DESIGN AND CONSTRUCTION OF A HIGH POWER ELECTRON GUN by GARY R. LEIKER, B.S. in Eng. Phy. A THESIS
IN PHYSICS
Submitted to the Graduate Faculty of Texas Tech University in Partial Fulfillment of the Degree of
MASTER OF SCIENCE
^O Approved
Accepted
May, 1984 7- ACKNOWLEDGEMENTS
I would like to thank Dr. M. Kristiansen, Dr. L. Hatfield, and Dr.
H. Thomas for serving on my committee. I would like to thank
Dr. K. Schoenbach for his instructive comments on this manuscript as well as supervision during the project. Also I am indebted to Chuck
Harjes, Kim Zinsmeyer, and the other students and technicians who worked on this project.
n ABSTRACT
A high power electron gun capable of producing an e-beam over a
100 square centimeter area has been designed and tested. Grid control of the e-beam provides 100 ns, 250 keV electron bursts through the foil in 100 ns intervals. Results of voltage and current measurements of the electron beam, as well as measurements of its spatial and temporal uniformity, are presented.
m TABLE OF CONTENTS Page
ACKNOWLEDGEMENTS ii ABSTRACT iii
LIST OF FIGURES v
LIST OF TABLES ' vii
I. INTRODUCTION 1
II. ELECTRON EMISSION FROM METALS 6 Richardson-Dushman Equation 7 Th-W Filaments 12 III. EXPERIMENTAL ARRANGEMENT 18
Electron Gun System 18 Vacuum System 29 Pulser System " 32 Cooling System 34 Grounding and Shielding System 39
IV. DIAGNOSTICS AND EXPERIMENTAL RESULTS 42
Voltage Measurements 42 Current Measurements 45 E-Beam Uniformity and Time Dependence 51
V. SUMMARY 56
LIST OF REFERENCES 58
APPENDIX 60
IV LIST OF FIGURES
Figure Page
1. Inductive Energy Storage System with E-Beam
Opening Switch 2
2. Effect of Gas Mixture on Current Fall time 4
3. Emission Current Density from Various
Thermionic Emitters 11
4. Activation Curves for Th-W Wire 14
5. Fraction of Surface Covered with Th(f) vs. Time of Activation for Data Shown in Fig. 4 .... 16 6. Block Diagram of the E-Beam Controlled Opening
Switch System 19
7. Cross Section of E-Beam Triode 20
8. Electron Number Transmission Coefficient (T^ vs. Incident Energy (EQ) for Various Foils 22 9. Energy Transmission Coefficient (Egv/Eo) vs. Incident Energy (EQ) 23 10. Cross Section of the Thermionic Cathode 24
11. Calculated Electron Emission from Pure Tungsten and Thoriated Tungsten 27
12. Test Circuit for Emission Data Shown in Fig. 11 .... 28
13. Vacuum System 30
14. Pneumatic System 31
15. Schematic of FRP-250 Pulser 33
16. Gas System of FRP-250 Pulser 35
17. Spark Gap Pressure vs. Total Charging Voltage for FRP-250 Pulser in Self-Fire Mode 36 18. Spark Gap Pressure vs. Total Charging Voltage for FRP-250 Pulser in Triggered Mode 37
19. Water Cooling System 38
20. Grounding and Shielding System 40
21. Voltage Probe 43
22. Voltage Probe Measurements 44
23. Schematic of Rogowski Coil 46
24. Reconstruction of Current Pulse 48
25. Rogowski-Coil Signal of E-Beam Current 49
26. Rogowski-Coil Signal of E-Beam with Pulsed Grid 50
27. Experimental Arrangement for Scintillator Tests .... 52
28. Scintillator Fluorescence Due to E-Beam Excitation ... 53
29. Scintillator Pulse as Recorded by a PMT 55
30. Cross Section of Triode and Switch Chamber 57
31. Energy Level Diagram Showing Fluorescence Process ... 61
32. Range of Electrons and Other Particles in NE-102A ... 65
33. Response of NE-102A to Electrons and Other Particles . . 66
34. Emission Spectrum of NE-102A 67
VI LIST OF TABLES
Table Page
1. Representative Values of Thermionic-Emission Constants for Pure Metals 9
2. Scintillator Properties 64
VII CHAPTER I INTRODUCTION
Current interest in inductive energy storage systems has motivated the design and testing of high power opening switches. One such approach is the electron-beam controlled opening switch 2,3 shown in
Fig. 1. A volumetric discharge is produced in the switch chamber by electron-impact ionization of the high pressure gas (p ^ 1 atm) between the electrodes. The low energy, secondary electrons produced by this ionization migrate to the positive electrode and constitute the primary current (Ip>10 x le-beam) i" ^^^ switch. When the e-beam is shut off, volume recombination and attachment processes in the gas cause the resistance of the switch to increase by a factor of a 100 or more in less than 100 ns. Opening of the switch is assured by designing the discharge circuit such that the open circuit voltage across the switch is less than the self breakdown voltage of the gas.
As shown in Fig. 1, the opening of the e-beam controlled switch,
Sj, must be simultaneously accompanied by the closing of switch S2 in order to transfer current to the load. With the current changing so rapidly during switch opening (di/dt ~ 10^2 A/s), even a small inductance in the load circuit can cause a huge driving voltage to develop across switch Si (L di/dt effect). The success of switch Si depends on its ability to interrupt direct current against such a high driving voltage and to do so repetitively (10-10,000 pps). 1 (a)
(b)
Fig. 1 Inductive Energy Storage System with E-Beam Opening Switch (a) Charging Cycle, (b) Discharge Cycle. The rep-rate requirement of switch Si puts strong emphasis on minimizing the voltage drop across the switch during the "on" phase.
Energy dissipated in the switch during operation heats the gas volume, causing a lowering of its dielectric strength. This increases the probability of breakdown during the "off" phase. Gas mixtures with high electron drift velocities and high ionization efficiencies have served to diminish the problem of gas heating.^ Also, secondary emission by ion bombardment of the cathode^ and ionization of metastable states in gases such as argon and methane^ have been studied as a possible means of reducing the discharge voltage and subsequent switch losses.
Other gas properties also play a major role during the opening of the e-beam controlled switch. By addition of small amounts of an electron attaching gas to the switch chamber, opening times on the order of a few hundred nanoseconds have been predicted.5 Shorter current falltimes (tp < 100 ns) have been demonstrated by Scherrer et. al^ with mixtures of 30-50% O2 in N2 and e-beam current densities ranging from 17-34 A/cm2 (Fig. 2). Current falltimes of less than
100 ns were also obtained for gas mixtures of over 60% O2 in Ar. Such high concentrations of an attaching gas, however, increase switch losses during the "on" phase of operation. Opening times for both the
O2-N2 and 02-Ar cases shown, in Fig. 2, are limited by the e-beam fall time of approximately 120 ns. 1000 " ' ' I' " I—II 1 M I I I 1 T TT 1 I M i >| ' M I M M 11 I 1 I I I 1 I M .
900 Je= 17-34 A/CM^AND 10-17 A/CM*
800
700
600 +F -I fns) 500
400
300
\ \ Jg= 10-17 A/CM 200 h.--;?,---.-.^.. 0,.N, 100 -
— » _ "•• ••! .M.liii I It M I liM I li!M Im I Im I liiM Im i " 0 10 20 30 40 50 60 70 80 90 100 % ATTACHING GAS
Fig. 2 Effect of Gas Mixture on Current Falltime (tp) Another switch parameter of interest is the switch current gain, defined as the ratio of the discharge current to the e-beam current.
For a recombination dominated discharge the current gain is proportional to v 1/Jb where Jb is the e-beam current density. Small e-beam current densities yield high current gains in the switch.
Bletzi nger^-lO has measured current gains in methane as high as 3000 when J5 = 6 mA/cm^ but decreasing to 1000 for J5 = 1000 mA/cm^. Larger e-beam currents are often used, however, to shorten the closing time of the switch.
This thesis is concerned with the design and construction of a high current (I ~ 100 A) electron source and the development of diagnostic techniques to measure the total e-beam current and uniformity. Chapter II gives a review of thermionic emission processes for Th-W filaments. Chapter III describes the experimental arrangement.
Chapter IV contains a brief description of the diagnostic methods used in the experiment and the experimental results. Chapter V is the con clusion followed by an Appendix on scintillators. CHAPTER II
ELECTRON EMISSION FROM METALS
Electrons are emitted from metals through four fundamental processes. In all of these processes, the electrons must obtain a suf ficient amount of kinetic energy normal to the metal surface to surmount the potential barrier of the metal. In thermionic emission
(Richardson-Dushman), the energy necessary for electron escape is obtained by heating the metal to sufficiently high temperatures
(T >^ 1500OK). The escape energy can also be obtained by bombarding the surface of the metal with incident radiation (photoelectric emission) or with incident ions and electrons (secondary emission).
The fourth method of electron emission from a metal (field emission) involves lowering the potential barrier at the surface of the metal by application of an intense electric field.
Photoelectric emission produces, in general, only small current densities. Conversely, secondary emission currents approaching 60 mA per watt of incident primary-electron energy can be obtained, a value which is comparable to some of the more efficient thermionic emitters.
However, energy values of the secondary electrons, as they leave the surface, are distributed over a wide range, depending on the target material and its surface condition. Both photoelectric and secondary emission suffer from the necessity of providing an external photon or particle source. Field emission is capable of delivering extremely high current densities (j ^lO^-lO^ A/cm^); however, the cathode tends to be short-lived due to the heat produced during operation. Also, exploding whiskers at the surface of field emitters produce a plasma which migrates to the anode and shorts the electron gun. Thermionic emission using Th-W filaments was chosen as the electron source because the current density (j - 1 A/cm^) can be easily varied with filament temperature and because the pulse lengths are long enough to consider rep-rated operation by use of a control grid.
Richardson-Dushman Equation
The basic equation describing thermionic emission (derived in many texts) is the Richardson-Dushman equation which has the form
J = CT2 e -b/T (2-1)
where J is the emission current density in A/cm2. The constant C is given by
C = 4TTmek^ A/cm2/OK2 (2-2) h3 where e is the electronic charge in Coulombs,
m is the mass of the electron in kilograms, 8
k is Boltzmann's constant in Joules/^K,
h is Planck's constant in Joules-sec.
The constant b is given by
b = e
Numerically, C is found to be 120 A/cm2oK2 when appropriate values are substituted into Eq. (2-2), However, Eq. (2-1) is not an exact derivation, but must be regarded as an empirical equation in which C and b (both held constant) are determined experimentally to fit the data. Actual values of C and b for some pure metals are shownH in
Table 1. Discrepancies in values of C are due to assumptions made in deriving Eq. (2-1) and to the fact that the work function (and consequently b) is actually a function of temperature for some metals, and not a constant as assumed in the empirical fit.
The current density, J, is dominated by the exponential term in
Eq. (2-1). Metals with low work functions operated at high temperatures would appear to give maximum current densities. The metals with the lowest values of b, however, outgas excessively even at moderate temperatures and destroy the vacuum necessary for thermionic emission. Tungsten, which has the lowest vapor pressure of any metal. TABLE 1 REPRESENTATIVE VALUES OF THERMIONIC-EMISSION CONSTANTS FOR PURE METALS.^1
J = CT2e-b/T
Metal C in b in o K A/cm2/OK2
Calcium 60.2 26,000
Carbon 60.2 46,500
Cesium 16.2 21,000
Molybdenum 60.2 51,500
Nickel 26.8 32,100
Platinum 60.2 59,000
Tantalum 60.2 47,200
Thorium 60.2 38,900
Tungsten 60.2 52,400 10 can be operated at high enough temperatures to give emission current densities which exceed any of the other pure metals, in spite of its large b value.
Composite materials tend to be the best thermionic emitters.
Fig. 3 compares the emission current densities as a function of temperature for oxide-coated cathodes, thoriated-tungsten (Th-W) fila ments, and for pure tungsten filaments. A typical oxide-coated cathode consists of Ni as the base metal, coated with the oxides of Ba and Sr, the emission from a mixture of the two oxides being greater than from either alone. Although oxide-coated cathodes are efficient in terms of emission current per input power, they are not used in high power devices because of the low melting point of Ni (17 250K).
Oxide-coated cathodes are extremely susceptible to deactivation by positive-ion bombardment and must be used at low plate voltages. The tendency of these cathodes to evolve gas during operation makes it dif ficult to maintain a high vacuum and therefore more positive ions are present than in a tube with tungsten filaments. Also, the evaporation of barium, which plates out on other electrodes in the system, leads to problems with primary and secondary emission from these surfaces. For these reasons Th-W filaments were used in the construction of the electron gun. 11
2.0
1.8 o j o O o o m 1 c» o o 1 C » "ro ] q. II 1 f If .o 1 s^ 1 M II .o ro" 1 -Q ^m, II * CVJ o o 1 o o o z d o II 1 UJ o 1.2 O O tn 1 rsi TUNG J STE N
E ATED ; ATED ; r > r\ 1 .. LTJ C > O 1 UI z Ui / < 1 hZ-) ID E Xo // 0.8 C§I / ... Q ^. ^ 1 X / H/ 0.6
0.4 —y.—
0.2
0.0 800 1200 1600 2000 2400 2800 3200 TCK)
Fig. 3 Emission Current Density from Various Thermionic Emitters. II 12
Th-W Filaments
Thoriated-tungsten filaments (1-2% Th) have emissive properties which exceed those of pure tungsten filaments under certain conditions.
Tungsten has a melting point of 3655 ^K and a work function of 4.52 volts. At a temperature of 2000 ^K a tungsten filament can be operated for tens of thousands of hours without appreciable evaporation, but its emission at this temperature is only lO"^ A/cm^. Thorium has a low work function of 3.35 volts but it melts at 2100 ^K. Thus either metal, by itself, is not particularly suited as a thermionic emitter.
Th-W filaments, on the other hand, can be operated at 2000 °K, which is near the melting point of pure thorium, and produce current densities as high as 3.0 A/cm^, lOOO times that of pure tungsten at the same temperature. 12 According to Langmuir, the increased emission is due to a layer of thorium atoms adsorbed on the surface of the tungsten. That the work function of Th-W filaments (2.63 volts) is lower than that of either thorium or tungsten is attributed to the fact that the adsorbed layer of thorium on the filament surface is electropositive with respect to the tungsten. Consequently, it forms a dipole layer with its positive side away from the tungsten, and thus lowers the potential barrier at the surface.
To obtain the high emission current densities characteristic of
Th-W filaments, the filaments must first be activated; that is, metallic thorium must be brought to the surface by diffusion from the 13 interior. Most of the thorium in the filaments is in the form of Th02
(thoria) which dissociates apart only at high temperatures. For this reason, the filaments must be heated to approximately 2800 ^K for a few minutes to reduce some of the Th02 to metallic thorium. However, at 2800 OK thorium atoms evaporate from the surface at a faster rate than they can diffuse from the interior. The temperature of the filaments must be lowered to a suitable activation temperature
(1800-2200 OK) where diffusion occurs faster than evaporation, and a surface layer of thorium is formed. At operating temperatures (T ~
1800 OK) diffusion approximately equals evaporation and the fraction of the surface covered with thorium atoms remains nearly constant.
Activation curves, such as those shown^^ in Fig. 4, are obtained by first flashing the filament at high temperatures to remove any : residual film of metallic thorium on the surface and then rapidly lowering the temperature of the filament to an activation temperature
(1845 OK and 1940 OK in Fig. 4). Periodically, the electron emission current (J in A/cm2, j^^ is the maximum current density obtained) is measured at a testing temperature (1549 OK^ in this case) where diffusion and evaporation are negligible. From the figure one can see that the electron emission current rises to a maximum value (in less time in the case of the hotter filament) and then falls off to an equilibrium value at later times. The equilibrium value for the run at
1845 OK is somewhat lower than for the 1940 OK run. 14
V 0—0 -0.4 r
-0.8
-1.2 0 RUNATI <340 ^ • RUNA T \i 345«'K hi -1.6 ID 5 -2.0 /
9 -2.4 T 1
-2.8 7
-3.2 I -3.6
-4.0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 TIME OF ACTIVATION IN MINUTES
13 Fig. 4 Activation Curves for Th-W Wire • 15
Figure 5 plots^^ f versus the time of activation for the two temperatures in Fig. 4, where f = t/t^; t^ being the time required for
log(J/Jm) to reach its maximum value. There is some evidence that the maximum current density (J/Jm = 1) occurs when the thorium forms a
monolayer on the surface of the tungsten.^^ Comparing Figs. 4 and 5
one can see that the maximum current density for both temperature runs-
occurs when f is approximately unity. Therefore, the quantity f can be
taken as a measure of the amount of thorium adsorbed on the tungsten
surface. Initially, f increases linearly with time but gradually
approaches an equilibrium value for later times. This equilibrium
value is somewhat higher for the run at 1845 ox than for the run at
1940 OK. The finite value of f at t = 0 is due to the fact that one
can never completely deactivate the filament by flashing at high temp
eratures because the filament then has to cool through the activation
regime.
A qualitative explanation of Figs. 4 and 5 is given by Brattain
and Becker13 as follows:
"Thorium diffuses to the surface at a rate which is constant for a particular run; this rate depends on the activation temperature and on the amount and distribution of thorium in the tungsten. At first, the rate of evaporation of thorium from the surface is negligible, so that the amount of thorium accumulated increases linearly with time. As the surface concentration of thorium increases, the rate of evaporation increases rapidly. When the evaporation becomes comparable with the diffusion rate, the rate of accumulation of thorium on the surface is no longer equal to the rate of diffusion, but gets less and less until equilibrium is reached between 16
1.4 Equil. Values ^ ^ .-O--- -^- *-•*"" • r^^" 1.2 Q-'t) 4 / u • 1.0 / ^» A 0.8 f • RUN AT I940*»K 0 RLi N AT 18 45*K
0.6 /
/ 0.4
0.2
0 TIME OF ACTIVATION IN MINUTES
13 Fig. 5 Fraction of Surface Covered with Th(f) vs. Time of Activation for Data Shown in Fig. 4. 17
evaporation and diffusion. The rate of evaporation increases more rapidly with temperature than the rate of diffusion, and therefore as the activation temperature increases, the equilibrium value of f decreases".
The linear regime of Fig. 4 can be fitted with an empirical equation,13
log J = log Oyf + a(l-e-bt) (2-4)
where 0^ is the current from clean tungsten (A/cm^) and a and b are constants to be determined by experiment. To explain the nonlinear regimes of the log J versus t or f curves, Brattain and Becker maintain j that the effectiveness of an individual electropositive atom in j reducing the work function of the surface is not constant, but decreases as f increases. This in turn means that the fraction of ad sorbed thorium which is ionized decreases with surface concentration.
This is similar to results obtained with cesium or barium adsorbed on tungsten.1^ CHAPTER III EXPERIMENTAL ARRANGEMENT
The major components of the e-beam controlled opening switch system are shown in Fig. 6. The main emphasis of this thesis is on the e-beam and diagnostics portion of the diagram.
Electron Gun System
The mainstay of the entire system is the thermionic cathode shown in Fig. 7. The cathode is housed in a Pyrex chamber under high vacuum conditions (p 110"^ Torr) to prevent destruction of the Th-W filaments by positive ion bombardment and oxidation. A Pyrex chamber was chosen because it is an excellent vacuum material, has a high melting temperature, and provides optical access to the e-beam gun. It was necessary to see the electron gun in order to measure the filament temperature using an optical pyrometer. It was found during operation that tungsten from the filaments coated the inside of this Pyrex chamber and, if not cleaned periodically with acetone, breakdown would occur along the glass surface. Glow discharge cleaning of the cathode, using low pressure argon gas, also succeeded in raising the breakdown voltage.
The foil, shown in Fig. 7, acts as an interface between the vacuum in the cathode region and the high pressure gas of the switch chamber (not shown). The foil is clamped between two aluminum rings and rests
18 19
TRIGGERING
TRIGGERING GRID SYSTEM PULSER I I SWITCH r -"I r 1 I GAS GRID FRP-250 BIAS PULSER SYSTEM P.S. I i I FILAMENT S I SWITCH P.S. e-BEAM SWITCH I PULSER I SPREADER ii i. I Z J BIAS P.S. 1 VACUUM COOLING SCREEN SYSTEM SYSTEM PROBES ROOM
e-BEAM L DIAGNOSTICS
Fig. 6. Block Diagram of the E-Beam Controlled Opening Switch Sys tem. 20
WIRE MESH GRID GRID J— FILAMENTS
HIBACH! STAINLESS STRUCTURE STEEL
PYREX
-TO FILAMENT •GRID • VACUUM POWER VOLTAGE SYSTEM
Fig. 7. Cross Section of E-Beam Triode. 21 on a stainless steel Hibachi structure for added support. A wire mesh grid located under the Hibachi structure (and at the same potential as the foil) prevents the foil from being ruptured should a breakdown occur. An 0-ring in a groove in the lower aluminum support ring provides a vacuum seal.
The foil used was 1 mil titanium because of its structural strength and high melting temperature. For incident electrons of
250 keV (EQ), 90% of the electrons pass through the foil (Fig. 8) and exit with a mean energy (Egv) of ~ 200 keV (Fig 9)^5. Aluminum has higher number and energy transmission coefficients (T|^ and Eav/Eo> respectively) than titanium (Figs. 8 and 9), but aluminum is more likely to rupture at high pressures. The 1 mil titanium foil has been j used up to pressures of 4 atm in the switch chamber. For higher pres sures (p ^10 atm) it will be necessary to use thicker foils. Number and energy transmission coefficients for 2 mil aluminum and titanium are also shown in Figs. 8 and 9.
A more detailed view of the thermionic cathode is shown in
Fig. 10. Stainless steel was used in the construction of the main body of the cathode due to its low vapor pressure even at temperatures exceeding 500 OK in the chamber. The sensitivity of Th-W filaments to ion bombardment and oxidation is well documented and justifies the use of stainless steel. 22
100 200 300 400 Eo (keV)
Fig. 8 Electron Number Transmission Coefficient (T.. vs Incident Engergy (E^) for Various Foils.15 ^ 23
100-
^av(%) 80.
70 __
60 — 50 — 40 30" 20- 10-4- L
100 200 300 400 Eo(keV)
Fig. 9 Energy Transmission Coefficient (E/E) vs. Incident Energy (EQ).15 av o 24
-a o
I—
+-> M- O
+-> U
oJ/) <_>
CD
O 5 O O 25
A DC power supply capable of 200 V, 200 A, was constructed to supply heating current to the Th-W filaments. Power is introduced through a Ceramaseal, vacuum feedthrough rated at 150 A (leftmost feedthrough in Fig. 10). The filaments (15 mil Th-W) are mounted at one end on stainless steel posts which have grooves milled on top to keep the filaments separated. At the other end, the filaments are tied to tungsten posts which hold the filaments taught by spring-action. The electrical connection is made to the center post, providing two sets of ten filaments each, operating in parallel. This method minimizes the voltage necessary to heat the filaments, which in turn ensures that electron trajectories from the gun will not be significantly altered by the potential at the center post.
The center feedthrough in Fig. 10 is connected to a stainless steel spreader plate which is biased to -1000 V. This negative potential, along with the positive accelerating voltage applied at the foil, causes electrons emitted from the filaments to be repelled from the cathode. The spreader plate also provides a shield for the more sensitive parts of the cathode (0-rings, insulators, etc.) from the radiated heat of the filaments.
The third feedthrough provides power for the grid. The grid structure provides control of the e-beam, similar to the action of the grid in a conventional triode. The grid is pulsed on and off to obtain repetitive pulses of electrons through the foil. 26
A water-cooled aluminum heat sink was incorporated into the design of the cathode to keep the 0-rings nearest to the filaments from melting. Although having a lower melting point and vapor pressure than stainless steel, aluminum has a better thermal conductivity and thus transports heat more rapidly. Water is piped in through vacuum tight, silver solder joints. The insulators used on the cathode are slabs or rods of boron nitride, chosen because of its low vapor pressure even at high temperatures (10-6 jorr at 1800 OK).
In order to verify filament activation, the emission current at low temperatures (T < 1800 OK) was measured, as shown in Fig. 11.
Theoretical values were calculated for 15 mil filaments assuming an emitting length of 10 cm for each of the 20 filaments. The entire ! surface of the filament (top and bottom) was assumed to emit electrons.
From these values, and using the Richardson-Dushman formula (Eq. 2-1), the solid line curves for Th-W and pure W were calculated. The lower experimental points were obtained using the circuit shown in Fig. 12.
Data points were limited by the current capabilities of the power sup ply. The upper experimental point was taken during an actual pulse firing of the e-beam gun. The measured values of the emission current can be seen to follow the calculated values for activated Th-W filaments. Unactivated Th-W filaments display emission characteristics similar to pure tungsten. 27
le(A)
1100 1200 1300 1400 1500 1600 1700 1600 T(K)
Fig. 11. Calculated Electron Emission from Pure Tungsten and Thoriated Tungsten (dots: experimental results). 28
10 M A ir- t- BEAM ? I CURRENT METER I' HVPS
Fig. 12 Test Circuit for Emission Data Shown in Fig. 11. 29
Vacuum System
The primary components of the vacuum station are shown in Fig. 13.
The main chamber is a Pyrex glass tube, 30.5 cm in diameter with 22.3
liters total volume. This chamber is pumped by an Edwards Diffstack
system which includes a 100/300 series (280 liters/sec) diffusion pump,
a manually-operated, backing/roughing valve, and a manually-operated,
butterfly valve at the pump inlet. For safety reasons, the butterfly
valve has been refitted with an electro-pneumatic control system, as
shown in Fig. 14. If power is lost to the solenoid(S), the butterfly
valve on the top of the diffusion pump is closed by the air cylinder
(A), thus isolating the main chamber. Simultaneously, the diffusion
pump heater is shut off through the opening of a relay.
Loss of power to the solenoid is caused by any one of four
automatic alarms: high cooling water temperature on the diffusion
pump, loss of cooling water, low air pressure in the building air-line,
or overpressure in the main chamber. The differential pressure switch
(DPS in Fig. 14) shuts down the vacuum system if the building air
pressure drops below 40 psig (the minimum pressure necessary to close the air cylinder). The air bottle (B) provides a reservoir to ensure complete extension of the air cylinder. The butterfly valve is also closed automatically when the electron gun is fired to prevent the diffusion pump from receiving an air blast should the foil rupture. 30
THERMAL EXPANSION VALVE
lATIC CONTVIOL VALVe
lACKIWI ROIMHIIIt MALVe
Fig. 13 Vacuum System. 31
BUILDING AIR I (50 psig) CONTROL VALVE— CLOSED WHEN ROD FULLY EXTENDED (N.a) DPS (N.O.) 1 -ixi— s
^
^^VENT
Fig. 14 Pneumatic System. 32
A cold trap, using R-12 refrigerant (freon), was installed to prevent oil vapor migration from the diffusion pump to the main chamber
(Fig. 13). The cold trap consists of a chevron-type optical baffle, maintained at -20OC by means of an thermal expansion valve on the dis charge of the compressor. Santovac 5 was used as the diffusion pump fluid due to its lesser tendency to "creep" than most silicon oils. The ultimate vacuum obtained by this system was 5 x 10"7 Torr, limited primarily by the outgassing of materials.
Pulser System
The accelerating voltage to drive the e-beam diode is provided by a Physics International, FRP-250 pulser. This pulser is designed to work into a 100 Q parallel plate transmission line and is capable of producing a 250 kV pulse with a risetime of < 10 ns (10-90%) and an exponential decay of approximately 1 ys when working into a matched load. At the 250 kV charging voltage, the output energy is approximately 350 J.
The circuit diagram for the pulser is shown in Fig. 15. Each capacitor bank consists of four each, 5 nF, cylindrical capacitors connected in parallel, or 20 nF for each bank. The RC charging time for each bank is on the order of 0.06 sec. using the 3 M^ charging resistors. Each bank is dc-charged from a Universal Voltronics power supply with a charging voltage of + 125 kV (bottom bank) and -125 kV
(top bank), and a charging current of < 50 mA. 33
OJ 00
UJ Q_
O ID
PU T C\J o iPLi r to en H CVJ 1- 3 Q- CO o r n O U
rtJ E QJ
LD
O) 34
The pulser container is made of fiberglass and holds a slight overpressure (1.0 psig) of a high breakdown strength gas (such as SF5) when operating at high voltages. In the rear of the pulser is a trig gered, pressurized spark gap which connects the capacitor banks in series. Figure 16 shows the gas system for the FRP-250 pulser and
Figs. 17 and 18 show plots of the spark gap pressure vs. the total charging voltage for the self-fire and triggered modes of operation.
Cooling System
A closed loop, water cooling system was constructed to provide cooling water for the diffusion pump and the aluminum heat sink in the cathode. A block diagram of the system is shown in Fig. 19. Water from the reservoir is continuously pumped through an air-cooled radiator, a particle filter, and a deionizing column. A thermostat control senses the water temperature in the reservoir and turns the fan on and off as needed. A flow switch is provided at the top of the reservoir which shuts off the e-beam filaments and the diffusion pump in the event the water pumps become unprimed or a leak develops. The use of deionized water and plastic tubing insures electrical isolation of the cooling system from high voltages. The filtered and deionized water is also less corrosive on the copper tubing in the diffusion pump and the short sections of copper tubing leading to the cathode. 35
Vessel Switch 0-100 n psig gauge Bleed 1 psig Valves^ pop-off ^-50 3sig regulator 100 psig Vessel 0-10 pop-off psig gauge ^Vessel Gas Input
10-2 Ipsig •regulator Note: Do not exeed 1 psig in vessel. GAS SUPPLY SF^ Regulator
Fig. 16 Gas System of FRP-250 Pulser. 36
75 90 105 120 135 150 165 180 195 210 225 250 265 TOTAL CHARGING VOLTAGE . kV
Fig. 17. Spark Gap Pressure vs. Total Charging Voltage for FRP-250 Pulser in Self-Fire Mode. 37
75 1 1 1 1 1 1 1 70 65 60
55
50
Q. A5 "
^ 30h cUrJ Q- 25 h 3 20 I 15 ^ 10
0 -
30 50 70 90 110 130 150 170 190 210 230 250 265 TOTAL CHARGING VOLTAGE . kV
Fig. 18. Spark Gap Pressure vs. Total Charging Voltage for FRP-250 Pulser in Triggered Mode. 38
DEIONIZING COLUMN DIFFUSON E-BEAM PUMP CATHODE
FILTER
RADIATOR PUMP n RESERVOIR 1 ® VALVES COOUNG THERMOSTAT FAN
Fig. 19. Water Cooling System. 39
Grounding and Shielding System
The grounding and shielding system for the experiment is shown in
Fig. 20. Thin aluminum sheets were placed under the FRP-250/switch area and also under the operator area where the controls for the filament power supply and pulser are located. These sheets serve as a low impedance ground plane to protect operators and equipment in the event of a major system fault. All power supply and control cabinets are connected to the ground planes at a single point using conductors of minimum length and large surface areas. The two ground planes
(shown in dashed lines in Fig. 20) are connected to one another by a conductor in the trench. A single point from the FRP-250/switch ground plane is connected to the aluminum bus bar in the trench.
Power to all the sensitive equipment, including power supply controls, the ion gauge controller, and diagnostics in the screen room, is supplied through isolation transformers. As added protection for the diagnostics in the screen room, EMI filters were placed on the power lines after the isolation transformers. These filters pass 60 Hz but attenuate higher frequencies. Power lines to the screen room are also encased in conduit to shield against stray E and B fields.
The screen room is provided with a pneumatic operated disconnect on the ground connection (isolation switch in Fig. 20). This disconnect is triggered by a push button switch inside the screen room once the door is closed. Noise problems on the ground line were eliminated by floating the screen room in this manner. When the 40
E OJ +-> CO >^ en
•o
•r—
CD •u o
O CVJ
a> 41 screen room door opens, the disconnect closes automatically. During actual firing of the e-beam the screen room is grounded through cables to the voltage and current monitors. CHAPTER IV
DIAGNOSTICS AND EXPERIMENTAL RESULTS
Voltage Measurements
A two-stage resistive voltage divider^^ was designed to measure the voltage pulse from the FRP-250. The probe, shown in Fig. 21, is a
5 1/2 in. diameter by 12 in. long Plexiglass tube filled with a solution of CUSO4. The electrode plates are made of Cu because of its good conductivity and because it does not deteriorate in the CUSO4 solution. For the initial pulser tests, a probe impedance of 100^ was used since the voltage divider also serves as a matched load for the pulser. The attenuation ratio of the probe, for the lOOf^ impedance case, is approximately 1500:1, as shown in Fig. 22a. The #1 electrode is mounted on a teflon rod and provides the first 10:1 attenuation of the initial pulse. The pulse is further reduced in amplitude by a stack of resistors mounted inside the hollowed-out center of the teflon rod. This resistor chain is terminated by a BNC connector and the voltage pulse is recorded on a Tektronix 519 oscilloscope.
A typical voltage pulse from the FRP-250 is shown in Fig. 22b. The risetime (10-90%) of the pulser is approximately 10 ns with a 1 ys exponential decay. This picture was taken at a charging voltage of
-200 kV.
42 43
0) 3 o
o CL 0; a> CU O E 5 Of o c a. ._ c o CL- o 5 Q: ^ Q. zr *•- -^ CD CL 6 CD o .— o "*-rMm-^Lnvoc*^oocrf— LL »>— .o o
+->
CVJ
CT^ 44
90^ 500X1
ion 3.3il son CABLE
(a)
H200nsh- (b)
Fig. 22 Voltage Probe Measurements (a) Circuit Diagram of Voltage Probe, (b) Voltage Trace of FRP-250 Pulser. 45
Current Measurements
A Rogowski coil 17-20 v/as designed and built to measure the e-beam current. A schematic of the coil is shown in Fig. 23. Physically, the coil consists of 122 turns with C' (capacitance/unit length) equal to
1.3 pF/cm and L' (inductance/unit length) equal to 0.3 yH/cm. These values were obtained using a Tektronix Type 130 L-C meter. From these measured values the total impedance of the coil is ZQ = v L'/C
~500 ^. The sensitivity of the coil was measured to be 0.1 V/A.
Physical dimensions of the coil include: diameter - 11.5 cm, circumference - 36 cm and cross-sectional area -2.25 cm2. The coil 15 wound on a Plexiglass core and is enclosed in an aluminum electrostatic shield. A 1 cm slit is cut azimuthally around the inside circumference of the shield to allow for magnetic field coupling.
Because the risetime of the current pulse is on the order of the transit time of the signal induced in the coil windings (2r =
4 rTTV^'C ~ 50 ns), the operation of the Rogowski coil must be modeled 1 Q pi as a transmission line."^ If the output impedance is much less than the characteristic impedance (ZQ) of the coil, the sensor integrates the signal and provides a voltage trace proportional to the current.
Because of noise problems, RL was chosen equal to ZQ, thus matching the system. The current pulse must, therefore, be reconstructed numerically. 45
ELECTROSTATIC TOROIDAL SHIELD COIL
ELECTROSTATIC SHIELD BROKEN AT POINT OF CONNECTION
TERMINATING IMPEDANCE
Fig. 23 Schematic of Rogowski Coil. 47
Reconstruction of the current pulse can be seen in Fig. 24. The top figure represents the input current I(t). From calculations due to 21
Krompholz,^"^ it can be shown that the signal current Is(t) is the input current minus the current at a time 2T earlier (where 2T is the two- way transit time of the coil, shown in the middle figure). The bottom figure shows that the input current can be obtained by adding the current I(t - 2T) to the signal current Is(t).
A reconstructed e-beam current pulse is seen in Fig. 25. The e-beam current is obtained by subtracting the displacement current, measured when the pulser is fired with no e-beam, from the current measured with an e-beam. The e-beam current pulse is almost rectangular, with an amplitude of approximately 60 ±_ lOA. The accuracy of the e-beam current measurement can be improved by reducing the magnitude of the displacement current. A new, lower inductance voltage monitoring circuit has been designed to reduce the displacement current contribution.
The Rogowski coil signal shown in Fig. 26 was obtained by applying a negative square pulse to the grid of the triode. For the first 100 ns the grid is on and only displacement current is measured. The e-beam current is then allowed to flow for ~ 50 ns. Since this time is equal to the two-way transit time (2T) of the Rogowski coil, the opposing
"humps" of the signal appear together, with no delay between them (see
Fig. 24). The pattern is repeated when the grid turns back on. 48
INPUT I(i)
OUTPUT SIGNAL
s(^)=I(^)-I(1:-2x)
•w -t TRANSIT TIME 2x
INPUT RECONSTRUCTION 7 ACCORDING TO \ I("b)=S(-b)+I(t-2'er)
-t
Fig. 24. Reconstruction of Current Pulse. 49
III!
n '\ \ r,-''/ / \v. / ^"\ \ .1 / .-, /\ \ \ r^ N \ \ V —^ ^—\j^ I\\jt \r V \ V /
1 1 1
Fig. 25. Rogowski Coil Signal of E-Beam Current, 20 ns/div. (top). E-Beam Current (upper trace) Evaluated by Subtracting the Rogowski Coil Signals of the Currents with an without E-Beam (lower traces). Solid line: Current with heated Cathode; Dashed line, Current with Cold Cathode. 50
-HlOOns K
Fig. 26. Rogowski Coil Signal of E-Beam Current with Pulsed Grid. 51
E-Beam Uniformity and Time Dependence
An NE-102A scintillator (see Appendix) was used for two purposes in this experiment: (1) to check the spatial variation of the e-beam that passed through the foil, and (2) to obtain information as to the time dependence of the e-beam current. The experimental arrangement for this technique is shown in Fig. 27. The scintillator is mounted about 1 cm from the Ti foil and intercepts the e-beam. The resulting fluorescence is captured on Polaroid 107 film using a Tektronix C-5A oscilloscope camera. The camera was mounted inside a small black plastic dark room so that the shutter could be left open during the e-beam shot. A small part of the light pulse is diverted through an optical fiber to a photomultiplier tube in the screen room. The output of the PMT is then displayed on a Tektronix 7834, storage oscilloscope.
A typical picture from the camera is shown in Fig. 28. The e-beam passing through the foil almost fills the 10 x 10 cm window. The horizontal dark lines running through the photograph are regions where no electrons penetrate the foil. These dark lines are caused by the scattering of electrons due to the Hibachi structure which holds the foil. The e-beam, although somewhat more intense near the center of the picture, is otherwise uniform. Bowing of the filaments could account for the higher energy electrons near the center since the higher resistance would cause the filaments to heat more in this region. This bowing effect can be minimized by tightening the springs which hold the filaments in pi ace. 52
-C-5 CAMERA BLACK '^ OPTICAL LINK PLASTIC FROM SCINTILLATOR TO PMT IN SCREEN ROOM ^ e-BEAM FRP - 250 J>100il PULSER
Fig. 27. Experimental Arrangement for Scintillator Tests. 53
Fig. 28. Scintillator Fluorescence Due to E-Beam Excitation. 54
Figure 29 shows the time dependence of the light pulse from the scintillator, as seen by a photomultiplier tube. The two pulses are approximately 75 ns in width and separated by 120 ns. A grid pulser is used to obtain the electron bursts. The PMT output is proportional to the light output of the scintillator which, in turn, depends on the energy and number of electrons striking the scintillator. The charging voltage on the FRP-250 pulser was 170 kV, corresponding to electron energies of -130 keV for the first pulse in the picture (the electrons 15 lose 25% of their energy passing through the foil). The reduction in amplitude of the second pulse can be explained by a loss in the number and average energy of the electrons. By the time the second pulse arrives the accelerating voltage at the anode has decreased to ~ 150 kV making the electron energies after passing through the foil ~ 100 keV.
Also, only 60% of the incident number of electrons will pass through foil for the 150 keV case as opposed to 70% for the 170 keV electrons. 55
Fig. 29. Scintillator Pulses as Recorded by a PMT. CHAPTER V
SUMMARY
A high current electron gun, utilizing thermionic emission from
Th-W filaments, has been designed and tested. The electron gun acts as a conventional triode with a negatively pulsed grid providing 100 ns bursts of ~ 200 keV electrons through the foil. The total electron current can be varied depending on the filament temperature.
The accelerating voltage of the electron beam was measured using a two-stage (CuSO.) resistive voltage divider with a 1500:1 attenuation ratio. The voltage pulse shows a risetime (10-90%) of ~ 10 ns and a lys exponential decay, characteristic of the FRP-250 pulser. A Rogowski coil was designed to measure the e-beam current. A rectangular shaped, e-beam current pulse (I^^^K ~ 60 A) was measured using this coil. peaK
Information about the temporal and spatial variation of the e-beam current was obtained by observing the fluorescence produced by electron bombardment on a plastic scintillator (NE-102A). The electron beam is 2 shown to be uniform over most of the 100 cm window area. The electron gun is part of the e-beam controlled opening switch system shown in Fig. 30. This system will be used to study e-beam sus tained, high pressure discharges in various gas mixtures.
56 57
SWITCH PULSER V 25 kV max VOLTAGE 12.5 IcA PROBE max T 1 us
SWITCH CHAMBER
TITANIUM SWITCH FOIL ELECTRODES H WINDOW
^fall= 2 us
CATHODE ASSEMBLY ROGOWSKI COIL
E-BEAM CHAMBER VOLTAGE PROBE
Fig. 30. Cross Section of Triode and Switch Chamber, LIST OF REFERENCES
1. Kristiansen, M., "Fundamentals of Inductive Energy Storage." ARO Workshop on Repetitive Opening Switches, Tamarron, CO (1981). Appendix A, pp. 313-321.
2. Hunter, R.O., "Electron Beam Controlled Switching." Proc. 1st IEEE Int. Pulsed Power Conf., Lubbock, TX (1976) IC8-1; Also see U.S. Patent 4,063,130.
3. Kovaltchuk, B.M. and Mesyats, G.A., "Current Breaker with Space Discharge Controlled by Electron Beam." Proc. 1st IEEE Int. Pulsed Power Conf., Lubbock, TX (1976). IC7-1.
4. Hallada, Marc R. et. al., "Application of Electron-Beam Ionized Discharges to Switches -- A Comparison of Experiment with Theory." IEEE Trans. Plasma Sci., PS-10. 218 (1982).
5. Kline, Laurence E., "Performance Predictions for E-Beam Controlled On/Off Switches." ARO Workshop on Repetitive Opening Switches (M. Kristiansen and K. Schoenbach, editors), Tamarron, CO (1981), pp. 121-127 (DTIC No. AD-A110770).
6. Scherrer, V.E. et. al., "The Control of Breakdown and Recovery in Gases by Pulsed Electron Beams." Third International Symposium on Gaseous Dielectrics, Knoxville, TN (1982) pp. 34-39.
7. Kline, Laurence E., "Performance Predictions for Electron-Beam Controlled On/Off Switches." IEEE Trans. Plasma Sci., PS-10, 218 (1982).
8. Bletzinger, P., "I-V Characteristics of Externally Ionized Plasmas." Bull. Amer. Phys. Soc. 26^, 718 (1981).
9. Bletzinger, P., "Switching Experiments with a Small E-Beam." ARO Workshop on Repetitive Opening Switches (M. Kristiansen and K. Schoenbach, editors), Tamarron, CO (1981), pp. 128-142 (DTIC No. AD-A110770).
10. Bletzinger, P., "Electron Beam Switching Experiments in the High Current Gain Regime." Proc. 3rd IEEE Int. Pulsed Power Conf., Albuquerque, NM (1981), pp. 81-84.
11. Gray. Truman S., Applied Electronics. 2nd ed. New York: John Wiley and Sons, 1954.
12. Langmuir,!., "The Electron Emission from Thoriated Tungsten Filaments," Phys. Rev. _22, 357 (1923).
58 59
13. Brattain, Walter H. and Becker, Joseph A., "Thermionic and Adsorption Characteristics of Thorium on Tungsten." Phys. Rev. 43. 428 (1933).
14. Becker, Joseph A., "Thermionic and Adsorption Characteristics of Caesium on Tungsten and Oxidized Tungsten." Phys. Rev. 28, 341 (1926). ~
15. Seltzer, S.M. and Berger, M.J., "Transmission and Reflection of Electrons by Foils." Nucl. Instr. and Meth., l^, 159 (1974).
16. Bishop, A.E. and Edmonds, G.D., "Electrolytic Resistors in Plasma Physics Research." Plasma Physics (Journal of Nuclear Energy, Part C), 7_, 423 (1965).
17. Anderson, J.M., "Wide Frequency Range Current Transformers." Rev. Sci. Instr., 42, 915 (1971).
18. Cooper, J., "On the High-Frequency Response of a Rogowski Coil." Plasma Physics, 5_, 285 (1963).
19. Pellinen, D.G., et. al., "Rogowski Coil for Measuring Fast, High-Level Pulsed Currents." Rev. Sci. Instr., 5J_, 1535 (1980).
20. McKnight, R.H. and Hebner, R.E., ed.. Measurement of Electrical Quantities in Pulse Power Systems (NBS Special Publication 6 28, June 1982), Session C- Current Measurements, pp. 175-193.
21. Hermann Krompholz, Texas Tech University, private communication (1983).
22. Schram, E., Organic Scintillation Detectors. New York: Elsevier Publishing Co., 1963
23. Birks, J.B., The Theory and Practice of Scintillation Counting. New York: Pergamon Press, 1964.
24. Frisch, O.R., ed.. Progress in Nuclear Physics, vol. 5, New York: Pergamon Press, 1956.
25. Scintillators for the Physical Sciences, EMI Gencom Inc., 80 Express St., Plainview, N.Y. 11803. APPENDIX SCINTILLATORS
Theory
Simply stated, a scintillator is any material which emits a brief pulse of fluorescent light when it interacts with a high-energy
particle or photon. Only a limited number of molecules show an ability to fluoresce; it is an inherent molecular property and arises from the electronic structure of these molecules. Scintillating materials can be composed of either organic or inorganic substances and the
application often determines which is used. Inorganic scintillators
[Nal being the most common] are often used to detect high-energy
radiation (x-rays, Y -rays, etc.) because of their higher densities and correspondingly greater stopping power. The disadvantage of inorganic
scintillators is that they must be used in the crystalline state whereas organic fluors can be crystalline, liquid, or, as in our case,
plastic. Plastic organic fluors are rugged, can be machined into desired shapes, and are, in general, more versatile.
The mechanisms involved in the scintillation process can be described briefly. The energy of an incident particle is initially consumed in ionization and excitation of the electrons in the scintil lating material. This excitation energy may be divided into three main categories, namely, electronic(S), vibrational (V), and rotational (R)
(Fig. 31). De-excitation of the molecules occurs through the following processes: 60 61
•n 'Radiation less Transition (100-efficient)
1 ' S1 ^^^^ Internal ^ Conversion
i^ FLUORESCENCE
Yz s \ Rn • ^ ^ Internal • i .Degradation • n c / r^ ^ / V 1 \ J ^1 ^ / > •
Fig. 31. Energy Level Diagram Showing Fluorescence Process 62
(1) internal degradation - vibrational energy is dissipated as
heat by collisions with surrounding molecules or lattice
vibrations, (2) internal conversion - through coupling between electronic and vibrational levels, electronic excitation energy may be converted into vibrational energy and then dissipated as heat,
(3) emission of 1ight - fluorescence caused by transitions from the first excited state to the ground state of the molecule.
Excitations to higher electronic states decay rapidly (t ~ 10" s) by radiationless processes to the first electronic state and then fluoresce. Only 10% or less of the entire excitation energy is given off as visible light, meaning that most of the energy is dissipated as heat through processes (1) and (2).
One important consequence of the strong coupling which occurs between electron motion and atomic vibrations (internal conversion being an example of the coupling) is the shift observed between the absorption and emission spectra of scintillating substances. The emitted quantum has a smaller energy than the absorbed one, meaning there is a shift towards longer wavelengths. This phenomenon is directly responsible for the high efficiency of most scintillators since the emitted light is only partially reabsorbed by the scintillator itself. 63
Experiment
The scintillator used for e-beam detection in this experiment was
NE-102A (obtained from LANL). NE-102A is a pi astic, scintill ator whose important physic properties are listed in Table 2 along with some of 25 the more common scinti 11 atars . Anthracene is an organic crystal with the highest known light output and is therefore used as the standard (100%). Figures 32, 33, and 34 give other pertinent data 25 available on NE-102A. It can be seen that plastic scintillators have some yery useful properties including \jery fast rise and decay times.
The NE-102A obtained from LANL had small scratches and patches of haze on the surface and was badly crazed. (Crazing has the appearance of thousands of tiny scratches when viewed at an angle in light.
Actually, these are fractures under the surface.) Micro-Mesh
(obtained from Micro-Surface Finishing, Inc., Wilton, Iowa) etc., a series of graded, sandpaper-type abrasives, was used to repair the damage and restore a polish to the scintillator. The polish was necessary since light otherwise would be lost due to reflections at the rough surface. TABLE 2 SCINTILLATOR PROPERTIES. 25
Light Pulse Decay Rise Wavelength Output Width Time Time Max. Emission Type (% Anth.) (FWHM ns) (ns) (ns) (nm)
NE-102A 65 2.7 2.4 0.9 423
NE-104 68 2.2 1.8 0.6 406
NE-104B 59 3.0 3.0 1.0 406
NE-110 60 4.2 3.2 1.0 434
NE-114 50 5.3 4.0 434
Pilot U 67 1.2 1.4 0.5 391
64 65
100
0.001
ENERGY(MeV)
25 Fig. 32. Range of Electrons and Other Particles in NE-102A. 66
25 Fig. 33. Response of NE-102A to Electrons and Other Particles. 67
100 1 /\^ 1 1 I -
80-— —
(/) 60-— 1 o 1 X \ \ \ \
40 - \ \ UJ
u 20 — / -- 5 -I UJ
1 1 1 1 400 420 440 460 480 500 WAVELENGTH nm
Fig. 34. Emission Spectrum of NE-102A.^^