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Response of Low-density KC1 Foils to Multi-MeV Electrons†
Response of Low-density KCl Foils to Multi-MeV Electrons? S t u tifo i tl
E. 1,. (:AR\.\’IN s r i t l . I , EL)(:E(‘UMHE I , i ricit 1, d ccr 1e 1 at u r C‘cI I t o ) (+Sf,.-I( ’) , Str r t tfo rtl I 1tr I LY r s i t y ~ ~ t ~ l l L f t J l ‘ f~! !‘, ~ I ~ f I f1 ;O f L .I I ‘.,\‘.A.
INTROD ITCTI~N The high gain and fast response time relwrted by Goetze et ~ i l . l - ~ for low density KCl dynodes make their applic*ation to direct particle detectioii as envisaged by Alvarez and Cone3 seem promising. A t multi-C:eV primary energies, determination of the r a t mass of a “counted” particle becomes very difficult hecause of the multiplicity of possible final states of an interaction. If the secondary yield of low density foils is greater than approximately 5 for minimum ionizing particles, and if it follows the relativistic rise in the ionization loss and has a reasonable statistical number distribution, then velocity determination (and hence rest-mass tleterniination if the momentum is known) of primaray radiation would be readily avhievable with a n array of foils. A single such foil, coupled to a multiplier section, would act not only as a particle detector but could give excrlleiit space- and time-resolution . The present work was undertaken to investigattx the possibility of using low-density deposits of KCl and CsI in relativistic-rise detectors. It was found t hat 5-6 secondaries per 1)rirnary are obtained, on the average, from low-density KCI for primary electrons of energy from 100 t o 1000 MeV, and th at thc gain follows vlosely the relativistic rise in the ionization loss. DVNOD IC Y R EPBRATI( )N A cross-section of a low-density dynode is shown in Fig. 1 . The substrate cmisists of a 1 in. diameter, 1000 A thick self-supporting aluminum oxide (A1,03) film formed by a n anodization-etching technique similar to th at of Hauser and K e r l c ~ .A~ 500 -4thick aluminum conductive backing is deposited h y t hrrmal evaporation on at room the L41z0J. Idow-dtwsity KC:l is d(’J)OSjt(d 011 a s ~ ib s t~ r a te tempc nt ur e l)y clvaporation from a taiititlum h a t in argon a t a pressure ‘I’ypic.ally, 25 mg of KC’I is e\aporatcd a t 4 torr static of a fkw argon pressure. using a %in. boat-to-substrate sparing; the KCll is t This work waH supported by the U.S. Atonill. Energy COII~II~ISAIOII. 1,
4-3
636
E. L. OARWIN A N D J. EDGECUMBE
first melted and is then evaporated in approximately 20 sec. This gives a dynode with a thickness of approximately 25 pm and a density of 2 to 3% of the bulk density. Low-density CsI can be prepared in essentially the same way except a molybdenum boat is used and the evaporation time is longer (- 100 sec). These dynodes are about 30 pm thick and have approximately 2% of the bulk density. Density and thickness of the dynodes
login,
Sapphire windows (0.1in.)
/ ,A L supports 7 d
Sapphire o w s (0.1i n . )
A u photocathode ( ~ 5 a )
Ip ,"down-stream" orientation
Au c o l l e c t o r
Ag mask ( ~ 5 0 0 0 % )
A 1 2 0 3 s u p p o r t i n g f i l m (lo0081
Low-density K C L
(pNo.02 pbulk)
AL conductive backing ( 5 0 0 i % )
PIO. 1 . Cross-section of a low-density dynode and the experimental arrangement for measuring the secondary yield.
was determined by weighing and measurement with an optical microscope in a separate calibration using a standard evaporation procedure. All evaporations were performed in a diffusion-pumped, freonrefrigerator-baffled, unbaked varuum system with a base pressure - 2 x 1 0 - 7 tom. Dynodes were transferred to small ion-pumped, all-metal, vacuum systems (base pressure - 5 x torr) for measurement of the gain. The transfer of the dynodes was carried out in a n atmosphere of dry nitrogen. The gain a t low primary energies, -10 keV, was measured (see Fig. 1 ) using a gold photocathode illuminated by a type BH-6 high-pressure mercury lamp.? Typical curves of gain (6) plotted against collector potential ( V , ) are shown in Fig. 2. These results for low-density KC1 are in good agreement with the data reported by Goetze et aL2 The low-energy characteristics were found to be unaffected by bombardment with multi-MeV electrons.
t
Supplied by PEK Labs, Sunnyvale, California, U.S.A.
RESPONSE OF I,O\V-I)ENSITY KCI FOILS TO MULTI-IIEV ELECTRONS
637
-
50
40
-
0
C
2
30
K C L 144
A C s I 28
-
-
EP=10 keV ~
-
50
100
I50
200
250
300
3 0
V , , Collector potential ( V )
MEASUREMENTTEVHNIQUE Gain measurements for multi-MeV primary electrons were carried out a t thc Stanford Mark 111 linear electron accelerator. A photograph of the experimental arrangement is shown in Fig. 3. The Faraday cup, which was used to measure the primary current, was designed to be more than S U ~ efficient o over the range of primary energies investigated (100-loo0 The value of S was determined from the slope of the curve (as plotted on a n X-Y recorder) of integrated current leaving the dynode plotted against integrated current collected b y the Faraday cup. Integrated current was measured. instead of the current itself, not only because of the large amount of elec*tricalnoise associated with the accelerator, but also to remove cffects of small heam-current fluctuations. A collector potential of 165 V was w e d in the present experiment because at higher potentials the gain of KCI was not sufficiently stable. Sporadic increases in 6 manifested themselves as steps in the curves. These stells occurred very infrequently at I’, = 166 V and probably correspond to the scGntillations reported by (foetze Pi ri1.2 The. measured valiirs of‘ S prcsc~ntrtlIwre are averages of 6 or A separatv t3et~c~minat,iotis at ii fixed valui, of ~)rimaryenergy ( K p ) ,while the eri*ors shown correspond to the range of values obtained. The beam was left on during the measurements at a fixed E , t o minimize
63s
E. L. GAR\VIN A N D 3 . E D G E C U M B E
FIG.3. Photograph of t h e experimerltd arrangement for ineaauririg t h o sacoiidary yield of low-tlcrisity dynodes at multi-MeV primary energies. 1, Vacuum system CDIItainiiig dynode; 2, Paraday cup; 3, end of the linear accelerat,or; 4 m d 5, television cammas for observing the heam spot, on ZnS acreem.
discharging of the exit, surface, but had t o be turned off to change energy. After changing energy, the exit surface was given sufficient time to charge before the measurements were continued. The data were taken by monotonically decreasing the energy from maximum, and possible deterioration of 6 under bombardment was checked by repeating the measurement a t maximum energy. The energy of the primary beam was known to an accuracy of about 5 MeV.
D~SCLTSS~ON OF EXPERIMENTAL RESULTS The gain of low-density KCl dynodes was found to he strongly dependent on beam intensity as shown in Pig. 4 (average beam current in amperes N _ 10-l' x electrons/pulse). This effect is believed to be due to bombardment-induced conductivity limiting the internal field. The few per cent difference in 6 for Yc = 165 and 200 V (apparent in Fig. 4) is consistent with the low-energy data in Fig. 2. To minimize changes in 6 resulting from changes in beam intensity, the energy
IiESPONSE OF LOW-DENSITY KC‘I FOILS TO MULTI-MEV ELECTRONS
-
0
-0-
- - -oftr-t3-
-0-
--0
K C I 144 0
-- . \
o \
v;.t2oov
A V,
=t I65 V
\
Ep = 9 6 5 M e V Down-streom orientation spot s i z e 5 1.25cm2 10’
631)
I
I
108
lo9
\
\
0
I
10 0
Beam intensity ( e l e c t r o n s / p u l s e )
Frc:. 4. Intensity dependence of the gain for a low-delisity KCL dynode.
dependence of S was determined at a beam intensity of (1.5 f0.2) x lo8 electrons/pulse. The results of these measurements are presented in Fig. 5 . The logarithmic increase in 6 with increasing energy is evident and arises from the relativistic rise in the ionization loss. The experimental points a t the lowest primary energies may have been shifted
d r’
4.5
A “U p -slream”orientation 0 “Down-stream”
orientation
V, =+165V, spot s i z e ~ l . 2 5 c m ‘ B e a m intensity- (1.5 5 0 . 2 ) x108 electrons/pulse 100
200
300
400
600 700 800900 1000
500
E P , P r i m a r y energy ( M e V )
FIG.3. L),vtiodu gain as a
fiitirtioti of priiiitlry cmwgy for
EI
low-tloirsity KCI
tiyllotlt,.
loy a few per cent to higher values of S by multiple scattering in the sapphire windows, uhich may cause elec*tronsto miss the Faraday cup. It is obvious t ha t the “down-stream” d ata (i.e. with tjhe primary beam incident on the A1,0, side of the dynode, see Fig. 1) arc lower, and the relativistic rise is slightly suppressed relative to the “ u p s tr e a m” data. This is in agreement with the theoretical considerations of Aggsori on the density effect.6 This effect should become important when the
640
E. L. QARWIN AND J . EDCBCUMBE
field-forming distance I , is of the order of the film thickness T , that is, when
where y = (1 - p 2 ) - : = (1 - w 2 / c 2 ) - : , m and e are the rest-mass and charge of the electron, n is the atomic electron density of the material, and c is the velocity of light. For KCI, a t E, = 500 MeV, 1, = 45 pm, which is comparable with T N 25 pm. As shown in Fig. 6, these data are in good agreement with the slope of the experimental results of Richter7 and the theoretiral work of
0)
v
g
4.5
-
k
D
i 100
2 Bethe-Blach formula (theor.) 3 V a n h u y s e a n d Van d e V i j e r ( ~ 2 0 0 ) f o r a s ~ n g l e A t f o i l sem ( t h e o r . ) 4 Richter ( x f 0 0 ) for a single A t foil
200
sem ( e x p t . ) 300
400
500
-
i
6 0 0 7 0 0 8 0 0 900 1000
E P , P r i m a r y energy IM e V ) 14'1~.
6. A coinparisoil of pi'esci~tdata with other exp~ritr~mtrtl ant1 thcoretiral work.
Aggson6 on aluminum foil secondary emission monitors (sem), but in only fair agreement with the theoretical work of Vanhuyse and Van De Vijer.E Curve 3 of Fig. 6 (with arbitrary normalization) was. taken from the work of Vanhuyse and Van De Vijer and was calculated by them for an aluminum foil sem. This calculation involved the theory of Baroodyg for secondary emission from metals; the agreement with the present work on insulators reflects the insensitivity of the slope of the ionization loss formula to material parameters which appear as arguments of logarithms. Curve 2 of Fig. 6 was calculated using the Bethe-Bloch formula, without density effect, for collisions involving energy transfers less than a predetermined value 7:
where 7 is tlie maximum energy transfer and 1 is the mean ionization potential. According to Aggson,' q is given by the solution of:
641
RESPONSE O F LOW-DENSITY KCl FOILS TO MULTI-MEV ELECTRONS
where I?(?) is the range-energy relation for keV electrons, cl, the maximum depth from which secondaries can escape, and cos 0 the angle between the secondary and primary electron trajectories given by relativistic kinematics. Using the range-energy relation from the work of Kanter,lo and the recently measured l 1 value d, = 7 pglcm2, Eq. ( 3 ) yields 77 = 4.6 keV. The value I = 10 eV has been used in evaluating Eq. (2), giving curve 2 in Fig. 6. This calculation, normalized to S = 5-1 at 100 MeV, is in excellent agreement with the present experimental results. The ability of two differing theories to give reasonable agreement with the present data is merely a result of the logarithmic dependence of dEldx on material parameters. However, the value of I used in the above calculation does not agree with the value calculated from the low energy data. Using the results reported by KanterlO it is estimated that I 2: 2 evlsecondary. This apparent low value of I may be due to internal multiplication in the field-enhanced emission process.12 Use of the tabulated13 collision loss of a 500 MeV primary, the value of d, given above, and I = 2 eV, allows calculation of S(500 MeV) = 5.95, which is very close to the measured value of 5.67. Because Z appears only within the logarithm in the Bethe-Bloch formula, the calculation of 6(500 MeV) is nearly independent of whether I = 2 eV with no internal multiplication, or I = 1 0 eV with internal multiplication by a factor of 5. Very limited results on low-density CsI dynodes were obtained for multi-MeV primary electrons. It was observed that S for CsI dynodeu was as much as 40% higher than for low-density KCl dynodes.
CONCLUSIONS The statistics of a single-particle detection device are determined almost entirely by the initial number of secondaries produced by the primary and a yield of at least 5 is required for accurate counting purposes. The data presented above indicate that an ensemble of lowdensity KC1 foils can be used for velocity determination and detection of relativistic particles unless the secondary emission process is statistically “pathological” such as to give, for example, 50 secondaries for 10% of the incident particles and 1 secondary for the rest, instead of 6 secondaries per primary with Poisson distribution. The question of internal multiplication is of crucial importance to the future applications of these dynodes to direct particle detection, and an experiment is planned at SLAC t o determine the statistics of the field-enhanced secondary emission process with relativistic primaries. P E l D.
‘11
642
E. L. QARWIN AND J. EDQECUMBE
ACPNOWLEDGMENTS The authors wish to thank Dr. H. DeStaebler, Dr. F. F. Liu, and Mr. J. Grant for assistance in performing the experiments at the Mark I11 accelerator, and Mr. A. Roder for preparation of the A1,03 substrates used in this work.
REFERENCES 1. Sternglass, E. J . and Goetze, G. W., I R E Trans. Nucl. Sci. NS-9,97 (1962). 2. Goetze, G. W., Boerio, A. H. and Green, M., J . Appl. Phys. 35, 482 (1964). 3. Cone, D. R., Univ. of California Radiation Laboratory Rept No. 1030 (1950), unpublished. 4. Hauser, V. and Kerler, W., Rev. Sci. Instrum. 29, 380 (1958). 5 . Brown, K. L. and Tautfest, G. W., Rev. Sci. Instrum. 27, 696 (1956). 6. Aggson, Th. L., Report LAL 1028, Laboratoire de 1'Accelernteur Lineaire, ORSAY, France (1962). 7. Richter, B., private communication. 8. Vanhuyse, V. J. and Van De Vijer, It. E., Nucl. Instrum. Methods 15, 63 (1962). 9. Baroody, E. M., Phys. Rev. 78, 780 (1950). 10. Kanter, H., Phys. Rev. 121, 461 (1961). 11. Edgecumhe, J. and Garwin, E. L., J. A p p l . Phys. (June, 1966). 12. Garwin, E. L. and Edgecumbe, J., Bull. Arner. Phys. SOC.10, 725 (lB65). 13. Berger, M. J. and Seltzer, S. M., NASA Report SP-3012.
DISCUSSION F . ECPART:
excitation?
Is the dynode gain variable with the duration of the high energy
E. L. CARWIN: We have not yet been able to determine whether the gain varies during the 1-psec beam pulse. We have determined that there is less than 109" decrease in gain for an integrated incidcnt high-energy current amounting to 0.5 C, on an area of less than 1 cm2.
E. 1,. (:AR\.\’IN s r i t l . I , EL)(:E(‘UMHE I , i ricit 1, d ccr 1e 1 at u r C‘cI I t o ) (+Sf,.-I( ’) , Str r t tfo rtl I 1tr I LY r s i t y ~ ~ t ~ l l L f t J l ‘ f~! !‘, ~ I ~ f I f1 ;O f L .I I ‘.,\‘.A.
INTROD ITCTI~N The high gain and fast response time relwrted by Goetze et ~ i l . l - ~ for low density KCl dynodes make their applic*ation to direct particle detectioii as envisaged by Alvarez and Cone3 seem promising. A t multi-C:eV primary energies, determination of the r a t mass of a “counted” particle becomes very difficult hecause of the multiplicity of possible final states of an interaction. If the secondary yield of low density foils is greater than approximately 5 for minimum ionizing particles, and if it follows the relativistic rise in the ionization loss and has a reasonable statistical number distribution, then velocity determination (and hence rest-mass tleterniination if the momentum is known) of primaray radiation would be readily avhievable with a n array of foils. A single such foil, coupled to a multiplier section, would act not only as a particle detector but could give excrlleiit space- and time-resolution . The present work was undertaken to investigattx the possibility of using low-density deposits of KCl and CsI in relativistic-rise detectors. It was found t hat 5-6 secondaries per 1)rirnary are obtained, on the average, from low-density KCI for primary electrons of energy from 100 t o 1000 MeV, and th at thc gain follows vlosely the relativistic rise in the ionization loss. DVNOD IC Y R EPBRATI( )N A cross-section of a low-density dynode is shown in Fig. 1 . The substrate cmisists of a 1 in. diameter, 1000 A thick self-supporting aluminum oxide (A1,03) film formed by a n anodization-etching technique similar to th at of Hauser and K e r l c ~ .A~ 500 -4thick aluminum conductive backing is deposited h y t hrrmal evaporation on at room the L41z0J. Idow-dtwsity KC:l is d(’J)OSjt(d 011 a s ~ ib s t~ r a te tempc nt ur e l)y clvaporation from a taiititlum h a t in argon a t a pressure ‘I’ypic.ally, 25 mg of KC’I is e\aporatcd a t 4 torr static of a fkw argon pressure. using a %in. boat-to-substrate sparing; the KCll is t This work waH supported by the U.S. Atonill. Energy COII~II~ISAIOII. 1,
4-3
636
E. L. OARWIN A N D J. EDGECUMBE
first melted and is then evaporated in approximately 20 sec. This gives a dynode with a thickness of approximately 25 pm and a density of 2 to 3% of the bulk density. Low-density CsI can be prepared in essentially the same way except a molybdenum boat is used and the evaporation time is longer (- 100 sec). These dynodes are about 30 pm thick and have approximately 2% of the bulk density. Density and thickness of the dynodes
login,
Sapphire windows (0.1in.)
/ ,A L supports 7 d
Sapphire o w s (0.1i n . )
A u photocathode ( ~ 5 a )
Ip ,"down-stream" orientation
Au c o l l e c t o r
Ag mask ( ~ 5 0 0 0 % )
A 1 2 0 3 s u p p o r t i n g f i l m (lo0081
Low-density K C L
(pNo.02 pbulk)
AL conductive backing ( 5 0 0 i % )
PIO. 1 . Cross-section of a low-density dynode and the experimental arrangement for measuring the secondary yield.
was determined by weighing and measurement with an optical microscope in a separate calibration using a standard evaporation procedure. All evaporations were performed in a diffusion-pumped, freonrefrigerator-baffled, unbaked varuum system with a base pressure - 2 x 1 0 - 7 tom. Dynodes were transferred to small ion-pumped, all-metal, vacuum systems (base pressure - 5 x torr) for measurement of the gain. The transfer of the dynodes was carried out in a n atmosphere of dry nitrogen. The gain a t low primary energies, -10 keV, was measured (see Fig. 1 ) using a gold photocathode illuminated by a type BH-6 high-pressure mercury lamp.? Typical curves of gain (6) plotted against collector potential ( V , ) are shown in Fig. 2. These results for low-density KC1 are in good agreement with the data reported by Goetze et aL2 The low-energy characteristics were found to be unaffected by bombardment with multi-MeV electrons.
t
Supplied by PEK Labs, Sunnyvale, California, U.S.A.
RESPONSE OF I,O\V-I)ENSITY KCI FOILS TO MULTI-IIEV ELECTRONS
637
-
50
40
-
0
C
2
30
K C L 144
A C s I 28
-
-
EP=10 keV ~
-
50
100
I50
200
250
300
3 0
V , , Collector potential ( V )
MEASUREMENTTEVHNIQUE Gain measurements for multi-MeV primary electrons were carried out a t thc Stanford Mark 111 linear electron accelerator. A photograph of the experimental arrangement is shown in Fig. 3. The Faraday cup, which was used to measure the primary current, was designed to be more than S U ~ efficient o over the range of primary energies investigated (100-loo0 The value of S was determined from the slope of the curve (as plotted on a n X-Y recorder) of integrated current leaving the dynode plotted against integrated current collected b y the Faraday cup. Integrated current was measured. instead of the current itself, not only because of the large amount of elec*tricalnoise associated with the accelerator, but also to remove cffects of small heam-current fluctuations. A collector potential of 165 V was w e d in the present experiment because at higher potentials the gain of KCI was not sufficiently stable. Sporadic increases in 6 manifested themselves as steps in the curves. These stells occurred very infrequently at I’, = 166 V and probably correspond to the scGntillations reported by (foetze Pi ri1.2 The. measured valiirs of‘ S prcsc~ntrtlIwre are averages of 6 or A separatv t3et~c~minat,iotis at ii fixed valui, of ~)rimaryenergy ( K p ) ,while the eri*ors shown correspond to the range of values obtained. The beam was left on during the measurements at a fixed E , t o minimize
63s
E. L. GAR\VIN A N D 3 . E D G E C U M B E
FIG.3. Photograph of t h e experimerltd arrangement for ineaauririg t h o sacoiidary yield of low-tlcrisity dynodes at multi-MeV primary energies. 1, Vacuum system CDIItainiiig dynode; 2, Paraday cup; 3, end of the linear accelerat,or; 4 m d 5, television cammas for observing the heam spot, on ZnS acreem.
discharging of the exit, surface, but had t o be turned off to change energy. After changing energy, the exit surface was given sufficient time to charge before the measurements were continued. The data were taken by monotonically decreasing the energy from maximum, and possible deterioration of 6 under bombardment was checked by repeating the measurement a t maximum energy. The energy of the primary beam was known to an accuracy of about 5 MeV.
D~SCLTSS~ON OF EXPERIMENTAL RESULTS The gain of low-density KCl dynodes was found to he strongly dependent on beam intensity as shown in Pig. 4 (average beam current in amperes N _ 10-l' x electrons/pulse). This effect is believed to be due to bombardment-induced conductivity limiting the internal field. The few per cent difference in 6 for Yc = 165 and 200 V (apparent in Fig. 4) is consistent with the low-energy data in Fig. 2. To minimize changes in 6 resulting from changes in beam intensity, the energy
IiESPONSE OF LOW-DENSITY KC‘I FOILS TO MULTI-MEV ELECTRONS
-
0
-0-
- - -oftr-t3-
-0-
--0
K C I 144 0
-- . \
o \
v;.t2oov
A V,
=t I65 V
\
Ep = 9 6 5 M e V Down-streom orientation spot s i z e 5 1.25cm2 10’
631)
I
I
108
lo9
\
\
0
I
10 0
Beam intensity ( e l e c t r o n s / p u l s e )
Frc:. 4. Intensity dependence of the gain for a low-delisity KCL dynode.
dependence of S was determined at a beam intensity of (1.5 f0.2) x lo8 electrons/pulse. The results of these measurements are presented in Fig. 5 . The logarithmic increase in 6 with increasing energy is evident and arises from the relativistic rise in the ionization loss. The experimental points a t the lowest primary energies may have been shifted
d r’
4.5
A “U p -slream”orientation 0 “Down-stream”
orientation
V, =+165V, spot s i z e ~ l . 2 5 c m ‘ B e a m intensity- (1.5 5 0 . 2 ) x108 electrons/pulse 100
200
300
400
600 700 800900 1000
500
E P , P r i m a r y energy ( M e V )
FIG.3. L),vtiodu gain as a
fiitirtioti of priiiitlry cmwgy for
EI
low-tloirsity KCI
tiyllotlt,.
loy a few per cent to higher values of S by multiple scattering in the sapphire windows, uhich may cause elec*tronsto miss the Faraday cup. It is obvious t ha t the “down-stream” d ata (i.e. with tjhe primary beam incident on the A1,0, side of the dynode, see Fig. 1) arc lower, and the relativistic rise is slightly suppressed relative to the “ u p s tr e a m” data. This is in agreement with the theoretical considerations of Aggsori on the density effect.6 This effect should become important when the
640
E. L. QARWIN AND J . EDCBCUMBE
field-forming distance I , is of the order of the film thickness T , that is, when
where y = (1 - p 2 ) - : = (1 - w 2 / c 2 ) - : , m and e are the rest-mass and charge of the electron, n is the atomic electron density of the material, and c is the velocity of light. For KCI, a t E, = 500 MeV, 1, = 45 pm, which is comparable with T N 25 pm. As shown in Fig. 6, these data are in good agreement with the slope of the experimental results of Richter7 and the theoretiral work of
0)
v
g
4.5
-
k
D
i 100
2 Bethe-Blach formula (theor.) 3 V a n h u y s e a n d Van d e V i j e r ( ~ 2 0 0 ) f o r a s ~ n g l e A t f o i l sem ( t h e o r . ) 4 Richter ( x f 0 0 ) for a single A t foil
200
sem ( e x p t . ) 300
400
500
-
i
6 0 0 7 0 0 8 0 0 900 1000
E P , P r i m a r y energy IM e V ) 14'1~.
6. A coinparisoil of pi'esci~tdata with other exp~ritr~mtrtl ant1 thcoretiral work.
Aggson6 on aluminum foil secondary emission monitors (sem), but in only fair agreement with the theoretical work of Vanhuyse and Van De Vijer.E Curve 3 of Fig. 6 (with arbitrary normalization) was. taken from the work of Vanhuyse and Van De Vijer and was calculated by them for an aluminum foil sem. This calculation involved the theory of Baroodyg for secondary emission from metals; the agreement with the present work on insulators reflects the insensitivity of the slope of the ionization loss formula to material parameters which appear as arguments of logarithms. Curve 2 of Fig. 6 was calculated using the Bethe-Bloch formula, without density effect, for collisions involving energy transfers less than a predetermined value 7:
where 7 is tlie maximum energy transfer and 1 is the mean ionization potential. According to Aggson,' q is given by the solution of:
641
RESPONSE O F LOW-DENSITY KCl FOILS TO MULTI-MEV ELECTRONS
where I?(?) is the range-energy relation for keV electrons, cl, the maximum depth from which secondaries can escape, and cos 0 the angle between the secondary and primary electron trajectories given by relativistic kinematics. Using the range-energy relation from the work of Kanter,lo and the recently measured l 1 value d, = 7 pglcm2, Eq. ( 3 ) yields 77 = 4.6 keV. The value I = 10 eV has been used in evaluating Eq. (2), giving curve 2 in Fig. 6. This calculation, normalized to S = 5-1 at 100 MeV, is in excellent agreement with the present experimental results. The ability of two differing theories to give reasonable agreement with the present data is merely a result of the logarithmic dependence of dEldx on material parameters. However, the value of I used in the above calculation does not agree with the value calculated from the low energy data. Using the results reported by KanterlO it is estimated that I 2: 2 evlsecondary. This apparent low value of I may be due to internal multiplication in the field-enhanced emission process.12 Use of the tabulated13 collision loss of a 500 MeV primary, the value of d, given above, and I = 2 eV, allows calculation of S(500 MeV) = 5.95, which is very close to the measured value of 5.67. Because Z appears only within the logarithm in the Bethe-Bloch formula, the calculation of 6(500 MeV) is nearly independent of whether I = 2 eV with no internal multiplication, or I = 1 0 eV with internal multiplication by a factor of 5. Very limited results on low-density CsI dynodes were obtained for multi-MeV primary electrons. It was observed that S for CsI dynodeu was as much as 40% higher than for low-density KCl dynodes.
CONCLUSIONS The statistics of a single-particle detection device are determined almost entirely by the initial number of secondaries produced by the primary and a yield of at least 5 is required for accurate counting purposes. The data presented above indicate that an ensemble of lowdensity KC1 foils can be used for velocity determination and detection of relativistic particles unless the secondary emission process is statistically “pathological” such as to give, for example, 50 secondaries for 10% of the incident particles and 1 secondary for the rest, instead of 6 secondaries per primary with Poisson distribution. The question of internal multiplication is of crucial importance to the future applications of these dynodes to direct particle detection, and an experiment is planned at SLAC t o determine the statistics of the field-enhanced secondary emission process with relativistic primaries. P E l D.
‘11
642
E. L. QARWIN AND J. EDQECUMBE
ACPNOWLEDGMENTS The authors wish to thank Dr. H. DeStaebler, Dr. F. F. Liu, and Mr. J. Grant for assistance in performing the experiments at the Mark I11 accelerator, and Mr. A. Roder for preparation of the A1,03 substrates used in this work.
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DISCUSSION F . ECPART:
excitation?
Is the dynode gain variable with the duration of the high energy
E. L. CARWIN: We have not yet been able to determine whether the gain varies during the 1-psec beam pulse. We have determined that there is less than 109" decrease in gain for an integrated incidcnt high-energy current amounting to 0.5 C, on an area of less than 1 cm2.