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The efficiency of the anthracene scintillation counter for low energy electrons
Physica X V I I I , no 12
D e c e m b e r 1952
THE EFFICIENCY OF THE ANTHRACENE SCINTILLATION COUNTER FOR LOW ENERGY ELECTRONS by D. K. B U T T London
Introduction. W e s t , Meyerhof and H . o f s t a d t e r 1°) have reported counting 8 and 2 keV. X-rays with 100% and 70% efficiency respectively, using a scintillation counter with a NaI(T1) crystal. R a m l e r and F r e e d m a n S ) on the otherhand found t h a t for an anthracene crystal there was a rapid fall of counting efficiency for electrons with energies b e l o w 15 keV. They indicated that this fall could be due to a rapid decrease of the physical efficiency of the crystal with energy for low energy electrons. This interpretation was disproved by T a y 1 o r et al. 8) who observed that the pulse height for anthracene, although not a linear function of the electron energy, did not undergo the rapi d fall that was. required to explain Ramler and Freedman's results. West, Mey-erhof and H o f s t a d t e r hinted that the low counting efficiencies at electron energies less than 15 keV obtained by R a m 1 e r and F r e e d m a n were probably due to poor light collection. On the basis of these results, an attempt has been made to redetermine the counting efficiency of the anthracene scintillation counter for low energy electrons, with improved light gathering power between the crystal a n d photo-cathode. Method. To achieve this end, electrons ~rom a Thorium active deposit (on a thick platinum button 2 m m in diameter) were focused, by means of a lens r-spectrometer, on to a n anthracene crystal of area about I sq. cm and thickness 2 mm, which was in direct optical contact with the uncooled photo-cathode of an - -
1 1 4 2
m
T H E E F F I C I E N C Y OF T H E A N T H R A C E N E S C I N T I L L A T I O N C O U N T E R
1143
E.M.I. 5311 photo-multiplier tube, and all except a n area of 2 m m diameter was masked from the incident electron beam with the aid of a perspex shield. The photo-cathode of the tube had a sensitivity of 38/z amps per lumen for light emitted from a filament of colour temperature 2848°K, and the 11 stage electron multiplier, a gain of 3.6 × l0 T at an interstage potential of 160 volts. The multiplier in this experiment was worked at an intefstage potential of 75 volts and the pulse clipping time was about 1 micro-second.
Results. Integral pulse amplitude curves were-taken for electron energies of 8 and 4 keV. These are shown in figure 1. The 8 keV curve becomes almost horizontal at small pulse amplitudes. This indicates that there are very few small pulses produced by electrons incident at this energy. The interpretation of the 4 keV curve is less obvious. By comparing it to that of the single photo-electron curve, it will be seen that its average pulse height is about three times larger than that due to the pulses of the single photo-electron Curve. This being so, it is reasonable to assume electrons which are not backscattered are fairly certain t ~ produce at least one photo-electron. Linearly extrapolating this pulse curve to zero pulse height (dotted in figure I), it is seen that about 80~/o of the pulses are counted, at a bias of 50 divisions. Interpretation of results. To interpret these curves, the fraction. of back-scattered electrons causing pulses large enough to be counted must be estimated. Experiments performed by S c h o n 1 a n d 7) and P a 11 u e 1 5) o n the back-scattering of electrons from metallic foils, indicate that for anthracene, it can be expected that approximately 10% of the incident electrons will be back-scattered *). This percentage *) I n making this estimate it has been assumed that the back-scattering coefficient for an insulator such as anthracene will lie on the curve given by the conductors of the experimentsreferred to above. For the low current densities used in this work (about 104 electrons per sq, cm L, per sec) this assumption appears legitimate. Since in the experiments of S c h o n 1 a n d and P a 11 u • 1, a suppressor grid was placed in front of the back-scattering foil, the back-scattering coefficients includbd only back-scattered electrons with 6nergies greater than 220 eV.
1144
D.'K. BUTT
was shown to be independent of the incident electron energy over a range of at least 3 to 80 keV (the range investigated).
sf 0
80
160 2 4 0 3 : ) 0 4 0 0 Pulse omplitude
Fig. l. A : 8 keV electrons, B : 4 keV electrons, C : single photo-electrons•
The results obtained by W a g n e r g ) and C h y l i n s k i a ) show. t h a t for electrons incident on a thick foil at an angle of 45 ° to the normal, the energy spectrum of back-scattered electrons
o
8
I IOO
I 150
I 200
i 250
t 300
I 350
HP Gauss-cm
Fig. 2. The low e n e r g y electron s p e c t r u m of t h e T h o r i u m active deposit.
leaving the foil at the same angle has quite a sharp peak at .6 to .8 of the incident electron energy. The shape of the spectrum does
T H E E F F I C I E N C Y OF T H E A N T H R A C E N E S C I N T I L L A T I O N C O U N T E R
1145
not change markedly with incident electron energy over a range of 2.1 to 30 keV (the range of energies investigated), or with atomic number between Z = 13 to 79. Although in this present experiment the electrons are incident normally on the crystal and leave it at all angles between 0 ° and 90 ° to the normal, the results of W a g n e r and C h y l i n s k i can be expected to give an indication of the average energy distribution of the back-scattered electrons. Frorh figure 1 it will be seen that electrons which lose 5 keV and more in the crystal will be reasonably certain of being counted at the minimum workable bias of 50 divisions." From Chylinski's curves it is evident that for 8 keV electrons, about 60% of the back-scattered electrons will lose at least this amount of energy. As well as this, many of the electrons which lose less than 5 keV will also be counted. Thus of the I0% back-scattered electrons, it Call be expected that about 8% will be counted, giving an overall counting efficiency of approximately 98%. As the energy of the incident electrons increases, the fraction of the back-scattered electrons which lose less than 5 keV will decrease and thus the counting efficiency can be expected to approach 100%. Assuming that none of the back-scattered electrons from the 4 keV curve are counted, the overall counting efficiency will be 7 2 ~ . This figure must be taken as a minimum. a
The low energy electron spectrum o/ the Thorium active deposit. The spectrum of the Thorium active deposit, obtained below 370 H e is shown in figure 2. The prominent group of lines above 230 He (4.7 keV) are the L Auger lines of Thorium (C + C") (see B i a c k 1) F 1 a m m e r s f e 1 d 4), and B u t t 3). The lowest possible energy for an L Auger line in this group is 5.26 keV (for Thorium C") *). The rises below 2 0 0 H e (3.5 keV) and in particular at 154H0 probably are due to the presence of imperfectly resolved M Auger lines. Received 6-9-52.
*) This is excluding the Coster-Kronig transitions of the types Lj .-)- L3M~ and L, --~ La M 6. Their m a x i m u m energy is at .27 keV.
1146 T H E E F F I C I E N C Y OF T H E Al~/.THRACENE SCINTILLATION C O U N T E R REFERENCES I) B l a c k , D. H., Proc. Camb. phil: Soe. 22 (1925) 838. 2) B u t t, D. K., Proc. phys. Soc. A., 63 (1950) 986. 3) C h y l i n s k i , S., Phys. Rev. 42(1932) 393. 4) F l a m m e r s f e l d , A., Z. Phys. 114 (1939) 227. 5) P a l l u e l , F., C. R. Ac. Sc. Paris 224 (1947) 1492. 6) R a m l e r , W. J. and F r i e d m a n , M.S., Rev. sci. Inst. 21 (1950) 7)Schonland, B. F. J., Proc. roy. Soc. (London) A 104 (1923).235; Soc. (London) A. 108 (1925) 187. 8) T a y l o r , C.J., Jentschke,*W. K., R e m e l y , M.E., E b y , K r u g e r, P. B., Phys. Rev. 84 (1951) 1034. 9) W a g n e r , P . B . , Phys. RevrR5 (1930) 98. I0) W e s t , H. I. (Jr~), M e y e r h o f , W. E. and H o f s t a d t e r , Rev. 81 (1951) 141.
784. Proc. roy. 1~. S. and
R.,
Phys.
D e c e m b e r 1952
THE EFFICIENCY OF THE ANTHRACENE SCINTILLATION COUNTER FOR LOW ENERGY ELECTRONS by D. K. B U T T London
Introduction. W e s t , Meyerhof and H . o f s t a d t e r 1°) have reported counting 8 and 2 keV. X-rays with 100% and 70% efficiency respectively, using a scintillation counter with a NaI(T1) crystal. R a m l e r and F r e e d m a n S ) on the otherhand found t h a t for an anthracene crystal there was a rapid fall of counting efficiency for electrons with energies b e l o w 15 keV. They indicated that this fall could be due to a rapid decrease of the physical efficiency of the crystal with energy for low energy electrons. This interpretation was disproved by T a y 1 o r et al. 8) who observed that the pulse height for anthracene, although not a linear function of the electron energy, did not undergo the rapi d fall that was. required to explain Ramler and Freedman's results. West, Mey-erhof and H o f s t a d t e r hinted that the low counting efficiencies at electron energies less than 15 keV obtained by R a m 1 e r and F r e e d m a n were probably due to poor light collection. On the basis of these results, an attempt has been made to redetermine the counting efficiency of the anthracene scintillation counter for low energy electrons, with improved light gathering power between the crystal a n d photo-cathode. Method. To achieve this end, electrons ~rom a Thorium active deposit (on a thick platinum button 2 m m in diameter) were focused, by means of a lens r-spectrometer, on to a n anthracene crystal of area about I sq. cm and thickness 2 mm, which was in direct optical contact with the uncooled photo-cathode of an - -
1 1 4 2
m
T H E E F F I C I E N C Y OF T H E A N T H R A C E N E S C I N T I L L A T I O N C O U N T E R
1143
E.M.I. 5311 photo-multiplier tube, and all except a n area of 2 m m diameter was masked from the incident electron beam with the aid of a perspex shield. The photo-cathode of the tube had a sensitivity of 38/z amps per lumen for light emitted from a filament of colour temperature 2848°K, and the 11 stage electron multiplier, a gain of 3.6 × l0 T at an interstage potential of 160 volts. The multiplier in this experiment was worked at an intefstage potential of 75 volts and the pulse clipping time was about 1 micro-second.
Results. Integral pulse amplitude curves were-taken for electron energies of 8 and 4 keV. These are shown in figure 1. The 8 keV curve becomes almost horizontal at small pulse amplitudes. This indicates that there are very few small pulses produced by electrons incident at this energy. The interpretation of the 4 keV curve is less obvious. By comparing it to that of the single photo-electron curve, it will be seen that its average pulse height is about three times larger than that due to the pulses of the single photo-electron Curve. This being so, it is reasonable to assume electrons which are not backscattered are fairly certain t ~ produce at least one photo-electron. Linearly extrapolating this pulse curve to zero pulse height (dotted in figure I), it is seen that about 80~/o of the pulses are counted, at a bias of 50 divisions. Interpretation of results. To interpret these curves, the fraction. of back-scattered electrons causing pulses large enough to be counted must be estimated. Experiments performed by S c h o n 1 a n d 7) and P a 11 u e 1 5) o n the back-scattering of electrons from metallic foils, indicate that for anthracene, it can be expected that approximately 10% of the incident electrons will be back-scattered *). This percentage *) I n making this estimate it has been assumed that the back-scattering coefficient for an insulator such as anthracene will lie on the curve given by the conductors of the experimentsreferred to above. For the low current densities used in this work (about 104 electrons per sq, cm L, per sec) this assumption appears legitimate. Since in the experiments of S c h o n 1 a n d and P a 11 u • 1, a suppressor grid was placed in front of the back-scattering foil, the back-scattering coefficients includbd only back-scattered electrons with 6nergies greater than 220 eV.
1144
D.'K. BUTT
was shown to be independent of the incident electron energy over a range of at least 3 to 80 keV (the range investigated).
sf 0
80
160 2 4 0 3 : ) 0 4 0 0 Pulse omplitude
Fig. l. A : 8 keV electrons, B : 4 keV electrons, C : single photo-electrons•
The results obtained by W a g n e r g ) and C h y l i n s k i a ) show. t h a t for electrons incident on a thick foil at an angle of 45 ° to the normal, the energy spectrum of back-scattered electrons
o
8
I IOO
I 150
I 200
i 250
t 300
I 350
HP Gauss-cm
Fig. 2. The low e n e r g y electron s p e c t r u m of t h e T h o r i u m active deposit.
leaving the foil at the same angle has quite a sharp peak at .6 to .8 of the incident electron energy. The shape of the spectrum does
T H E E F F I C I E N C Y OF T H E A N T H R A C E N E S C I N T I L L A T I O N C O U N T E R
1145
not change markedly with incident electron energy over a range of 2.1 to 30 keV (the range of energies investigated), or with atomic number between Z = 13 to 79. Although in this present experiment the electrons are incident normally on the crystal and leave it at all angles between 0 ° and 90 ° to the normal, the results of W a g n e r and C h y l i n s k i can be expected to give an indication of the average energy distribution of the back-scattered electrons. Frorh figure 1 it will be seen that electrons which lose 5 keV and more in the crystal will be reasonably certain of being counted at the minimum workable bias of 50 divisions." From Chylinski's curves it is evident that for 8 keV electrons, about 60% of the back-scattered electrons will lose at least this amount of energy. As well as this, many of the electrons which lose less than 5 keV will also be counted. Thus of the I0% back-scattered electrons, it Call be expected that about 8% will be counted, giving an overall counting efficiency of approximately 98%. As the energy of the incident electrons increases, the fraction of the back-scattered electrons which lose less than 5 keV will decrease and thus the counting efficiency can be expected to approach 100%. Assuming that none of the back-scattered electrons from the 4 keV curve are counted, the overall counting efficiency will be 7 2 ~ . This figure must be taken as a minimum. a
The low energy electron spectrum o/ the Thorium active deposit. The spectrum of the Thorium active deposit, obtained below 370 H e is shown in figure 2. The prominent group of lines above 230 He (4.7 keV) are the L Auger lines of Thorium (C + C") (see B i a c k 1) F 1 a m m e r s f e 1 d 4), and B u t t 3). The lowest possible energy for an L Auger line in this group is 5.26 keV (for Thorium C") *). The rises below 2 0 0 H e (3.5 keV) and in particular at 154H0 probably are due to the presence of imperfectly resolved M Auger lines. Received 6-9-52.
*) This is excluding the Coster-Kronig transitions of the types Lj .-)- L3M~ and L, --~ La M 6. Their m a x i m u m energy is at .27 keV.
1146 T H E E F F I C I E N C Y OF T H E Al~/.THRACENE SCINTILLATION C O U N T E R REFERENCES I) B l a c k , D. H., Proc. Camb. phil: Soe. 22 (1925) 838. 2) B u t t, D. K., Proc. phys. Soc. A., 63 (1950) 986. 3) C h y l i n s k i , S., Phys. Rev. 42(1932) 393. 4) F l a m m e r s f e l d , A., Z. Phys. 114 (1939) 227. 5) P a l l u e l , F., C. R. Ac. Sc. Paris 224 (1947) 1492. 6) R a m l e r , W. J. and F r i e d m a n , M.S., Rev. sci. Inst. 21 (1950) 7)Schonland, B. F. J., Proc. roy. Soc. (London) A 104 (1923).235; Soc. (London) A. 108 (1925) 187. 8) T a y l o r , C.J., Jentschke,*W. K., R e m e l y , M.E., E b y , K r u g e r, P. B., Phys. Rev. 84 (1951) 1034. 9) W a g n e r , P . B . , Phys. RevrR5 (1930) 98. I0) W e s t , H. I. (Jr~), M e y e r h o f , W. E. and H o f s t a d t e r , Rev. 81 (1951) 141.
784. Proc. roy. 1~. S. and
R.,
Phys.