Neutron capture gamma ray facility using an electron linear accelerator

NUCLEAR INSTRUMENTS

AND M E T H O D S 72 (1969)

254-268; ©

NORTH-HOLLAND

PUBLISHING

CO.

N E U T R O N C A P T U R E G A M M A RAY~FACILITY USING AN E L E C T R O N L I N E A R A C C E L E R A T O R * V. J. ORPHAN, C. G. HOOT, A. D. CARLSON, JOSEPH JOHN and J. R. BEYSTER

Gulf General Atomic Incorporated, San Diego, California 92112, U.S.A. Received 14 March 1969 A versatile facility for the study of resonance neutron capture gamma-ray spectra with high energy resolution is described. An electron linear accelerator is used to provide a pulsed beam of neutrons. Gamma-ray spectra are measured using a Ge(Li)NaI(TI) spectrometer. The gamma-ray pulse height and the

neutron time-of-flight information are stored using an on-line computer. The salient features of the capture facility are described and spectra resulting from the capture of neutrons in natural tungsten are displayed.

1. Introduction

parities of nuclear states and average widths of transitions from many initial states to individual final states can be obtained from an analysis of the average spectra. A versatile facility has, therefore, been constructed at Gulf General Atomic Inc., to study resonance neutron capture gamma-ray spectra. It is possible to study the gamma-ray spectra from the capture of neutrons of any set of energy ranges. These ranges, chosen at the time of analysis, may scan just one resonance or several resonances, may correspond to a small region between two resonances, or cover the entire range from thermal energy to a few hundred keV. An electron linear accelerator is used to produce a pulsed beam of neutrons. The capture sample is located at the end of a 16-metre long evacuated flight path. G a m m a - r a y spectra are measured using a coaxial Ge(Li) detector surrounded by a NaI(TI) annulus. G a m m a - r a y spectra resulting from each of many distinct capturing states are observed in the same timeof-flight experiment. This facility combines the high neutron energy resolution resulting from small burst widths and the high gamma-ray energy resolution from the use of the Ge(Li) spectrometer. This paper describes the principal features of this facility. Operating characteristics of the linear accelerator neutron source and the Ge(Li) spectrometer are summarized. The spectra resulting from the resonance capture of neutrons in natural tungsten are displayed. The performance of this system is compared with that of a fast chopper assemblyS).

An investigation of the g a m m a radiation emitted following the capture of neutrons provides information concerning the states of the resultant nucleus over a wide range of excitation energy. Radiative capture of thermal neutrons has been investigated in detail for several years. Only recently have efforts been directed toward the study of resonance neutron capture spectra. Radiative capture is usually a very complex process and detailed study of the resulting gamma-ray spectra is interesting from several points of view. It is desirable to determine the distribution of the partial radiation widths F~j. for transitions to the final state j from a series of initial states ,,, of a particular spin and parity formed by the resonance capture of neutrons. Some such measurements made in the past are in good agreement with the Porter-Thomas 1) description of the distribution of the widths associated with a single exit channel. Several other measurements 2) suggest that the partial widths have a distribution corresponding to more than one degree of freedom. The neutron-capture mechanism also requires detailed investigation. The existence of narrow resonances in the low-energy capture cross section suggests a compound nuclear process in which a complex unbound state is formed. However, the study of the intensities of high energy g a m m a rays emitted from thermal neutron capture led to the description of the capture mechanism as a simple direct 3-5) type in which the capture state consists of a neutron coupled to the target nucleus. The emitted g a m m a ray results from a single particle transition made by the captured neutron. The mechanism of the (n,'/) reaction can be investigated 6) by examining the interference effects between resonance and off-resonance amplitudes and the partial width correlations. Spectroscopic information can also be obtained by studying the average 3:-ray spectra 7) from the capture of neutrons over a wide energy range. Spins and 254

2. The neutron source The neutron source for this facility is the Gulf General Atomic Electron Linear Accelerator. In this application the Linac typically provides an electron * Supported by the U.S. Atomic Energy Commission and performed under subcontract No. 3032 with Union Carbide Corporation.

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Fig. 3. T h e relative n e u t r o n flux m e a s u r e d with a BF3 counter. T h e flux, as a function o f flight-time, is plotted against n e u t r o n energy. These m e a s u r e m e n t s were m a d e with the Linac operating at reduced power. T h e data points are averages over a r a n g e A E . T h e rapid drop below 0.8 eV is the effect of the c a d m i u m shutter in the flight path.

beam of 40 MeV to 60 MeV, the pulse width being adjustable from 3 nanosecond to 4.5/~sec. It has been operated for extended periods with repetition rates as high as 600 pps. The Linac target consists of a water-cooled fansteel (a tungsten alloy) converter surrounded by a 15.3-cm dia. right circular cylinder of depleted uranium. The electron target assembly is illustrated in fig. 1. The uranium cylinder is encapsulated in an aluminium can to provide containment of radio-active material. A thin titanium window is provided on the aluminium can for the entry of the electron beam. The electrons slowing down in the fansteel target produce bremsstrahlung which in turn produces high energy neutrons, principally through the (?,n) reaction, with a broad maximum near 1 MeV. These neutrons are slowed down to the epithermal and thermal region by a 2.54cm thick piece of polyethylene, shaped to fit around the side of the target facing the neutron flight path. Water surrounding the aluminium can provides

cooling for the uranium and contributes additional moderation. This target arrangement is chosen because it gives a favourable neutron-to-gamma-flash ratio without seriously sacrificing the neutron yield. The neutron beam traverses an evacuated flight path 16 metre long, at the end of which is located the capture sample. A plan view of this arrangement is shown in fig. 2. Shielding employed around the neutron flight path to minimise the background at the spectrometer is also displayed in this figure. Five irises are located along the flight path to prevent neutrons from scattering off the wall of the flight tube into the capture sample. Each iris consists of a 30.5-cm thickness of boric acid in epoxy and 10.2 cm of lead. The beam defining collimator is positioned at the terminus of the flight path and allows a 15.3-cm diameter capture sample to view all of the Linac target. This collimator consists of 33.0 cm of LizCO3 in epoxy and 10.2 cm of lead. A cadmium shutter is provided at 14 m fi'om the neutron source. This shutter can be closed

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in this energy range. The e x p o n e n t ~ is found from the slope o f the curve and the c o n s t a n t K d e t e r m i n e s the absolute value o f the flux, g0(t). F o r the present arrangement o f the target and the m o d e r a t o r , the value o f = 0.585 _+ 0.003. The neutron flux as a function o f the neutron energy, go(E), is given by go(E) = go(t)dt/dE = k'[E(t)] ~- ~,

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and d r o p s off a p p r o x i m a t e l y as l/E for the a b o v e value o f c~. BF 3 m e a s u r e m e n t s described a b o v e are not employed at 16 m to measure neutron flux for energies greater than 1000 eV because o f the finite time taken for the detection system to recover from the g a m m a flash. However, m e a s u r e m e n t s °) m a d e at 200 m verify that the expressions (2) and (3) for relative flux shape can be extended to a b o u t 100 keV neutron energy. The relative flux shape is converted to a b s o l u t e values by measuring the counting rate in an energy region in which all the incident neutrons are c a p t u r e d . A convenient m e t h o d is to use a " b l a c k " I°B abs o r b e r and measure the yield o f the 478 keV g a m m a

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rays from the I°B(n,c07Li * reaction as a function of the neutron time-of-flight. From the known efficiency o f the g a m m a - r a y spectrometer for the 478 keV g a m m a ray, the area of the I°B sample and the fraction o f 478 keV g a m m a rays emitted for every neutron captured in I°B, the absolute flux can be easily calculated.

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The Ge(Li) detector is mounted in a "dip-stick" cryostatt at the end of a 38.2 cm long horizontal copper cold finger inside a 5.7-cm dia. aluminium cap. The spectrometer may be simultaneously operated in the following three modes and the data stored in the CDC-1700 on-line computer: 1. Singly-operated Ge(Li) detector;

3. The Ge(Li)-NaI(TI) spectrometer The gamma-ray spectrometer consists o f a 30-cm 3 Ge(Li) detector located at the centre of a large annulus of NaI(TI) split into two optically decoupled halves. This annulus has 6.3 cm i.d., 20.3 cm o.d. and is 30.5 cm long. Three R C A 8053 photomultiplier tubes are optically coupled to each half o f the NaI(TI) annulus. /

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The spectrometer assembly is mounted on a movable carriage. This allows the distance between the spectrometer and the capture sample to be varied. It also facilitates the rotation of the spectrometer assembly about the centre of the sample. An annular shield consisting of a 5.1-cm layer of lead surrounded by a 5.1-cm thick layer of LizCO3-epoxy mixture protects the NaI(TI) on all sides except the backface. A conical gamma-ray collimator is centred along the axis of the NaI(T1) annulus in the front face of the shield. Details of this arrangement are contained in fig. 2. 3.1. THE Ge(Li) DETECTOR A 30-cm 3 coaxial Ge(Li) crystal, fabricated in our laboratory and similar to a detector described by Orphan and Rasmussen1°), is used as the primary gamma-ray detector. The response of the Ge(Li) detector to low energy gamma rays is studied using several radio-active sources. Its response to higher energy g a m m a rays is investigated using the 4432 keV line from a Pu-Be source and the lines resulting from the

radiative capture of thermal neutrons in chlorine, iron, and mercury. Using optimum time constants in the pulse-shaping amplifier, the average energy resolution of the Ge(Li) detector is 3.3 keV (fwhm) for the 122 keV and 136 keV lines from 57Co and 5.3 keV (fwhm) for the 2754 keV g a m m a ray from ZgNa. During actual data taking using the Linac, it is necessary to shorten the time constants to insure fast recovery from the g a m m a flash. This worsens the over-all resolution of the detector somewhat. For gamma rays of energies above 7 MeV a resolution of 8 keV (fwhm) is typical for data obtained with the Linac as is demonstrated by the partial resolution of the 14 keV doublet at 7632 and 7646 keV in the capture of thermal neutrons by iron. This is illustrated in fig. 4. The response of the Ge(Li) detector to the 2614 keV g a m m a ray from ThC" is shown in the top portion of fig. 5. The full-energy peak, the single-escape peak and the double-escape peak are identified. The energy resolution is about 7 keV (fwhm). The ratio of the

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The operation of the spectrometer in the Compton suppression mode is illustrated by the block diagram of the electronics given in fig. 6. The signals from the three photomultiplier tubes associated with each half of the NaI(TI) annulus are amplified and shaped into a bipolar pulse and passed through a timing single channel analyzer operating as an integral discriminator. Every gamma-ray event in the Nal(TI) above a bias set at 100 keV generates a timing pulse. When a timing pulse from either half of the NaI(TI) annulus is in coincidence with a pulse from the Ge(Li) detector, the linear signal from the Ge(Li) is "tagged". All Compton scattering events occurring in the Ge(Li) detector in which the scattered gamma rays are detected by the NaI(TI) annulus are rejected from the Compton suppression spectrum, The central spectrum in fig. 5 is

measured in the Compton suppression mode. It illustrates the improved peak-to-background ratio ill comparison with the spectrum measured in the singles mode shown at the top of the same figure. Note that the average Compton background is reduced by about a factor of 5.3. At the Compton edge, however, the reduction is only about 4.0. The reduced effectiveness in the region near the Compton edge is the result of large-angle scattering events in which the scattered photon passes through little or no Nal. The relative intensity of the single- and double-escape peaks is significantly reduced in comparison to the full-energy peak. The ratio of the full energy peak to the average Compton background is about 20: 1. The ratio of the intensities of the full energy peak to the double escape peak is improved by a factor of 11.5. The full energy peak efficiency of the spectrometer in the Compton suppression mode, measured over the energy range 500 to 2754 keV using calibrated radioactive sources, is shown in fig. 7. Below 500 keV the exact shape of the efficiency curve is not determined,

262

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This curve includes the attenuation of 1.6 mm of lead and two 2.5 cm pieces of °LiH in 1 mm thick aluminium cans placed in the gamma-ray collimator. The lead greatly reduces the intensity of the gamma flash scattered from the capture sample and the 6LiH helps shield the detectors from sample-scattered neutrons. 3.3. THREE CRYSTAL PA1R SPECTROMETER The logic used for the operation of the pair spectrometer is also shown in fig. 6. The timing single channe! analyzers used in this segment of the circuitry are used as differential discriminators. Windows are set on the 511-keV peaks that result from the absorption of annihilation radiation in the Nal crystals. A crossover pick-off circuit is used in each Nal channel to generate a timing logic pulse whenever a pulse is within the 511-keV window. The timing signal from the Ge(Li) detector is obtained from a time pick-off unit. When the two timing pulses from the NaI halves and the timing pulse from the Ge(Li) detector are in

triple coincidence, the linear signal from the Ge(Li) detector is "tagged" as a pair event prior to storage in the on-line computer. This arrangement yields spectra in which only double-escape peaks appear; full energy and single escape peaks and the Compton background are rejected. The performance of the spectrometer in the pair mode can be seen in the bottom portion of fig. 5 which shows its response to a ThC" source. A single line corresponding to the double-escape peak of the 2614 keV g a m m a ray is seen. Except for a small tail on either side of the pair peak, the background is almost completely eliminated. The three spectra contained in fig. 5 are measured simultaneously using the on-line computer. A comparison of the peak in the pair spectrum and the double-escape peak in the singles spectrum denotes a loss of efficiency in the pair spectrometer by about a factor of 3.1. The efficiency of the pair spectrometer for g a m m a rays between 1370 keV and 8500 keV is shown in

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1.25 MILLION WORD MAGNET C DISK UN YJ Fig. 9. Block diagram of the electronics required for two parameter data acquisition. The electronics associated with the operation of the spectrometer are not shown. The time sequence, where the system is made inoperative during the gamma flash and a short period thereafter, and turned on for the required duration when data is accumulated, is illustrated.

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fig. 8. The efficiency curve is determined from the known j ~) intensities of g a m m a rays from the capture of thermal neutrons in chlorine and from the two g a m m a rays from a calibrated Z4Na source. A sample of CC14, 15.2 cm dia. and 5 cm thick placed at 45" to the beam axis, is used as the source of capture g a m m a rays for these efficiency measurements. The uncertainty in the measured values of the efficiency is about + 10°;,.

4. Two-parameter data acquisition and sorting The gamma-ray energy and the corresponding neutron-time-of-flight are digitized and stored sequentially on the magnetic disc of the CDC-1700 on-line computer. Hence for every event detected in the Ge(Li) spectrometer, two computer words are written on the disc, the first word contains the pulse height information and the second word makes up the time-offlight information. In addition to the pulse height information, the first word contains two bits, one of which identifies events detected in the pair mode (bit 14) and the lother labels events in the Compton suppression mode (bit 13) of the gamma-ray spectrometer. At regular intervals, or at the end of an experimental period, data are transferred on to magnetic tapes for subsequent analysis. A block diagram of the two parameter data acquisition system is given in fig. 9. Only the electronics directly connected with data acquisition are shown in this figure. For simplicity, the electronics associated with the operation of the Ge(Li)-NaI(TI) spectrometer, described in sections 3.2 and 3.3, are omitted. For every gamma-ray event in the Ge(Li) detector two pulses are derived: a linear pulse (gamma-ray energy) and a timing pulse (neutron time-of-flight). The gamma-ray energy signal passes through a linear gate that is normally open. This gate is closed, by gate generator labelled # 1, for about 4 l~sec during the burst of electrons and a short period thereafter to prevent the gamma-flash pulses from being analyzed. A second gate generator (labelled # 2 ) is set to cover the time interval after a Linac pulse corresponding to neutron energies of interest. Those timing pulses that are in coincidence with this gate pulse are used to enable the pulse-height analyzer. In this way a one-to-one correspondence between the pulse-height and the time-of-flight variables is ensured. An El Dorado time analyzer, which has channel widths adjustable from 5 nsec to 80 nsec, is used to measure the time interval between the pre-injector pulse, used as a reference, and a timing pulse from a gamma-ray event. The pre-injector pulse occurs about 2/~sec before the

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actual Linac pulse. The timing of the Linac pulse is determined by observing the time duration between the trigger pulse and the arrival of the gamma-flash at the detector. 4.1. DATA ACQUISITION CODE

The data acquisition code contains a digital gain and zero stabilization routine which corrects the incoming data for any electronic gain or zero-intercept shift. Two very stable pulser peaks, one at either end of the spectrum, are used as reference peaks. The computer code recalibrates the energy scale after a preset number of counts have been accumulated in the reference pulser peaks and adjusts the incoming pulse height data. The smallest incremental shift currently used is 0.25 channel out of the spectral range of 4096 channels. Since the test pulse is injected at the pre-amplifier, the reference peaks are sensitive to gain shifts throughout the system of electronics. The stabilizing routine can be interrogated at any time during a run to determine the amount of gain and zero shift correction that the programme is currently applying to the data. Use of the stabilization routine has permitted the successful addition of four sets of spectra accumulated over a onemonth period without any serious energy shift or loss of energy resolution. A pulse in coincidence with each reference pulse, is used to turn bit 15 on to identify these reference pulses so that the data-accumulation programme may exclude them from the gamma-ray pulse-height spectra. The data acquisition programme allows singles, pair and Compton suppression events to be accumulated simultaneously. External logic circuitry (fig. 6) is used to set bits 13 and 14 to identify Compton suppression and pair events respectively. This code classifies all events as singles data, events having bit 14 set to 1 as pair data and those having bit 13 set to 0 as Compton suppression data. In addition, different thresholds can be pre-set for separate modes of detection, the particular event being registered only if the pulse height falls above the corresponding limit. The code also generates and stores in the computer memory a time-of-flight spectrum (corresponding to all pulse-heights) and a pulse-height spectrum (for all neutron energies). Either of these spectra is available for live display during the experiment. 4.2. THE DATA SORTING PROGRAMME The data are sorted off-line using the Univac 1108 computer located at the central computing facility. The main function of the Fortran IV Programme SORTER is to generate gamma-ray pulse-height

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Fig. 10. Time-of-flightspectrum using natural tungsten sample. This spectrum is measured in the singles mode of tile spectrometer. The low energy resonances are clearly resolved. spectra resulting from the capture of neutrons of energies between two specified limits. The programme can handle 10 neutron-energy limits at a time. For a given neutron energy interval, the programme generates three pulse height spectra, one spectrum corresponding to each operating mode of the spectrometer. In addition, three time-of-flight spectra are also developed, one spectrum corresponding to each mode of detection of the gamma ray. The two 16-bit words from the on-line computer, corresponding to each event, are treated as one word by the Univac 1108 central processor in which the basic computer word consists of 36 bits. The programme SORTER splits each word into the original 16-bit words by suitable masking. It then examines the timeof-flight information to determine to which of the neutron energy ranges the capture event belongs. Once this is ascertained, the programme inspects the first word which contains the pulse-height information and the spectrum label. The spectrum label determines to which of the three pulse-height spectra corresponding to this particular neutron energy range the event should be added. One count is then registered in this spectrum at the channel number deduced from the pulse height. Each magnetic disc has the capacity to store information for about 0.63 x 10 6 events. The programme, on the average, uses 3½ rain to analyze the contents of one disc and generate thirty spectra. It can handle the

contents of several discs. In addition, the programme generates plots of these spectra.

5. Capture gamma-ray spectra Capture gamma-ray data have been recorded for natural tungsten covering the range from thermal energy to 100 keV neutrons. These measurements have been made using a cylindrical disc of natural tungsten with its axis at 45 ° to the axis of the neutron beam. The spectrometer was located at 90 ° to the beam, viewing the front face of the sample. The sample was 15.2 cm in dia. and had an effective thickness of 0.0071 nuclei/barn. A time-of-flight spectrum measured for all gamma rays detected in the singles mode of the spectrometer is shown in fig. 10. The channel width for this display is 1.2psec. The low-energy resonances in the various isotopes of tungsten are well resolved. The peak at the lowest energy corresponds to the 4.16 eV resonance in 1S2W, the next one is due to the 7.65 eV resonance in l s3W. There are two resonances in the neighbourhood of 20 eV; one at 18.84 eV in ~S6W and the second at 21.2 eV in 182W. The display of these two resonances is somewhat distorted by the effects of the thick capture sample used in this measurement. Fig. 11 shows the pair spectra for neutron capture in the energy range 0.02 eV to 1.0 eV and three neutron energy ranges covering the four low energy resonances in natural tungsten. These spectra are dominated by

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programme. Fig. 13 shows the gamma-ray spectrum resulting from the capture of neutrons from 1.5 eV to 100 keV measured with the pair spectrometer. This gamma-ray yield is averaged by an approximately lIE spectrum of incident neutron flux. Such averaged spectra may be used to obtain spectroscopic information. The spectra shown above are only a part of over 30 spectra from tungsten that have been generated and analyzed. A computer code, G A M A N L ' 5), developed at the Massachusetts Institute of Technology has been modified for the analysis of these data. The observed

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characteristics

The c a p t u r e g a m m a - r a y facility has been in o p e r a t i o n for a b o u t a year. The Linac has been o p e r a t e d with TABLE l Performance characteristics of the Linac capture gamma-ray facility. Narrow pulse Wide pulse Pulse width (#sec) Electron energy (,MeV) Electron current (A) Repetition rate (,pps) Flux* at 1 eV [n/(.cmZ.sec.lnE)] Flux* atl00keV [n/(cm"-.sec.lnE)] Timing resolution (nsec/m)

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repetition rates up to 600 pps. Typical p e r f o r m a n c e characteristics o f the system are s u m m a r i z e d in table 1. Figures are quoted for two different modes o f o p e r a t i o n - one for n a r r o w pulses and the other for wide pulses. It is o f interest to c o m p a r e the flux and timing resolution o f the Linac a r r a n g e m e n t with those of a fast c h o p p e r facility 8) which is also used to study c a p t u r e g a m m a - r a y spectra. There is a distinct difference between the two systems. In the case of the Linac the pulse width and the pulse-repetition rate can be varied independently. Hence, high average beam intensity can be m a i n t a i n e d without sacrificing the neutron energy resolution. In the fast chopper, however, the burst width and the repetition rate are intimately c o n n e c t e d by the nature o f the c h o p p e r o p e r a t i o n . In that case high average beam intensity necessarily implies p o o r neutron energy resolution. in the wide pulse mode, with a pulse width o f 5.0 /~sec, the c h o p p e r provides a flux per unit lethargy o f 8 x 103 n e u t r o n / c m Z - s e c which is quite c o m p a r a b l e to the flux per unit lethargy o f 4 . 2 x 103 to 1.12x 104 n e u t r o n / c m z. sec o b t a i n e d with the Linac for a 4.5 ~tsec pulse width at 180 pps. The c h o p p e r with a 22-m flight

268

v . j . ORPHAN et al.

path has a timing resolution of 227 nsec/m compared to 280 nsec/m for the 16-m Linac system. However, in n a r r o w pulse operation, the Linac outperforms the chopper facility. The chopper operated with a 1.0 psec pulse width has a flux of 4.4 x 10z n e u t r o n / ( c m 2. sec. • ln E interval). F o r a 0.1/~sec pulse width, the Linac provides a flux of 1.56 x 103 to 4.14 x 103 n e u t r o n / (cm 2• sec- In E i n t e r v a l ) at 600 pps which is at least three times the flux from the chopper. The timing resolution for the Linac system is 6.25 nsec/m c o m p a r e d to 4 5 . 5 nsec/m for the chopperS). It is therefore evident that the Linac system is superior to the chopper assembly for high resolution studies in the epithermal energy range. This superiority is m a i n t a i n e d even with the new chopper flight path of 48 m. We are grateful to Y. D. Naliboff for p r o g r a m m i n g the CDC-1700 C o m p u t e r a n d to Prof. N. C. Rasmussen for supplying us with the G A M A N L code. The assistance of the Linac O p e r a t i o n s staff, the electronics personnel and the w o r k s h o p staff in the development, o p e r a t i o n and m a i n t e n a n c e of this facility is acknowledged.

References 1) c. E. Porter and R. G. Thomas, Phys. Rev. 104 (1956) 483. 2) e.g., D. L. Price, R. E. Chrien, O. A. Wasson, M. R. Bhat,

M. Beer, M. A. Lone and R. Graves, Nuclear Physics AI21 (1968) 630. a) A. M. Lane and J. E. Lynn, Nuclear Physics 17 (1960) 586. 4) C. K. Bockelman, Nuclear Physics 13 (1959) 205. a) H. Morinaga and C. Ishii, Prog. Theor. Phys. (Kyoto) 23 (1960) 161. ~) O. A. Wasson, R. E. Chrien, M. R. Bhat, M. A. Lone and M. Beer, Phys. Rev. 173 (1968) 1170. 7) L. M. Bollinger and G. E. Thomas, Phys. Rev. Letters 18 (1967) 1143; ibid. 21 (1968) 233. s) R. E. Chrien and. M. Reich, Nucl. Instr. and Meth. 53 (1967) 93. •~) W. M. kopez, M. P. Fricke, D. A. Costello and S. J. Friesenhart, Gulf General Atomic Report GA-8835 (Sept. 1968) unpublished. lo) V. J. Orphan and N. C. Rasmussen, Nucl. Instr. and Meth. 48 (1967) 282. 11) N. C. Rasmussen, Y. Hukai, T. lnouye and. V. J. Orphan, Massachusetts Institute of Technology Report M1TNE-85, to be published. va) E. R. Rae, W. R. Moyer, R. R. Fullwood and. J. L. Andrews, Phys. Rev. 155 (1967) 1301. 13) R. R. Spencer and K. T, Faler, Phys. Rev. 155 (1967) 1368. 14) W. V. Prestwich and R. E. Cote, Phys. Rev. 160 (1967) 159. 15) T. Harper, T. Inouye and N. C. Rasmussen, Massachusetts Institute of Technology Report MIT-3944-2 (August 1968).