuum. The evaporation of the photoconductor could not be carried out after sealing the tube off the pump on account of gas being generated in the tube, since suitable "getters"• were not available. The photoconductive material should have a resistivity greater than. 1012 ohm cm in order to give frame charge-storage and to avoid lateral spread of the charge image. Amorphous selenium,lead monoxide, and antimony trisulphide have been 68. used for light sensitive vidicon tubes (Weimer, Cope, 1951; Porgue, Goodrich and Cope, 1951; Heijne, 1957). A deposit of lead monoxide attacks glass at elevated temperatures and attempts at preparing an evaporated X—ray sensitive layer of lead monoxide were not successful. No X—ray sensitivity could be obtained from a commercial vidicon having antimony trisulphide as the photoconductor. Commercial selenium vidicons did give a television picture from an X—ray image and since selenium had been used with success- by Keller and Ploke (1955), amorphous selenium was chosen as the photo— conductive material. When selenium is heated, it melts to a viscous liquid at 217°C and in a vacuum it boils with a tendency to form bubbles at the free surface. The surface tension of molten selenium is 105.5 dynes/cm at 220°C and 53.2 dynes/cm at 310°C so that the excess pressure inside the bubble at the aperture of the evaporating cup could rise to 1.0 torr. A metal evaporating cup directly heated by the radio— frequency heater reduced the tendency of the selenium to form bubbles at the aperture; while a glass cup with a tungsten heating coil was more likely to form bubbles which burst spraying molten-selenium at the target. The rate of effusion of selenium frana small cup may be expressed by the following equation (Champion and Davy, 1947) 69. A = j2ir RT

where Q is the mass of selenium of molecular weight M, which effusec through an aperture of area A in 1 second at an absolute temperature T and at a low pressure, P being the pressure difference across the aperture and R the gas constant. To evaporate 1 gm of selenium with a pressure difference across the aperture of the evaporating cup of 10-4 torr a time of 7 hours would be required. More typical of the values actually used in the processing of the tubes is the evaporation of 0.6 gm in 2 minutes, which represents one evaporating cup of selenium; the pressure difference P would then be about 6 x 10-2 torr during the evaporation. A vapour pattern of selenium was produced on a glass plate mounted in a vertical plane parallel to the axis of a cylindrical evaporating boat, by evaporation in a high vacuum through a small hole level with the top of the boat in a screen separating the glass plate from the boat. The vapour pattern showed that selenium vapour was scattered sideways in the region 1 cm above the top of the boat, so that here vapour pressure was greater than 10-2 torr. This accounts for the observed spreading of selenium over the inside walls of the tube (Smith, 1960). 70. Keller and Ploke (1955) gave results of measurements on sandwich cells of amorphous selehium which indicated that the optimum layer thickness for 50 kV unfiltered X—radiation was about 150 microns. Under conditions of cathode potential stabilization (CPS) each picture element is successively brought to cathode potential as the electron beam lands on it. The signal plate, a conducting electrode in contact with the photoconductive layer, is held at a postive poten— tial relative to the cathode of the tube and hence to the photoconductive surface. During the interval between scans, charge leaks through the photoconductive layer making the free surface slightly positive. At the next scan the beam supplies sufficient charge to return it to cathode potential, and since the target is in effect a parallel plate condenser, an equal charge will be induced in the signal plate to which the amplifier input is connected. While X—rays are being absorbed in the photoconductive layer its resistance is reduced, and a larger voltage change is induced on the signal plate at each scan. This induced voltage is ampli— fied to modulate the brightness of the spot on .a picture tube which is scanning a similar raster to the electron beam in the camera tube. Thus a visual image is produced in which the brightness is proportional to the X—ray intensity at corresponding points in the X—ray image. The amplifier 71. in the television channel was linear for input currents from the camera tube up to 10-7 amp and its root-mean- square noise current was approximately 10-9 amp or 6.25 x 109 electron charges/sec. The X-ray photocurrent corresponds to about 500 electron charges passing through the layer for each X-ray quantum absorbed in it (Keller and Ploke, 1955). If the target area is 100 square cm and the 150 micron thick photoconductive layer absorbs 35% of the X-rays of 70 kVp effective wavelength which have already been reduced by 25% in passing through the end window of the tube, and if 1 roentgen of 70 kVp X-rays corresponds to 2 x 1010 Quanta per square cm (Mulvey and Ballinger, 1959), then an X-ray intensity at the outside of the front face of the tube of 0.0015 rimin will give a signal equal to the root mean square noise from the amplifier. If the thorax transmits about 10% of the incident X-rays (Sturm and Morgan, 1949) and if a picture with a signal-to-noise ratio of 10:1 is of acceptable quality then the dose rate to the patient will be 0.15 r/min which is at least one-tenth of the dose rate without image intensification.

Description of the Tube.

The tube is shown in figure 1. Electrons from the cathode 1 of the electron gun are accelerated with a potential in the range 500 to 2,000 volts applied to the AMPLIFIER WALL ANODE 7. CONNECTION 10. DECELERATING ELECTRODE $.

ANODE 3.

GRID 2,

CATHODE I. FOCUS COIL4

SIG NA L PLATE 9. PUMPING STEM LINE & FRAME I2, DEFLECTION COILS PHOTO -CONDUCTIVE 5. LAYER I I. LINE $ FRAME SHIFT COILS 6.

X-RAY SENSITIVE CAMERA TUBE. 73. anode 3, 7. The current in the electron beam is about 1% of the current flowing to the anode and is adjusted by varying the potential of electrode 2. The beam which emerges from a small hole in the anode cylinder is focused by a short magnetic lens 4, and is then deflected magnet- ically by coils 5 to give a 405 line and 25 frames per second interlaced television raster, while coils 6 enable the position of raster relative to the target to be adjusted. The beam is decelerated so that it scans the surface of the :carved target orthogonally with low energy and stabilizes it at the potential of the cathode 1. The wall electrode is kept at anode potential as close to the target as possible while allowing for one earthed electrode 8 to prevent leakage from the high-voltage wall electrode 7 to the signal plate 9. Connection 10 is made from the signal plate to the amplifier input. The selenium photoconductive layer 11 is formed on the signal plate by evaporation in high vacuum, after outgassing the tube. The selenium in the evaporating cup is introduced through the pumping stem 12 and positioned near the centre of curvature of the front face; it is evaporated by radio-frequency heating the cup. When the evaporation is complete the selenium covers the wall electrodes as well as the signal plate but this does not affect their performance. 74.

Tube Constmotion and :Processin.

1. The front face of the tube was made from the base of a conical flask or a pyrex watch glass, its inner surface was ground to remove any grit particles and polished, the front face was then sealed in the glass lathe to the cone formed by cutting the base off the conical flask. The pumping stem and the platinum tape or tungsten seals were inserted, and a graded seal from the pyrex to kodial glass was added at the neck. 2. The tube was thoroughly cleaned and dried. 3. The electrodes were gold or platinum paint or colloidal graphite. These materials were applied to the clean glass with a paint brush on a long cranked handle manipulated through the neck of the tube. 4. The wall electrodes were connected to the metal-through- glass seals by platinum paste. Platinum tape seals were coated with silver paste on the outside of the tube to enable connecting leads to be soldered on after the tube had been baked. The connecting leads were clipped on to the pro- jecting wires when metal-through-glass seals were used. 5. The anode of the electron gun was connected to a spare pin on the base pinch, two getter bars were welded to the anode cylinder, and the electron gun was sealed into 75. position with the pins of the base pinch supported in a jig. A mixture of hydrogen and nitrogen was passed through the tube while it was hot to prevent oxidation of the electron gun. 6. The assembled tube was given a preliminary evacuation and was baked to 400o09 after which it was checked for leaks, and if satisfactory it was sealed off under vacuum until required for processing. The arrangement of side arms to contain the selenium, the evaporating cup, iron slugs for loading the cup with selenium pellets, and the glass button for sealing the tube off the pump are shown in figure 2. . 7. When required for processing, the tube was allowed to fill with dry air and sealed on to the pump immediately. 8. The tube was evacuated, baked at 40000 until the pressure gauge recorded 10-6 torr at that temperature. The oven was switched off and the tube was allowed to cool; if the pressure fell rapidly it was an indication that no leaks were present. The oven was removed when the temperature reached 10000. 9. The electron, getter bars, and selenium evaporating cp.') were radio-frequency heated to a dull redness, with care to ensure that the pressure in the vacuum system did not rise appreciably during this process. FIGURE 2, MIIM• — — — PAGE 76

TUBE BEING PROCESSED. SEAL-OFF--*

PUM P TABLE.

t\\\\\\\\1( IRON CYLINDERS IONISATION TO ENGAGE HOOK GAUGE. ON EVAPORATOR. USED SLUGS.

7 TO: LOAD Se...STORE WITH CUT-OFF VALVE, IRON SLUG AFTER DIFFUSION & EVERY TENTH PELLET MECHANICAL OF SELENIUM. PUMPS. EVAPORATION CUP COLD TRAP, (LIQUID NITROGEN)

PICK UP COIL FOR HEATER WHEN GLASS CUP IS USED. SEAL-OFF BUTTON JOINED TO IRON LIFTING COIL BY TUNGSTEN WIRE.

IRON COIL TO RAISE & LOWER EVAPORATOR & — HOOK TO SUPPORT EVAPORATOR IN RAISED ACT AS SHOCK POSITION. ABSORBER.

MAGNET TO RAISE & ?,1 --LOWER EVAPORATOR_

X-RAY CAMERA TU BE - PROCESSING ARRANGEMEN T 77. 10. The cathode of the electron gun was slowly warmed by passing a current through the heater; the gas evolved was allowed time to pump away. The filament current was increased to 0.9 amps, the anode voltage was slowly brought up to 2,000 volts. An emission current between 100 µA and 1 mA was usually obtained. 11. The tube was then given a short bake until the pressure became less than 10-6 torr at the elevated temperature. 12. Strips of 0.002" thick copper foil were soldered to the silver paste to form the connections to the wall electrodes and signal plate. 13. If the front face was td be cooled for the evaporation of the selenium, a double walled cylinder in which the space between the walls had been evacuated was placed over the front face of the tube and packed with pieces of solid carbon dioxide or ice. The actual temperature of the front face when the selenium was evaporated was not critical, except that when liquid nitrogen was used, the target was found to be crazed after it had warmed up to room temperature. 14. The evaporating cup was constructed to hold 10 pellets (0.6 gm) of selenium. This avoided having the cup so long in relation to the diameter that there was a risk of bubble formation at the aperture. 15. The method of loading the. cup is shown in figure 2 . The selenium pellets were pushed from the side arm into the cup by means of small iron slugs, one of which was placed after every tenth selenium pellet, and which were stored in another side tube after use. The evaporating cup was raised by means of a powerful permanent magnet acting on the iron wire coil at the lower end of the stalk, the length of which was determined by the fact that the magnet could not safely be brought close to the iron frame of the pump table when glass tubing was between the pole pieces. 16. The selenium was evaporated with the coil of the radio— frequency heater in a fixed position relative to the raised position of the evaporator so that it could be lowered, recharged, raised again, and the heater operated for the same period as that required to complete the first evapor— ation. This was necessary because after one evaporation the tube walls had become opaque. 17. When the evaporation had been completed, the cup was lowered and the pumping stem warmed with a gas torch to give a small region free from selenium. The eletron gun emission was checked, one of the getters was fired by radio— frequency heating, the sealoff button was raised into the pumping stem and the tube was sealed off by collapsing the wall of the puMping stem on to the button which was of 79.

the same glass and a loose fit in the pump).ng stem. The button was kept in a side arm until required for the seal— off. The tube was shielded from the heat with asbestos paper which was removed after the completed seal had been annealed in a small gas flame.

Tube Construction Programme.

Altogether eighteen tubes were constructed, ten of these were made from i litre conical pyrex flasks and had targets which were 2" square. The first tubes were made to develop the techniques of construction and processing, and on test most of them gave an X—ray television picture. After this, successive tubes were.made with thicker photo— conductive layers in an attempt to reach what was thought to be the optimum thickness of 150 microns (Keller and Ploke, 1955). The greatest thickness actually obtained was about half this. All the tubes showed a background of bright spots which became more troublesome as the voltage across the target was increased (Weimer and Cope 1951). Replacing the base of the conical flask by a watch glass resulted in a smoother surface for the target but did not reduce this background. Six tubes were made with a 4" square signal plate using a 1 litre conical flask as the basis for the glass 80. work. X-ray images were obtained from the whole target area, although at high beam currents some instability was observed at the edges of the signal plate. Two tubes were constructed with a 6" square target but with these only the central area within a diameter of 2" would stabilize at cathode potential and give a television picture.

Camera Unit.

The tubes were tested in a camera unit constructed for use with the Department's television channel (Industrial Television Equipment Type 10270 C and Receiver HMV 1824 by Messrs. Electrical and Musical Industries Ltd.). Since the channel was in general use in the Department, the X-ray camera was designed to be completely interchangeable with the normal channel-camera by using the same 34-way connecting cable, power supplies and waveforms (figure 3 ), All the controls were in the channel unit end, so that the camera would be remotely controlled with both the X-ray set and the camera behind lead screening. The channel design has not been published but it is similar to the channel for the large CPS Emitron camera tube described by White and Harker (1950). FIGURE 3 . PAGE ~t -4C E.H.T. LINE SCAN SUPPLY GENERATOR CATHODE POTENTIAL CONTROL FOCUS COIL CURRENT

AMPLIFIER 10 CAMERA MIL UNIT CM"'-' -CABLE A

gf GAIN SHIFT LIFT FOC US BEAM CATHODE CURR ENT POTENTIAL FRAME SCA N GENERATOR MAI N FRAME AMPLIFIE I. -TRIG. A

LINE A LINE BLACKOUT 11-1kHCLAMP1-4(CLAMP TRIG. CATHODE FOLLOWER t MIXED 1 PEAK WHITE l i4 LIM I E R SYNCH. WAVEFORM MIX ED GENERATOR VIDEO SUP PN. OUTPUT CHANNEL UN 1 T

PICTURE MONITOR

X - RAY TELEVISION CAMERA &CHANNE 82,

Circuits Included in the Camera Unite

The line scan geAerator circuit is the same as that used in the original camera unit; it embodies a resonant return scanning circuit with separate feedback circuits for the linearity control and for the storage of energy during flyback. The circuit is triggered by a line pulse from the waveform generator. Line scan flyback waveforms were picked up by the signal plate but they were removed by providing an electrostatic screen between the neck of the camera tube and the scanning coil assembly. The line scan circuits are situated as far as possible from the signal plate and are well screened and decoupled. The focus control circuit uses a pentode valve with the focus coil in the anode circuit; the focus current is controlled by the grid potential from a suitable potentiometer in the channel unit. The cathode of the camera tube is fed by a cathode follower and the mean potential is controlled by a suitable potentiometer in the channel. This, together with the blackout pulses from the waveform generator, is connected to the grid of the cathode follower. The signal plate of the camera tube is d.c. connected to the grid of the first 83. valve of the head amplifier which is near earth potential. The potential of the surface of the photoconductive layer is determined by the potential of the cathode of the camera tube which therefore determines the potential difference across the layer. The line and frame alignment controls on the channel are suitable for supplying the shift coils, and the beam current potentiometer is suitable for cont- rolling the grid of the camera tube, 2 in figure 1, and hence its beam current. The high voltage supply is taken from a commercial power unit but additional filtering is required to prevent radio-frequency ripple on this supply being picked up by the signal plate. The head amplifier circuit is the same as that used for the normal camera unit although the layout was altered to give a compact unit which can be mounted close to the signal plate connection on any size of camera tube and so keep stray capacitance to a minimum. Where possible valves with flying leads wired into the circuits were used to give more reliable operation. The first stage used a high slope (24 mA/V) triode (R5559 or VX5049) instead of a (Z77 or CV138) pentode connected as a triode (White and Harker, 1950). It is designed for a camera tube representing a constant current source of about 0.1 µA peak-white signal. 84.

The high slope, low noise, triode input stage is followed by two amplifier stages and a cathode follower, a high value of grid leak resistor is used to improve the signal- to-noise ratio and reduce the effects of low frequency noise, hum and microphony. The input resistor and the stray capacitance in the grid circuit of the first stage result in a fall in the high frequency response. This is compensated by negative D.C. feed-back to reduce the effective value of the first stage grid leak and by tuned filter couplings in the main amplifier' situated in the channel unit. The input time constant of the head amplifier is determined by the capacitance at the input, for the X-ray camera with a 4" tube in position there is a 25% increase in the input time constant as compared with that given by the circuit when used in the normal camera. This increase is compensated by reducing the negative feedback at high frequencies with the circuit adjustments provided in the original design. The shot noise from the first stage of the amplifier is the predominant source of noise in the system. For a triode it can be expressed in terms of the Johnson noise that the equivalent noise resistance would generate at room temperature; the latter is inversely proportional to the mutual conductance. The high slope triode, 85559, has a 85.

mutual conductance of 24 mA/V and an equivalent noise res— istance of 110 ohms, the equivalent noise resistance of the Z77 connected as a triode is about 320 ohms and connected as a pentode is about 1,000 ohms. The noise can be expressed as an equivalent signal current In from the camera tube as shown by White and Harker (1950) 2 4kTRnCo2w 30 1.6 x 10-20 RnCo2 w 03 In , 2n .3 2n .3 where Rn = equivalent noise resistance Co = total input capacitance wo = 2n x frequency range k = Boltzmann's Constant T = absolute temperature. Using the values Rn = 110 ohms, Co = 62 pF(measured value for amplifier and an X—ray camera tube with a 4" square target), 2)-1. = 3 Mc/s. The noise current from the first valve is calculated as a 1.55 milli—microamp, the input resistor contributes a further 0.13 milli—microamp, the anode load 0.2 milli—microamp, and the second stage 0.07 milli—microamp, and if the noise sources are not correlated they are equivalent to a signal current of 1.57 milli— microamp. The noise current was estimated from the noise output 86. of the amplifier displayed on an oscilloscope, which had been calibrated by feeding a signal generator output at 100 kc through a 10 MCI resistor connected to the amplifier input (figure 3 ). The value obtained for the noise current was 1.5 milli-microamp. James (1952) describes a technique which could be used to give a further reduction in the noise from the head amplifier. An inductance is inserted in 41te series with the connection from the signal plate to the head amplifier, and its value is chosen to resonate near the high frequency end of the video band with the stray capacitances to ground on either side, thereby increasing the shunt impedance across the amplifier input and increasing the signal voltage in relation to the valve noise. For capacities measured with a 4" tube in the camera the improvement would be less than 6 dB. The head amplifier delivers a peak white signal to the main amplifier in the channel unit at a level of 0.5 volt, through a coaxial line in the camera cable. The main amplifier raises the signal level and corrects for the high frequency characteristics of the head amplifier and the camera cable. The waveform generator produces the line waveforms given in figure 4, and the field waveforms given in FIGURE 4. ...:4.... I • 5/4S -"PAGES MASTER OSC.

-4( 49"S CLAMP & LINE TRIGGER.

98 AS

BROAD PULSE. 39AS

LINE SYNCH. & BLACKOUT. ---0

9 8/t.S

LINE SUPPRESSION-÷I

98.itS

LINE SCAN (CURRENT WAVEFORM ---____,j

WHITE

VIDEO WAVEFORM 10% LIFT BLACK SYNCH. 14-- ONE COMPLETE LI NE LINE WAVEFORMS 88. figure 5. During the blackout period there should be no signal from the amplifier and the signal level corresponding to black is determined by the setting of the "lift" control (figure 3). The clamp trigger pulse connects the "lift" potential to the main amplifier by means of a gating circuit for the duration of the blackout period. In the following stage, the peak amplitude of the signal is limited to improve the amplifier recovery after an overload signal. During flyback, the suppression waveform disconnects the main amplifier output and synchronisation pulses are added in the opposite polarity to the signal. The output from this stage is amplified to feed the video picture monitors and to modulate a 45 Mc carrier wave for feeding to the aerial input of a television broadcast receiver.

Waveform Generator.

The waveform generator produces the waveforms shown in figures 4Emd 5 which are made use of as shown in figure 3 . The master oscillator generates a 1.5 microsecond pulse at a repetition frequency of 20,250 per second, which is twice the line frequency on the 405 line system, and from this pulse all the waveforms are timed and locked in phase to the 50 cycles mains. The master oscillator pulses are divided by two for the production of the clamp trigger, the line FIGURE 5 PAGE 89

MASTER 0 SC. DIV IDER OUTPUT

I•41nS FIELD (- SUPPRESS 104

I7n.S *RI FRAME TRIG. -+ f- & SY NCH.

2 0 wi,S 405 MASTER 0 S C. INTERLACE PULSES TIMING 0 10 2021 2124: 405 LINE S.

ONE COMPLETE F RAM E

FIELD WAVEFORMS 90. suppression and the line scan trigger. These are timed by the front edge of the master oscillator pulse. The broad pulses. are timed from the back edge of the mastEr oscillator pulse and they control the line synchronization and the frame interlacing pulses for the field suppression. The trigger waveform for the frame scan circuit and the frame synchron— ization is produced by dividing the master oscillator pulses by the number of lines in one frame (405) and since the master oscillator frequency is twice the line frequency these pulses occur every 2022 lines giving a picture with double interlace.

Measurements on Photoconductive Tubes.

Theory of Photoconduction (Moss, 1952).

The essential difference between metals and insulators lies in the number of electrons which are free to conduct. In metals this is of the order of one free electron per atom whereas in insulators there are few electrons free to conduct. If free electrons can be produced in an insulator then these are able to move under an electric field until "trapped". The secondary electrons produced in the processes of X—ray absorption give rise to X•-ray photoconduction. Electrons may also be freed from their bound states by light, giving photoconduction and by, thermal agitation giving semi—conduction. 91.

In a crystal, the ions are considered to be fixed at their normal lattice positions giving a periodic electric field within the crystal lattice. The electron is only permitted to use certain values of allowed-energy, these are separated by forbidden-zones of energy level, and in a crystal the permitted energy values form bands of allowed energies which lie so close to each other as to be a con- tinuamcf energy levels; the number of allowed-energies in each band is a small multiple of the number of atoms in the lattice. If the highest band has some vacant energy levels then the electron in it will conhct in the direction of an applied electric field (n-type conduction). An insulator may be made to conduct if by some means electrons can be raised from the highest filled band beyond the forbidden- zone into the next vacant band where they can give conduction. Additional conductivity will also b2 provided by the movement of the positive holes left behind in the previously full ' band which drift through the crystal in the opposite direction to the electrons (p-type conduction). If the electrons originate from the full-band, conduction is said to be intrinsic: if the electrons originate at impurity centres having localized energy levels within the forbidden--zone conduction is said to be extrinsic. Impurity levels are termed donor or acceptor levels, the donor levels 92. are close to the conduction band and are full at the absolute zero of temperature, the acceptor levels are close to the full band and are empty at absolute zero, but they trap electrons excited from the full band giving rise to p-type conduction. The diffusion length of a carrier is the distance travelled in the life time of the free carrier T due to the process of diffusion; it is a path length travelled in random direction. The drift length in an electric field is the distance A travelled in time T by a carrier due to the velocity imparted by an electric field E. If µ ,the mobility is the velocity imparted by a unit electric field, then A = µT E. A is frequently referred to as the "range" of the carriers. In amorphous selenium the range of the carriers is given (Weimer and Cope, 1951) as 1 micron for electrons and 10 microns for holes at field of 5 x 104 V/cm. For thin films of amorphous selenium4holes = 0.15 cm2 sec-1 v-1 , -3 2 -1 -1 and 11 electrons= 4°7 - 5°5 x 10 cm sec v . (Spear, 1957). 4electron is predominantly controlled by thermal release from trapping states lying at a depth 0.25 eV below the conduction band and µ hole from states at 0.15 eV. Thus for holes hole = 1.3 x 10-7 sec. -7 electron = 3.63 - 4.25 x 10 sec. 9J,

Volume Excited Photocurrents.

If in a given photoconductor when there is a total increase N in the free carriers throughout the volume of the material due to F excitations/sec when rate of removal of free carriers equals their rate of production, then the photocurrent I is given by N, = (Rose, 1955) Tr

where e is the electronic charge and Tr is the transit time

between the electrodes under the applied electric field E, or eFT I = Tr but 2 T = = . = Ell VIA ' hence I for plane parallel electrodes with separation L and a poten— tial difference V. But T —A— E

= _A I . Tr L and I = eF,A/L that is the photocurrent is inversely proportional to the thickness of photoconductor between the electrodes expressed 94.

as a multiple of the range of the carriers. It is also directly proportional to the applied electric field.

Space-charge-limited Photocurrents.

The space-charge-limited photocurrent Ism is given by

ISCL -A. -g- (Rose, 1951) where Q is the charge required to alter the potential of the "interior" of the photoconductor by V, and if C = Q/V is the capacitance of the "interior" of the photocolthctor then subsituting for Q and Tr v2 CV. 114 E2C 11 SOL"' L2 = L2 C 11 That is the space' charge limited current is proportional to the square of the applied field.

Rise and Decay Times for the Photocurrent.

Only the time spent in the conduction band is counted as the life time of a free electron. It may be repeatedly trapped in shallow states before being finally trapped in a deeply bound state. The shallow trapping states do not affect the steady state photocurrent, but they do affect the time required for its build up and decay, which is proportional to the ratio of the shallow trapped to the free carriers, 95.

In polycrystalline and amorphous materials, even though chemically pure, bound state densities of 1019/cm3 can be expected. It is found that very small photocurrents persist in insulating photoconductors for hours or days at room temperature; these are due to the slow emptying of traps. Life times of the order of - 10-7 sec are needed to account for the small photocurrents observed in amorphous selenium.

Measurement of Current in the Photoconductive Layer.

Because of the scanning system the results are not complicated by the small regions of high conductivity which influence measurements on photoconductive cells. Method 1. The peak signal current from the video output of the amplifier (figure 3) is compared with a calibration signal using an oscilloscope. The peak X-ray signal current is noted and then without the X-rays the calibration signal is switched on and adjusted until equal to the former level. The calibration signal is given by a suitably terminated signalgenerator at 100 kilocycles which feeds the input of the head amplifier through a selected 10 megohm resistor of negligible self-capacitance. The calibrated attenuator on the signal generator gives the r.m.s. voltage applied to the 10 megohm resistor. This method was used for the 96.

measurements on tubes numbered 18, 20„ 25 and 27 and gave results in agreement with the following method.

Method 2. This method was used for tubes 25, 27 and 28. The average value of photocurrents which are less than the amplifier noise current can be measured. In this second method the free surface of the target is stabilized at cathode potential. Since the signal plate is nearly at earth potential the cathode potential only differs by a small constant amount from the potential V across the photo— conductive layer. After this, the potential of the cathode is made more positive so that no electrons can land on the target, but the free surface of the photoconductive layer loses charge to the signal plate by leakage through the photoconductor. After a suitable time interval the cathode is taken negative until a signal appears on the picture monitor showing that electrons are landing on the layer; the potential of the cathode (V — IS V) at this instant is noted. The dark current is so small that no change of target pot— ential can be measured when the target is left without electrons landing on the layer for times shorter than 2 minutes. If the target is exposed to X—radiation for 10 seconds a convenient change of potential results, but if, 97. dark current is being measured the layer needs to remain for 10 minutes without electrons landing cn it for a measurable result. The rate at which charge leaks through the photo— conductive layer is given by, 8V i =mv rt- where i is the current flowing through unit area of the layer, V is the change in the potential of the free surface of the target in the time Jot and C is the capacitance of unit area of the target. If V is the potential across the photoconductive layer, and R the resistance per unit area of the layer:

• V 6V 1 = =C gt or CR = V -6ot7

The target is effectively a parallel plate condenser so that the capacitance may be written in terms of the per— mittivity E of the photoconductor and the thickness of the layer d. For unit area of the target C = e /d. The resistance of the layer may be expressed in terms of the resistivityp and the thickness d, so for unit area 98.-

of target R = d p Thus CR = ep = V8V — and 6t.V P e 6V If the change in potential 8V is not negligible compared with V9 then the equation 6V = St ep may be integrated to give (t/ep) Vt = Voexp_ where V t is the value of V at time t and Vo is the value at time t = 0. Whence

- t = 1nVt e p Vo _ 0 = 2 + Vt o Vt where < 1 9 Vo and t(Vo Vt ) e p (V0 + Vt \ 2 I taking the first term of the expansion only. Thus the 99. change of potential 6V becomes (Vo Vt) and the potential V is expressed by the mean potential

Vo + Vt 2

Results of Measurements.

Dark Current.

The dark currents were found to correspond to a resis— tivity of the order of 1055 ohm cm, which is in general agreement with other results for amorphous selenium (Moss, 1952). The dark current increased as the square of the voltage across the layer, figure 6 , and decreased as the cube of the thickness of the layer. These results are consistent with the dark current being space charge limited. Bright spots corresponding to small regions of high conductivity in the photoconductive layer were observed for all tubes when a sufficiently high potential difference was applied across the photoconductive layer, this limited the tube sensitivity which could be obtained. The bright spots have been discussed by Weimer and Cope (1951) who attribute them to regions of partially crystalline selenium in which the crystal size 'is a small fraction of the layer thickness. Layers of selenium which had been treated as described on page 120 etmq.to reduce the photoconductive time lag were FIGURE 6 PAGE 100

.11 10 TY SI DEN NT RE R CU K R A D

VOLTAGE ACROSS TARGET OF TUBE 20, THICKNESS 30 MICRONS.

DA RK CURRENT 101

found to show fewer spots for a given potential across the layer and thus could be operated with a higher potential difference to compensate for their lower sensitivity.

X-ray Photocurrent.

The curves in figure 7 show that the X-ray photocurrent is proportional to the X-ray tube current. The relative positions of the curves show that thin layers of selenium give more useful charge to the signal plate than the thicker layers, although these have absorbed more of the X-radiation. In figure 8 , the estimated number of X-ray quanta per square cm per second absorbed in the target is plotted against the X-ray photocurrent per square cm of the target. The curves appear in order of the thickness of the photoconductive layer except in the case of tube 28 which was doped with tellurium as described on page 112 . The X-ray dose rate was estimated from the X-ray tube current, and its distance from the camera tube. This estimate was checked with an ionization chamber. The number of X-quanta per roentgen was taken from Mulvey and Ballinger (1959). Figure 9 shows the variation of the X-ray photocurrent with the voltage across the target. The two lines on the graph drawn with slopes of unity and two respectively, FIGURE 7 PAGE 102 -9

10 1 1

VIM ■

2 —

>- -fic -10 •-'10 N/M•

_AMPLIFIER _ IMO .mp, R.M.S. NOISE U 5 cn

et U D I- TUBE NO. LAYER VOLTAGE 0 THICKNESS AC ROSS ( MICRONS) TARGET CL I 0 18 V 2.4 10 — I 9 D 10 10

NEM 20 x 30 17 X 25 • i I 60 27 • 74 60 5 28 A 60 60 -- A

I I'll . 1 1 1 1 1 1 1 1 1 I -5 -6 .7 -8 I 2 3 4 5 6 7 8 9 10

X-RAY TUBE CURRENT, M ILLIAMPS AT 65 KV.P. APPROXIMATE X-RAY INTENSITY 0.3 R./MA/MIN. 103. correspond to the regions of volume—excited and space—charge— limited photocurrents. The experimental points show that the transition takes place at about 20 volts for a target of thickness 74 microns. The measurements given in figure 7 lead to sensitivities which are about 1000 times greater than those predicted from the results of Keller and Ploke (1955), closer examination of which shows an inconsistency between the X—ray photow current deduced from the photoconductive cell results and other data given in the same paper. They related the X—ray photocurrent to the X—ray dose rate, the electric field in the layer and -the area of the cell, by a constant a for which a graph was given showing the values of a as a function of the thickness of the layer based on cell measurements. The X—ray photocurrent in amps Ir was given by

Ir = aI.E.F I was the X—ray dose in mr/sec = 10 mr/sec E was the field strength in the layer in kV/cm = 50 kV/cm 2 2 F was the surface area in cm = 30 x 30 cm . .-1 The constant a was given as 10-14 (ohm.cm.mr/sec) for a layer 150 microns thick, and inserting these values in the equation for Ir -14 -9 Ir = 10 .10.50.900 = 4.5 x 10 amps.

X-RAY PHOTOCURRE NT IN SELENIUM LAYER (AMPS/CM2) _ O!i - 61 6, —i O t *0 N.) — 6 N IV K.) K.) _ — C 4) 03 ca co %.,1 Ul 0 m x O Ill . • X a i z cr 00• c › z -rte.p. z>--- cs. -“,, v —w 5 n) Kmm o it. - 0 —1a0 4 0

Oxcu 0 zFio -4 c.nx 0 zz ma) tn m o ci

z <-1 0> —I o• O. m.— — — 'LI 71 0 00v 00-iam > — >m 0 = CI 0 a-1 m u me m -i a) 13 — m 0 4 a+ 105. This is not the 4.5 microamps arrived at in the above paper. If it is assumed that the physical dimensions given for a -1 are correct as (ohm.cm.mr/sec) then E should have been given by Keller and Ploke in volts/cm and not kV/cm. When this assumption is made and a is calculated from the values of X—ray photocurrent measured with the writer's tubes, an agreement as to the order of magnitude is obtained as shown by the following table 12.

Table 12.

Tube No. Layer a from writer's a from graph Thickness measurements by Khller and taking E in Ploke (1955) volts/cm. (microns) (ohm.cm.mr/sec) (ohm.cm.mr/sec)

'15 -15 18 2.4 1.0 x 10 1 x 10 -15 -15 19 10 7.2 x 10 2 x 10 15 -15 20 30 5.7 x 10 4 x 10 -15 -15 25 11 1.5 x 10 2 x 10 15 27 74 2.0 x 10 6 x 10-15

Cope and Rose (1954) give the X—ray quantum sensitivity of a layer of amorphous selenium, 25 microns thick as between 170 and 250 charges reaching the signal plate for.each 65 kVp X—ray quantum absorbed. Hence 107 quanta/m2/sec absorbed in

F IGURE PAGE 106

10 100 VOLTAGE ACROSS TARGET

TUBE 27 . TARGET 74 MICRONS THICK

X—RAY TUBE, 65 KVP 10 MA 107. the photoconductor would represent an X-ray photocurrent of between 2.7 x 10-10 and 4 x 10-10 ampsAm 2. This comes in the appropriate position relative to the curves in figure 8 and further confirms the order of magnitude of the X-ray photocurrents obtainable from a target of amorphous selenium. The maximum performance of high resistivity photoconductors is discussed by Redington (1958) and Rose (1957).

Layer Thickness.

The thickness of the layer was calculated from the known weight of selenium evaporated and the distance of the evaporating cup from the front face of the tube. The weight of an average pellet of selenium was found and the number of pellets evaporated was counted. The distance of the top of the cup from the front face of the tube was measured before evaporation commenced. Scattering of the selenium at the mouth of the cup and the curvature of the front face helped to give a uniform layer, the thickness of which was calculated on the assumption that evaporation took place uniformly over a hemi- sphere. This agreed with the thickness deduced from the measurement of the photocurrent in the same tube by both the two methods described on pages95-99, which give the resistivity and the resistance of the layer, the area of which can be measured directly. 108; Variation of Resistivity with Temperature.

The resisitivity t-'f the layer of amorphous selenium in a tube at room temperature was measured by Method 2. The layer was cooled by packing the front face of the tube with solid carbon dioxide and after the temperature had been allowed to reach equilibrium the resistivity was measured, then the target was heated to 50°C with an electric heater and the resistivity again measured. An attempt to measure the resistivity at 60°C resulted in a permanent change to the high conductivity form of selenium. The measurements given in table 13 are mean values of the readings taken.

Table 13.

Temperature Dark Resistivity oc. ohm.cm.

+ 50 6.3 x 1014 + 23 3.0 x 1014 — 78.5 11.0 x 1014

Amorphous selenium is an intrinsic semiconductor with mainly p—type conduction. For an intrinsic semiconductor the conductivity is proportional to exp (—E/2kT) where E is the activation energy, T is the absolute temperature, and k is Boltzmannis constant (Moss, 1952). 109.

By tak±ng the ratio of the conductivities at 23°C and 78.5°C the activation energy E for this range is found to be 0.13 electron volts (eV), which is in reasonable agreement with the value 0.16 eV given by Spear (1956) for the depth of the trapping states for the positive holes. Between 23°C and 50°0 the conductivity decreased with rise in tem— perature. The activation energy corresponding to this change in conductivity is about 0.5 eV, and may be associated with the conversion of the selenium from the amorphous to the metallic form. Evidence for this transition taking place near room temperature is reported by Keck (1952). The decrease in activation energy of 0.4 eV associated with this transition is mentioned by Moss (1952) who quotes a study by Gilleo (1951) of the optical absorption in selenium livers before and after the transformation from the amorphous to the metallic type.

Time Lag.

The time—lag appeared as the slow build—up and decay of the television signal which 'cook many seconds to reach equilibrium. The lag was sufficient to render the system useless for the television of objects which moved. One source of lag can be the inadequate discharge of the target capacitance by the scanning beam of electrons. 110.

This effect, capacitance lag, is discussed by McGee (1950), and Meltzer and Holms (1958); its presence was demonstrated for these tubes as follows: (a) Reducing the area of the target scanned resulted in the decrease in the time lag as shown in figure 10a. The bean current was kept at a constant low value. (b) When the beam current was reduced by decreasing the voltage applied to the heater of the electron gun, the lag increased as shown in figure 10a. (c) The lag was found to decrease to a minimum value as the beam current was increased, as shown in figure 10b this minimum time lag was about 2 seconds for a 90% response, or second for a 50% response to the application or removal of an X—ray image. During the tube construction programme the following tests were carried out to see whether this time—lag could be modified in any way. Test 1. During the evaporation of the selenium, the external front faces of the tubes were kept at temperatures in the range 2000 to — 78°C. Test 2. The evaporating cups were made from pyrex glass with a tungsten heating coil inside the cup, pyrex glass heated from outside the cup and from nickel foil heated directly. In each case an eddy current heater was used. FIGURE 10 PAGE III

0 8 w I I 1 1 1 1 1 ; I I I z Z < t.) Li) (/) 6 — . w HEAT ER VOLTAGE 4.0 F-= _ w o • ------.i, . .... 0 Z .....'1 ix < — 4 — I-. w _ • u_ c $ 0 D 2 —. <0 ,t• HEATER VOLTAGE 2.3 tal (1) IX $ • <

1 I

-j • __ 0' I 1 I I I

I I I I 1 I 1 I 1 I 1 1 I I I 10 20 30 TIME IN SECONDS FOR THE SIGNAL TO DECAY TO SOME ARBITRARY LEVEL AFTER THE X-RAYS HAVE BEEN SWITCHED OFF 112.

Test 3. Platinum, gold, silver, aluminium and colloidal graphite were used for the signal plate. Test 4. The rate of evaporation was altered so that the times for complete evaporation of the selenium took from 15 seconds to 45 minutes (Spear, 1956). Test 5. A tube was tested with an X—ray image while the target was cooled with solid carbon dioxide. Test 6. For the first batch of selenium used (Johnson Matthey's spectroscopically pure selenium) the spectroscopic analysis supplied with it reported 1 part in 5,000 of tellurium as the greatest impurity present. About 1 % of tellurium was added to the selenium used for tube 28 to determine the effect of increasing its concentration. The time lag was not altered but the sensitivity was reduced as seen from figures 7 and 8, The analysis sheets supplied with later samples of this selenium reported less than 10 parts per million of the various impurities listed. None of the above tests gave any significant change in the photoconductive time lag.

Properties of Amorphous Selenium.

Selenium is prepared commercially (Thorpe, 1954) by its precipitation from aqueous selenous acid with a reducing 113.

agent such as sulphur dioxide, hydrazine hydrate, or by the electrolysis of aqueous selenous acid. Since selenium and sulphur form a complete system of mixed crystals in which any proportions of sulphur and selenium are possible (Gemlin, 1949), it. seems likely that all selenium precipitated from selenous acid with sulphur dioxide will be mixed crystals of the Se—S system. The analysis sheets supplied with the spectroscopically standardized selenium used for the tubes did not include sulphur among the impurities listed. Johnson Matthey stated over the telephone that the spectros— copically pure selenium J.M. 781 contained 0.005% sulphur, which was estimted by chemical analysis, and that since the sulphur did not show up in the spectroscopic analysis it was not included; however, subsequent analysis sheets did include the sulphur. The crystallization of amorphous selenium is exothermic and occurs rapidly at temperatures above 7000, but below this temperature crystallization will still take place slowly (Keck, 1952). As catalysts for the crystallization of selenium, Grison (1951) mentions tellurium, and Henisch (1957) includes alkali metals, halogens, amines, phenyl— hydrazine, and quinoline, while the presence of sulphur ions on the surface of amorphous selenium can cause the rapid crystallization of the surface. Phosphorous and arsenic are 114. reported to inhibit the crystallisation (Krebs, 1951).

Demountable Tube.

A demountable tube was constructed to provide a quick and effective means of testing a large number of targets, which was free from capacitative lag and the limitations imposed by a scanning system. In this tube a.flood-gun provided a uniform flux of electrons over the whole area of the target. The gun anode was a nickel cylinder, the dimensions of which were adjusted empirically with the demountable system using a fluorescent screen. The assem- bied gun was welded to a base pinch and sealed into the demountable tube, figure 11. The target was prepared.on'a curved aluminium foil and inserted into the tube for test, the front face was sealed to the tube by an "0" ring seal, the tube evacuated, and the cathode activated.

Experiments with the Demountable Tube.

The results of the experiments with the demountable tube are summarized in the following table 14. SCALE 1 / 2 . FIGURE II PAGE 115

CLAMPING RINGS PYREX (REMOVED WHEN GLASS TUBE IS UNDER VACUUM). FLOOD OF GRADED SEAL

KODIAL GLASS,

HEATER,

ANODE,

WALL ANODE

Ae END PLATE EITHER USED EARTHED ELECTRODE AS SIGNAL PLATE OR AS INSULATED FROM SUPPORT FOR At FOIL OF SIGNAL PLATE BY SAME CURVATURE, °O-RING" SEAL

PUMPING STEM. (TUBE PUMPED CONTINUOUSLY),

DE MOUNTABLE TUBE 116. Table 14.

Experiment Time in seconds Target materials No. for a fall in tested in demountable signal level by tube approximately 90% 50%

1 QUM J.M. 781 No. 592 selenium

2 2 4 J.M. 781 No. 592 selenium, first half was evaporated to waste 3 2 J.M. 781 No. 592 selenium, + 10 atomic % arsenic 4 40 10 Selenium + 2% sulphur 5 10 Selenium shaken with carbon disulphide 6 2 "Vidicon selenium" 7 50 10 New Metals " Chemicals 99.99% selenium 8 0.5 Mining & Chemical Products Distilled Selenium 9 2 J.M. 781 No. 592 selenium, zone refined

0.15 MIN J.M. 781 No. 592, 9 x vacuum 10 distilled

0.15 OOP J.M. 781 No. 592, 9 x vacuum distilled (repeat experiment)

11 0.30 ON. J.M. No. 592 fractionated in Vigreux column under high vacuum 0.10 J.M. 781 No. 592 sublimed 12 under high vacuum 0.15 The residue which had not sublimed 1 when the experiment was completed

13 0.04 OMB J.M. No. 592 heated in high vacuum to 1000C for 16 hours 117.

When the target was an aluminium plate at cathode potential, the response time to an applied transient was 0.4 m.sec for a 50% response. 1. With a selenium layer, the response speed to X—rays was the same as had been observed with the sealed—off tubes, 0.25 sec for a 50% response. Hence the demountable tube gave further confirmation that the remaining time—lag was photoconductive rather than capacitative. 2. A layer prepared from the selenium left in the evaporating boat after half the original charge had been evaporated to waste gave no change in the time—lag. 3. A target was prepared using selenium with, the addition of 10' atomic per cent of arsenic, this gave slightly shorter time lag than the selenium itself, but the experiment was not conclusive. 4. A target was prepared from selenium to which had been added 2% of sulphur. This gave a time lag which was about twenty times longer than was observed with the same selenium to which the sulphur had not been added. 5. Shaking some selenium with carbon disulphide before evaporating it gave a similar increase in time lag. 6. A sample of selenium which was reported to produce light sensitive photoconductive camera tubes with a time lag shorter than 1/25 sec was used twice to produce a layer for 118. test in the demountable tube, and on both occasions the layers gave time lags several times greater than that given by the Johnson Matthey spectroscopically pure selenium; the method of preparing the layers must have differed in some significant detail from that used.by the firm who supplied the sample. This result is entered in table 14 under "Vidicon selenium". 7. A sample of 99.99% pure selenium obtained from New Metals and Chemicals Ltd. gave a time-lag of about 1 minute for a 90% response. 8. A sample of distilled selenium obtained from Mining and Chemical Products Ltd. gave a time-lag of 0.5 sec. for a 90% response. A silica crucible was used for the evaporation of the selenium to check for contamination from the molybdenum boats used in the above experiments. The first layer prepared by evaporation from a silica crucible gave a time lag of over one minute. The experiment was repeated with some small pieces of paraffin wax attached to the signal plate. The wax was melted, showing that the signal plate had attained a temperature greater than the melting point of the wax, 420C. A special bell jar was constructed for the demountable pump which had a Dewar flask section at the top. The inside of the flask section and the reverse 119.

side of the signal plate were coated with colloidal graphite to increase the heat transfer. The flask section was filled with liquid nitrogen before the selenium was evaporated and this kept the signal plate below room temperature throughout the evaporation of the selenium. A layer of selenium prepared in this way showed the same time lag as that prepared using a molybdenum boat. Hence the heating of the signal plate was responsible for the increase in the time-lag at first observed and contamination of the selenium by the evaporating boat was unlikely. The latent heats of selenium were considered as a possible source of time-lag by heating the layer as it was formed. The latent heat of vaporization of selenium is 750 cal/gm and the latent heat of fusion is 20 cal/gm, these would produce a thermal gradient of only 1/10°C during evaporation, so that there is no reason to attribute the lag to the heating of the layer by the latent heats. 9. Chemical purity was further considered since it had been shown that the addition of sulphur to the selenium (experiment 2).gave an increase in the time lag. Purifi- cation of selenium by zone refining was tried, but the time- lag of the refined was the same as for the original selenium. 10. A quantity of selenium was distilled nine times in a high vacuum. When a layer was prepared from this selenium 120,

and tested in the demountable tube, t-Le time lag was found to have been reduced to 0.15 seconds. A repeat of the experiment gave the same result. 11. In an attempt to extend the distillation to a continuous process; a Vigreux reflux still was constructed (Carney, 1949). It was sealed off under vacuum and operated for 6 hours. A layer prepared from that selenium which had not been retained in the column gave a time lag of 0.3 seconds when tested in the demountable tube. 12. The process of sublimation in high vacuum was tried. Some selenium was placed in a carefully cleaned Pyrex glass tube which was sealed on to the vacuum pump and heated to 150°C in a high vacuum for 42 hours. Some of the selenium sublimed to the cooler end of the tube and a layer prepared from the sublimate gave a time lag of about 0.1 sec when tested in the demountable tube. 13. Reference to vapour pressure tables by Honig (1957) showed that the vapour pressure of sulphur is 10-2 torr while selenium is 10-7 torr at 100°C. The rate of evaporation is proportional to the vapour pressure and inversely proportional to the square root of the molecular weight (Champion and Davy, 1947). Hence sulphur could be expected to evaporate 1.6 x 105 times as quickly as selenium when heated in a high vacuum to 100°C. 121.

A sample of selenium which had been prepared by precipitation from selenous acid with sulphur dioxide and had not been purified further was obtained from Mining and Chemical Products, Ltd. This was expected to have a high sulphur content and was used to enable the separation to be observed more readily. Some of this selenium was placed in a carefully cleaned Pyrex glass tube and sealed on the vacuum pump. The tube was evacuated to 10-6 torr, outgassed and freed from small particles of selenium by heating the glass with an air-gas flame. A steam jacket was placed over the end of the tube containing the selenium. When steam was passed through the jacket the pressure recorded by the ionization gauge rose to 5 x 10-6 torr and then gradually dropped to 2.5 x 10-7 torr after about three hours. Within half an hour, a white deposit was observed just outside that part of the tube covered by the steam jacket. After three hours the deposit was opaque; in addition, a red deposit having every appearance of selenium had formed under the cork(ne:.12a); The white deposit was separated by cutting the glass tube. Since some of the red deposit extended into the region of the white deposit a chemical test for sulphur in the presence of selenium was carried out: the sample was extracted by boiling 122.

Figure 12a. The deposits in the tube resulted from the heating of a sample of selenium for 3 hours in a high vacuum with the steam jacket shown. The red deposit on the left formed in that part of the tube covered by the cork, the white depOsit on the right formed just outside the cork.

Figure 12b. The white deposit obtained by heating sulphur for the same time and under the same conditions as above. 123.

with pure pyridine which was filtered and allowed to cool to room temperature. Then one tenth of its volume of twice normal sodium hydroxide solution was added (80 gm/litre) and the mixture was shaken. The test gave the blue colouration with a small quantity of sulphur (an olive-green or red-brown colouration is also possible), and remained colourless when a blank test with the pyridine was carried out. When the white deposit was tested, the small amount of selenium present coloured the pyridine a pale yellow, and while it is thought that the colour chaAged to a yellow-green the experiment was not convincing. A physical test was devised in which some sulphur was heated in the same apparatus and under the same conditions that had produced the deposits from the selenium. A white deposit formed in the same place and after the same time interval as the previous white deposit (figure 12b). If this white deposit was not sulphur it must have been a substance with nearly the same vapour pressure, molecular weight and colour. This test suggested a method for reducing the sulphur content in the Johnson Matthey spectroscopically pure selenium even though the amount of sulphur present would be too small to produce a visible deposit. Some of this selenium (J.M. 781) was heated,to 100°C in a high vacuum for a total of 16 hours. No white deposit was observed but 124.

eventually the selenium deposit extended beyond the region where the white deposit might have been expected to appear. The selenium remaining was used to prepare a photoconductive layer which was tested in the demountable tube and gave a time lag of about 1/25th second. In view of the importance of this result the experiMent was repeated several times with freshly prepared targets and the time lag was similar in each case. When the flood electron gun was replaced by a gun giving a narrow beam of electrons and focus and scanning coils used to obtain a television signal, the tailing of a rapidly moving high contrast object was just visible on the two half frames behind the main image. The " effective sensitivity of a 10 micron thick layer of this selenium was about the same as previously observed for a 10 micron thick layer because it was found possible to operate the layer with a potential difference of over 100 volts before the background of bright spots became significant. This is explained if both the bright spots and the photoconductive time-lag are due to the presence of a small quantity of sulphur in the layer. Heating the layer increases the time-lag and like sulphur accelerates the crystallization of amorphous selenium. Thus the time-lag may be due to trapping states provided by crystalline selenim, However, removal of the sulphur from a sample of selenium is

125. sufficient to reduce its time—lag under the conditions of tube processing.

Sensitivity and Noise Performance.

The root—mean—square noise current due to the head amplifier was approximately 1.6 x 10-9 amps (page 85 ). The root—mean—square noise current due to statistical fluctuations in the X--ray photocurrent is obtained as follows: Suppose that n X—quanta are absorbed by unit area of the target per second, and each quantum encounters a large and fixed number p electrons, when there is a small chance q that an electron will be excited and a small chance r that conduction through the layer will result. If the number of electrons encountered is x = np, and = rip where the bar denotes time average, if y the number of electrons excited y = qx = npq, and y = npq, if the number of charges giving rise to conduction is z = npqr and B. = Epqr, then if the mean—square—deviations are equal to the mean values,

/ \ 2 — A x)2 = ( A n) = n

(A y)2 =

(A z)2 =

126.

and if the fluctuations due to the probabilities p, q, r are not correlated, the total mean square fluctuation in the number of charges conducted through the layer will be

N2 2 k Aztotal/ = ( Az1) "27 + ( Lz3)3 + where ( Az1 )2 = 7 = pqr

2y = Epqr2 ( z2)e = (r 4y)2 = r

( Az3)2 = (qr Ax)2 = er251 = Eper

( Az4)2 = (pqr An)2 = iipyr2

2 ( Aztotal)= Epqr(pqr + qr + r + 1) If every X-ray quantum in the selenium gives on the average a electrons, then pqr = a ; and if q << 1 and r << 1.

2 ( Aztotal) 4---6- R a( d + )) (de Haan, 1960)

Mean values of a derived from the X-ray photocurrent measurements (figure 8) are given in table 15. In the X-ray vidicon the charge image on the target is read off by a scanning beam of electrons with a frame time of to and if the number of picture elements is p, the scanning time T for each picture element is to 1 = T = — p 2f 127. where f is the highest frequency in the signal current. Let the number of quanta absorbed by one picture element between two scans be no. Then the average signal current

s with uniform irradiation of the target is - ct noe Is =

where e is the electronic charge. The mean squared fluc— tuation ( 1is)2 is given by e2 ( pis) = no of o 1) = 2efis( a + 1)

Values of p/ ( Ais)2 are given in table 15 for a video bandwidth of 3 Mc and for values of is corresponding to an X—ray intensity of 1 r/min at the photoconductive layer. The photoconductive time—lag of 2 secs (50 frames) reduced the observed quantum noise by a factor of 4!TO below that given in' table- 15. Thus the quantum noise would have been noticable only with the fast response layers in the demountable tube. For the other tubes, the quantum noise was masked by the head amplifier noise. The ratio of the signal to the combined photocurrent and amplifier noise in table 15 shows that even if the time—lag is removed, the tubesare only of use where a high dose—rate is allowed. The photocurrent would be increased by increasing the area of the target, but then the amplifier 128.

Table 15.

Tube No: 18 19 20 25 27 Layer thickness in microns 2.4 10 30 11 74- % X-ray absorption 3 13 34 14 63 (65 kVp) 4 Cr, the approx. number of charge carriers pro- . duced by one 500 250 50 375 25 absorbed X- quantum (65 kVp) Calculated r.m.s. noise in X-ray photocurrent. 1.4 1.9 0.8.- 3.8 0.3 i . N 2 •(‘-415) amPs. x 10-9 x 10- x 10- x 10-9 x 10-9 (calculated for no time lag). Signpl to;X-ray photocurrent ' noise ratio for 1 r/min at layer 4 7 10 4 (calculated for no time lag). i Ratio of signal to photocurrent and amplifier 1.3 3 3 9 0.7 noise for 1 r/min at layer (calculated for no time lag). r 129. noise would increase in proportion to the area duesto additional input capacitance. Increasing the thickness of the photoconductive layer would increase the signal with respect to both the amplifier noise and the quantum noise because of the increased X-ray absorption, providing that the additional charge carriers so produced could be extracted. The X-ray photocurrent is proportional to the voltage across the photoconductive layer (figure 9 ); raising this voltage could be used to improve the charge extraction, providing that the background of white spots was suppressed without loss of sensitivity. The fast response layers in the demountable tube had less sensitivity for a given electric field in the layers but higher field strengths could be used before the background of white spots appeared. Thus there appeared to be no het loss in performance, but neither was there any prospect of improvement. 130.

Section 5. The Intensification of Faint Light Images from

X-r ...21uorescent Screen.

Change of Project.

This failure to produce a photoconductive layer having a high X-ray absorption, from which the charge extraction was efficient; an appreciation of the high value of the X-ray absorption obtainable with fluorescent screens; and reduction in the background brightness of image intensifier tubes (McGee, 1956),led to the abandonment of the X-ray sensitive photoconductive camera tube in favour of a system in which the faint light image from an X-ray fluorescent screen is intensified to the level at which a television signal can be generated. The background brightness of image intensifier tubes is largely due to field emission which is enhanced by the lowering of the work function of surfaces in the tube by the action of the caesium vapour used to activate the photocathode. It can be reduced by processing the photocathode on a plate in an end compartment of the tube; after the tube is sealed off from the pumps, the plate is reversed so that the photocathode faces the main section of the tube. The plate and the shelf are not gas tight but form an effective barrier to the passage of caesium vapour (McGee, 1961). 131.

The Intensifying and Producing of Television Signals from

the Weak Light ImagesiL,..enLloi/ an X—ray Fluorescent Screen.

If a lens is used to focus light emitted according to Lambert's Cosine Law from a fluorescent screen, approXimately twice the light will reach the image compared with that which would reach it if the emission was isotropic (for practical lens apertures). Considering a source which emits light isotropically, the fraction a of the usefully emitted light which reaches the focal plane of a lens of relative aperture F when the magnification of the system is M and the transmission coefficient of the lens is c (about 80%), is given by M2F2e a 8(1+M)2 and for Lambertian emission the fraction A will be double the above value of a . Lenses designed for use with 35 mm cameras are available with relative apertures of F/1.5. Copying lenses of relative aperture F/1.0 are available corrected for fixed magnifi— cations. There is an F/1.5 lens of focal length 32n used with television camera (Hay, 1958) and an F/ 0.71 lens which can be corrected for use either with an object at infinity or with a 12" x 15" fluoroscopic screen (Wynne, 132. 1951). In the latter case the resolution is reported to be 28 line pairs per mm over the 24 mm diameter field, and 80 line pairs per mm at the centre of the field. If the F/0.71 lens is used so that the diagonal of a 12" x 15" fluoroscopic screen fills the image field, the magnification is approximately 1/20 and the fraction of the light reaching the focal plane is only 1/1100. If the fluoroscopic screen size is kept the same and the F/1.5 lens is used with a 4" diameter image field, the magnification is 1/5 and the fraction of the light reaching the focal plane is 1/400. In radiological practice the whole area of the screen is examined for a short time, and is usually followed by the detailed examination of a smaller area with the X-ray beam screened from unwanted regions by adjustable lead shields. Under these conditions the F/1.5 lens at unit magnification would allow 1/45 of the light to reach the 4" diameter image field. Making use of available techniques it is possible to obtain 5000 light quanta from an X-ray quantum absorbed in a fluoroscopic screen, which can have an X-r•ay absorption of 80%. If these light quanta are focused with an F/1.5 optical system at unit magnification, 110 would reach the image which; if formed on a multi-alkali photocathode (Sommer, 1956), would emit 34 photoelectrons. Considering 133, a single stage image intensifier which has an aluminium backed phosphor screen of 25% intrinsic efficiency emitting 66% of the light in the output direction with a Lambertian distribution, the mean number of useful photons from the output phosphor for each incident 20 keV electron is about 1300. Hence each absorbed X—ray quantum would produce 44,000 photons from the output phosphor. With the output phosphor mounted on a thin transparent support and the target of a photoconductive camera tube section formed on the reverse side, each X—ray quantum absorbed in the fluoroscopic acreen would produce approximately 6000 electron charges in the corresponding picture element of a photoconductive layer of porous antimony trisulphide having a sensitivity of 100µAiltunen (Cope, 1956). A porous layer is made by evaporating the material in an inert gas at a pressure of a few torr and gives a lower capacity target which results in less capacitative time lag. The amplifier r.m.s. noise current is of the."order of 10-9 amps or 1000 electron charges per picture element, so that, with 6000 electron charges per absorbed X—quantum, a single stage of image intensification in front of a photoconductive camera tube would reach the limit set by X—ray quantum noise. However, the processes must be

134.

considered statistically, and to determine the signal—to— noise ratios of such systems the following result by Mandel (1959) is used to give a more general treatment than that due to de Haan (1960) who arrives at similar conclusions in respect of the importance of high X—ray absorption in the primary detector. In any interaction in which a primary particle gives rise to X secondary particles where X is a stochastic variate, the number of secondaries 4 produced by n incident particles will be such that

and (A4 )2 = 2 (Mandel, 1959) X .(611) 1-1.(AX )2 where a bar is used to denote the average and A the deviation from the mean. Consider the system in which n X—ray quanta are incident on a fluorescent screen which absorbs p X—ray quanta to emit q light photons; let these light photons be focused by a lens on to a photocathode at which r photons arrive to produce s photoelectrons. If the gain after this point is high, the fluctuations will not be appreciably increased. If the fraction of the X—rays absorbed by the fluorescent screen is a, the number of photons

135a producc,d by one X—ray quantum is b, the fraction of the light reaching the photocathode is c, the quantum sensitivity of the photocathode is d, then

(A p)2 = a2(A n)2 H(A a)2

= 101-3

r = eq r)2 = c2(ti a)2 q( b, 02

3= (IF

(A s )2 = d2(A r)2 + r( Ad) F = abcdii

(A s)2 = a2b2c2d2(A n)2 + c2c2d2a717

+ ac2d2E(A b)2 + abd2E(A c)2

+ abcii(46 d)2

Further it will be assumed that the incident X—ray quanta and the photons from the screen obey a Poisson distribution:

(A n)2 = E and (A b)2 = b

For the other distributions, it will be assumed, following Mandel (1959), that

136,

(A a)2 , a(1 - a) (A 02 = c(1 —

(A d)2 = d(1 d) which apply to first order interactions where one primary particle gives rise to not more than one secondary particle. From the above

(A s)2 = ab2 c2d21.1 + abcdE

s.2 4. 1 1 = Jn.a .bcd. ' bed and the signal-to-noise ratio is given by

4/ s)2

+ bed j1 + bcd1

Thus so long as the factor (bcd) is large compared with unity, the signal-to-noise ratio is determined by the X-ray quanta and the absorption of the fluoroscopic screen. Thus, signal-to-noise ratio is ET, for (bcd) >> 1. In the design of a complete system, the fluoroscopic screen should be chosen to have the highest practicable X-ray absorption since this will determine the values of a and b, while the values of the quantities c and d should be a practical compromise.

137.

The effect of the cfuantities a, b, c, d on the signal—to--noise ratio is shown in the following table 16 and notes.

Table 16.

X—ray Photons per Light Efficiency Signal to absorption X—ray quad= reaching noise in screen absorbed photo— ratio cathode a b d

1. 80% 5000 2% 30% 0.88 2. 8o% 5000 2% 10% 0.85 3.. 80% 5000 0.2% 30% 0.78 4. 80% 500 2% 30% 0.78 E2 5. 80% 500 0.2% 30% 0.43 E,2 1 6. 20% 5000 2% 30% 0.44 EP 1 7. ' 24% 500 100% 10% 0.49 E 2

1. Represents the best performance which might be expected, from an image intensifier tube with a multi—alkali photocathode using an f/1.5 lens at unit magnification to form an image on the photocathode of the image on the fluoroscopic screen. 2. Represents the use of an Sb—Cs photocathode instead of a multi—alkali photocathode. 138.

3. The television of the whole of a 12" x 15" screen, using an intensifier with 4" photocathode instead of operating at unit magnification. 4. The use of a screen with lower efficiency. 5. The effect of 3 and 4 together. 6. A screen having a lower X—ray absorption so as to give about the same signal—to—noise ratio as the previous combination. 7. A Philips image intensifier tube for which Ardran reported the effect of quantum noise (Ardran, 1956).

Image—Vidicon.

The diagram of a tube based upon the above calculations is shown in figure 13 and is referred to as an Image—Vidicon. Light from an external fluoroscopic screen is focused with a wide aperture optical system on to the first photocathode 1 from which the photoelectrons are accelerated by a uniform electric field to an energy of 20 keV and focused by an axial magnetic field on to a phosphor screen 3 with a reflecting backing of aluminium. The phosphor is settled on a thin sheet of mica 4 and the photoconductive layer is formed on the reverse side of the mica on the transparent conducting coating of tin oxide 5 to which the amplifier is connected SCALE I/2 FIGURE 13. PAGE 139

FLYING SPOT SCANNER 11. & LENS 10. CNOT TO SCALE).

END PLATE 12. Cs &Sb SIDE ARM & PUMPI NG STEM. Cs TRAP 14. TURN-OVER PHOTOCATHODE PRECISION ASSEMBLY, THIC K BORE 13. PHOTOCATHODE 7.

SPIRAL ELECTRODES 2

(ACCELERATING) MESH 8.

SHELF & REVERSAL (DECELERATING) MECHAN ISM 9. STAINLESS STEEL FRAME TO SUPPORT LAYERS OF : PHOTOCONDUCTOR 6. "NESA- 5 . SPIRAL MI CA 4. ACCELERATING ALUMINISED PHOSPHOR 3 ELECTRODE 2.

PUMPING STEM

PRECIS! 0 N BORE 13. TURN-OVER PHOTO CATHODF ASSEMBLY I.

Cs TRAP 14.

Cs & Sb SIDE ARM. & PUMPING STEM. ci

IMAGE VI DICO END PLATE 12. 140. by way of the sunport ring 9. The free surface of the layer is cathode potential stabilized by the scanning beam of low energy electrons which generates the television signal. A flying spot scanning system was incorporated as a means of obtaining a more efficient discharge of the target to reduce the capacitative time lag. The beam of electrons produced by scanning a photocathode with a beam of light does not reach the target with the tangential energy imparted by the deflection.processes (Lubszynski 1936). The beam in the cathode ray tube 11 is scanned to produce a 405 line raster on its screen of short persistence blue phosphor ("gehleniten), the light from which is focused by lens 10 to form an image of the raster on the photocathode. This photocathode 7 is formed with a slightly thicker layer of antimony than is usual so that although the photosensitivity is reduced, the light penetrating this photocathode is insufficient to affect the photoconductive layer 6 beyond. The photoelectrons produced from 7 are accelerated by a uniform electric field so that the free surface of the photoconductive layer 6 is scanned orthogonally by low energy electrons which stabilize it at the potential of the photocathode 7. The camera unit described on pages 80-90 was modified for use with the Image Vidicon as follows: the scanning 141. coils were removed and replaced by a solenoid of inside diameter 4" and length 10"; a glass cylinder inside the solenoid provided electrical insulation between the solenoid and the.Image Vidicon tube; an aluminium cooling fin was inserted between each of the 10 coils from which the solenoid was assembled. A partly completed flying—spot scanner chassis was available for use, but since its line time—base circuit gave a very non—linear waveform, it was replaced by the circuit from the camera chassis; the deflection coils were re—wound to match the impedance of the output transformer. dimciasionr,; The diomsnions of the Image—Vidicon were chosen to make use of existing parts and facilities so as to integrate this work with that of the rest of the Department. The tube had a sensitive area of 30 x 40 mm, so that for a 405 line picture a line pair separated by 0.074 mm had to be resolved. With a solenoidal magnetic field for focusing, the disc of confusion should have a diameter given by d 1.611V 0 (Slack 1958) where V is the potential corresponding to maximum emission energy of the electrons, about 1 eV for a photocathode, L is the distance from photocathode to screen, and 0 is the accel— erating potential. 142.

The minimum value for the electric field 0/L which can be used if the resolution is not to be worse than 0.074 mm is 22,000 volts per metre. The magnetic field B required for an image tube focused by a long solenoidal field is determined by the condition that the transit time for an electron between the photocathode and the screen shall be integrally related to its period of orbit in the magnetic field. For a long solenoid. B = 4 nni.10-7 weber/metre2 where n is the number of turns per metre, i is the current in amps flowing through the solenoid. Equating the transit time to the period of orbit of the electrons gives

21, 2 nm = k eB 492 whence L - k Tc (2)2 where k is the number of orbits made in the L metres between the photocathode and the screen. For a tube 0.15 metres, long, the. minimum value. of 0 is 3,300 volts, taking the minimum value of °A, as 22,000 volts/metre. Focus conditions will be realised with k = 3 for accelerating potentials between 3.3 kV and 33 kV and magnetic fields between 0.012 and 0.038 wb/m2 (120 to 380 gauss). Since 0.05 metres of the tube is required for operating the photocathode 143. turn—over mechanism the lengths of the image section and the camera section are conveniently made in the ratio 3:2 so that each section can be focused with the same magnetic field. Fine control of.,the focus would be obtained by a separate adjustment of the potentials for each section. The Image Vidicon tube was constructed from 58 mm inside diameter pyrex glass tube. The glass in the region of the skirts on the photocathode shelves 13 was tooled down on to a mandrel to a precision bore. The stainless steel discs for 1, 7, 8 and 9 were clipped to tungsten pins sealed into the wall of the tube, the electrode connections were made through platinum tape seals (Davis, 1958). The skirts on the photocathode shelves were coated with colloidal graphite 14 to absorb caesium and prevent it from entering the high voltage part of the tube from the photocathode processing compartment. The phosphor 3 was the blue emitting silver activated zinc sulphide. The optimum coating density for viewing from the opposite side to that on which the electrons are incident depends upon the phosphor particle size and upon the electron energy which determines the penetration, table 17. 144,

Table 17. (Leverenz, 1950, p.444)

Average particle Electron energy Optimum coating size for ZnS density

3 microns 17 keV 1.4 mg/cm2 3 47 3.6 8 17 5.0 8 50 10.0

A suspension of the phosphor was made up in N/100 magnesium sulphate (Analar) in Analar water, to which some hydrogen sulphide had been added. This provided some S-- ions to reduce dissociation of the ZnS phosphor and so prevent its aggregation (Leverenz, 1947). The concentration of phosphor in the suspension was 1 mg/cm3 so that the level of the liquid in cm would give the number of mg/cm2 of the resulting screen. The cleaned surface on which the screen was to be settled was immersed below about 1 cm of 11/100 MgSO4 (analar) in analar water in a beaker. The requisite amount of the phosphor suspension was added and allowed to settle. The beaker -was tilted so that the screen made an angle of o between 5o and 10 with the horizontal. This made the liquid run off the phosphor with approximately zero angle 145,

of contact, thereby reducing the chapce of the retreating line of liquid disturbing the phosphor. The liquid was syphoned off using a glass capillary about 1 mm inside diameter with a difference in height of 2 cm between the twc limbs= When the liquid had been completely removed the whole beaker was placed in a desiccator over silica gel until the screen was dry. Drying the screens at room temperature improved the adhesion of the phosphor. The screen could be safely moved either if there was at least a few cm of liquid covering it or if the surface was no more than damp. After the screen had dried, it was baked for one hour at a temperature between 200° — 300°. After this the screen was moistened by holding over steam and then carefully covered with N/100 magnesium sulphate in analar water. One drop of a filming solution (nitrocellulose in a suitable solvent) was placed on the surface of the liquid from the point of a glass rod. The thickness of the film was such that a yellow interference colour was seen as the solvent evaporated. When the beaker was very clean, and the sides were wet, the edges of the film would rise as much as 1" above the level of the liquid, pushing any scum of phosphor particles clear of the screen. The screen was tilted, the liquid was syphoned off as before, and the screen was dried in a desiccator over 146.

silica gel. It was then placed under the bell jar on the demountable pump and the film coated with an evaporated layer of aluminium (Strong, 1948). The thickness of aluminium was determined by evaporating a known weight from measured distance. Bril and Kiasens (1952) give the optimum. thickness of an aluminium backing as 0.02 to 0.08 microns for 10 keV electrons and 0.1 micron for 20 keV electrons. After this, the film of nitrocellulose was removed by heating in air at 350°C for an hour. If during the final sealing of the tube the screen was also liable to become heated, nitrogen gas was passed through the tube to prevent the aluminium backing from being oxidized. The glass plates were sealed to the ends of the assembled tube with silver chloride which melts at 450°C. It is soluble in an aqueous solution of sodium thiosulphate which may be used tore—open joints. The surfaces to be joined with silver chloride were either ground and polished, or else the original edges of a clean cut. All surfaces of the joint were coated with platinum paint to protect the silver chloride from the action of light. Pyrex glass end plates were sealed to the end of a tube by heating the plate on an electric boiling ring, with the plate standing on a clean piece of mica. Small pieces of silver chloride were stuck to the point of a glass rod and applied to the joint when the temperature of the 147. glass was above 450°C To obtain a vacuum tight seal, the point of a glass rod was run around the joint while the silver chloride was molten. The completed seal was coated with picein wax to protect from water vapour and light. Tubing was joined, end-to-end by heating the join with a carbon ring heated by a radiofrequency heater. The carbon ring was less than i cm away from the outside of the joint. The tube was sealed on to the pump and evacuated immediately after assembly. As soon as the pressure was better than 10-4 torr the oven was switched on, the tube was baked for as long as possible at the highest temperature permitted by the glass, after which it was sealed off the pumps. When required for processing, the side arms with the nickel capsule containing pellets of caesium chromate and zirconium and the antimony evaporator were sealed to the tube and the assembly was sealed on to the pump.. It was evacuated and baked for a day at a temperature not greater than 285°C to avoid evaporating the antimony. The metal parts were radio-frequency heated to redness after which a further short bake at about 10000 was given. The caesium was produced by strongly heating the capsule with the radio-frequency heater, driven into the side tube and the bulb with the spent capsule was sealed off. 148. The antimony was evaporated using the radio frequency heater to give a layer with about 40% optical transmission, either from a small piece of antimony inside a coil of tungsten wire or from a layer of antimony evaporated on to a 'VI section strip of molybdenum in a side tube immediately beforehand. The latter technique gave a large area source which enabled the evaporator to be placed close to the photocathode plate and still give a uniform layer. A lamp was arranged to give a spot of light of intensity 0.1 lumen on the photocathode plate. A heating coil was placed over the caesium in the side arm. A multi-range galvanometer (0.2 to 125 microamps f.s.d) was connected in series with the photocathode, a 1 megohm limiting resistor, a potential sufficient to give saturated photoemission 100 - 300 volts, and an anode. The oven was lowered and the heater bars switched on to a suitable tapping on the transformer to take the temperature slowly up to 145°C. Because of the vacuum on each side of the photocathode plate control of the temperature was less accurate and the usual photocathode processing procedure modified. Photo- cathode sensitivity began to appear at about 110°C, at 145°C the caesium side-arm heater coil was switched on and the current adjusted until the sensitivity of the photocathode passed through a maximum and dropped to 50% of that maximum. 149.

The oven was switched off and allowed to cool to 90oCI while caesium heater clrrent was adjusted to keep the photosensitivity constant at 50% of the maximum value attained. The caesium heater was switched off when the oven temperature reached 90°C, but the photocathode was at a higher temperature so caesium was lost to the walls of the processing compartment and as the tube cooled to room temp— erature the sensitivity increased. The photocathode was then treated with oxygen to give a further increase in sensitivity, after which the tube was sealed off the pumps. The photoconductive layer of porous antimony trisulphide was prepared by evaporation in an inert gas at 1.25 torr. The saturation vapour pressure of xenon is about 10-1 torr at the temperature of the liquid nitrogen cold trap and the use of a solid carbon dioxide cold trap resulted in loss of photocathode sensitivity. Argon was used as it has a saturation vapour pressure of 380 torr at the temperature of the liquid nitrogen trap; it was introduced into the system by breaking off the glass point of a sealed capsule containing the appropriate volume of pure argon at about atmospheric pressure. Three tubes were made in which only the Vidicon section was assembled and a television picture of a light image 150.

projected on to the photoconductive target was obtained, the layers in the other two tubes were not photosensitive. More sensitive photocathodes were obtained by processing after the antimony trisulphide photoconductive layer had been formed; the latter was kept cool during the photocathode processing. The method was to wrap rubber tubing around the tube, apart from the photocathode processing compartments. Cold water was passed through the rubber tubing while the oven was hot. A layer of aluminium foil protected the rubber from radiation from the heating elements. It became clear that the combination of an image intensifier and a vidicon in the same envelope would involve further development of techniques and that this would be best done separately. The development of the image intensifier section was based on an attempt to increase the gain by raising the accelerating potential and to overcome the background glow of the output phosphor. For single stage image intensifiers the accelerating potential was limited to about 20 kV by the appearance of the background glow on the output phosphor even though a turn-over photocathode was used. In an image intensifier tube the accelerating field is usually provided by a number of conducting annular rings along the axis of the tube, these are insulated from each other and are connected to an 151.

external potential dividing network. Increasing the number of rings results in a more uniform electric field which in turn gives better resolution and geometry to the image. A limit is set by closeness with which the seals through the glass envelope can be placed. One way of overcoming this limitation is to replace the electrodes by a high resistance spiral connected between cathode and anode. If the spiral can be allowed to dissipate 1 watt then with the tube operating at 10 kV a resistance not less than 108 ohms is required; for a spiral of 100 turns with each turn containing 100 squares of resistance track, the coating should have a resistance of not less than 10,000 ohms per square. The first material tried was a conducting layer of tin oxidelE prepared by vapour reaction at the glass surface (Holland, 1956). The resistivity could be varied by adjusting the thickness and composition of the layer. It was found to be possible to apply the layer as a spiral by using colloidal graphite as a mask which was afterwards washed off with detergent and water. The spiral of colloidal graphite was applied by means of a screw cutting lathe which had been modified to hold glass

Footnote: This is often referred to as "nesa". 152. tubing from 1" to 3" diameter. The colloidal graphite was contained in a cup which was spring mounted so that the point of a hollow needle would remain in contact with the glass. It was attached to a long arm coupled to the lead screw of the lathe which could be inserted into the glass tubing. A small rubber balloon tied over the cup gave the slight excess pressure needed for a steady flow of the mixture down the needle. Careful adjutment was needed to prevent vibration as the needle passed over die marks along the length of the glass tube, but a continuous 6" to 8" length of spiral could be drawn, the pitch of the spiral could be made as small as 1/32". The effect of caesium vapour on the tin oxide coatings was investigated. Coatings were prepared which had resis- tances between 50 ohms/square and 100,000 ohms/square. After exposure to caesium vapour at 145oC they were all approximately 1,000 ohms/square. There was no further change of resistance upon reheating so long as the temperature remained below 200°C. Above this temperature the resistance became too high to measure after the tubes had cooled to room temperature. The use of tin oxide for the spiral accelerating electrodes in image tubes is not satisfactory owing to the 153.

difficUlty in obtaining a uniform coating on the inside of the tube which has a sufficiently high resistance, yet is not open circuit. A more satisfactory material for high resistance spiral electrodes is a mixture of colloidal graphite and hydrolyzed ethyl silicate. This gives a hard coating which is not easily scratched during tube assembly. Hydrolysis of Ethyl Silicate.

Mix 3 ml of analar water with 5 ml of absolute alcohol. Add 2 drops of concentrated hydrochloric acid (analar). Mix in 20 ml of ethyl silicate and stir continuously until the temperature, which should be continuously observed, begins to fall, indicating that hydrolysis is completed. With these quantities the maximum temperature reached was found to be in the region of 30°0 — 40°C beginning at a room temperature of 20°C. The hydrolyzed ethyl silicate should be allowed to reach room temperature before use and may be kept for 24 hours after which it is liable to solidify. Ethyl silicate does not mix with water but is miscible with alcohol and can be hydrolysed with a mixture of water and alcohol (adapted from Monsanto, 1958). The resistance can be varied from 105 to 109 ohms per square by altering the proportion of colloidal graphite in 154.

alcohol from 75% to 35% by weight. The dry coating has to be stabilized by a bake for one hour at 400°0 either in air or in vacuum. Spiral accelerating electrodes were tried out in the single stage image intensifier tubes shown in figure 14. Pyrex glass tubing of 58 mm inside diameter with the ends ground and polished, and Pyrex glass end plates were used. The photocathode assembly 1 was fitted into the precision- bore section 2 after the application of a graphite coating to form the caesium trap. The spiral accelerating electrode 4 was applied between the photocathode 1 and the aluminium backed phosphor screen 5. The final seal was made with silver chloride. The tubes were pumped and baked, the metal parts were radio-frequency heated, the photocathodes were processed and the tubes were sealed off the pumps. Where they were tested in a solenoidal magnetic field, a bright background was observed on the phosphor for accelerating potentials greater than 8 to 10 kV. This background was not due to gas in the tubes. An image could be obtained with potentials up to about 10 kV, above this potential the background became brighter than the image; although the background increased in brightness, the tubes did not break down When 50 kV was applied. To exclude the possibility that caesium had leaked past the photocathode shelf, tubes were FIGURE 14. SCALE 1/2 PAGE 155

FINAL SEAL SPIRAL ACCELERATING C SILVER CHLORIDE) 6. ELECTRODE 4,

PHOTOCATHODE I. PRECISION BORE 2 •

Cs t Sb SIDE ARMS

ANODE FOR ACTIVATION PHOSPHOR SCREEN

OF PHOTO CATHODE. 3. BACKED WIT H ALUMINIUM 5.

SINGLE STAGE IMAGE INTENSIFIER 156, assembled, pumped, baked and sealed-off with no caesium present. These gave a similar background for an accelerating potential of 8 to 10 kV when examined in s solenoidal magnetic field. Remembering that the wall electrodes of the X-ray sensitive photoconductive camera tubes functioned although covered with a layer of selenium, three tubes were made as shown in figure 14 except that spiral electrode 4 on the inside wall was replaced by six equally spaced ring electrodes of platinum on the outside wall of the tube. The inside wall was carefully cleaned and the tubes were pumped,, baked and sealed off with no caesium inside. The external electrodes were connected to a potential dividing chain of resistors and the tubes were tested in a solenoidal magnetic field. The background was observed at the same potential as before. It was found that the background could be reduced by immersing the high voltage end of the tube in oil to suppress corona discharges anithat the background was less when the cathode end of the tube was at earth potential, with the screen at the high positive potential. Some of the background was thus due to corona discharges outside the tube at the cathode end. To improve the control of the external electrodes over the potential of the inside wall, a series of tubes was constructed from lime-soda glass 157. instead of pyrex glass; the conductivity of lime—soda glass is about 1000 times that of pyrex. Three tubes 6" long were made from 1" diameter lime— soda glass tubing, a ring electrode of platinum paint was applied to the inside wall at each end and contact was made through platinum tape seals. The tubes contained neither photocathode nor phosphor, they were pumped, baked at 400°C, and sealed-•off. When they were immersed in oil and a high voltage, but no magnetic field, was applied, the glass walls began to show a slight fluorescence at 30 kV, visible only to the dark adapted eye. The first 2" of the tube from the cathode end showed no fluorescence. A spiral electrode of the colloidal graphite—hydrolized ethyl silicate mixture was applied to the outside of one of these soda glass tubes, the resistance of the spiral was 5 x 109 ohms. When the tube was tested in the dark room, no fluorescence of the glass was visible at potentials up to the limit of the power supply, 50 kV, in the absence of an applied magnetic field, although previously the same tube had shown fluorescence of the glass walls at 30 kV with no magnetic field. Four tubes were made as shown in figure 15. The two pieces of tubing 1 were G.E.C. glass No. X8 (expansion coefficient 9.65 x 10-6/°C), the plates 2,4 were cut from FIGURE 15. SCALE 1 / 2 PAGE 158

X8. GLASS TUBING I.

ALUMINISED PHOSPHOR SC REEN, 3.

END PLATE.2, END PLATE. 2.

ACTIVATION ANODE. 7.

SEALED-OFF TUBE SPIRAL ACCELERATING TO PUMP, Sb EVAPORATOR ELEC TRODE APPL I ED & Cs SIDE ARM. TO OUTSIDE WALL AFTER SEALING-OFF. 6.

PHOTOCATHODE PLATE . 5 . PHOTOCATHODE SHELF. 4. CA HOOK OF IRON WIRE SEPARATED\ PLATE FROM SHELF UNTIL TUBE HAD BEEN BAKED, IT WAS THEN REMOVED THROUGH THE PUMPING STEM.)

IMAGE INTENSIFIER TUBE -SODA GLASS. 159.

Schott White Spectacle Crown Glass (expansion coefficient 9.55 x 10-6/°C). The photocathode plate 4 also had a central hole cut in it (Strong, 1948). The tubes were sealed to the plates with C93 glass which is a soft lead glass available in powder form. The X8 glass has the maximum of the expansion curve at 550°C * 10°C and an annealing range from 520°C - 400°C. The corresponding temperatures for C93 glass were about 100°C lower. The tube ends were ground flat and heated in a gas flame until they were flame polished, then they were pressed, while hot, on to powdered C93 glass spread thinly over a sheet of paper. Some of the powder adhered to the end of the tube; this was fused by further heating in the flame, after which the tube was annealed. The plates were painted with a ring of bright platinum where electrical contact was required under the seal. The parts were assembled vertically in an electric oven so that their weight would press the joints together. The temperature was raised slowly to the upper annealing point of the glass and then rapidly raised to 650°C so that the C93 glass flowed and made a good seal before the X8 had time to distort. The temperature was quickly lowered to the upper annealing point of the X8 a British Thompson Houston Ltd., Rugby. N*General Electric Co. Ltd., Wembley. 160.

glass and the full annealing schedule was carried out:- 52000 to 460°C at 1°Cblin, 460°C to 400°C at 2°C/Min. After pumping, baking, processing the photocathode and sealing the tube off the pump, the spiral accelerating electrode 6 was applied to the outside of the tube with the mixture of colloidal graphite and hydrolized ethyl silicate. When the tubes were examined in a uniform magnetic field with dark adapted vision, the bright background could just be discerned for an accelerating potential of 8 kV. One tube, which had a photocathode giving 15 microamps per lumen, gave a gain of 8.3 at a potential of 10 kV. This was measured using a calibrated selenium cell to compare the intensity of a small spot of blue light projected on to the photocathode with the increase in the light intensity from the blue emitting silver activated zinc sulphide phosphor. An image could be obtained for accelerating potentials of 25 kV, but at higher potentials the background became brighter than the image on the output phosphor; the geometry of the image was good, apart from some defects due to a poor aluminium backing on the phosphor. Two tubes similar to the above were made in which there was no caesium, a coating of platinum was used to give a conducting surface to the photocathode plate. The brightness of the background was measured with a selenium cell for 161.

various positions of a short magnetic lens to determine whether the background was increased by the application of a magnetic field to a particular part of the tube. There was least ' background when there was no applied magnetic field, the background brightness was greatest when the magnetic field was applied to the cathode end of the tube as shown in figure 16 . When this tube was examined in the long solenoid the background brightness at 5 kV with a magnetic field was the same as the background brightness at 15 kV with no magnetic field. If any electrons strike the walls of the tube with sufficient energy, they will produce positive ions from the glass which will not be appreciably deflected by the magnetic field and will travel back to the cathode plate where they produce more electrons which with the secondary electrons produced at the glass walls will be accelerated to the screen, resulting in the bright background glow observed. The spiral electrode on the out$ide of the tube reduces the number of electrons striking the walls of the tube by improving the linearity of the electric field, but the addition of the solenoidal magnetic field increases the number of electrons striking the walls of the tube (McGee, 1961). FIGURE 16. PAGE 162

SCREEN BRIGHT4ESS (ARBI TPARY SCALE) 3

- -... 1 4.0" ma."

SP 1 RAL ACCELER ATI N G ALL MIN I SED ELECTRODE ON OUTSIDE PH C SPHOR SC RE EN OF TUBE. RES I STANCE 7.5 x101 OHMS.

f -10 KV.

CATI- O DE- END- P LATE COATE D SELENIU P CELL & WITH PLATINU M GALVA NO MET ER.

• a -5 0 5 10 15 CM, POSITION OF A SHORT MAGNETIC LENS RELATIVE TO THE CATHODE OF THE ABOVE TUBE. EFFECT OF MAGNETIC FIELD ON PHOS PH OR BACKGROUND. 163. Recent Developments Applicable to X—ray Image Intensification.

An image orthicon preceeded by image intensifier stages is described by Morton and Reudy (1960). X—ray quantum noise is reported with one stage of intensification in front of the image orthicon section when a good optical system is used. Cascade image intensifiers comprising a succession of phosphor—photocathode stages in optical contact can produce an image of a single primary photoelectron bright enough to be photographed (Zavoiskii, 1957; Butslov et al, 1958). Another method of electron image intensification uses thin composite membranes which are bombarded with electrons of energy such that while few penetrate the film each release a number of secondary electrons from the opposite face of the membrane. Five stages of this type in cascade with an average gain of about five electrons per stage give an overall elec— trdn gain of 3,000 and, taking the performance of the photo— cathode and output phosphor into account, the photon gain is of the order of 105. This is sufficient for recording of the effect of-single electrons emitted from the input photo— cathode. However, there is a possibility that the life of the thin films under electron bombardment is limited. The resolution is fairly uniform across the whole field and the image definition corresponds to about 600 picture points across the picture width (McGee, 1961). 164. Such tubes are being produced commercially. Techniques for thg intensification of weak light images were discussed at the "Second Symposium on Photoelectronic Image Devices as Aids to Scientific Observation", held at Imperial College in September 1961 (McGee, Wilcock and Mandel, 1962). 165.

Conclusion.

A theoretical limit to the size of an object which can be detected by its X-ray image has been derived and is given by: (i12 + 1,2A2 kTsi: (AAy2) (µ2'"µ1) gnats where A is the area presented to the incident radiation by the object, Ay is the thickness of the object, k is the signal-to-noise ratio which enables the image just to be resolved and is taken as k 1ft 2, G is a factor representing the effect of the detection process on the image contrast, na is the number of X-ray quanta/cm2/sec forming the X-ray image, is is the storage time of the detector, (11:2-1µ 1) is the difference in the linear X-ray absorption coefficients of the material of the object and its surrounding medium, Is is the ratio of the scattered radiation to the primary radiation at the detector. The value of (A Ay2) depends upon the shape of the object, and is evaluated for some simple shapes (table 18).

Table la

Shape of Object (A A

Cube of side a a4 2 Rectangular solid sides abc abc 3 Cylinder diameter a, length 1 a 1 3 4 Sphere diameter a n a 166.

From this equation it was shown that the limit set by the quantum nature of the image can be attained by existing film-screen combinations, fluoroscopic screens and image intensifier systems. Further improvement in resolution or dose reduction is still possible: by a reduction in the scattered radiation reaching the detector; by control of the contrast of the image, which can be done most readily if the image is in the form of a television signal; by an increase in the X-ray absorption of the primary detector; and by the development of contrast media and techniques. Reduction in overall genetic dose to thepopulation will not be achieved until the majority of X-ray units are implementing the Adrian Committee Report: improved apparatus is unlikely hositals to be used with success by the 75% of Itoptlitais in which current practice is not already of the highest standard. Two techniques were investigated in detail for the intensification of the X-ray image by its conversion to a television signal for display on picture monitors, cinephotography, or image-storage. From the work on the X-ray sensitive photoconductive camera tube it was concluded that the thickest layers from which the X-ray photocurrent could be efficiently extracted by an applied electric field were of the order of 10 microns and had an X-ray absorption of the order of 10% at 65 kVp. Even after difficulties due 167.

to photoconductive time lag had been overcome, a dose rate of 1 r/min was required to give a signal-to-noise ratio of 9. This was due to the head amplifier noise and the small values of the X-ray photocurrent. The tube is of little use for low dose rates unless the electric field in the photoconductor can be raised, and the resulting background of white spots can be eliminated without .loss of sensitivity. This tube was abandoned in favour of a system in which the faint light image from an X-ray fluorescent screen is intensified to a level at which a television signal can be generated. Development of techniques of light image intensification for this purpose was described. It is shown that for a good signal-to-noise ratio, the fluoroscopic screen must have a high X-ray absorption and that the product of the quantum efficiencies at the optical coupling from the screen to the intensifier, the photocathode, the output phosphor, should be large compared to unity. Using the best of present techniques, such an intensifier system would reduce the signal-to-noise ratio of the X-ray quanta by a factor of 0.88 when viewing a 4" diameter of the fluoroscopic screen and using a 4" diameter photocathode in the image tube, and when viewing a 12" x 15" screen with a 4" diameter image tube the signal-to-noise ratio of the X-ray Quanta would be reduced by a factor of 0.78. 168.

A one-stage image intensifier preceding a vidicon camera section in the same vacuum envelope or a two-stage image tube with optical coupling to a conventional camera tube would suffice to bring the X-ray image close to the limitation set by the X-ray quanta. The development of techniques for the vidicon section produced difficulties in processing a high sensitivity photocathode and a porous photoconductive layer in the same vacuum envelope, which were not overcome. Itvw shown that the application of the solenoid magnetic focusing field to the image intensifier tubes increased the brightness of the background on the output phosphor of the image tube and that similar tubes without either caesium or a photocathode behaved likewise. This has subsequently been confirmed as due to electrons striking the walls of the tube and producing secondary emission. A tube design in which this effect is greatly reduced is described by McGee (1961). 169.

Acknowledgements.

The author is graleful to Professor J.D. McGee for his interest and encouragement, to Mr. R. Lupton and Mr. E.D. Newton for technical assistance and to the other members of the Instrument Technology Department for helpful discussions. The work was supported by the Paul Instrument Fund of the Royal Society. 170.

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PHOTO - ELECTRONIC IMAGE DEVICES

held at London, September, 1958

Published by Academic Press Inc., New York, An X-Ray Sensitive Photoconductive Pick-up Tube

C. W. SMITH

Instrument Technology Section, Physics Department, Imperial College, University of London, England

INTRODUCTION

While every effort is now being made to reduce the amount of ionising radiation to which people are exposed,' the direct fluoroscopic examination in medical diagnosis may give a patient an X-ray dose of between 2.5 and 17 roentgen per minute for a period from 3 to 10 minutes.2 The Philips image intensifier3 enables the dose rate to be reduced to 0.3 roentgen per minute. In this instrument, an X-ray fluorescent screen 13 cm. diameter has a photocathode formed next to it. The electrons emitted are accelerated and focused on to another fluorescent screen one-ninth the diameter; this is viewed with a nine times magnifying eyepiece. This final screen has a brightness of about 3 foot- lamberts although the background brightness is often 10% of this, giving loss of contrast. Viewing the X-ray image through an eyepiece is inconvenient for the radiologist who likes to be free to palpate . the patient at the same time; the presentation of the reproduction of the X-ray image as a television picture would overcome this difficulty. At the Johns Hopkins University Hospital, Morgan and Sturm4 televised an X-ray image by focusing the image from a fluorescent screen on to the photocathode of an image orthicon camera tube. Hut and Garthwaitet are working on this method of intensifying the X-ray image. Work on an X-ray television system using a scanning source of X-rays with a crystal scintillator and a photomultiplier as the detector, described by Moon,5 is reported by Greatorex. § Work is also in progress on the intensification of X-ray images by means of solid-state image- intensifier screens. A brightness amplification by a factor of 30 over the normal X-ray fluorescent screen has been reported.6. 7, 8 However, it seems that far too much time-lag is introduced into the image.

G. A. Hay. X-Ray Image Intensifier Using Optical Television Methods. See p. 363. E. Garthwaite. X-Ray Image Intensifier Using Image Orthicon Tubes. See p. 379. § C. A. Greatorex. Image Intensification Using a Flying Spot X-ray Tube. See p. 327. 345

346 C. W. SMITH It has been shown by Cope and Rose9 and also by Heinje and his collaborators," that the photoconductive type of television camera-tube —using selenium photoconductive and lead oxide photoconductive targets respectively—can give a televised picture of an X-ray image falling directly on to the target. Their tubes had targets of only 2 cm. diameter and 1 x 12 in. respectively, and required an X-ray intensity of 5 mA at 100 kV. to produce the picture. The effective area of such targets was too small for practical radiology. Keller and Ploke described" an X-ray television system with a photoconductive pick-up tube (Rontgenikonoscope) which had a target 30 cm. diameter. The target (amorphous selenium) was operated under conditions of anode potential stabilisation. The results seemed very promising although the sensitivity was similar to that of a conventional fluoroscopic screen. A similar tube has been described by Jacobs, Berger and Pace.'", This tube had an 18 cm. diameter target of photoconductive lead oxide. It was reported that a 0.5 mm. mesh could be resolved at an X-ray intensity of 0.01 r/min. while the speed of response was determined by the 30 frames/sec. scanning rate of the system.

TIII3E DESIGN It was hoped that a tube similar to that described by Keller and Ploke, but operated with cathode potential stabilisation of the target, would be much more sensitive. In order to stabilise the free surface of the target by a scanning electron beam to the potential of the cathode from which that beam originates, it is necessary that the beam shall arrive on that surface with an energy such that the secondary emission ratio is less than unity. For amorphous selenium the secondary emission ratio equals unity for about 40eV electrons. Even if the electrons arrive with substantially zero energy normal to the target, they may have considerable energy in the plane of the target, which can still result in secondary emission. If an electron beam of energy Ve electron-volts is deflected so as to make an angle 8 with the normal to the target, then the energy component normal to the surface of the target is Ve cos' 0 and parallel to the surface of the target is Ve sin2 0; thus at the point where the electron beam makes an angle 0 with the normal, the target will stabilise Ve sin 0 volts more positive than the cathode.14 The angle of incidence must, therefore, be such that Ve sin2 0 is not greater than the potential at which the secondary emission ratio reaches unity. It was pointed out by Lubszynski and Rodda" that this difficulty could be overcome by making the target as a part of a sphere of AN X-RAY SENSITIVE PHOTOCONDUCTIVE PICK-UP TUBE 347 which the centre of curvature should coincide with the centre of deflection of the electron scanning system: the electron beam would then land normally on the whole area scanned. The objection of complicating the optical system, that arises in applying this to a television camera- tube for light images, does not apply for X-ray images which have no plane of optimum definition. Accordingly, the tube as illustrated in Fig. 1 was designed with a

Amplifier connection Decelerating electrode Wall anode Anode Grid

(8) Cathode Focus coil Signal plate Line and frame 4in. square deflection coils Photo-conductive layer Line and frame shift coils (thickness not to scale) FIG. 1. X-ray sensitive camera tube. spherical front face, the centre of curvature of which was at the point of deflection of the electron beam. The electron beam was accelerated to an energy of about 2 keV before deflection, and then decelerated in an approximately linear field, produced by the wall electrodes, to cathode potential.

TUBE CONSTRUCTION AND PROCESSING After unsuccessful attempts to make a sealed-off tube with an aluminium front window, it was noticed that the X-ray absorption in Pyrex glass is less than in the same thickness of aluminium; Pyrex glass was therefore used for most of the tube envelope. The base pinch supporting the electron gun was of Kodial glass. The graded seal from Pyrex to Kodial was located in the region covered by the deflection coils in Fig. 1. The wall electrodes, of either colloidal graphite, "Nesa", or a liquid metal paint, were applied direct to the glass walls. One of these same materials, or alternatively an evaporated aluminium layer, applied to the glass end of the tube, formed the signal plate. The signal plate and wall electrode connections were made through either tungsten to glass seals or platinum tape seals." 348 c. w. SMITH The tube was out-gassed by a bake up to 400°C., and the electron gun by eddy-current heating. Because selenium in the amorphous form crystallises rapidly above 70°C.,17 the target could not be prepared before the tube had been out-gassed. The selenium, in the form of small pellets, was kept in a side tube outside the oven during the bake. After the tube had cooled to room temperature, the target was prepared by evaporating the selenium in high vacuum on to the inner surface of the front face of the tube, where it condensed in the amorphous form. The selenium was evaporated 10 pellets at a time; it was loaded into the evaporating cup by means of a small iron slug behind every tenth pellet; these slugs were afterwards disposed of into a spare side tube. The evaporating cup was lifted up into the tube through the pumping stem with a powerful permanent magnet, and held in position with an iron peg while the selenium was evaporated by eddy-current heating the cup; the cup was then lowered and the process repeated until a layer of the required thickness had been formed. In practice, the selenium found its way over the whole of the inside of the tube. owing to the high vapour pressure over the surface of the cup (Fig. 2). The amorphous selenium

Selenium evaporated from electrically heated boat

Photograph of the deposit of Aluminium screen selenium on the glass plate Glass plate with pin-hole

FIG. 2. Evaporation of selenium. Sectional diagram.

had too high a resistance to short circuit the electrodes, yet it did not prevent the beam from being decelerated. In order to allow the evaporating cup to pass through the pumping stem, the latter was made of 12 mm. bore tubing. The tube was sealed off the pump by making a drop seal on to a close-fitting glass disc which was kept in a side tube until needed. AN X-RAY SENSITIVE PHOTOCONDUCTIVE PICK-UP TUBE 349

TESTING THE TUBE The Camera Unit A camera unit was constructed for use in conjunction with the Industrial Television Equipment Type 10270 C made by Research Laboratories Ltd, in which it replaced the conventional camera. The picture was displayed on a television receiver with a 14 in. rectangular tube. The camera unit, shown in Fig. 3, comprised the head amplifier, the

Head amplifier Signal input plate

X -ray sensitive pick-up tube

Sealed -off Head pumping amplifier stem

Cathode potential and focus current control circuits

Line time-base Fm. 3. Camera unit. line time-base generator and circuits to permit the remote adjustment from the control unit of the tube cathode potential, the focus-coil current and the shift-coil currents. The tube was operated with the signal-plate connected directly to the grid of the input stage in the head amplifier which was close to earth potential, while the potential of the cathode of the electron gun could be adjusted between earth and —130 volts. Hence, in operation the surface of the selenium could be stabilised from 0 to 130 volts negative relative to the signal-plate. Electrostatic screening was provided by aluminium panels bolted to the sides of the chassis (not shown in Fig. 3). Earthed strips of aluminium foil were

350 C. W. SMITH glued to the neck of the tube to prevent line fly-back pulses from being picked up at the signal-plate. The camera unit was operated inside a lead box which gave adequate protection against X-rays, as all the controls were remotely operated.

MEASUREMENT OF SIGNAL CURRENT Method 1 (Tubes 18, 20, 27) The peak signal current at the output of the video amplifier was noted when X-rays were incident on the target. The output of a signal generator at 100 kc. was fed into the amplifier through a 10 M S2 resistor. The output of the signal generator was adjusted until the peak amplitude measured at the output of the video amplifier was the same as that noted. The peak current flowing in the 10 M LI resistor gave the signal current produced by the area being scanned. The tube signal plate and the 10 M S2 resistor were connected to the amplifier input all the time to keep the capacity constant. The resistor had negligible self-capacity.

Method 2 (Tubes 2,5, 27, 28) In this method the signal current was measured by allowing the free surface of the target to drift in potential for a certain period of time in the absence of the scanning beam of electrons, but while X-rays were incident on the target (unless the dark current was being measured). In detail, the free surface of the target was first stabilised to cathode potential; since the signal-plate was always effectively at earth potential, the difference between the cathode potential and earth gave the potential across the selenium layer. The cathode was then earthed, thus prevent- ing any electrons from landing on the target while it was exposed to X-rays for a period,—usually 10 seconds. The cathode potential was then slowly taken negative. The potential at which the X-ray image first appeared on the picture tube was the potential to which the free surface had drifted during the interval. The above measurements enabled the time constant of the layer to be calculated and hence the resistivity and the current density. The target could be left for up to 2 minutes in the absence of the scanning beam without any change which could be measured in the potential of free surface.

RESULTS OF MEASUREMENTS ON THE TUBES Dark Current The signal current that was obtained in the absence of any X-ray or light input to the tube, was termed the dark current; it could be measured by Method 2 if the target was left for 10 minutes in the absence AN X-RAY SENSITIVE PHOTOCONDUCTIVE PICK-UP TUBE 351 of the scanning beam. The dark current was found to vary as the square of the voltage applied across the target, indicating a space charge limited current. Fig. 4 shows the dark current of tube 20 as a function

'KT" I ' I ' "I I " I

N 2 E 44 ci 10-12 5 I 171 2 Dark current

10-13 U 5 Tube 20 Target thickness 30F -

I I I I I I I II I I I „I 100 1000 Voltage across target FIG. 4. Dark current of tube 20 as a function of the applied voltage.

of the applied voltage. The values of the dark resistivity, measured with 80 volts across the target by Method 2, are given in Table 1.

TABLE 1

DARK RESISTIVITY

Tube 20 1.4 X 10" ohm. cm. Tube 27 6.6 x 10" Tube 28 2.5 X 10"

X-RAY PHOTO-CURRENT Variation with Voltage For low voltages across the photoconductive layer the X-ray photocurrent varied as the square of the applied voltage, as did the dark current. Above 20 volts the photocurrent varied linearly with the 352 C. W. SMITH voltage across the target. For tube 27, Fig. 5 shows that the transition from square-law to linear dependence on voltage occurs at 7 x 10-11 amp./cm.2

E 1° 1 1 1111111 r t1lli11 1 1 1 1-1111.

E a fi. L >, 4... O E 10-io ,o .7 N ____ T.) if) Tube 27 C X-ray tube current — / c S _ 10mA 65K.V.P. E 10-11 O O-c a - 10-12 I X 1 10 100 1000 Voltage across target

Flo. 5. Transition from square-law to linear dependence on voltage for tube 27.

Variation with X-ray Intensity Whether the results are plotted in terms of the X-ray intensity incident on the target (Figs. 6 and 7) or in terms of the X-ray quanta calculated to be absorbed in the target (Fig. 8), it is seen that saturation occurs for high X-ray intensities.

Variation with Target Thickness More important, however, are the relative positions of the curves for the tubes in Figs. 7 and 8. The thickness of the target has far more influence on the maximum X-ray photocurrent than any other parameter. Thicker layers both give, and saturate at, smaller photocurrents. The result of increasing the target thickness to absorb more X-rays is a net loss in sensitivity. The optimum thickness of target is about 10 microns, which is actually equal to the reported range's. 19 of positive holes in amorphous selenium. The X-ray absorption of the selenium targets in the tubes (Table 2) was calculated from the mass absorption coefficients given by Vie- toreen.2° An effective wavelength of 0.5 A was assumed for 65 kVp X-rays. The quantum gain represents the number of carriers reaching the free surface of the target for each absorbed X-ray quantum. Target E volts - eL ••••;;;--. 60 volts __ EeL 10 10 O L •--A 20 volts - 0 • E ,A • r- 'E / •• 10-1

0 0 10-12

L X 4 6 8 10 12 X-ray tube current (mA) Approx. X-ray intensity 0.3r/mA/min FIG. 6. X-ray photocurrent as function of X-ray intensity.

109

E 0 - 1

/.._____--: E 10-1° - A 4. : / A Amplifier rt.-• t / r.m.s. noise current iy • Layer Voltage across rube No. thickness target 18 • 2.4p 10 19 a 10p, 10 0 25 a. 11p 60 0 20 • 30/1. 17 0_10 11 27 A 74p, 60 a 28 • 60µ 60

0 2 4 6 8 10 12 X-ray tube current (mA at 65 K.V.P.) Approx. X-ray intensity 0.3 r/mWmin FIG. 7. X-ray photocurrent as function of X-ray intensity. AA 354 C. W. SMITH

10 9 I " I 0.0°qa° E ■

L IT 10

E "E a) <7, tr, C

5 10-11 10p. Voltage across ( Tube No Target target 16 V 24A 10 O ♦ 19 10 20 9 30A 17 25 ■ 111lµ 60 27 • 74p, 60 X 28 A 60p, 60 I, I ilr I 10-12 106 107 106 X-quanto/sec absorbed in target FIG. 8. X-ray photocurrent as function of X-ray quanta absorbed.

Keller and Ploken give results of measurements made on cells consisting of a layer of amorphous selenium sandwiched between a pair of electrodes. The X-ray photocurrent in the selenium was measured,

TABLE 2

Tube Target thickness Calculated X-ray Quantum microns % absorption gain

18 2.4 3.2 800 20 30 33.5 44 25 11 13.8 445 27 74 63 26 Cope and Rose s 25 — 190 and the parameter a, a measure of the sensitivity, was defined by their equation iR = aIEF, where iR is the X-ray photocurrent in amp, I is the dose rate in mr/see., E is the layer field strength in kV/cm., F is the cell surface (area) in cm2. AN X-RAY SENSITIVE PHOTOCONDUCTIVE PICK-UP TUBE 355 A curve of a as a function of the layer thickness given by Keller and Ploke indicates an optimum thickness at about 150 microns. The writer found the optimum thickness to be about 10 microns and the calculation of a from tube measurements gave values 1000 times smaller than those given by Keller and Ploke. The writer also estimated a value of a from the published figures for the selenium Vidicon of Cope and Rose,' and obtained a value in agreement with his own results. It is found that if the values given on page 564 of the paper by Keller and Ploke" are substituted in their equation (1) quoted above, iR is 4.5 x 10-9 amp. and not 4.51LA as stated therein. Keller and Ploke do mention that their observed photocurrent was of the order of 0.1 ILA, rather than 4.5

Variation with Temperature The resistivity of a tube was measured between 50°C. and —78-5°C. in the hope that greater sensitivity might be found. The dark resistivity was 3 x 10'4 ohm cm. at 23°C., 1.1 x 10" ohm cm. at —78.5°C. and 6.3 x 1014 ohm cm. at 50°C. At —78.5°C. the photocurrent reached saturation at a smaller value than at 23°C.

Variation with Time The tubes showed an objectionable amount of time-lag which appeared as a slow building up of the picture intensity after the X-rays had been switched on, and the persistence of the image for some seconds or minutes after the X-rays had been switched off. The lag also meant that when an object was moved in the X-ray beam there was either considerable blurring in the image or even a complete failure to repro- duce it. The effect of beam current on the time taken for the picture to decay to an arbitrary level, judged by the visual inspection of the television picture, is shown in Fig. 9(b). The current to the limiter electrode on the electron gun was measured. Experiments with a monoscope indicated that the beam current was about 1% of the limiter current. Beam currents greater than 0.02 ILA had no effect on the lag. If the X-ray intensity was increased, the decay time also increased unless the beam current was increased as well, as shown in Fig. 9(a). The time for the picture to decay was also measured as a function of the area of the signal-plate being scanned. This area was determined by counting the number of holes visible in the television picture of a piece of perforated zinc, the X-ray image of which fell on the target. The reduction in the lag as smaller areas of the target were scanned is shown in Fig. 9(c) and indicates that the beam current in that particular 356 C. W. SMITH tube was inadequate to completely discharge the target. The effect was further demonstrated by reducing the cathode heater voltage, which reduced the beam current, when the lag increased tenfold.

Time lag in tube 25

6 1 I I 1 1 I I i 4 • T t' 4 0 L- < X o E (a) a) 2 z

0 1111111 4-;

L L (b) o 1

0 I I I tv a c 6 N

c_0v v —.-c 4 - Heater voltage (c) -ta 4.0 4.• Heater voltage - 2 2.3 x Q7

11 f I I O. 10 20 30 Time in seconds for signal to decay to some arbitrary level after the X-rays have been switched off Fin. 9. Picture lag as a function of (a) X-ray intensity, (b) beam current, (c) area scanned.

These measurements showed that the capacitive time-lag could be made predominant, but that another source of lag was present: this had about one or two seconds duration, and was subsequently found to be photo-conductive lag in the selenium target.

PHOTO-CONDUCTIVE LAG IN SELENIUM Many features of the tube-processing procedure were altered in the attempt to find something which would produce any change at all in the photo-conductive lag of the selenium layers. It did not matter whether the electron gun was activated before or after the evaporation of the selenium. The temperature of the front face of the tube during the evaporation did not matter between —78.5°C. and 25°C. At liquid AN X-RAY SENSITIVE PHOTOCONDUCTIVE PICK-UP TUBE 357 nitrogen temperature the layer was found to craze on warming to room temperature. Glass or metal evaporating cups made no difference, neither did the presence of gas in the tube, nor the rate of evaporation of the selenium. A tube was constructed having strips of four different materials forming the signal plate; this made no difference to the time lag although the dark current was lowest opposite the colloidal graphite strip. Impurity elements were added to the selenium in the hope of producing an increase in the lag so that it would be known which element, already present as an impurity, should be removed. The elements were selected on the basis of the physical and chemical properties of amorphous selenium given by Henisch.21 Tellurium and arsenic made no difference to the lag. Sulphur and CS2 produced a twenty-fold increase in the time lag. Thus effort was concentrated on the removal of sulphur from selenium. A short time-lag was taken as the criterion for the purity of a sample of selenium, and various preparations of selenium were examined for time-lag, using a demountable tube. The tube, similar in principle to that illustrated in Fig. 1, was fitted with an electron gun experimentally designed to give a uniform flood of electrons over the whole diameter (4 in.) of the target without the need for a scanning system, as this would have limited the measurements to time-lags of the order of a frame period. The target was produced by evaporation of the selenium on to an aluminium disc so curved that the electrons would land orthogonally at all points. The time-lag was observed on an oscilloscope connected to the aluminium disc serving as the signal plate. By chang- ing the potential of the electron gun cathode it was confirmed that there was sufficient beam current available to give a response to an X-ray photocurrent rising from zero to maximum in one millisecond, which was substantially free from lag. Several methods of removing sulphur from the selenium were examined. Zone refining, which relies upon a differential solubility of the impurity between the solid and liquid phases, gave no improvement. Distilling the selenium successively nine times in a high vacuum gave a reduction in the X-ray photo-conductive lag from about 2 seconds to 150 m sec. Fractionating in a Vigreux column22 under high vacuum, reduced the lag to 300 m sec. Sublimation at temperatures between 100 and 160°C. in a high vacuum reduced the lag to 100 m sec., while merely heating the selenium in a high vacuum with an external steam jacket for 16 hours, gave a lag of about 40 m sec. Since the X-ray tube was self-rectifying and was operated from a 50 cycles mains, a rise time of 5 m sec. is the shortest which could be measured by switching the X-ray set on and observing the rate of rise of photo-conductive current.

358 C. W. SMITH

THE SEPARATION OF SULPHUR FROM SELENIUM That sulphur is separated from selenium by heating to 100°C. in a high vacuum is based on the following experimental result. Some chemically pure selenium (precipitated with S02) which had not been distilled, was obtained.t Reference to Honig" showed that the vapour pressure of sulphur at 100°C. is P5 x 10-2 mm. Hg., while that of selenium is 10-i mm Hg. at the same temperature. A Pyrex glass tube containing a heap of selenium powder was pumped to 10-6 mm. Hg. Steam was then passed through a jacket enclosing that part of the tube containing the selenium. A milky white deposit began to build up in the tube just beyond the cork at the end of the steam jacket; after three hours, when the white deposit had become quite opaque, the experiment was stopped. On sliding the cork back along the tube it was seen that there were two deposits: the white deposit had formed just outside the cork and a red deposit, having every appearance of selenium, had formed from just inside the cork. The evidence that the white deposit was sulphur is not quite conclusive. Owing to the small quantity available it was not possible to obtain a really convincing result with a chemical test—the pyridine- sodium hydroxide test for sulphur in the presence of selenium—though this test was satisfactory when tried with a small grain of sulphur. Unfor- tunately, selenium which was present from the adjacent deposit on the tube gave the pyridine a pale yellow colour, and while it is thought that the colour did change to a yellow-green on adding the NaOH, it was by no means a convincing experiment. When some sulphur was heated to 100°C. in vacuo, in the same apparatus, a similar white deposit was obtained in the same place; there was no red deposit under the cork this time. When some distilled and spectroscopically pure selenium was heated in this way, the red deposit appeared under the cork but no white deposit was visible up to the time that the red deposit had extended beyond the region previously occupied by the white deposit. This was repeated several times with up to 20 gm. of selenium in the tube. This selenium probably did not contain enough sulphur to give a visible deposit but, heating it to 100°C. for 16 hours did produce a marked reduction in the time lag when a layer was prepared from it and tested in the demountable tube, modified to give a television picture. When an object was moved rapidly in the X-ray beam, a very slight tail was visible extending two half-frames behind the image in the television picture. The process of heating selenium to 100°C. in a high vacuum probably

t From Dr. Ludekens of Mining and Chemical Products Ltd. AN X-RAY SENSITIVE PHOTOCONDUCTIVE PICK-UP TUBE 359 removes sulphur which acts as a catalyst for the crystallisation of amorphous selenium.21 Both the addition of sulphur and evaporating selenium on to a surface warmer than 42°C. produced increased time lag and the latter favours the crystallisation of the selenium."

CONCLUSION An X-ray television system has been made to work using a photoconductive pick-up tube with an X-ray sensitive target 4 in. square. The target material was amorphous selenium in which the X-ray image was absorbed directly. At first, photo-conductive time lags of 1 to 2 seconds were persistently encountered. A method of processing the selenium was devised, resulting in a reduction of the lag to 1/25 sec. The sensitivity seemed to be further reduced in the process.24, 25 The sensitivity was not high enough to give a television system, using these pick-up tubes, any advantage over the visual observation of a fluoroscopic screen. The best tubes (Nos. 19 and 25) were only able to give photocurrents up to 10 times the amplifier r.m.s. noise current, whereas photocurrents 100 times the noise current are required to give an adequate rendering of contrast. Since the r.m.s. amplifier noise current and the amplifier input capacity are both proportional to the target area, there will be no improvement in the ratio of signal-to-noise from larger area targets. An X-ray image intensifier with a 6-in. diameter sensitive area would seem to be acceptable for radiology. The additional facility of being able to examine an area 12 in. x 15 in. is necessary if the instrument is to replace completely the fluoroscopic screen. Any X-ray image intensifier which has a large area X-ray image detector inside a vacuum envelope is subject to a loss of X-ray quanta from the image by absorption in the end window: this loss occurs at a point where the information carried by the quanta cannot be replaced by subsequent amplification.

ACKNOWLEDGMENTS The writer wishes to express his thanks to the Paul Instrument Fund Committee of the Royal Society for their financial support and to Professor J. D. McGee for his direction, help and advice.

REFERENCES I. "Code of Practice for the Protection of Persons exposed to Ionizing Radia- tions." H.M.S.O. (1957). 2. Ardran, G. M., Brit. J. Radiol. 29, 266 (1956). 3. Teves, M. C., Tol, T., Philips tech. Rev. 17, 69 (1955). 4. Morgan, R. H., Sturm, R. E., Radiology 57, 556 (1951). 5. Moon, R. J., Science 112, 389 (1950). 360 C. w. SMITH 6. Diemer, G., Klasens, H. A., Van Stanten, J. G., Philips Res. Rep. 10, 401 (1955). 7. Kazan, B., Proc. Inst. Radio Engrs, N.P. 45, 1358 (1957). 8. Orthuber, R. K., Ullery, L. R., J. opt. Soc. Amer. 44, 297 (1954). 9. Cope, A. D., Rose, A., J. appl. Phys. 25, 240 (1954). 10. Heinje, L., Schagen, P., Bruining, H., Philips tech. Rev. 16, 23 (1954). 11. Keller, M., Ploke, M., Z. angew. Phys. 7, 562 (1955). 12. Jacobs, J., Berger, H., Elect. Eng., N.Y. 75, 158 (1956). 13. Berger, H., Pace, A. L., Non-destr. Test. 15, 26 (1957). 14. McGee, J. D., Proc. Instn elect. Engrs 97, III, 377 (1950). 15. Lubszynski, H. G., and Rodda, S., Brit. Pat. No. 442666 (1934). 16. Davis, E. J., J. sci. Instrum. 35, 308 (1958). 17. Keck, P. H., J. opt. Soc. Amer. 42, 221 (1952). 18. Weimer, P. K., Cope, A. D., R.C.A. Rev. 12, 314 (1951). 19. Spear, W. E., Proc. phys. Soc. B. 70, 669 (1957). 20. Victoreen, J. A., J. appl. Phys. 20, 1141 (1949). 21. Henish, H. K., "Rectifying Semiconductor Contacts," p. 75 et seq., Oxford University Press (1957). 22. Carney, T. P., "Laboratory Fractional Distillation." Macmillan (1949). 23. Honig, R. E., R.C.A. Rev. 18, 195 (1957). 24. Rose, A., Hely. phys. Ada. 30, 242 (1957). 25. Redington, R. W., J. appl. Phys. 29, 189 (1958).

DISCUSSION B. W. MANLEY: Was a grid used in the photo-conductive tube to produce normal landing; and how was the pick-up from line fly-back pulses avoided? c. W. SMITH: No grid was used. Since the target was 4 in. square and it was hoped to increase the size still further, a grid was not practicable. Moreover, in order to be able to "zoom" for the examination of a small area in detail, a very fine mesh would have been necessary. Pick-up from the line flyback pulse was avoided by screening the whole of the front end of the camera chassis with an electrostatic screen consisting of strips of aluminium foil glued to that part of the tube which was inside the coil assembly, and by adequate decoupling and screening in the construction of the head amplifier. A. E. ENNOS: In connection with the apparent variation of photoelectric time-lag of selenium with its purity, the following facts are important: The distillation of selenium, if carried out under an insufficiently high vacuum, will give rise to oxidation of the selenium. This oxide has an evaporation point somewhat higher than that of selenium and may have been responsible for the white deposit obtained. Furthermore, our experiments on the sensitivity and time-lag in thick layers of selenium sandwiched between aluminium indicate no appreciable time-lag (in confirmation of Keller and Ploke's work) at the X-ray intensities used in fluoroscopy. The conclusion is that the time-lag is more likely to be connected with the mechanism of the beam acceptance or with the interface between the selenium and the original plate electrode. c. w. sittrrn: If the white deposit were due to selenium oxide with an evaporation point higher than selenium, the deposit would have appeared on the high AN X-RAY SENSITIVE PHOTOCONDUCTIVE PICK-UP TUBE 361 temperature side of the selenium deposit and not on the low temperature side as found. If the white deposit is not sulphur it must be something which has an almost identical vapour pressure, as the experiment with sulphur showed. The work on the purification of selenium was carried out because the results of Keller and Ploke and Weimer and Cope could not be repeated with the materials and procedure used. Even now that they have been repeated, it is possible that effects at the interface between the selenium and the plate electrode would cause lag effects under suitable conditions. There is evidence of the injection of holes into selenium from supporting electrodes or from semiconducting films in contact with the selenium. Inadequate beam acceptance must always be remembered as a likely source of lag. P. A. EINSTEIN: Did you ever find that the tube reverted to operating at anode potential, and was the sensitivity with this mode of operation comparable with the sensitivity when the target was at cathode potential. c. w. SMITH: If the signal plate potential was increased too far, the surface could be seen to go over to anode potential stabilization. The effect usually spread in from the edges of the signal plate. Because of the long time-constant of the target, the potential of the free surface was lowered by switching on the X-rays in the absence of beam current so that the C.P.S. state could be obtained. The sensitivity with anode potential stabilization was not specifically measured but it was of the same order of magnitude as the sensitivity under C.P.S. conditions.