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Fast-timing characteristics of some channel electron multipliers
NUCLEAR INSTRUMENTS
A N D M E T H O D S 99 (I972) 445-45t; © N O R T H - H O L L A N D
PUBLISHING
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F A S T - T I M I N G C H A R A C T E R I S T I C S OF S O M E C H A N N E L E L E C T R O N M U L T I P L I E R S * M. I. G R E E N , P. F. KENEALY and G. B. BEARD
Department of Physics, Wayne State University, Detroit, Michigan 48202, U.S.A. Received 26 July 1971 and in revised form 10 November 1971 The pulse shapes and fast-timing characteristics of two types of Bendix channel electron multipliers have been studied. We also show a relationship between pulse widths tin the saturation mode of operation) and the dispersion in transit times of the secondary electrons across the multipliers. Using this knowledge, a method
was devised which reduced the fwhm of the prompt time resolution curve by approximately a factor of 2. Application of these techniques in the measurement of short lifetimes using a delayed coincidence system is illustrated in lifetime measurements of levels in 59C0 and 17°yb.
I. Introduction A channel electron multiplier (CM) is a non-magnetic device consisting of a glass tube whose inside surface is a high-resistance semiconducting coating used as a secondary electron emitting surface. When a high voltage is applied, the coating provides a continuous potential gradient along the tube. The semiconducting coating can detect electrons 1-5) ( ~ 5 0 eV to > 1 MeV), protons 4-6) ( ~ 2 keV to > 50 MeV), positive ionsS'7), metastable thermal moleculesS), energetic atomic hydrogen9), and photons from the soft X-ray region to the vacuum ultraviolet 3'1°'11) ( < 2 ~, to 1500 /~). The coating is not sensitive to very low energy electrons (such as thermal electrons), but they can be detected by a CM if they gain sufficient energy from the accelerating field before impinging upon the coating. We have examined the pulse shapes, pulse rise times and electron transit-time variation in several multipliers and devised a method to improve their timing performance in delayed coincidence systems. Only one other use of these detectors as timing devices in a coincidence circuit has come to our attention12). In that application, multipliers were used as detectors at the ends of two hemispherical electrostatic analyzers. The best resolving time prompt-coincidence curve in this case had a fwhm of 4.3 x 10 -9 S. The electrostatic analyzers enabled precise selection of electron energies (to within ~ 140 eV) and fixed the input angles of electrons incident on the CM detector. Our primary interest in these devices is their use in experiments to measure short ( ~ 1 0 - 9 S) lifetimes of nuclear energy states where no prior energy selection of the electrons is used and where the input electron
flux is determined by the source-detector solid angle.
* Work supported by the National Science Foundation.
2. Equipment Two types of multipliers, Models CEM 4010 and CEM 4028, manufactured by the Bendix ElectroOptics Division were investigated. Both models are manufactured from a tube (1 m m i.d., 2 m m o.d. and 10 cm long) which is curved in order to reduce afterpulsing caused by the detection of accelerated positive ions produced near the anode by the primary electron pulse 3, 7). The CEM 4010 is curved into a 270 ° section of a 2.2 cm radius circle and requires a separate collector, biased > 200 V positive with respect to the anode and positioned about 1 m m from the anode. The CEM 4028 is curved into a helix and incorporates a capped collector and an input cone which increases the detection area. The multipliers are windowless devices and must be operated at pressures < 1 x 10 -4 torr to reduce spurious signals which occur in the form of afterpulses3'7). The vacuum enclosure was a Nonex glass tube with a front window of 0.127 m m aluminum foil. The system was enclosed in a brass tube for electromagnetic shielding. The vacuum was about 1 x 10 - 7 torr. Since the applied voltages were high (2-6 kV), care was taken in designing the system to prevent arcing between the various components. Two methods (electroplating or cementing) were used to fasten the radioactive sources to the end of a 0.4 m m platinum wire placed within 5 m m of the CM input. The source wire and the CM input were held at the same potential. The distance from the collector to the 50 f2 output cable was about 3.5 cm. The pulse shapes were obtained with a Tektronics Model 661 sampling oscilloscope and recorded 445
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Fig. I. Block diagram of delayed coincidence circuit. photographically. Some of the traces were obtained with an amplifier (gain X 10 and a 1.5 ns rise time) inserted between the CM and the oscilloscope to increase the oscilloscope triggering stability for low pulse heights. Since the rise time of the amplifier was faster than the CM pulses, there was no observable difference in pulse shapes due to the addition of the amplifier. Fig. 1 shows a block diagram of the delayed coincidence circuit (with provisions for pile-up rejection) used in this study. The system was tested with Pilot B plastic scintillators in both arms of the circuit, and a p r o m p t resolution curve with a fwhm of 570 ps and slopes of 70 ps was obtained for a 6°Co source.
to electrons collected by the collector. These properties cause non-uniform pulse amplitudes and rise time~. The best fwhm obtained with the Bendix Model CEM 4028 multiplier was 1.8 ns which is almost twice as large as that obtained with the C E M 4010. 3.1.2. C E M
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3. Experimental results .
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between 200 and 400 V). Tests were made both with and without a 50 12 terminating resistor at the collector end of the output cable. Pulse heights were doubled without the terminating resistor, and there was no noticeable effect on the pulse rise time because of the low capacitance of the system. Fig. 3 is a picture of an output pulse which indicates afterpulsing caused by ion feedback. Ions may be formed from residual gases within a vacuum system or from gases desorbed from the inside surface of a multiplie.r by the electrons in the pulse. The ions accelerate toward the input end and cause an afterpulse upon being detected. Afterpulsing usually occurs following exposure of the multiplier to atmospheric pressure or when the cathode-anode potential difference is raised significantly. The afterpulsing decreases with multiplier use and disappears within a few days depending upon
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the counting rate. Since we have observed extensive afterpulsing following the initial operation of a new CM, it is probable that most of the initial multiplier gain decrease found by other authors 3- 5, ~3-15) is the result of a decrease in afterpulsing. Fig. 4 shows the output pulse shape obtained under typical operating conditions ( ~ 3-6 kV anode--cathode potential). The chief characteristics are the uniformity of the leading edge, the relatively flat top, and the wide distribution of pulse widths. Pulse heights from 10 to > 70 mV were observed depending upon the individual characteristics of the multiplier, its previous history, the counting rate, and the operating voltage. The 10% to 90% rise time is about 5 ns and the 20% to 180~/o rise time is about 3 ns. These rise times were found to be relatively independent of the multiplier voltage and its
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channel analyser opened a gate allowing time spectrum pulses to be recorded by the multichannel analyser. In fig. 7 the horizontal axis is the time centroid channel position, the vertical axis represents single channel analyser baseline settings with a constant window width selecting various pulse widths. A higher value of the time centroid channel corresponds to a shorter multiplier transit time. Fig. 7 implies that the larger pulse widths have shorter transit times with approximately a three nanosecond time difference between pulses of maximum and minimum width. The pulsewidth variation does not result from the detection of fl particles at varying distances inside of the CM, since one would then expect larger pulse widths to have longer transit times. It can be seen that, if pulses of a given width (selected by the method described above) are allowed as timing signals, a significant improvement in the time resolution of a system employing a CM can be obtained, it should also be possible to develop a simple "pulsewidth" compensation system by summing a fraction of the charge-integrated CM pulse with the time spectrum pulse. Such a compensation system would increase collection efficiency without loss of time resolution. 3.3. TIME RESOLUTION 6°Co was electroplated on the end of a 0.4 mm platinum wire over a length of 2 ram, and the source was placed 2l mm into the input of the CM. The CM counting rate was 450 cts/s.
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FAST-TIMING
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TABLE 1 Experimental results for half-lives.
In o r d e r to o b t a i n a p r o m p t time resolution spect r u m with this source, a C E M 4010 detected betaparticles and a Pilot B scintillator, optically coupled to an A m p e r e x 56AVP p h o t o m u l t i p l i e r , detected
Nucleus
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p h o t o n s . The p h o t o n w i n d o w was set at the C o m p t o n edge o f the g a m m a spectrum while the beta w i n d o w was set at the m a x i m u m o f the C M " p u l s e - w i d t h " spectrum. The f w h m was 1.10 ns; the fwtm 2.1l ns; the left and right h a n d slopes were 164 ps a n d 107 ps, respectively. These n u m b e r s a p p r o x i m a t e l y d o u b l e when pulse-width d i s c r i m i n a t i o n is not used on pulses from the CM. The a s y m m e t r y of the p e a k is p r o b a b l y
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1952 20 1956 21 1959 22 1962 23 1963 24 1965 25 1965 26 1905 27 1966 28 1966 29 present work
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59Fe was electroplated on the end o f a 0.4 m m plat i n u m wire over a length o f 1.5 m m a n d was p l a c e d 1 m m from the C M input. Fig. 8 shows the result o f a m e a s u r e m e n t o f the half-life of the 1.29 M e V level o f 59Co. The inner plot is o f a 6°Co p r o m p t spectrum o b t a i n e d with identical instrument settings. Beta-particles from the disinlegration of 59Fe were detected by a C E M 4010, while g a m m a s from b o t h the 1.29 a n d 1.10 M e V levels o f Sgco were detected by a Pilot B scintillator. The g a m m a energy window was set such t h a t there was a b o u t a 15% p r o m p t c o n t r i b u t i o n f r o m the 1.10 MeV level. The beta w i n d o w was set at the m a x i m u m of the multiplier " p u l s e - w i d t h " spectrum. The m e a s u r e d half-life o f the 1.29 M e V level is 5 6 4 + 2 0 ps, in g o o d agreement with the previous m e a s u r e m e n t s listed in table 1. The t7°Tm source was m a d e by n e u t r o n i r r a d i a t i o n o f 0.15 m g o f thulium metal which was subsequently a t t a c h e d to the end o f the 0.4 m m p l a t i n u m wire by means of h i g h - v a c u u m cement. Fig. 9 shows the result o f the m e a s u r e m e n t o f the half-life of the 84 keV level o f 17°yb. I n this e x p e r i m e n t the Pilot B scintillator detected betas f r o m the disinteg r a t i o n o f 17°Tm. The beta energy w i n d o w was set a b o v e 100 keV to insure t h a t only beta-particles were detected by the scintillator. The C E M 4010 was used
450
M.I. GREEN
to detect conversion electrons from the 84 keV level and "pulse-width" discrimination was used. The measured half-life of the 84 keV level is 1.60+0.02 ns which is in very good agreement with the previously measured values listed in table 1. The 17°yb half-life plot has a significant p r o m p t c o n t r i b u t i o n (approximately 50%). The geometrical a r r a n g e m e n t of the experiment eliminates the possibility that the p r o m p t c o n t r i b u t i o n is a result of the CM detecting electrons backscattered from the Pilot B scintillator. We believe that the p r o m p t c o n t r i b u t i o n is a result of ~7°Tm beta-particles detected by the scintillator in coincidence with the CM detecting soft X-rays, Auger electrons, and shakeoff electrons acc o m p a n y i n g atomic rearrangement of ~7°Yb. It should be noted that there are three times as m a n y beta decays to the g r o u n d state of ~7°yb as there are to the 84 keV level. In two other measurements a significant p r o m p t c o n t r i b u t i o n was indicated2~'26), and it appears that our explanation is not ruled out by their description of experimental technique.
5. Summary and discussion It has been demonstrated that channel electron multipliers can be used as detectors of beta-particles and electrons in fast timing experiments to measure halflives of excited nuclear levels as short as 500 ps. It has been shown that the o u t p u t pulses of a CEM 4010 in the saturation mode are of u n i f o l m amplitude but varying width. By using a charge-integrating circuit, the pulse-width variation is transformed to a pulseheight distribution which (by gating on a particular region of this distribution) enables selection of pulses having more nearly equal transit times. This eliminates a major source of time jitter and improves the p r o m p t time-resolution spectrum by a factor of two. This technique also points to a way to apply the ordinary pulse-height c o m p e n s a t i o n methods to correct the time dispersion associated with the varying pulse widths. These detectors have a particular advantage over conventional plastic scintillators when the only possible sources of a t i m i n g signal are low energy electrons and/ or low energy photons. A n o t h e r advantage is that these detectors are relatively insensitive to p h o t o n s above 10 keV and, therefore, the possible 7-ray b a c k g r o u n d from a complicated decay scheme is almost non-existent. There are also some obvious disadvantages. The decay scheme must be favorable so that conversion electrons and atomic rearrangement electrons and photons, leading to or from metastable levels other than the one of interest, do not interfere. Also, the slow but con-
et al.
stant gain degradation of these detectors found by m a n y authors 2 - 4 ' 8 ' ~a,3o) at moderate c o u n t rates is a drawback for long term operation. The gain degradation mechanisms have been investigated by others~O, 13-~6,3o,3~). The problems of the small i n p u t geometry can be alleviated in some applications by using a C M with an i n p u t cone (with a consequent decrease in time resolution). We wish to acknowledge the help of Bendix Research Laboratories in this investigation and t h a n k them for their cooperation. We also wish to t h a n k R. Martin and the staff of the Ford Reactor at the University of Michigan, Phoenix Memorial Laboratory for their valuable assislance with the irradiations.
References 1) R. J. Archuleta and S. E. DeForest, Rev. Sci. instr. 42 (1971) 89. ~) L. A. Frank, N. K. Henderson and R. L. Swisher, Rev. Sci. Instr. 40 (1969) 685. a) K. C. Schmidt, Bendix Tech. Appl. Note no. 9803 (Bendix Electro-Optics Div., 1969). ~) A. Egidi, R. Marconero, G. Pizzella and F. Sperli, Rev. Sci. lnstr. 40 (1969) 88. '~) B. Tatry, J. M. Bosqued and H. Reme, Nucl. Instr. and Meth. 69 (1969) 254. ~) R. D. Andresen and D. E. Page, Nucl. Instr. and Meth. 85 (1970) 137. 7) E. Yellin, L. I. Yin and 1. Adler, Rev. Sci. Instr. 41 (1970) 18. s) D. P. Donnelly, J. C. Pearl, R. A. Heppner and J. C. Zorn, Rev. Sci. Instr. 40 (1969) 1242. ~) J. A. Ray and C. E. Barnett, IEEE Trans. Nucl. Sci. NS-17 (1970) 44. J0) U. Mayer, M. Mozer and M. v. Reinhardt, Appl. Optics 8 (1969) 617. ll) C. S. Weller and J. M. Young, Appl. Optics 9 11970) 505. ~) U. Amaldi, Jr., A. Egidi, R. Marconero and G. Plzzella, Rev. Sci. Instr. 40 ~1969) 1001. ~:~)A. F. Timothy and J. G. Timothy, J. Sci. Instr. (J. Phys. E) 2, Ser. 2 (1969) 825. H) B. D. Klettke, N. D. Krym and W. G. Wolber, IEEE Trans. Nucl. Sci. NS-16 t1969) 72. 1~) W. G. Wolber, B. D. Klettke and H. K. Lintz, Rev. Sci. Instr. 40 (1969) 1364. 6) R. D. And resen and D. E. Page, Rev. Sci. lnstr. 42 ( 1971) 37 I. Jr) y. K. Agarwal, C. V. K. Baba and S. K. Bhattacherjee, Nucl. Phys. A99 (1967) 457. is) N. P. S. Sidhu and U. C. Gupta, Nucl. Phys. A91 ( 1%71 55?. I!~l I. Arens and H. J. K6rner, Z. Physik 242 (1971) 138. e~) R. L. Graham, J. L. Wolfson and R. E. Bell, Can. J. Phys. 30 (1952) 459. Zl) H. De Waard and T. R. Gerholm, Nucl. Phys. 1 (1956) 281. ee) S. Gorodetzky, R. Manquenouille, R. Richert and J. Lefort, C. R. Acad. Sci. (Paris) 248 (1959) 2202. ~:~) M. S. EI-Nesr and E. Bashandy, Z. Physik 168 (1962) 349. ,.,r) D. B. Fossan and B. Herskind, Nucl. Phys. 40 (1953) 24. ca) W. Meiling and F. Stary, Nucl. Phys. 74 (1965) 113. e~) R. Rougny, J. J. Samueli and A. Sarazin, J. Phys. Radium 26 1965) 63.
FAST-TIMING CHARACTERISTICS
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27) j. W. Tipple and R. P. Scharenberg, Phys. Letters 16 (lq65t 154. 28) D. V. Narasimha Rao and Swami Jn,qnal~anda, Nucl. Phys. 75 (1966) 109.
MULTIPLIERS
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29) E. G. Funk, H. J. Plask and J. W. Mihelich, Phys. Rev. 141 (1966) 1200. 30) R. H. Prince and J. A. Cross, Rev. Sci. lnstr. 42 (1971) 66. :3¢) B. D. Klettke, W. L. Wilcock, R. K. Muel!er and W. G. Wolber, Appl. Phys. Letters 16 (1970) 421.
A N D M E T H O D S 99 (I972) 445-45t; © N O R T H - H O L L A N D
PUBLISHING
CO.
F A S T - T I M I N G C H A R A C T E R I S T I C S OF S O M E C H A N N E L E L E C T R O N M U L T I P L I E R S * M. I. G R E E N , P. F. KENEALY and G. B. BEARD
Department of Physics, Wayne State University, Detroit, Michigan 48202, U.S.A. Received 26 July 1971 and in revised form 10 November 1971 The pulse shapes and fast-timing characteristics of two types of Bendix channel electron multipliers have been studied. We also show a relationship between pulse widths tin the saturation mode of operation) and the dispersion in transit times of the secondary electrons across the multipliers. Using this knowledge, a method
was devised which reduced the fwhm of the prompt time resolution curve by approximately a factor of 2. Application of these techniques in the measurement of short lifetimes using a delayed coincidence system is illustrated in lifetime measurements of levels in 59C0 and 17°yb.
I. Introduction A channel electron multiplier (CM) is a non-magnetic device consisting of a glass tube whose inside surface is a high-resistance semiconducting coating used as a secondary electron emitting surface. When a high voltage is applied, the coating provides a continuous potential gradient along the tube. The semiconducting coating can detect electrons 1-5) ( ~ 5 0 eV to > 1 MeV), protons 4-6) ( ~ 2 keV to > 50 MeV), positive ionsS'7), metastable thermal moleculesS), energetic atomic hydrogen9), and photons from the soft X-ray region to the vacuum ultraviolet 3'1°'11) ( < 2 ~, to 1500 /~). The coating is not sensitive to very low energy electrons (such as thermal electrons), but they can be detected by a CM if they gain sufficient energy from the accelerating field before impinging upon the coating. We have examined the pulse shapes, pulse rise times and electron transit-time variation in several multipliers and devised a method to improve their timing performance in delayed coincidence systems. Only one other use of these detectors as timing devices in a coincidence circuit has come to our attention12). In that application, multipliers were used as detectors at the ends of two hemispherical electrostatic analyzers. The best resolving time prompt-coincidence curve in this case had a fwhm of 4.3 x 10 -9 S. The electrostatic analyzers enabled precise selection of electron energies (to within ~ 140 eV) and fixed the input angles of electrons incident on the CM detector. Our primary interest in these devices is their use in experiments to measure short ( ~ 1 0 - 9 S) lifetimes of nuclear energy states where no prior energy selection of the electrons is used and where the input electron
flux is determined by the source-detector solid angle.
* Work supported by the National Science Foundation.
2. Equipment Two types of multipliers, Models CEM 4010 and CEM 4028, manufactured by the Bendix ElectroOptics Division were investigated. Both models are manufactured from a tube (1 m m i.d., 2 m m o.d. and 10 cm long) which is curved in order to reduce afterpulsing caused by the detection of accelerated positive ions produced near the anode by the primary electron pulse 3, 7). The CEM 4010 is curved into a 270 ° section of a 2.2 cm radius circle and requires a separate collector, biased > 200 V positive with respect to the anode and positioned about 1 m m from the anode. The CEM 4028 is curved into a helix and incorporates a capped collector and an input cone which increases the detection area. The multipliers are windowless devices and must be operated at pressures < 1 x 10 -4 torr to reduce spurious signals which occur in the form of afterpulses3'7). The vacuum enclosure was a Nonex glass tube with a front window of 0.127 m m aluminum foil. The system was enclosed in a brass tube for electromagnetic shielding. The vacuum was about 1 x 10 - 7 torr. Since the applied voltages were high (2-6 kV), care was taken in designing the system to prevent arcing between the various components. Two methods (electroplating or cementing) were used to fasten the radioactive sources to the end of a 0.4 m m platinum wire placed within 5 m m of the CM input. The source wire and the CM input were held at the same potential. The distance from the collector to the 50 f2 output cable was about 3.5 cm. The pulse shapes were obtained with a Tektronics Model 661 sampling oscilloscope and recorded 445
446
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Fig. I. Block diagram of delayed coincidence circuit. photographically. Some of the traces were obtained with an amplifier (gain X 10 and a 1.5 ns rise time) inserted between the CM and the oscilloscope to increase the oscilloscope triggering stability for low pulse heights. Since the rise time of the amplifier was faster than the CM pulses, there was no observable difference in pulse shapes due to the addition of the amplifier. Fig. 1 shows a block diagram of the delayed coincidence circuit (with provisions for pile-up rejection) used in this study. The system was tested with Pilot B plastic scintillators in both arms of the circuit, and a p r o m p t resolution curve with a fwhm of 570 ps and slopes of 70 ps was obtained for a 6°Co source.
to electrons collected by the collector. These properties cause non-uniform pulse amplitudes and rise time~. The best fwhm obtained with the Bendix Model CEM 4028 multiplier was 1.8 ns which is almost twice as large as that obtained with the C E M 4010. 3.1.2. C E M
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3. Experimental results .
3.1.
GENERAL
3.1.1. C E M
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between 200 and 400 V). Tests were made both with and without a 50 12 terminating resistor at the collector end of the output cable. Pulse heights were doubled without the terminating resistor, and there was no noticeable effect on the pulse rise time because of the low capacitance of the system. Fig. 3 is a picture of an output pulse which indicates afterpulsing caused by ion feedback. Ions may be formed from residual gases within a vacuum system or from gases desorbed from the inside surface of a multiplie.r by the electrons in the pulse. The ions accelerate toward the input end and cause an afterpulse upon being detected. Afterpulsing usually occurs following exposure of the multiplier to atmospheric pressure or when the cathode-anode potential difference is raised significantly. The afterpulsing decreases with multiplier use and disappears within a few days depending upon
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CHANNEL
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the counting rate. Since we have observed extensive afterpulsing following the initial operation of a new CM, it is probable that most of the initial multiplier gain decrease found by other authors 3- 5, ~3-15) is the result of a decrease in afterpulsing. Fig. 4 shows the output pulse shape obtained under typical operating conditions ( ~ 3-6 kV anode--cathode potential). The chief characteristics are the uniformity of the leading edge, the relatively flat top, and the wide distribution of pulse widths. Pulse heights from 10 to > 70 mV were observed depending upon the individual characteristics of the multiplier, its previous history, the counting rate, and the operating voltage. The 10% to 90% rise time is about 5 ns and the 20% to 180~/o rise time is about 3 ns. These rise times were found to be relatively independent of the multiplier voltage and its
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channel analyser opened a gate allowing time spectrum pulses to be recorded by the multichannel analyser. In fig. 7 the horizontal axis is the time centroid channel position, the vertical axis represents single channel analyser baseline settings with a constant window width selecting various pulse widths. A higher value of the time centroid channel corresponds to a shorter multiplier transit time. Fig. 7 implies that the larger pulse widths have shorter transit times with approximately a three nanosecond time difference between pulses of maximum and minimum width. The pulsewidth variation does not result from the detection of fl particles at varying distances inside of the CM, since one would then expect larger pulse widths to have longer transit times. It can be seen that, if pulses of a given width (selected by the method described above) are allowed as timing signals, a significant improvement in the time resolution of a system employing a CM can be obtained, it should also be possible to develop a simple "pulsewidth" compensation system by summing a fraction of the charge-integrated CM pulse with the time spectrum pulse. Such a compensation system would increase collection efficiency without loss of time resolution. 3.3. TIME RESOLUTION 6°Co was electroplated on the end of a 0.4 mm platinum wire over a length of 2 ram, and the source was placed 2l mm into the input of the CM. The CM counting rate was 450 cts/s.
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FAST-TIMING
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449
MULTIPLIERS
TABLE 1 Experimental results for half-lives.
In o r d e r to o b t a i n a p r o m p t time resolution spect r u m with this source, a C E M 4010 detected betaparticles and a Pilot B scintillator, optically coupled to an A m p e r e x 56AVP p h o t o m u l t i p l i e r , detected
Nucleus
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p h o t o n s . The p h o t o n w i n d o w was set at the C o m p t o n edge o f the g a m m a spectrum while the beta w i n d o w was set at the m a x i m u m o f the C M " p u l s e - w i d t h " spectrum. The f w h m was 1.10 ns; the fwtm 2.1l ns; the left and right h a n d slopes were 164 ps a n d 107 ps, respectively. These n u m b e r s a p p r o x i m a t e l y d o u b l e when pulse-width d i s c r i m i n a t i o n is not used on pulses from the CM. The a s y m m e t r y of the p e a k is p r o b a b l y
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0.586-+0.020 o.6o_+o.o5 0.564-+0.0o5 0.564_+0.020
1967 17 1967 18 1971 19 present work
17°yb
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1952 20 1956 21 1959 22 1962 23 1963 24 1965 25 1965 26 1905 27 1966 28 1966 29 present work
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59Fe was electroplated on the end o f a 0.4 m m plat i n u m wire over a length o f 1.5 m m a n d was p l a c e d 1 m m from the C M input. Fig. 8 shows the result o f a m e a s u r e m e n t o f the half-life of the 1.29 M e V level o f 59Co. The inner plot is o f a 6°Co p r o m p t spectrum o b t a i n e d with identical instrument settings. Beta-particles from the disinlegration of 59Fe were detected by a C E M 4010, while g a m m a s from b o t h the 1.29 a n d 1.10 M e V levels o f Sgco were detected by a Pilot B scintillator. The g a m m a energy window was set such t h a t there was a b o u t a 15% p r o m p t c o n t r i b u t i o n f r o m the 1.10 MeV level. The beta w i n d o w was set at the m a x i m u m of the multiplier " p u l s e - w i d t h " spectrum. The m e a s u r e d half-life o f the 1.29 M e V level is 5 6 4 + 2 0 ps, in g o o d agreement with the previous m e a s u r e m e n t s listed in table 1. The t7°Tm source was m a d e by n e u t r o n i r r a d i a t i o n o f 0.15 m g o f thulium metal which was subsequently a t t a c h e d to the end o f the 0.4 m m p l a t i n u m wire by means of h i g h - v a c u u m cement. Fig. 9 shows the result o f the m e a s u r e m e n t o f the half-life of the 84 keV level o f 17°yb. I n this e x p e r i m e n t the Pilot B scintillator detected betas f r o m the disinteg r a t i o n o f 17°Tm. The beta energy w i n d o w was set a b o v e 100 keV to insure t h a t only beta-particles were detected by the scintillator. The C E M 4010 was used
450
M.I. GREEN
to detect conversion electrons from the 84 keV level and "pulse-width" discrimination was used. The measured half-life of the 84 keV level is 1.60+0.02 ns which is in very good agreement with the previously measured values listed in table 1. The 17°yb half-life plot has a significant p r o m p t c o n t r i b u t i o n (approximately 50%). The geometrical a r r a n g e m e n t of the experiment eliminates the possibility that the p r o m p t c o n t r i b u t i o n is a result of the CM detecting electrons backscattered from the Pilot B scintillator. We believe that the p r o m p t c o n t r i b u t i o n is a result of ~7°Tm beta-particles detected by the scintillator in coincidence with the CM detecting soft X-rays, Auger electrons, and shakeoff electrons acc o m p a n y i n g atomic rearrangement of ~7°Yb. It should be noted that there are three times as m a n y beta decays to the g r o u n d state of ~7°yb as there are to the 84 keV level. In two other measurements a significant p r o m p t c o n t r i b u t i o n was indicated2~'26), and it appears that our explanation is not ruled out by their description of experimental technique.
5. Summary and discussion It has been demonstrated that channel electron multipliers can be used as detectors of beta-particles and electrons in fast timing experiments to measure halflives of excited nuclear levels as short as 500 ps. It has been shown that the o u t p u t pulses of a CEM 4010 in the saturation mode are of u n i f o l m amplitude but varying width. By using a charge-integrating circuit, the pulse-width variation is transformed to a pulseheight distribution which (by gating on a particular region of this distribution) enables selection of pulses having more nearly equal transit times. This eliminates a major source of time jitter and improves the p r o m p t time-resolution spectrum by a factor of two. This technique also points to a way to apply the ordinary pulse-height c o m p e n s a t i o n methods to correct the time dispersion associated with the varying pulse widths. These detectors have a particular advantage over conventional plastic scintillators when the only possible sources of a t i m i n g signal are low energy electrons and/ or low energy photons. A n o t h e r advantage is that these detectors are relatively insensitive to p h o t o n s above 10 keV and, therefore, the possible 7-ray b a c k g r o u n d from a complicated decay scheme is almost non-existent. There are also some obvious disadvantages. The decay scheme must be favorable so that conversion electrons and atomic rearrangement electrons and photons, leading to or from metastable levels other than the one of interest, do not interfere. Also, the slow but con-
et al.
stant gain degradation of these detectors found by m a n y authors 2 - 4 ' 8 ' ~a,3o) at moderate c o u n t rates is a drawback for long term operation. The gain degradation mechanisms have been investigated by others~O, 13-~6,3o,3~). The problems of the small i n p u t geometry can be alleviated in some applications by using a C M with an i n p u t cone (with a consequent decrease in time resolution). We wish to acknowledge the help of Bendix Research Laboratories in this investigation and t h a n k them for their cooperation. We also wish to t h a n k R. Martin and the staff of the Ford Reactor at the University of Michigan, Phoenix Memorial Laboratory for their valuable assislance with the irradiations.
References 1) R. J. Archuleta and S. E. DeForest, Rev. Sci. instr. 42 (1971) 89. ~) L. A. Frank, N. K. Henderson and R. L. Swisher, Rev. Sci. Instr. 40 (1969) 685. a) K. C. Schmidt, Bendix Tech. Appl. Note no. 9803 (Bendix Electro-Optics Div., 1969). ~) A. Egidi, R. Marconero, G. Pizzella and F. Sperli, Rev. Sci. lnstr. 40 (1969) 88. '~) B. Tatry, J. M. Bosqued and H. Reme, Nucl. Instr. and Meth. 69 (1969) 254. ~) R. D. Andresen and D. E. Page, Nucl. Instr. and Meth. 85 (1970) 137. 7) E. Yellin, L. I. Yin and 1. Adler, Rev. Sci. Instr. 41 (1970) 18. s) D. P. Donnelly, J. C. Pearl, R. A. Heppner and J. C. Zorn, Rev. Sci. Instr. 40 (1969) 1242. ~) J. A. Ray and C. E. Barnett, IEEE Trans. Nucl. Sci. NS-17 (1970) 44. J0) U. Mayer, M. Mozer and M. v. Reinhardt, Appl. Optics 8 (1969) 617. ll) C. S. Weller and J. M. Young, Appl. Optics 9 11970) 505. ~) U. Amaldi, Jr., A. Egidi, R. Marconero and G. Plzzella, Rev. Sci. Instr. 40 ~1969) 1001. ~:~)A. F. Timothy and J. G. Timothy, J. Sci. Instr. (J. Phys. E) 2, Ser. 2 (1969) 825. H) B. D. Klettke, N. D. Krym and W. G. Wolber, IEEE Trans. Nucl. Sci. NS-16 t1969) 72. 1~) W. G. Wolber, B. D. Klettke and H. K. Lintz, Rev. Sci. Instr. 40 (1969) 1364. 6) R. D. And resen and D. E. Page, Rev. Sci. lnstr. 42 ( 1971) 37 I. Jr) y. K. Agarwal, C. V. K. Baba and S. K. Bhattacherjee, Nucl. Phys. A99 (1967) 457. is) N. P. S. Sidhu and U. C. Gupta, Nucl. Phys. A91 ( 1%71 55?. I!~l I. Arens and H. J. K6rner, Z. Physik 242 (1971) 138. e~) R. L. Graham, J. L. Wolfson and R. E. Bell, Can. J. Phys. 30 (1952) 459. Zl) H. De Waard and T. R. Gerholm, Nucl. Phys. 1 (1956) 281. ee) S. Gorodetzky, R. Manquenouille, R. Richert and J. Lefort, C. R. Acad. Sci. (Paris) 248 (1959) 2202. ~:~) M. S. EI-Nesr and E. Bashandy, Z. Physik 168 (1962) 349. ,.,r) D. B. Fossan and B. Herskind, Nucl. Phys. 40 (1953) 24. ca) W. Meiling and F. Stary, Nucl. Phys. 74 (1965) 113. e~) R. Rougny, J. J. Samueli and A. Sarazin, J. Phys. Radium 26 1965) 63.
FAST-TIMING CHARACTERISTICS
OF SOME C I I A N N E L E L E C T R O N
27) j. W. Tipple and R. P. Scharenberg, Phys. Letters 16 (lq65t 154. 28) D. V. Narasimha Rao and Swami Jn,qnal~anda, Nucl. Phys. 75 (1966) 109.
MULTIPLIERS
451
29) E. G. Funk, H. J. Plask and J. W. Mihelich, Phys. Rev. 141 (1966) 1200. 30) R. H. Prince and J. A. Cross, Rev. Sci. lnstr. 42 (1971) 66. :3¢) B. D. Klettke, W. L. Wilcock, R. K. Muel!er and W. G. Wolber, Appl. Phys. Letters 16 (1970) 421.