A facility for fast neutron time of flight experiments

NUCLEAR

INSTRUMENTS

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METHODS

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(1973) 4 7 9 - 4 9 1 ;

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A FACILITY FOR FAST N E U T R O N T I M E OF F L I G H T E X P E R I M E N T S G. I. C R A W F O R D ,

S.J. H A L L , J. M c K E O W N ,

J. D. K E L I , I E and D. B . C . B .

SYME

Keh'in Laboratoo', Department of Natural Philosophy, University of Glasgou', Glasgou', U.K. Received 22 December 1972 Nuclear reaction experiments involving n e u t r o n s as primary or secondary particles have been performed on a multi-angle time o f flight spectrometer with an electron linear accelerator source.

Essential features in the design o f a flexible a r r a n g e m e n t with good resolution are discussed, and examples are given to illustrate the quality o f the experimental results.

1. Introduction

100 mA are available depending upon the final energy selected (20 to 100 MeV), with pulse repetition rates up to approximately 1200 Hz. Accelerator dark current is negligible (less than I part in 106 of real current) under these normal operating conditions. Experiments in a test rig not connected to the accelerator have shown that the diode gun is capable of 2A and that the low current acceptance of the accelerator is due to a lack of drive on the deflector plates caused by the geometrical layout of the plates and the limitations of our present nanosecond modulator. Work is in hand on an improved injection system.

In recent years considerable use has been made of electron linear accelerators as intense sources of high energy g a m m a rays and neutrons, and the pulsed nature of the source led naturally to applications of neutron time of flight spectroscopy in studies of the photoneutron emission process and of neutron induced nuclear reactions. Examples may be found in the work of Bertozzi and co-workers 1"2) and Firk and associates3-6). Two of the main features of the techniques required are the compromise between energy resolution and counting rate discussed in their review of experimental methods by Bertozzi et al.2), and the suppression of background effects from the bremsstrahlung pulse which inevitably precedes the neutrons from such a source. Solutions to these and other problems are presented in describing a neutron time of flight facility designed to accumulate high resolution data on nuclear reactions with maximum utilisation of the neutron (or photon) flux available. For example, we have accumulated data on the neutron total crosssection and elastic scattering cross-section of 28Si simultaneously with measurements on other neutron beam lines of the neutron inelastic scattering crosssections of 2VAI, 31p and 32S. The quality of data is illustrated by examples for the various reactions.

2. General description The electron linear accelerator of the Kelvin Laboratory, University of Glasgow, satisfies most requirements for a pulsed electron source for neutron time of flight measurements. Electrons injected from a Pierce geometry cylindrical diode gun, at 30 keV are swept into the accelerator prebuncher by a 10 kV pulse applied across a pair of deflection plates from a thyratron driven spark gap, giving 3.5 ns wide pulses of up to 140 mA peak current in the first of three accelerator sections. After acceleration, peak currents of 50 to 479

The layout of the accelerator, beam deflection room and neutron flight paths is shown in fig. 1. For time of flight experiments, the undeflected electron beam is focussed by quadrupole magnets onto a target arrangement in the centre of the well shielded neutron cell. A ZnS scintillation screen is positioned some 15 cm before the target. The position and size of the beam spot are observed indirectly by a well shielded television camera. Beam diameters between 10 mm and 15 mm are obtained depending on the electron energy, which can be measured between runs using the 90 ° magnetic analysis system in the beam deflection room. During measurements of" photoneutron spectra the electron energy is confirmed several times per day to guard against long term drifts, but the normal operating characteristics of the accelerator are such that the peak energy has been shown to change by less than 100 keV in 48 h running at 27 MeV. The energy spectrum of the electron beam has fwhm of 4% at 23 MeV and 2% at 76 MeV. For use as a pulsed source of fast neutrons with maximum yield, an electron beam of high energy ( ~ 100 MeV) is converted to bremsstrahlung and thence to neutrons [mainly by ('~,,n) reactions] in a high Z target of up to 5 radiation lengths diameter placed at the focus of the flight path system. In the heavy

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lerator and beam transport system. The time zero " S T A R T " pulse for neutron timing is conveniently taken from one of these and " S T O P " pulses are obtained from suitable detectors in the experimental area. The time interval is converted to a form suitable for computer storage by a time to pulse height converter plus analogue to digital converter (fig. 2). This conventional arrangement allows maximum flexibility in simultaneous collection of data from many flight paths and makes a very small contribution to the total timing uncertainty. Such a system cannot analyse more than one event per start pulse however, so that the actual counting rate must be less than 1 per 10 beam bursts to permit accurate correction ot" spectra for counting losses. In our present arrangement this limitation becomes restrictive only in the measurement of neutron total cross-section by the transmission method and our solution to this problem will be

FAST NEUTRON

T I M E OF F L I G H T

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EXPERIMENTS

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presented in the section dealing with such experiments. Eight flight paths radiate from the neutron producing cell at angles between 23½" and 156½° . Details of the flight path lengths available on each angle are given in table 1, where it may be seen that the use of the 23½° angle is limited to flight paths of less than 12 m by the earth shielding wall which encircles the flight path area, but that six flight paths of 20 m plus one up to 100 m are available simultaneously for time of flight studies. This arrangement has obvious advantages for measurement of the angular distributions of photoneutrons because the angle to angle normalisation is not time dependent. In addition, several neutron physics

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experiments may be carried out simultaneously on different flight paths with a common neutron source and with some sharing of flux monitors. Only the long flight path is evacuated at present but the small effect of 20 m of air on the neutron spectrum has been measured by the present authors with good energy resolution. A simple transmission measurement was made with a neutron detector at the 50 m station and with 45 m then 25 m of the flight path evacuated in turn. The results are presented in fig. 3, where comparison is made with the magnitude of the effect, as calculated PRESENT

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from the available data on the total cross-section of nitrogen and oxygenS"9). The correction spectrum is stored in PDP-10 computer programmes which may be called during data analysis to remove air absorption effects, where necessary. The evacuated flight path may be continous from 2 m to 100 m or may be broken at 7 m, 25 m, or 50 m to insert collimators, detectors or scattering samples as required. The vacuum pipe is 30 cm diam. to 24 m and 4 0 c m diam. thereafter, and this large volume is evacuated by a rotary p u m p to a working pressure below 100 l~m Hg. The flanges on the neutron cell and the optional gap at 7 m are 1.25 cm thick steel except for a 7.5 cm diameter central window of 0.025 cm 6 ' M y l a r ' , while the 40 cm diameter window at the detector end is of 1 mm aluminium for safety reasons. The compromise between resolution and counting rate is illustrated by some examples in table 2, where values for a lead target and an overall time resolution of 5 ns are given. The components of the total timing resolution are specified in table 3 for the four main classes of time TABLE 2 Typical values at 20 m and 100 m for (a) fast neutron flux from a 5 r lead target at an accelerator p.r.f, of 1000 Hz, (b) energy resolution for neutrons of different energies corresponding to a total time resolution of 5 ns. The radiation length for brcmsstrahlung emission is denoted by r in this articlc. Distance (m)

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of flight experiment we have performed. It may be seen that other contributions are smaller than the 3.5 ns fwhm elcctron pulse, except in inelastic neutron scattering where the time resolution of the gamma ray detector dominates.

3. Collimation of flight paths Neutrons originating in the production target or elsewhere in the neutron cell may reach the detection apparatus by scattering from the wall of the neutron cell, the flight path tube or any shield or apparatus near the detector. The number of scattered neutrons of the first two types is minimised by use o f a collimator tit the cnd of the flight path tube nearest the neutron cell, as the scattered flux is proportional to the solid angle subtended by this aperture. Scattering from the detector area is reduced by a second collimator nearer the detector. This procedure ensures that target and detector are reciprocally entirely in view and that both are well inside the cone defined by the two collimators. The neutron beam at the detector or scattering sample is normally made 50% larger in diameter than the detector or sample. Measurement with a small plastic scintillation detector has shown that variations in the beam intensity profile are less than 5% over the expected area. Simultaneous alignment of several flight paths is simplified with this geometry since the position o f target or detector has been made non-critical. Such an arrangement is necessary to ensure that the neutron fluxes in different flight paths are in a constant ratio determined mainly by the angular distribution of the emitted photoneutrons and independent o f variations in the position or shape ot" the electron beam spot. Composite collimators consisting of alternate layers o f lead and borated paraffin wax are used. The total

TABLE 3 Contributions to the timing resolution in T O F experiments. Experiment

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FAST NEUTRON TIME OF FLIGHT EXPERIMENTS thickness of each material is 0.3 m and 0.6 m, respectively. These collimators have been calculated to give attenuation (including buildup factors) of about 10 3 for 5 MeV neutrons and 109 for 1 MeV photons. The collimator is constructed as a tight fit in the flight pipes in the neutron cell wall or the external shielding walls. Collimators with inner apertures of different diameters may easily be substituted to tailor the dimensions of the neutron beam to the requirements of the experiment. In inelastic scattering we often use ring geometry (see section I1) and the required neutron beam profile is obtained by inserting a 1 m steel shadow bar of the required diameter, concentrically into that flight path, at the appropriate distance. 4. Neutron detectors

483

from the accelerator. The same arrangement may be used to collect the charge pulse from the lead target as an indirect monitor in experiments using the facility as a pulsed neutron source. For such experiments, the neutron spectrum and intensity are monitored more directly by small scintillators used as T O F detectors (a) in the unused flight paths and (b) in the unused edge of the flat part of the neutron beam for the lines which are in use, far away from other detectors and scattering samples. In the latter case, careful measurements are made to ensure that the presence of the monitors does not affect the response of the detector and that the monitors themselves are insensitive to the presence of scattering samples. We have also used small scintillators to view a 0.15 cm thick polythene foil which covered the area o f the neutron beam and scattered a small percentage of neutrons into the monitor detector. Good correlation between these different methods is obtained over a large range of neutron flux values The computer which collects the data is programmed to print out the total number of counts from each detector and monitor per 5 rain interval throughout experimental runs and the ratios of these quantities are examined to provide a dynamic check on the stability of the apparatus.

Fast neutrons are normally detected in our experiments by a 12 cm diam., by 5 cm thick plastic or liquid organic scintillator mounted on an XPI040 photomultiplier. The timing signal is derived from a constant fraction of pulse height trigger in the photomultiplier base chain* or from a separate leading edge discriminator driven by the fast current pulse from the anode. A lower threshold is set on the photopeak of the 241Am 60 keY X-ray corresponding approximately to the maximum pulse height from a 0.5 MeV neutron in these scintillators1°'11). Under these conditions the detection efficiency for neutrons in the energy range 0.75 to 10.0 MeV is greater than 20%. This arrangement is conventional and exhibits simplicity, easy duplication and good timing resolution. (We normally obtain between 1 and 2 ns fwhm for radiation giving a 20:I pulse height range). Similar detectors with the scintillator dimensions chosen to suit the expected fluxes are used for monitoring and other counting purposes.

Our time of flight spectra are calibrated by the following procedure. The integral linearity is measured before and after each run, by the gated oscillator method, using a commercial unit t. The y-flash pulse is then allowed to appear on the linear portion of the T A C - A D C scale either by removal of the A D C threshold which otherwise prevented this or by insertion of a suitable and well-calibrated delay. This provides an absolute time calibration. A delay of up to 1000 ns,

5. Monitors

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For studies of photoneutron emission, a separate bremsstrahlung converter is used before the (),,n) target and the charge incident on this arrangement is measured by the pulse induced in a ferrite toroid before the targets in the neutron cell. Another signal proportional to the bremsstrahlung intensity is obtained from the charge collected from the bremsstrahlung converter which is made to present the same area to the electron beam as the photoneutron target. The electronic arrangement used to digitize the charge pulse is shown in fig. 4. The fast gating arrangement is necessary to discriminate against pick-up of radio frequency power * Type 271, Ortec Ltd., Oak Ridge, Tenn., U.S.A.

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calibrated to better than 0.5 ns, is required and we use a long 50 f2 coaxial cable, calibrated by insertion in the stop channel of a coincidence arrangement feeding a T A C - A D C combination whose gain (nanoseconds per channel) has been previously measured as above. Neutron detectors also have their energy range calibrated by reference to the well known resonances in the neutron scattering cross-section of ~2C, which cover the energy range of interestS"~2). Neutron flux monitors are also calibrated in such a fashion, and the number of counts between specified energies, as defined by carbon lines, enable accurate cemparisons to be made which are independent of TAC, ADC gains. Where possible, e.g. in monitors viewing the neutron producing target through other flight tubes, carbon absorbers are continuously in the beam, otherwise their insertion forms part of a cyclic data-taking process. Careful temperature control of the counting room enables drifts in time converters to be kept below 1 part in 2000 over typically a 6 h period. The flight path length, the gated oscillator spectrum and the absolute time calibration are accepted by a F O R T R A N programme on the PDP-10 computer, which calculates the time of flight corresponding to each ~hannel number before conversion of the time of flight spectrum to one in neutron energy. 7. "t-ray 'flash' effects and their suppression When a combined bremsstrahlung/photoneutron target is used the thick target bremsstrahlung produced may enter the detector directly or by scattering from any secondary targets. With a separate bremsstrahlung converter which is not viewed by the detector directly, a scattering from the photoneutron target is required and if, is tends to decrease the intensity and soften the spectrum of the '~-flash' pulse. The effect of the relatively large pulse of ionisation produced in the detector by the 7-flash depends mainly on the magnitude of the pulse and the characteristic response time of thc detector. NaI detectors have an inherently long scintillation decay timc (approximately 0.24 ys) and time constants of 1 /~s or longer are required in processing the linear signals for these or Ge(Li) 7-ray detectors so that the detection of a large "/-flash pulse may distort the baseline for pulses due to neutron events which typically begin a few hundred nanoseconds later. It is therefore prudent to ensure that detection of a ),-flash pulse inhibits the processing of neutron events in that beam burst and the probability of detection of ~-flash pulses

must be kept low in such experiments if the rate of interesting events is to be significant. Spectra and angular distributions of bremsstrahlung from thick targets have been calculated by Berger and Seltzer t3) who confirm the simple expectation of a predominantly low energy spectrum, (mainly below 2 MeV), peaked sharply forward in intensity. Therefore neutron inelastic scattering experiments are performed at angles backward of 90 ~ (fig. l) and normally include a high Z filter in the neutron flight path to attenuate the low energy photons. After the first 1 cm of lead filter, the photon beam is hardened considerably and further lead attenuates the photon spectrum by only a factor of 10 per 2.5 cm whilst attenuating the neutron beam by about a factor of 2. Therefore there arises an optimum thickness of filter which maximises the number of neutron induced events (at the expense of 7-inhibits) per beam burst, for a particular experimental configuration. Organic scintillators are also sensitive to 7-rays but have fast scintillation decay times (the light output falls by 3 orders of magnitude in 70 ns for some liquid scintillators*). For our applications long time constants are not used, so that quick recovery is possible. This permits running with a higher intensity of y-flash, for we require only that this large pulse must decay well below threshold before the arrival of the fastest neutrens. However, a more stringent limitation on ),-flash pulse height exists for any detector using a photomultiplier tube for signal amplification, due to satellite pulses which are produced some hundreds of nanoseconds after severe illumination of the photocathode. These are caused by the acceleration and arrival at the photocathode of positive ions (mainly hydrogen) liberated from the first dynode. When these 'after-pulses' are produced by the 7-flash pulses they occur in the time range of fast neutrons for flight paths between 6 and 40 m and may seriously distort the fast neutron spectra, especially in conditions where the neutron counting rate per beam pulse is IowA4). A quantitative survey of after-pulsing characteristics in our photomultipliers has been made and the results are to be published elsewhere. We have shown that the percentage of after pulsing depends mainly on the ratio of the "/-flash pulse to the discriminator levcl and may be made negligibly small ifthe ratio is kcpt below 3 to 1. We have developed a system which reduces the ;,,-flash intensity by a factor of 200 by reversing the accelerating voltage between the photocathode and first focus grid before the arrival of the ~,-flash and switching back * NE211, Nuclear Enterprises, Sighthill, Edinburgh, Scotland.

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'on' some 200 n s later with 50 ns risetimc. This system will also be more fully reported elsewhere, but the reduction achieved has been more than adequate to eliminate 7-flash in our present range of flux values.

8. Data acquisition system Although we occasionally use conventional multichannel analysers our data collection system is based on the use of two small computers, a PDP-8 of 4k twelve bit words and a PDP-7 of 16k eighteen bit words of memory. The first of these is normally used as an independent data terminal, servicing two 1024 channel A D C devices when unconnected experiments are being performed on different flight paths, but most data accumulation is done on the PDP-7 which provides more core and a wide variety of peripherals, as shown in tig. 5. Nine analogue to digital converters with up to 8192 channels are interfaced into the computer and these may be used in any combination to suit experimental requirements (subject, of course, to the memory capacity), Programmes written in PDP-7 assembler language service these and the other peripherals, leaving about 13k words for data storage. In onedimensional mode for example, we may run simultaneously with eight 1024 channel A D C ' s and one of 256 channels or with four 2096 channel ADC's, four of 1024 channels and one of 256 channels. Three distinct types of programme exist for two-] r. . . .

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parameter analysis. One provides direct in-core storage but with the limited resolution of a 128 × 100 channel matrix. A more complex programme '5) provides for continuous updating of a 512 × 300 channel matrix on Dectape, with the power to display any segment of the analysed data. Two more bi-dimensional acquisition lines are available simultaneously with this programme, but only the projections of this data on the axes can be displayed. For each of these two inputs the data is stored sequentially on Dectape as twc portions of an eighteen bit word, so that there is a limit of 512× 512 channels or 1024 × 256 channels, etc. At the same time, the programme services two 1024 channel A D C ' s used in single parameter mode, with in-core storage, mainly for monitor spectra. A third version of the bi-dimensional analysis programme operates with 512 × 512 input channels and stores in core the total spectra along each axis, Up to 7 digital pulse height windows can be set on one parameter (usually 7-ray energy) and the spectra of the other parameter (usually time of flight) in coincidence with each of these windows are stored directly in memory. In this mode, approximately 6k storage is available for monitor spcctra or for independent but simultaneous experiments. In future, sequentially stored data will be more economically stored on the disc or magnetic tape of the PDP-10 computer, connected to the PDP-7 by one of the links shown in fig. 5. The serial link operates at 48 kilobaud and is used at present for data transfer to the PDP-10 (48k, 36 bit word) for storage and analysis. Also shown in fig. 5 is a high speed parallel link now in construction between the 3 computers mentioned. This will pcrmit faster information exchange and will facilitate automatic assessment of experimental conditions by the PDP-10. The Camac system shown in fig. 5 is also under construction and will provide interfacing for a further ADC. a scaler system, and any future additions.

9. Data analysis

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puter in the Kelvin Laboratory. This includes basic calibration programmes, which find the time of flight corresponding to each channel number, given input oscillator spectra and carbon absorption lines.Although most of the range of the T A C - A D C combination is lincar, we allow for non-linearities by fitting time of flight to a high order polynomial in channel number. Background subtraction and dead time corrections

486

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are normally performed at this stage before conversion to neutron energy and further analysis such as transposition to the c.m. frame in photoneutron or elastic scattering studies, or correction for distortions to the energy spectrum by air or filters in the flight paths and by the efficiency curve of the neutron detector. These effects are stored as data tiles and may be called by pregrammes where required. The detector efficiency curves are calculated on the PDP-10 by a Monte Carlo programme. We also use Monte Carlo techniques for full multiple scattering corrections in neutron elastic and inelastic scattering work and again these programmes are available and working on our computer. Routines for further analysis convert angular distributions to Legendre series or perform phase shift analyses. We have written nuclear model programmes on the PDP-10 for necessary comparison of data with theory. These include an optical model code which gives total and shape elastic cross-sections and the transmission coefficients which are required by another programme which performs the Hauser-Feshbach calculation of compound elastic and inelastic cross-sections 16).

gap between ground and first excited state of the daughter nucleus. (7, n) targets are normally right cylinders less than 1.5 cm diameter to limit elastic scattering of neutrons• Inner collimators on the neutron flight paths subtend a small area some 3 cm in diameter round the photoneutron target to discriminate against background events (neutron from the bremsstrahlung converter, for example). With 27 MeV electrons, and our geometry and target thickness most photons strike the photoneutron target, but there is some rejection of the scattered electrons which form a wider core. Electro-production events in the photoneutron target are therefore less than 4% and are simply corrected for. Counting rates are somewhat limited by the available electron current at present. Target out background (mainly due to natural radioactivity and cosmic radiation) is of order 10%. This prevents the use of otherwise desirable techniques which lower the count rate such as the inclusion of an electron beam absorber or magnetic sweeping system after the bremsstrahlung converter. Organic scintillation detectors are used in each flight path at 20 m to accumulate time of flight spectra simultaneously at six angles from 40 '~ to 156 ~' from the same photon flux and target arrangement. Relative normalisation between different angles is then simplitied and is limited by the extent to which the detectors are truly identical. We have measured the response of our detectors placed simultaneously round a -'4~Am/Be neutron source and find that they are matched to better than 3% r.m.s, under normal biasing conditions. The incident photon flux is measured by storing the induced

I0. Measurement of photoneutron energy spectra and angular distributions For studies of photoneutron spectra, we use a thick (0.15 r) bremsstrahlung converter some 10 cm before the photoneutron target, which is placed at the intersection of the 7 flight paths. Photoneutrons corresponding to the top 'G" MeV of the bremsstrahlung are necessarily ground state, where 'G" is the energy 12'0 5 r--co

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nucleus in its ground state or in an excited state. In the latter case, prompt de-excitation to the ground state takes place by emission of a ),-ray or cascade of 7-rays, which are detected by Nal(TI) or Ge(Li) counters. Both the ';-ray energy and the time of flight of the incident neutron are obtained and stored by one of the two-parameter routines already described. We have measured cross-sections for excitation of inelastic scattering in both disk and toroidal target geometries and have demonstrated agreement and lack of systematic errors between the two methods by taking data for 31p in both. After multiple scattering corrections (we use 30% absorbing samples) the cross-sections found by the two methods agree to within the statistical accuracy obtained. This is 5% at 1 MeV and 15% at 8 MeV. Cross-sections have been measured for the production of de-excitation )'-rays following inelastic neutron scattering in 27AI, 2aSi, 3tp, 32S and 56Fe. Figs. 8 and 9 show the spectrum of de-excitation ),-rays from 27A1 for two ranges of incident neutron energy and the crosssection for the production of the 2.237 MeV de-excitation ),-ray from 32S. An examination of fig. 8 shows

40

DE-EXCITATION

PHOTON ENERGY(MeV)

Fig. 7, Legendre coefi%ients for the angular distributions of 2D (;,, n) p.

charge on the bremsstrahlung converter and absolute cross-sections for photoneutron production are obtained by direct comparison with that known for deuterium, using a water-heavy water subtraction. Data are accumulated on the PDP-7 and analysis is performed on the PDP-10. Conversion to the c.m. frame is followed by the derivation of Legendre coefficients for the angular distribution. We show as examples in figs. 6 and 7 the 90 ° differential crosssection for ~60(),,n);SO and the angular distribution coefficients of photoneutron from ZD(7,n) p respectively. In the deuterium results, good agreement is obtained with the theoretical results of Partovi ~7) and the ()',p) experiment of Weissman'8).

INELASTIC NEUTRONS

Z

SCATTERING BY Ag.27

FOLLOWING

OF UP TO 2.6 Me_V

0.84 MIV

o

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q ' :j

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PRODUCED BY

l o t Me.V

(2.6-90) 1"72 MeV M~V

Me.V NEUTRONS

t 2-21

"%:

r-. Z :3

oo.,

I I. Inelastic neutron scattering

The experimental arrangement allows for simultaneous use of three independent flight paths of 25 m at the end of which are the targets of the nuclei under consideration (fig. 1). An incident neutron interacts with a target nucleus to form a nuclear state which may decay by neutron emission leaving the residual target

GAMMA RAYS

!

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22

..

1

2-73 McV

[

i... ;'-./i

q

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GAMMA

ENERGY

•

Fig. 8. De-excitation gamma rays following inelastic scattering of neutrons from 27AI.

488

G.I.

CRAWFORD

target from a distance of 60 cm and spectra have been taken at eight angles by moving three detectors and leaving one in a constant position as an additional run-to-run monitor. Good resolution data (0.2 ns/m) have been obtained on the angular distributions of elastic scattering of neutrons in the energy range from 0.5 MeV to the first excited state of the target nuclei, viz. a ° C a , 2SSi and 32S. Cross-sections were obtained with reference to that known for ~2C using a carbon target of the same external dimensions as the target of interest, and with reference to monitors in other flight paths and other monitors viewing the incident flux by scattering from a 0.15 cm thick perspex foil in the main flight path. The results obtained are illustrated by a differential cross-section for elastic scattering on calcium in fig. 10 and by a typical angular distribution shown in fig. II.

clearly that the 1.72 MeV de-excitation ),-ray from 2 7 A l is a cascade y-ray between the 2.73 and 1.01 MeV levels. 12. Neutron elastic scattering

Neutron elastic scattering is studied by detecting the neutrons scattered at different angles from hollow cyclindrical samples placed in the neutron beam at about 20 m. The sample thicknesses are chosen to keep multiple scattering below 4%. Four neutron detectors of the type described in section 4 viewed the scattering

3~-S( n,n' ~) E~= 2.237 MeV

z 8

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,

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13. Measurements of fast neutron total cross-sections with high resolution

zia 9"0

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Total cross-sections are measured with the best energy resolution attainable in our system using a detector at the 100 m station of the evacuated flight path. The layout of collimators, detectors, monitors, and samples in the [tight paths for such measurements is shown in fig. 12. Transmission samples are placed after the 2 cm internal diameter collimator in the main [light path at about 7 m from the neutron source and hence these may be as small as 2.5 cm diameter. Target thicknesses are normally chosen to absorb about 50%

O

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5.0 NEUTRON

ENERGY

et al.

MtV

Fig. 9. C r o s s - s e c t i o n for the p r o d u c t i o n o f the 2.237 M e V de-exc i t a t i o n g a m m a ray f r o m :v-'S, by i n e l a s t i c s c a t t e r i n g o f n e u t r o n s .

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(MeV)

Fig. 10. Differential c r o s s - s e c t i o n at 55 for elastic s c a t t e r i n g o f n e u t r o n s by 'l°Ca.

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o f the incident neutrons. With this geometry inscattering is negligible because o f the small solid angles between source and sample and between sample and detector. The flux and the energy spectrum o f neutrons in the long flight path are observed by identical monitors I and 2 which are small organic scintillator detectors used to determine the time o f flight spectrum o f neutrons back scattered from a 7.5 cm diameter area a r o u n d the centre o f the 7 m collimator. This area is defined by a pre-collimator 5 m from the neutron source. The neutron source is also viewed by a similar

OF FLIGHT

EXPERIMENTS

489

monitor at 15 m in the 90 ° flight path. The main detector at 100 m is a 12 cm diameter by 5 cm thick liquid scintillator assembly in which the photomultiplier is switched "on' some 1 lls after the },-flash and l l~s before the arrival of neutrons of 9 MeV, the highest energy of interest. Data are accumulated in 2048 channels each 5 ns wide to cover the 10 ps flight time difference between 0.5 and 10 MeV neutrons. G o o d statistical accuracy is obtained in short runs by running at maximum p.r.f. and counting about one neutron per beam pulse. Rate dependent distortions are avoided by a system which can count up to four events per start pulse, using four time to amplitude converter units and the gating system shown in fig. 13. Dead time between stops due to gating is normally 20 ns or 0.2% of the time scale and the events are distributed a m o n g the 1-4 converters as 500 per s, 200 per s, 40 per s and 5 per s approximately, so that losses are small and are easily calculable. We keep the counting rate in T A C I constant to within 10% during runs, and adjust the electron current to make this rate equal for target in and out measurements. Residual losses and dead time effects cancel in first order in the comparison of the two runs. Monitor sizes are chosen so that their counting rates are less than I % of the pulse repetition frequency, and rate effects are therefore negligible but good statistical accuracy is obtained on the total number of counts accumulated. Experimental runs at many different flux levels have established good agreement a m o n g the three monitors. Data is accumulated in the cycle: carbon absorber in, no absorber, carbon absorber in, sample in, carbon absorber in, etc. The carbon runs check the time stability of the system and take only 15 min. Main runs last some 3 h, for about 5 million total counts. Crosssections with statistical accuracy of order 3% per channel are obtained in one complete running sequence. Strict temperature control of the counting room has been necessary to eliminate drifts in the time scale due to limitations in T A C stability. With this measure, the short acquisition time enables us to keep drifts to less than one channel in 2000 per run. Any small change between runs is observed under the frequent carbon and y-flash calibrations and is accounted for in the final conversion of time of flight to energy. Corrections are made for counting losses in the last (4th) T A C and the time of flight spectrum from each T A C is converted to an energy spectrum. The four energy spectra are then added. Comparison between the total spectra for target in and out is made with respect to unit flux determined by integration o f the

490

G. 1. C R A W F O R D et al.

p

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monitor spectra between known energies. tions between 0.7 MeV and 9 MeV are stored and plotted on the PDP-10 facility. As an example of the data taken by this show in figure 14 the total cross-section

Cross-seccalculated, system, we of natural

uranium for fast neutrons. The statistical accuracy and energy resolution are about 2% and 2 keV at 1 MeV and 4.5% and 54 keV at 9 MeV, respectively. We obtain good agreement with previous measurements (see ref. 8) at points of overlap.

FAST N E U T R O N

T I M E OF F L I G H T

491

EXPERIMENTS U~ TOTAL CROSS SECTION

)

.

%

!

) ,-

z

881

SAMPLE LENGTH DENSITY

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.

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-

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. . . . . . .

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IO NEUT!qON ENERGY M EV

Fig. 14a. T h e total cross-section o f natural u r a n i u m for fast neutrons. In the energy region 0.85 0.99 MeV illustrating the resolving power o f the system. T h e line merely connects the points and has no special physical signilicance.

References 1) W. Bertozzi, F. R. Paolini and C. P. Sargent, 1958 Phys. Rev. Letters I10 (1958) 790. ") W. Bertozzi, P. T. D e m o s , S. Kowalski, F. R. Paolini, C. P. Sargent and W. Turchinetz, Nucl. Instr. and Meth. 33 (1965) 199. :)) F. W. K. Firk and K. H. Lokan, Phys. Rev. Letters 8 (1962) 321. 4) F. W. K. Firk, J. K. Whittaker, E. M. Bowey, K. H. k o k a n and E. R. Rae, Nucl. Instr. and Meth. 23 (1963) 141. ~) F. W. K. Firk, Nucl. Phys. 52 (1963) 437. 6) F. W. K. Firk, Nucl. Instr. and Meth. 43 (1966) 312. 7) G. S. Mutchler, Thesis ( M a s s a c h u s s e t t s Institute o f Technology, 1966) unpublished. 8) D. J. H u g h e s and R. B. Schwartz, Brookhaven National Laboratory Report BNL-325 (1958).

60i

" i "1,'~d%0'~i.~

P 52~ 0 . . . . . . . . . . . . . . . . . . . . . . .

08

2O

4O

60

80

92

KEUTRON ENERGY ( M E V I

Fig. 14b. T h e total cross-section or natural u r a n i u m for fast n e u t r o n s in the energy region 0.75 8.8 MeV. The large crosses are from previous measurementsS). ~)) D. G. Foster, and D. W. Glasgow, Phys. Rev. C3 (1970) 576. 10) V. V. Verbinski, W. R. Burrus, T. A. Love, W. Zobel and N. W. Hill, Nucl. Instr. and Meth. 65 (1968) 8. ll) D. L. Smith, R. G. Polk and T. G. Miller, Nucl. Instr. and Meth. 64 (1968) 157. 1,,) R. S. Stehn, M. D. Goldberg, B. A. M a g u r n o and R. WeinerC h a s m a n , Brookhaven National Laboratory Report B N L 325, 2nd ed., Suppl. 2 (1964). 13) M. J. Berger and S. M. Seltzer, Phys. Rev. C2 (1970) 621. 14) j. M. Rawlins and Y. M. Shin, Nucl. Instr. and. Meth. 58 (1968) 353. is) j. D. K. Kellic and G. I. Crawford, Nucl. Instr. and Meth. 72 (1969) 329. ](1) W. H a u s e r and H. Feshbach, Phys. Rev. 87 (1952) 366. 17) F. Partovi, Ann. Phys. 27 (1964) 73. 18) B. Weissman, Thesis, EAL Internal Report 81 (University o f Yale, 1969).