University Microfilms, Inc., Ann Arbor, Michigan PAST SPECTKOPHOSEHÜRIMETER

MASTER'S THESIS M-642

HECKER, Irwin. FAST SPECTROPHOSPHORIMETER.

The American University, M.S., 1963 Physics, general

Irwin Hecker

Submitted to the Faculty of the College of Arts and Sciences of the American University in Partial Fulfillment of the Requirements for the Degree of Master of Science

Signatures of Committee

e Colle ge)eair o e College)eair Date : Date; /^i3

A:izmcm umversit 1^55 LIBRARY

The American University SEP 3 01964 Washington, D .G. WASHINGTON. D £

i:) ACKNOWLED GEMENTS

I wish to thank the faculty and fellow students of the American University Physics Department for their stimulating discussions and suggestions during the development of the phosphorimeter, Dennis Robin son, an undergraduate research assistant at the univer sity, was very helpful in the construction and testing of the phosphorimeter. I am particularly indebted to Professorial Lecturer Samuel Moss for his direction and valuable criticisms. This research was sponsored by the American Instrument Go. Inc. of Silver Spring, Md. Finally, I wish to thank my wife, Barbara Hecker, for typing all of the manuscripts and for help with the proofs. CONTENTS LIST OF ILLUSTRATIONS...... 1 1. INTRODUCTION...... 2

1.1 Phosphorescence...... 2 1.2 Present Phosphorimeters...... 4 1.5 Need For Fast Phosphorimeter...... 5

2. GENERAL DESCRIPTION...... 7

2.1 Design Of System...... 7 2.2 System Operation...... 7

3. LIGHT SOURCE SUBSYSTEM...... 10

5.1 Function Of Light Source...... 10 5.2 Explanation Of Circuit...... 10

4. PHOTOMULTIPLIER DETECTION SUBSYSTEM 16

4.1 Requirements On Detection Subsystem...... 16 4.2 Theory Of Photomultiplier Tubes,... 16 4.3 Explanation Of Circuit...... 17

5. SWITCH SUBSYSTEM...... 25 5.1 Timing ...... 25 5.2 Delay Line...... 25

5.5 Monostable Multivibrator And Amplifier...... 25

5.4 Shockley 4-Layer Diodes...... 27

5.5 High-Speed Switch...... 50

6. CONCLUSION...... 55

BIBLIOGRAHIY...... 34 ILLUSTRATIONS

1. Block Diagram Of System...... 8 2. Flash Tube Firing Circuit...... 11 3. Exciting Light Source Subsystem Schematic...... 12 4. Successive Light Pulse From Plash Source..... «...... 14 5. Gate Pulse Caused By Flash...... 14 6. Picture Of Flash Light Source.,...... 15 7. Operation Of Secondary-Emission Multiplier Tubes...... 17 8. Schematic Of Photomultiplier Detection System...... 19 9. Voltage Across Phototube ...... 21 10. Pictures Of Detection System Components...... 22 11. Timing...... 24 12. Delay Line, Multivibrator And Amplifier...... 26

1 3 . Characteristic V-1 Curve Of Shockley Diodes...... 28 14. Construction Of Shockley Diode...... 28

1 5 . Picture Of Shockley Diode String...... 32 INTRODUCTION

1.1 PHOSPHORESCENCE

When a substance absorbs energy a fraction of the absorbed energy may be re-emitted as electromagnetic radiation in the visible or near visible region of the spectrum. This phenomenon is called luminescence^. Luminescence involves at least two steps: the exci tation of the electronic system of the sample and the subsequent emission of photons. The initial excita tion may be caused by light, electron or ion bombard ment, mechanical strain, chemical reaction, or direct heating. Fluorescence is luminescence in which the light is emitted during excitation. Luminescence in which the light emission occurs after the excitation has ceased is referred to as phosphorescence or afterglow. The afterglow period may be from microseconds to hours in duration. A decay time of 10"^ seconds is frequent ly taken as the demarcation line between fluorescence and phosphorescence. This period is the approximate p lifetime for dipole radiation from an excited atom.

1 This term does not include the emission of block-body radiation, which obeys the laws of Kirchbbff and Wien. 2 Adrianus J. Dekker, Solid State Physics. (Englewood Cliffs, N.J., Prentice-Hall, Inc., 1961). Luminescent solids are usually referred to as phos phors . Several models have been successfully applied to the study of phosphorescence. The property of lu minescence is known to be due to the presence of small amounts of impurities, called activators, in the com pound. The phosphor chemists have been able to syn thesize, under carefully controlled conditions, much A. more efficient phosphors than those found in nature^. Photoluminescent phosphors are substances which absorb electromagnetic energy, usually ultraviolet light, subsequently re-emitting this energy in the form of visible light. The specific properties of a phosphor (e.g., the color of the light it emits, the wavelength of light it must absorb to be excited to fluorescence, the brightness of the emitted light, the duration of the phosphorescence, or afterglow) are a function of the physical nature of the material and the activator it contains. The decay characteristics of a phosphorescence substance are of great importance in the study of luminescence. According to the models, fluorescence and phosphorescence are both first-order

3 J.S. Prener, D.B. Sullenger, "Phosphors", Scientific American, (Oct. 1954). processes and follow exponential laws of decay. The identification of the luminescence centers and their energy levels is simplified if the decay characteris tics are knownt

1 .2 PRESENT PÏÏÜSPHÜRIMETERS

Some important types of apparatus designed for investigation of photoluminescence are the phosphorimeters (phosphoroscopes) and fluorimeters. These serve for measuring the duration of short decaying e- mission process. These instruments are based on the principle of permitting the observation of the phos phorescence for a short, and if desired, a variable time after the end of the excitation-^. Although there is a wealth of equipment now avai lable for measurement of decay times, there is a need for a phosphoroscope v/hich is accurate, inexpensive, flexible, and capable of measuring decay times as short as 1 microsecond. Most phosphoroscopes now avai lable are limited by the speed of their mechanical components^. These instruments cannot measure decay

4 Dekker, op. cit. 5 For a detailed history and description of these devi ces see Peter Pringsheim, Fluorescence and Phospho rescence , (Interscience Idblishers, Inc., N.Y., 1949) ■ times of less than 10~^ seconds. For the measurement of shorter decay periods (down to 10 ^ seconds) fluorimeters are used. These devices are not limited in speed but have other disadvantages 7 . For example, fluorimeters which make use of Kerr cells have the li mitation that Kerr cells absorb certain wavelengths of light. Other fluorimeters require the incident light beam to be diffracted periodically by means of supersonic waves. These "supersonic-cell" 8 fluorime ters are very space consuming. Both types of fluo rimeters mentioned above are difficult to operate and expensive.

1.3 NEED FOR FAST PHOSPHORIMETER

To more fully understand the structure a lumines cent compound, it is desirable to measure precisely the decay of both the fluorescence and the phosphores cence radiation. Since the fluorescence decay time for some substances is as long as 1 0 ”^ seconds, a fast, accurate phosphorimeter is required to be able to dis tinguish the phosphorescence radiation from the radia tion of fluorescence.

6 Such as the rotating discs used by Becquerel or the revolving mirrors used by Vavilov and Levshin. Ibid. 7 Ibid. 8 Ibid. Phosphors are widely used in fluorescent lamps, cathode ray oscilloscopes, radar and television pre sentation, and nucleon and radiation detectors. Better understanding of the nature of phosphors may lead to improvements of these devices. In addition, phosphorimeters have been applied to a number of problems involving both identification and quantita tive assay of many organic compounds. It was felt that it is within the state of the art to build a phosphoroscope which could measure decay times as short as lO"^ seconds and which would be inexpensive, easily operated, flexible, and accu rate. This device was designed to be used in conjunc tion with a system of two grating monochromators^ for selecting excitation and phosphorescence wave lengths. This paper describes the design and opera tion of the electronically switched phosphoroscope which resulted.

9 Such as the Aminco-Keirs Spectrophosphorimeter described in Instruction Book No. 816. (American Instrument Co. Inc., Silver Spring, Md., I960). 2 GENERAL DESCRIPTION

2.1 DESIGN OP SYSTEM

The system can be logically divided into three subsystems: a source of light to excite the sample, an electronic switch to handle the delicate timing problem, and a unit which detects the phosphorescent radiation of the sample. Pig. 1 is a block design of the general features of the system. The exciting light source (A) was a Xenon flash tube. This tube produced a flash of high intensity and short dura tion. The electronic switch (B) was designed to switch very high voltages on to a load in less than one microsecond. The detection system (C) was de signed so that it could be sensitized a short time after sample excitation.

2.2 SYSTEM OPERATION

The system functioned in the following manner: The Xenon flash light provided the electromagnetic energy to excite the sample. This source of light was sharply cut off. The electronic switch then ac tivated the detection system by impressing a high voltage across a phototube. Since the exciting light from the flash tube had ceased, radiation de tected at that time was solely luminescence decay of the sample. The detection system, a phototube, must 1

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8 be off while the exciting light is on, since the exciting light v/ould make it insensitive for many microseconds. The selection of exciting and phosphorescence wavelengths was made by reflective gratings By changing the angle of incidence of the exciting and phosphorescence light on gratings G1 and G2, respectively, (See Pig. 1), specific frequencies were selected.

^°Ibid. 5 LIGHT SOURCE SUBSYSTEM

$.1 FUNCTION OF LIGHT SOUECE

The light source subsystem provides the inci dent electromagnetic energy to the phosphor being studied. This light was of sufficient intensity over the pertinent frequency band to produce mea surable phosphorescence. The detection system was not turned on until the exciting light had ceased. These steps had to occur within one microsecond of each other, in order to

measure decay times of approximately 2 microseconds. A flash light tube which supplies an intense but short-lived flash of light was selected to provide the exciting radiation.

$.2 EXPLANATION OP CIRCUIT

The high speed electronic photo-flash tube cho sen was a Xenon E6&G FX12. This tube was designed with a high gas pressure so that it would not flash spontaneously when the electrodes were connected directly across a charged capacitor. See Fig. 2. Firing these tubes required a momentary high poten

tial of 10,000 to 1 5 ,000 volts peak applied to a grid wire attached to the tube envelope; this ionized the gas and made the flash tube conductive. The 22R44- ignition transformer was a small lightweight unit designed specifically for supplying the high

10 voltage pulse necessary to fire photoflash tubes which require a separate ionizing potential.

RA-TED OpiLRATXWe. V o l t a g e XENON thiERGV PLAS4-I zr^AjxTxoKi CoaooasoR

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Fig. 2 Basic Firing Circuit For High-Pressure Flash Tubes Which Require A Separate Ionizing Potential

The flash circuit (Fig.5) worked in the following way: The .22mf 4-OOV B.C. capacitor (01) is charged to 180V' by 2RCA V590 90 volt batteries in series. Across the flash tube, a ,1 mfd. Glassmike $KV

capacitor (0 2 ) was charged to -1500 volts by an ex ternal power supply. Closing the momentary contact switch (S) put a pulse through the primary of the photo-flash transformer (T). The resultant high vol tage pulse in the secondary ionized the Xenon and thus made the flash tube conductive. Capacitor 02

discharged through the tube in less than 1 microse cond. Fig. 4 snows several successive light pulses.

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12 These traces are the output from a photo diode de tector v/hich v/as available during testing. The position of successive traces horizontally was adjus ted manually. The time scale is .25 microseconds per cm. Note that the light pulse was quite reproducible and decayed in considerably less than one microsecond. In addition to providing the exciting light re quired, this subsystem provided a small (8 -lOV) pulse^^ at point (P) in Pig. 5* This pulse was used to gate on the electronic sv^itch, which in turn switched on the detection system phototube. Pig. 5 shov/s this oscillating pulse on a .2 microsecond/cm. time scale. The vertical scale is 5 volts/cm. Pig. 6 shows the flash light source subsystem ex cluding the battery and power supply units.

11 M.E. Bishop, (Private Communication)

13 . \ ' X ^ \ \

Pig, 4- Successive Light Pulses Prom Plash Tube Time Scale; ,25 Microseconds/cm,

Pig. 5 Pulse Caused By Plash Vertical Scale 2V. /cm. Horizontal Scale .2^s,/cm,

il. Fig. 6 Flash Light Source

15 4 PHOTOMULTIPLIER DETECTION SYSTEM

4.1 REQUIREI/IENTS ON DETECTION SUBSYSTEM

The detection system is a photomultiplier tube. As mentioned earlier, during detection it is impor tant that the only light incident on the phototube be the phosphorescence light from the excited sample. Since the exciting light from the flash source would render the photomultiplier insensitive for many micro seconds, it was turned on only after the exciting light had ceased. To accomplish this the 8-10 volt pulse obtained from the light source was delayed un til the light had turned off completely. This pulse was then used to gate on the phototube.

4.2 THEORY OP PHOTOMULTIPLIER TUBES

Phototubes are based on the principle of the pho toelectric effect, i.e. the emission of the electrons from metallic surfaces under the action of the light. The output current of a photomultiplier tube (photo tube) is a function of the electromagnetic energy in cident on the photocathode. The number of electrons released in a unit time by light of a definite wave length is directly proportional to the intensity of this light, while the energy of these electrons is directly proportional to the frequency. In a photomultiplier tube the mechanism of secon dary emission is utilized along with the photoelectric

16 effect. Fig. 7 shows the operation of a secondary- emission photomultiplier tube. The cathode emits electrons wheti lectromagnetic energy is incident on it. These initial electrons emitted from the cathode are directed to strike against a series of subsidiary plates called dynodes by increasing positive voltage between dynodes. Fach dynode has a surface treated for high secondary emission so that several secondary electrons will be released for each electron striking this surface. When 9 dynodes are used such as in the 1P28, the amplification which can be achieved is enormous. (-^1 ,25x10^)

X 300 VOLTS VDi_TS GL&CTAON Pa t VA

PMoToT ü I3 dywoqe p l a t e : VOLTS x*-+Oo VOLTS

Pig, 7 Operation Of Secondary-Emission Multiplier

4-,$ EXPLANATION OF CIRCUIT

Pig, 8 is a schematic of the photomultiplier de tection system. The pulse from the flash source gated

on the high speed switch (Sj, which switched 1000 4ML volts across the dynode resistor string of^photoelec tric tube.

17 In addition to having high amplification for the frequency band of interest, the tube chosen (1P28) was required to be energized in less than 1 micro second and maintain a constant amplification for at least 100 microseconds. The time required to raise the plate voltage of the tube to the operating vol tage is a function of the resistance (R^) and capa citance (C^) of the tube. The voltage rises as (V=V^(l-e""^/^t^t ). The interelectrode capacitance (C^) of the 1P28 is approximately 25 picofarads. After 4 time constants

(RC),V^V^ so the required constant was .25 microseconds. Since 0^=25x10 —12 farads, the maximum allow able dynode resistance (Pp across the tube was lo"^ ohms. See Fig. 8. To maintain this voltage (across the dynode re sistors) for 100 microseconds, a large current supply is needed. The supply plate voltage is 1000 volts and the resistance of the dynode string is 10,000 ohms. Therefore, by Ohms' Law, 100 ma. must be su pplied . Since the gain of the phototube is very sensitive to a change of voltage across the dynode resistors, the voltage must remain constant to within 1% for the 100 microseconds. This current is provided by the

discharge of the large capacitor 05 through the load,

^^t). The discharge is initiated by the closing of

18 FIG. 8 SCHFMATIC OF PHOTOMULTIPLIER DETECTION SYSTEM

IP2^ PHoroMUi-TIPL/et^ TmSe m u

SWITCH w

19 the high speed switch. The decay time is a function of the load , (the dynode resistors and the negli gible resistance of the closed switch), and the capacitance (C^) of 0 3 .

(V=V^e"^/®tS ). 03 a 2 mfd. 2000 volt capacitor and R^ is approximately IL 10 ohms. The long time constant, R^C^=2QOOOmicro seconds provided the current of 100 ma. i 1% for more than the required 100 microseconds. Fig. 9a shows the voltage across the phototube string when the tube is activated by the gate pulse. Note that the voltage switches from 0 to -1000 volts in less than one microsecond and starts a very slow decay. Pig. 9b shov/s that after 100 microseconds the voltage has decayed less than 1%. Pig. 9c shows the eventual complete decay and return to the "off” state. Fig, 10 shows some of the components of the detection system.

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21 Fig. 10 TOP AND BOTTOM VIEW OF DETECTION SYSTEM COMPONENTS

22 SWITCH SUBSYSTEM

5.1 Timing

Pigs. 11a and b show the light pulse from the flash tube, and the corresponding 8-10 volt pulse measured across the of ?^20 wire on one end of the flash tube. This oscillating pulse is a result of the current surge through the flash tube when the tube is conducting. Point A in Pig. 11a is the time at which it is safe to turn the detection system phototube on. It can be seen that in order to use the voltage pulse to gate "on" the phototube, it had to be delayed approximately .5 micro seconds. In addition a sharp, high voltage pulse is necessary to insure positive firing of the switch at the proper time. A monostable multivibrator and an am plifier were used to convert the oscillating 8-10 volt pulse into a sharp pulse. The high-speed switch itself v;as required to energize the phototube in less than 1 microsecond and be able to withstand the 100 milliamps steady current discharging through the load (phototube dymodes) from capacitor C3. Pig, 11c snows the oscillating pulse delayed .5 microseconds. Pig. lid shows the output of the multivi brator and amplifier circuit. Pig. lie shows the voltage across the phototube, indicating that the detection

23 FIG. 11 TIMING

Ll&HT Light pulse from flash tube.

Oscillating gate pulse.

VOLTS Gate pulse delayed by ,ÿ/s.

VOLTS Output of multivi brator and ampli fier circuit.

Voltage across VOLTS phototube.

S t.o /.r TIME /LÔ/

24 system is "on" at approximately point A, Fig. 12a shows the place of the delay line, multi vibrator and amplifier and high speed switch in the phosphorimeter system.

5.2 DELAY LINE

A delay line was introduced between the trigger pulse from the flash source and the mono-stable multi vibrator circuit, allowing a delay of up to five micro seconds between the light pulse and the energizing of the photomultiplier tube. This allows one to observe the decay of the phosphor at various times after exci tation and insured that light output from the exciting source had ceased before observation begrtn . The delay line is housed in a minibox with a selector switch to choose the desired delay.

5.5 MONOSTABLE MULTIVIBRATOR AND AMPLIFIER

This is designed to provide a large negative pulse to fire the Shockley diode string. The trigger signal from the light source after passing through the delay line enters the grid of the first tube. This tube is a dual triode operating as a single-shot multivibrator. The signal from the second cathode of this tube is fed into the grid of the second, tube, a single stage ampli fier. The result is a negative square pulse delayed by a minute amount from the light flash which fires the high voltage source. Fig. 12b is a schematic of the

2 5 Fig. 12 DELAY LINE, MULTIVIBRATOR AND AMPLIFIER

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26 multivibrator and amplifier. The single-shot circuit provides a reproducible trigger pulse which insured positive firing of the high voltage. Without this assembly use of the delay line, which alters the light source signal considerably, would be prohibited.

5.4 SHOCKLEY 4-LAYER DIODES

The Shockley 4-layer diode is a two terminal, si licon, semiconductor switch. It has two stable states as shown in the V-I Curve (See Fig. 1^). The "off" or high impedance state is shown in Region (%. To turn the device on, voltage across the terminals must exceed switching voltage (V^). The 4-layer diode is turned off by reducing the current flowing through the device below holding current (I^). Fig. 14 is a diagram of the construction of the Shockley 4-layer diode. Produced from single-crystal silicon, the four layers are obtained by the controlled diffusion of suitable impurities. The symbol for the Shockley 4-layer diode is a mo dified "4"; the slant line of the "4" indicates the for ward direction of current passing through the device when in the "on" state. In Fig. 1$ the voltage-current characteristic for the 4-layer diode shows three essential operating regions

l-"off" or high resistance state 11-transition or negative resistance state lll-"on" or low resistance state

29 /

FIG. 13 CHARACTERISTIC V-I CURVE OF SHOCKLEY DIODES

TERMS AND SYMBOLS

Switching Voltage Switching Current Ih Holding Current v% Holding Voltage Qon "On" Resistance (the slope of the V-I Curve Measured at Currents ) Leakage Current Vrt Reverse Breakover (Avalanche) Voltage

FIG. 14- CONSTRUCTION OF SHOCKLEY DIODE 1 O - N Conventional Lead for connection Current to positive supply N Flow is marked +.

28 This curve is shown on a very expanded scale (nonlinear) for illustration purposes only. Note that as the voltage rises and reaches the switching voltage (Vg), the device begins to switch "on". The current at this point (I ) is typically several microamperes. The device switches because of an internal feedback mecha nism allowing the diode to pass a steadily increasing current as the voltage decreases (negative resistance state, Region 11). When "on" (Region 111), the 4-layer diode passes a current which is limited principally by the external circuit. In the "on" state, the device has a dynamic resis tance of less than 4 ohms and a voltage drop of about one volt. As long as sufficient current is passed by the circuit, the device will remain in the "on" condition. At the point on the curve I^, the circuit is passing just enough to keep the device in the "on" condition. If the current drops below I^, the diode switches back to the high resistance or "off" condition. In its "off" condition, the 4-layer diode may be characterized by a capacitance and large resistance in parallel. This capacitance is similar to the collector capacitance of a normal transistor. It has a value of 4 to 60 pf depending on the nominal switching voltage of the diode and actual voltage across the device. In its "on" condition, the diode has such a low resistance that capacitance effects may be ignored.

29 In ins "off" condition the device will pass a ca pacitive current in response to a sharply rising vol tage wave. If the rise rate of this voltage wave is large enough (usually 10 to 100 volts microsecond ), switching occurs below the DC sv;itching voltage. This is called the rate effect. The load impedance in series with the 4-layer diode must prevent it from passing excessive current in the "on" state. The maximum "on" currents are determined by the thermal characteristics of the 4-layer diode package. To keep the device "on", the series impedance should pass sufficient current to exceed the holding current (I^). In other words, the load line of the cir cuit should intersect the curve to the right of the ne gative region for the device to remain on,

5.5 HIGH-SPEED SWITCH

The Shockley 4-layer diode series for this appli cation consists of 6 type 4E200-8 diodes providing a minimum stand-off voltage of 1060 volts. (See Eig.

12a). Each 4E200-8 has a switching voltage (V^s ) of 200 ^20 volts. The current carrying capacity is 150 ma. steady D.C. but these diodes can stand a short lived peak current of up to 10 amperes. The switching time is on the order of 100-200 nanoseconds. As shown in Fig. 12a equal voltage division is maintained across each diode by a string of 750 K re-

30 sistors connected in parallel with the 4-layer diodes. The triggering of the series switch can be accom plished with a low power pulse at point A (Pig. 12a) if it has a sufficient voltage amplitude to raise each diode above its switching voltage. However, a sharp trigger pulse is desirable to turn the 4-layer diodes on uniformly and for this reason the delayed gate pulse was put through the one-shot multiplier and amplified to approximately -100 volts. When the diodes are "off" they have a virtually infinite impedance. Since the phototube resistance 4 in series with the switch is only 10 ohms, almost all the voltage is across the switch, i.e. the phototube is "off". Since in its "off" state, there is -1000 volts, across the switch, the addition of the -100 volts, combined with the rate effect which lowers the switching voltage to about 1050 volts, switches all 5 diodes uniformly, reducing their total resistance to a few ohms. This puts almost the entire 1000 volts across the load resistance (phototube) and the detec tion system is "on". The switch will remain in its "on" (low resistance) state as long as enough current is supplied by capa citor 05. Fig. 9c showed the voltage being switched on to the phototube plate. This voltage decreases as capacitor 05 is discharged. (V=V^e“^'^^^) where R is the combined resistance of the phototube and the switch

51 and G is the interelectrode and stray capacitance. In the "on" state both the capacitance and resistance of the switch is negligible. Note that there is a sharp cut-off voltage about 50 milliseconds after switching. This is due to the capacitor supplying less than the holding current, (I^) to the Shockley switch, thus turning the switch "off" again. The voltage is again primarily across the switch rather than the tube. The capacitor is then charged up again by the power supply and the process may be repeated by pressing the manual momentary switch (S).

Fig. 15 is a picture of the Shockley Diode string.

Fig. 15 Snockley Diode String

5 2 6 CONCLUSION

When used with reflective gratings this system is called a spectrophosphorimeter. This device can measure decay times of as little as 2 microseconds with good accuracy. The operation of the system is extremely simple. The gratings are adjusted to study the sample at various exciting and phosphorescent frequencies. The output of the phototube can be displayed and photographed on an oscilloscope. The sample is placed between the flash tube and the detection photo tube (See Fig. 1). Pressing the momentary switch (S) will display the decay curve on the oscilloscope screen. It is hoped that this device will prove useful in the study of phosphorescence.

33 BIBLIOGRAPHY

Dekker, Adrianus J. , Solid State Physics, Englewood Cliffs, N.J.: Prentice-Hall, Inc. , 1961. Prener, J.S. and Sullenger, D.B., "Phosphors", Scientific American, Oct. 1954.

Pringsheim, Peter, Fluorescence And Phosphorescence, N.Y.: Interscience Publishers Inc" 1949. American Instrument Co. Inc., Amineo-Bowman Spectro phosphorimeter , Silver Spring, Md. , i960.

Shockley Transistor, Applications, AD-8 , Palo Alto, Calif., 1961.