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Fast neutron spectroscopy using the yale electron linac and a new nanosecond time-of-flight system
NUCLEAR INSTRUMENTS AND METHODS
43 (1966) 3x2-316 ; (D
NORTH-HOLLAND PUBLISHING CO.
FAST NEUTRON SPECTROSCOPY USING THE YALE ELECTRON LINAC AND A NEW NANOSECOND ' .NIEE-OF-FLIGHT SYSTEM F. W. K. FIRK Yale Üniverst6Y, New Haven, Connecticut
Received 4 March 1966 The Yale electron linac has been modified to provide pulses 10 ns wide, sand 3:1 .5 A peak current. The useful range of citron energy is from 10 to 70 MeV. A lime-to-time converter and synchronmed os :iUator provide a flexible timing system with channel widths between 2 and 10 ns. A-1 overall resolution of 0.2 nsjm is achieved. The favorable ratio of signal-to-background moved for the system permits investigations of a wide range of fast neutron experiments previously the prerogative of Van de
Graaff accelerators. The differential elastic scattering of neutrons from the 12C(n,n)12C reaction has been measured with high resolution at several angles for energies between 1 and 10 MeV. As a further example, polarization effects in the yield of photoneutrons from the giant dipole resonance region of the 160(y,n)IS0 reaction have been studied by left-right scattering from a 12C analyser.
l. Introduction In recent years, several high powered electron linear accelerators have been used for fast neutron studies using nanosecond time-of-flight techniques") . However, experiments performed with them have been generally limited to studies of (y,n) spectra or measurements of total neutron cross sections. So far, no results have been eported of partial neutron cross sections in the MeV region and polarization experiments are nonexistent . This paper reports modifications to the Yale electron linac to produce pulses < 10 ns in duration and peak currents of almost 2 A with a view to studying partial neutron cross sections from i to 10 MeV. Results are presented for the differential ellastic scattering yield of neutrons from 12C(n,n)12C at
several angles . This work supplements and extends the results obtained using Van de Graaff machines) and synchro-cyclotrons') . It clearly demonstrates the potentialities of the spectrometer in this field . Measurements of the polariza' ion of photoneutrons from the 160(y,n)"O reaction using a "C analyser also provide a stringent test of the system .
FILAMENT 6 .3 V a . c .
2. The electron gun modulator The electron gun is a triode operating with a grid bias of -250 V. Under normal running conditions, a hardtube modulator produces a trigger pulse 5 500 V amplitude and 0.1 to 6 Its wide . The entire gun modulator operates at a potential difference of 150 kV with respect to ground : this provides the necessary
VOLTAGE
5000 --l 20 TURNS ON I/4"DIA FERRITE CORE
TO GRID OF E L E ï. r O N GUN ( - 250V BIAS)
Fig. 1 . The electron gun modulator . 312
313
FAST NEUTRON SPECTROSCOPY
injection velocity for electrons entering the corrugated waveguide of the accelerator. In the present arrangernent a thyratron modulator is installed in the lßß kV housing and gives a trigger wise - I IN in amplitude and Z 10 ns wide . The Qb vOtage supply to the thyratron can be controlled ft -,)m the accelerator console. This enables the peak injected current and pulse width to be varied over a wide range the high while the accelerator is operating. If voltage is reduced so that the trigger pulse is 500 V in amplitude the observed current pulse is 5 ns wide: the peak current is then reduced by a factor of two. A circuit diagram of the modulator whichis shown in fig. 1 . It is a conventional systems in a small tbyrMron (2D2 1) is used to provide a fast, high current trigger pulse for a higher powered thyratron (3C45.) . The values of the inductances in the heater circuit of the 3C45 are critical factors in determining the output pulse width and amplitude. Optimurn conditions are obtained by adjusting the length of the ferrite cores. The amplitude of the slow rise at the beginning of the pulse (characteristic of the thyratron's ionization time) is insufficient to overcome the grid bias .
The modulator has performed reliably during the past six months. 3 . The nanosecond time-of--flight spectrometer The basic components of the spectrometer are shown in fig. 2. An Orman-type time-to-time converter') (or time expander) measures the inte!rval T between the electron pulse and the arrival of a neutron at the detector : T is typically in the range I to 5 ps . The device converts T, linearly, to an "`expanded" interval 200T. A synchronized oscillator operating at 0.5, 1 or 2 MHz measures the expanded interval digitally : the output is counted and stored in a 400 channel RIDL analyser. In previovis applications of this method, an unsynchronized oscillator has been used thereby introducing an additional timing uncertainty`). A suitable synchronized oscillator, used in the pr,-sent experiments, is shown in fig. 3. It is basically a tunnel diode monostable circuit which is biased in its negative resistance region during the period of the gating waveform, (the expanded time interval). The frequency of the oscillator is determined primarily by the value of the inductance L. Using a stabilized power supply, an overall stability of 10' Underground
01 .
à
10 il sec
- Electron Gun
and Modulator C 150 kV =rA, ground)
Address Overflow
Reset
Fig. 2. B1cKA diagram of the experiment (W=0,01") .
400 Chonmol RIDL Analyzer
314
F. W. K. FIRK P
+10 Fig. 3. Circuit diagram of the synchronized oscillator .
better than 1 :10' is achieved over a period of weeks. The neutron detector consists of a 6" dia. x 1" thick NE102 plastic scintillator viewed by a 58 AVP phototube. The output is fed directly to a Sugarman-type limiter and fast discriminator') ; the effective bias of the system corresponds to a neutron energy of 300 keV.
vail in the underground flight path with double-wall shielding as shown in fig. 2. Two experiments, outlined in the introduction, demonstrate the performance of the entire system .
4. Spectrometer performance A typical time distribution of the y-flash from the accelerator target obtained when operating under peak current conditions (- 1 .5 A) is shown in the lower part of fig. 4. This is observed in a plastic scintillator after attenuating the y-rays with 12" Pb. If the full intensity of the y-flash is observed the time distribution shown in the upper part of fig. 4 is obtained . This is a measure of the overall electronic resolving time of the spectrometer since the dead-time of the system is such that only the arrival time of the initial photons in the flash is recorded : it is seen to be 5 ns. The current pulse is therefore 9 ns wide, in excellent agreement with the value obtained by viewing the current pulse directly with a sampling oscilloscope . Using flight paths up to 45 rn, resolutions of 0.2 ns/m are therefore achieved .
12C
Results Measurements of the absorption of photoneutrons from a Pb(z,,n) target by ,a 12" thick carbon sample show that the background is less than 1% of the open beam spectrum for neutrons between 1 and 10 MeV. This is attributed to the good shielding conditions which pre-
5.1 . THE ' 2 C(n,n) 12C REACTION
Differential neutron scattering from a 1 cm thick target was measured at lab,)ratory angles of 32°,
4000 -Resolution of Electronic System Including Detector
Fig. 4. Time distribution of the y-flash and the overall time resolution .
31 5
FAST NEUTRON SPECTROSCOPY
45°, 90°, 135° and 148° using a 30 m flight path . Three typical spectra are shown in fig. 5. The background obtained with the carbon scatterer removed is shown for the 32° measurement. Angular distributions for the well-kown resonances at 2.08, 2.96 and 3.65 MeV are symmetrical about 90° and are consistent with their assignment as d-wave resonances- ). Above 4 MeV, pronounced forward peaking is observed over the resonances indicating values for the orbital angular momentum ofat least 1== 3. At energies above4.43 MeV the present results measure the total scattering yield since the detector is so close to the scatterer that the elastic and inelastic neutron groups are not resolved. In order to obtain the true elastic scattering yield it is necessary to adopt the wcll-establli shed procedure of increasing the detector gala:, thus rejecting the inelastic (lower cnergjr) neutrmrs . The data previously i~eporâed on differential scattering from "C between 4 and 10 MeV are sparse') . The present work shows that this region is now accessible with good resolution so that resonance parameters may be obtained with the ultimate aim of determining phase shifts and polarizations. 5 5.2 . POLARIZATION STUDIESINTM " 6O(y,n)" 0 REACTION
Polarization measurements of nuclear reaction products are notoriously difficult since they generally involve some form of double scattering experiment") . The resulting low fluxes and high backgrounds tend to discourage work in this field. The '6 0(y,n)' S0 reaction is of current interest and
has been studied in considerable experimental 2,4 ) and theoretical detail" , "). A number of problems remain unsolved, however : in particular, the spins and parities of many of the observed levels are not yet established. The polarization of photoneutrons provides a means of identifying the parities . One such xneasurement has been reported using a He spectrometer which is both an energy and polarization sensitive device"'). Unfortunately, the energy resolution is rather coarse compared with the known widths and spacings of levels in the giant dipole resonance region of "60. An attempt was therefore made to observe polarization of photoneutrons by combining the high resolving power of a time-of-flight system. with left-right . scattering from a " 1 C analyser. The experimental arrangement is shown in fig, 6. A bremsstrablung spectrum from a thick tungsten target irradiates a 1" thicl, water target . The resulting time-of-flight spectrum of photoneutrons from the "0(y,n)" - O reaction is observed by scattering from a "zC target placed 20 m from the (y,n) target . The neutron detector is mounted 0.4 m from the C scatterer and is placed at ± 30° with respect to the incident neutron direction. The observed time-of-flight spectra are shown in fig. 6. The :scattering of giant resonance photoneutrons is clearly observed at energies corresponding to the levels in ("2 C + n) at approximately 6.3 and 7.8 MeV. The strong level in ' 60 at 23 MeV (equivalent to a photoneutron energy of about 7 MeV) is not observed since there is no resonant scattering from ' 1 C at 7 MeV. The !eft-right scattering
v w
N O
E w o. Z
0 Z
-2000 m c c 0 t w ,. 1000
a
c
O U
.00
150
200 Çhonnes Nurnbir
250
300
Fig . 5. Titre-of-flight spectrum of seutrcn:4 from the reaction 12C n , r 012C at 32 °, 90° and 148°. Resolution 0.3 ns/m .
F. W. K. FIRK Ey,acx
-. 35 MeV
016 40° F _T_ 21m
300
v
(y,n)
200
c AR c c c r ü c
0
v
60
Chonnei
Number
Fig. 6. Polarization studies, by 30° left-right scattering from
yields are the same within the statistical accuracy of the measurements : the differences noted at neutron energies of 6.3 and 3.8 MeV are possibly instrumental in origin . In the arrangement of fig. 6, the photoneutrons are ob rved at 90° with respect to the direction of the incident photons. The polarization should therefore be zero for E l photon absorpti ~an forming 1 - states in `0: this is essentially the case in the present experiment . There is an obvious need for an accurate measurement ofthe polarization in the "C(n,n)"C reaction at energies above 4 MeV in order to interpret the results completely. f. Conclusions The work reported here clearly demonstrates the part which electron l:nacs can play in the field of fast neutron physics other than in the more straightforward experiments involving total neutron cross_ section measurements . With the continued development of high powered electron linacs used to provide ns pulses at peak currents in excess of 10 A and repetition rates > 1000 Hz it seems likely that difficult experiments such as (n,j}) spectra measurements in the MeV region will become feasible . It is a pleasure to acknowledge the support and encouragement given to this work by Professor H. L.
12C,
1 20
of photoneutrons from the
160(y,n)150
reaction .
Schultz and colleagues in the electron accelerator laboratory . References 1) W. Bertozzi, F. R. Paolini and C. P. Sargent, Phys. Rev. 110 (1958) 790. 2) F. W. K. Firk and K. H. Lokan, Phys. Rev. Letters 8 (1962) 321 . 3) P. F. Yergin, R. H. Augustson, N. N. Kaushal, H. A. Medicos, W. R. Mayer and E. J. Winhold, Phys. Rev. Letters 12 (1964) 733. 4) V. V. Verbinski, private communication (1965) . 5) J. Wills Jr., K. Bair, H. O. Cohn and H. B. Willard, Phys. Rev . 109 (1958) 891 . (1) A. Langsford, P. Bowen, G . C. Cox, F. W. K. Firk, :D. 113 . McConnell, B. Rose and M. J. Saltmarsh, Proc. Antwerp Conf. on Neutron Interactions (1965), unpublished . 7) A. T. G. Ferguson, P. R. Orman and J. H. IVIOUrag:lc, l'r('JC, of Belgrade Conf. on Electronics, Paper NE/ 109 (1961) . s) R. Sugarman, F. C. Merritt and W. A. Higinbotham, Brookhaven National Laboratory Report : BNL-711 096211j . 4) M. D. Goldberg, V . M. May and J. Ft . Stehn, Brookhaven National Laboratory Report : BN L-4(f), 2,td rd. (19%:2) . 10) W . Haeberli, Fast Neutron Physics 2 (Iritcrsc=cnce New 1963) p. 1379. 11) J. P. Elliott and B. H. Flowers, Proc. Roy . Soc. (London) A242 (1957) 57. 12) G. E. Brown, L. Castillejo and .l, A . Lvans, Nucl . Puys . 22 (1961) 1. 13) B. Buck and A. Hill, private communication (1965). 14) W. Bertozzi, C. P. Sargent and W. 'i:urchinetz, pi-wait communication (1965).
43 (1966) 3x2-316 ; (D
NORTH-HOLLAND PUBLISHING CO.
FAST NEUTRON SPECTROSCOPY USING THE YALE ELECTRON LINAC AND A NEW NANOSECOND ' .NIEE-OF-FLIGHT SYSTEM F. W. K. FIRK Yale Üniverst6Y, New Haven, Connecticut
Received 4 March 1966 The Yale electron linac has been modified to provide pulses 10 ns wide, sand 3:1 .5 A peak current. The useful range of citron energy is from 10 to 70 MeV. A lime-to-time converter and synchronmed os :iUator provide a flexible timing system with channel widths between 2 and 10 ns. A-1 overall resolution of 0.2 nsjm is achieved. The favorable ratio of signal-to-background moved for the system permits investigations of a wide range of fast neutron experiments previously the prerogative of Van de
Graaff accelerators. The differential elastic scattering of neutrons from the 12C(n,n)12C reaction has been measured with high resolution at several angles for energies between 1 and 10 MeV. As a further example, polarization effects in the yield of photoneutrons from the giant dipole resonance region of the 160(y,n)IS0 reaction have been studied by left-right scattering from a 12C analyser.
l. Introduction In recent years, several high powered electron linear accelerators have been used for fast neutron studies using nanosecond time-of-flight techniques") . However, experiments performed with them have been generally limited to studies of (y,n) spectra or measurements of total neutron cross sections. So far, no results have been eported of partial neutron cross sections in the MeV region and polarization experiments are nonexistent . This paper reports modifications to the Yale electron linac to produce pulses < 10 ns in duration and peak currents of almost 2 A with a view to studying partial neutron cross sections from i to 10 MeV. Results are presented for the differential ellastic scattering yield of neutrons from 12C(n,n)12C at
several angles . This work supplements and extends the results obtained using Van de Graaff machines) and synchro-cyclotrons') . It clearly demonstrates the potentialities of the spectrometer in this field . Measurements of the polariza' ion of photoneutrons from the 160(y,n)"O reaction using a "C analyser also provide a stringent test of the system .
FILAMENT 6 .3 V a . c .
2. The electron gun modulator The electron gun is a triode operating with a grid bias of -250 V. Under normal running conditions, a hardtube modulator produces a trigger pulse 5 500 V amplitude and 0.1 to 6 Its wide . The entire gun modulator operates at a potential difference of 150 kV with respect to ground : this provides the necessary
VOLTAGE
5000 --l 20 TURNS ON I/4"DIA FERRITE CORE
TO GRID OF E L E ï. r O N GUN ( - 250V BIAS)
Fig. 1 . The electron gun modulator . 312
313
FAST NEUTRON SPECTROSCOPY
injection velocity for electrons entering the corrugated waveguide of the accelerator. In the present arrangernent a thyratron modulator is installed in the lßß kV housing and gives a trigger wise - I IN in amplitude and Z 10 ns wide . The Qb vOtage supply to the thyratron can be controlled ft -,)m the accelerator console. This enables the peak injected current and pulse width to be varied over a wide range the high while the accelerator is operating. If voltage is reduced so that the trigger pulse is 500 V in amplitude the observed current pulse is 5 ns wide: the peak current is then reduced by a factor of two. A circuit diagram of the modulator whichis shown in fig. 1 . It is a conventional systems in a small tbyrMron (2D2 1) is used to provide a fast, high current trigger pulse for a higher powered thyratron (3C45.) . The values of the inductances in the heater circuit of the 3C45 are critical factors in determining the output pulse width and amplitude. Optimurn conditions are obtained by adjusting the length of the ferrite cores. The amplitude of the slow rise at the beginning of the pulse (characteristic of the thyratron's ionization time) is insufficient to overcome the grid bias .
The modulator has performed reliably during the past six months. 3 . The nanosecond time-of--flight spectrometer The basic components of the spectrometer are shown in fig. 2. An Orman-type time-to-time converter') (or time expander) measures the inte!rval T between the electron pulse and the arrival of a neutron at the detector : T is typically in the range I to 5 ps . The device converts T, linearly, to an "`expanded" interval 200T. A synchronized oscillator operating at 0.5, 1 or 2 MHz measures the expanded interval digitally : the output is counted and stored in a 400 channel RIDL analyser. In previovis applications of this method, an unsynchronized oscillator has been used thereby introducing an additional timing uncertainty`). A suitable synchronized oscillator, used in the pr,-sent experiments, is shown in fig. 3. It is basically a tunnel diode monostable circuit which is biased in its negative resistance region during the period of the gating waveform, (the expanded time interval). The frequency of the oscillator is determined primarily by the value of the inductance L. Using a stabilized power supply, an overall stability of 10' Underground
01 .
à
10 il sec
- Electron Gun
and Modulator C 150 kV =rA, ground)
Address Overflow
Reset
Fig. 2. B1cKA diagram of the experiment (W=0,01") .
400 Chonmol RIDL Analyzer
314
F. W. K. FIRK P
+10 Fig. 3. Circuit diagram of the synchronized oscillator .
better than 1 :10' is achieved over a period of weeks. The neutron detector consists of a 6" dia. x 1" thick NE102 plastic scintillator viewed by a 58 AVP phototube. The output is fed directly to a Sugarman-type limiter and fast discriminator') ; the effective bias of the system corresponds to a neutron energy of 300 keV.
vail in the underground flight path with double-wall shielding as shown in fig. 2. Two experiments, outlined in the introduction, demonstrate the performance of the entire system .
4. Spectrometer performance A typical time distribution of the y-flash from the accelerator target obtained when operating under peak current conditions (- 1 .5 A) is shown in the lower part of fig. 4. This is observed in a plastic scintillator after attenuating the y-rays with 12" Pb. If the full intensity of the y-flash is observed the time distribution shown in the upper part of fig. 4 is obtained . This is a measure of the overall electronic resolving time of the spectrometer since the dead-time of the system is such that only the arrival time of the initial photons in the flash is recorded : it is seen to be 5 ns. The current pulse is therefore 9 ns wide, in excellent agreement with the value obtained by viewing the current pulse directly with a sampling oscilloscope . Using flight paths up to 45 rn, resolutions of 0.2 ns/m are therefore achieved .
12C
Results Measurements of the absorption of photoneutrons from a Pb(z,,n) target by ,a 12" thick carbon sample show that the background is less than 1% of the open beam spectrum for neutrons between 1 and 10 MeV. This is attributed to the good shielding conditions which pre-
5.1 . THE ' 2 C(n,n) 12C REACTION
Differential neutron scattering from a 1 cm thick target was measured at lab,)ratory angles of 32°,
4000 -Resolution of Electronic System Including Detector
Fig. 4. Time distribution of the y-flash and the overall time resolution .
31 5
FAST NEUTRON SPECTROSCOPY
45°, 90°, 135° and 148° using a 30 m flight path . Three typical spectra are shown in fig. 5. The background obtained with the carbon scatterer removed is shown for the 32° measurement. Angular distributions for the well-kown resonances at 2.08, 2.96 and 3.65 MeV are symmetrical about 90° and are consistent with their assignment as d-wave resonances- ). Above 4 MeV, pronounced forward peaking is observed over the resonances indicating values for the orbital angular momentum ofat least 1== 3. At energies above4.43 MeV the present results measure the total scattering yield since the detector is so close to the scatterer that the elastic and inelastic neutron groups are not resolved. In order to obtain the true elastic scattering yield it is necessary to adopt the wcll-establli shed procedure of increasing the detector gala:, thus rejecting the inelastic (lower cnergjr) neutrmrs . The data previously i~eporâed on differential scattering from "C between 4 and 10 MeV are sparse') . The present work shows that this region is now accessible with good resolution so that resonance parameters may be obtained with the ultimate aim of determining phase shifts and polarizations. 5 5.2 . POLARIZATION STUDIESINTM " 6O(y,n)" 0 REACTION
Polarization measurements of nuclear reaction products are notoriously difficult since they generally involve some form of double scattering experiment") . The resulting low fluxes and high backgrounds tend to discourage work in this field. The '6 0(y,n)' S0 reaction is of current interest and
has been studied in considerable experimental 2,4 ) and theoretical detail" , "). A number of problems remain unsolved, however : in particular, the spins and parities of many of the observed levels are not yet established. The polarization of photoneutrons provides a means of identifying the parities . One such xneasurement has been reported using a He spectrometer which is both an energy and polarization sensitive device"'). Unfortunately, the energy resolution is rather coarse compared with the known widths and spacings of levels in the giant dipole resonance region of "60. An attempt was therefore made to observe polarization of photoneutrons by combining the high resolving power of a time-of-flight system. with left-right . scattering from a " 1 C analyser. The experimental arrangement is shown in fig, 6. A bremsstrablung spectrum from a thick tungsten target irradiates a 1" thicl, water target . The resulting time-of-flight spectrum of photoneutrons from the "0(y,n)" - O reaction is observed by scattering from a "zC target placed 20 m from the (y,n) target . The neutron detector is mounted 0.4 m from the C scatterer and is placed at ± 30° with respect to the incident neutron direction. The observed time-of-flight spectra are shown in fig. 6. The :scattering of giant resonance photoneutrons is clearly observed at energies corresponding to the levels in ("2 C + n) at approximately 6.3 and 7.8 MeV. The strong level in ' 60 at 23 MeV (equivalent to a photoneutron energy of about 7 MeV) is not observed since there is no resonant scattering from ' 1 C at 7 MeV. The !eft-right scattering
v w
N O
E w o. Z
0 Z
-2000 m c c 0 t w ,. 1000
a
c
O U
.00
150
200 Çhonnes Nurnbir
250
300
Fig . 5. Titre-of-flight spectrum of seutrcn:4 from the reaction 12C n , r 012C at 32 °, 90° and 148°. Resolution 0.3 ns/m .
F. W. K. FIRK Ey,acx
-. 35 MeV
016 40° F _T_ 21m
300
v
(y,n)
200
c AR c c c r ü c
0
v
60
Chonnei
Number
Fig. 6. Polarization studies, by 30° left-right scattering from
yields are the same within the statistical accuracy of the measurements : the differences noted at neutron energies of 6.3 and 3.8 MeV are possibly instrumental in origin . In the arrangement of fig. 6, the photoneutrons are ob rved at 90° with respect to the direction of the incident photons. The polarization should therefore be zero for E l photon absorpti ~an forming 1 - states in `0: this is essentially the case in the present experiment . There is an obvious need for an accurate measurement ofthe polarization in the "C(n,n)"C reaction at energies above 4 MeV in order to interpret the results completely. f. Conclusions The work reported here clearly demonstrates the part which electron l:nacs can play in the field of fast neutron physics other than in the more straightforward experiments involving total neutron cross_ section measurements . With the continued development of high powered electron linacs used to provide ns pulses at peak currents in excess of 10 A and repetition rates > 1000 Hz it seems likely that difficult experiments such as (n,j}) spectra measurements in the MeV region will become feasible . It is a pleasure to acknowledge the support and encouragement given to this work by Professor H. L.
12C,
1 20
of photoneutrons from the
160(y,n)150
reaction .
Schultz and colleagues in the electron accelerator laboratory . References 1) W. Bertozzi, F. R. Paolini and C. P. Sargent, Phys. Rev. 110 (1958) 790. 2) F. W. K. Firk and K. H. Lokan, Phys. Rev. Letters 8 (1962) 321 . 3) P. F. Yergin, R. H. Augustson, N. N. Kaushal, H. A. Medicos, W. R. Mayer and E. J. Winhold, Phys. Rev. Letters 12 (1964) 733. 4) V. V. Verbinski, private communication (1965) . 5) J. Wills Jr., K. Bair, H. O. Cohn and H. B. Willard, Phys. Rev . 109 (1958) 891 . (1) A. Langsford, P. Bowen, G . C. Cox, F. W. K. Firk, :D. 113 . McConnell, B. Rose and M. J. Saltmarsh, Proc. Antwerp Conf. on Neutron Interactions (1965), unpublished . 7) A. T. G. Ferguson, P. R. Orman and J. H. IVIOUrag:lc, l'r('JC, of Belgrade Conf. on Electronics, Paper NE/ 109 (1961) . s) R. Sugarman, F. C. Merritt and W. A. Higinbotham, Brookhaven National Laboratory Report : BNL-711 096211j . 4) M. D. Goldberg, V . M. May and J. Ft . Stehn, Brookhaven National Laboratory Report : BN L-4(f), 2,td rd. (19%:2) . 10) W . Haeberli, Fast Neutron Physics 2 (Iritcrsc=cnce New 1963) p. 1379. 11) J. P. Elliott and B. H. Flowers, Proc. Roy . Soc. (London) A242 (1957) 57. 12) G. E. Brown, L. Castillejo and .l, A . Lvans, Nucl . Puys . 22 (1961) 1. 13) B. Buck and A. Hill, private communication (1965). 14) W. Bertozzi, C. P. Sargent and W. 'i:urchinetz, pi-wait communication (1965).