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The polarization of neutrons from the 2H(d,n)3He reaction for deuteron energies less than 1 MeV
NUCLEAR
INSTRUMENTS
AND
THE POLARIZATION
METHODS
108
(1973)
OF NEUTRONS
DEUTERON
FROM
ENERGIES
H DAVIE* Physrcs Department,
581-586;
0
NORTH-HOLLAND
THE 2H(d,n)3He
LESS
THAN
PUBLISHING
REACTION
co.
FOR
1 MeV
and R B GALLOWAY Unrversrty of Edmburgh,
Scotland
Received 11 December 1972 A serx.s of measurements are reported of the polarlzatlon of neutrons emltted at a laboratory reactlon angle of 46” from the zH(d,n)sHe reactlon for deuterons from 0.3 to 0.9 MeV Incident on a thm target Partxcular attention IS given to the collection and analysis of data from the 4He scattering polarlmeter m a way
which detects and corrects for spurious scattermg to give confidence that the polarlzatlon values correctly chart the energy dependence through previously pubhshed discrepant values. A possible dlscrepancles IS discussed
1. Introduction
side counters. As was noted m a preliminary reportll) on the design and operation of the polarlmeter used m the present study, the helmm recoil spectrum associated with neutrons detected m comcldence between the gas scmtlllator and a side detector contamed not only the expected peak, but also a tall Unlike the peak, the tall showed negligible asymmetry effects associated with the polarlzatlon of the incident neutrons. A Duke University group working on various reactions has also reported such a tall, and have seemed uncertain as to its ongm12). In none of the papers referred to m fig 1 was mention made of helmm recoil spectra. Conse-
Hlstoncally the polanzatlon of fast neutrons emitted m a nuclear reaction was first predictedI) and measured’) for the case of the 2H(d,n)3He reaction. Subsequently many measurements of D-D neutron polarlzatlon have been made, as may be seen from the review of Galloway3), and yet there still remain ranges of deuteron energy over which these results show little or no measure of agreement. Fig. 1 shows a plot of results (post 1957) m the energy region with which this report 1s concerned, for laboratory reaction angles around 45”, this being close to the centre of a broad maximum m the polarization angular dlstnbutlon6). With the exception of Behof et al.*) all of the workers whose results are displayed m fig. 1 employed thm targets with stopping powers m the 50-150 keV range. Behof et al.*) used a thick target and reported measurements for incident deuteron energies from 60 to 380 keV, their results showing little or no variation with deuteron energy, and consequently provldmg some justlficatlon for their display along with thm target measurements Fig 1 shows that the dlscrepancles thrown up by the earlier workerss*6) have been compounded by more recent publications’*“). The present study set out to mvestlgate these dlscrepancles by taking accurate polanzatlon measurements and while doing so looking for possible sources of error which might have passed unnoticed in earlier work. The technique employed was similar to that used for all but three4*7,‘0) of the measurements listed m fig 1, namely the neutrons were scattered from a high pressure helium gas scmtlllator and detected m two * Present
address Department of Electromcs and ElectrIcal Engmeermg, Umverslty of Glasgow, Scotland.
581
effects, so as reported here the scatter of orlgm of past
-0 30
1 -0
25
t
t
01
T
I
1
02
04 MEAN
08
06 DEUTERON
ENERGY
IO
(Mb’)
Fig. 1. Polarlzatlon of neutrons emltted at a laboratory angle of about 45” from the 2H(d,n)3He reaction 0 Levmtov et al. (1957)3; 0 Pasma (195Q5) with the analysmg power recomputed usmg the phase shifts of Austm et al ls), n Boersma et al. (1963)‘j), 0 Mulder (1966)7), m Behof et al (1968)*); A Rodmg and Scholermann (1969)g); 0 Smith and Thornton (1972)l”), X present measurements.
582
H. DAVIE
AND
TAZGET
Fig. 2.Sketch
of polarlmeter
m plan.
quently m the present work particular emphasis 1s placed on a thorough mvestlgatlon, both experimentally and by computation, of possible tall producing mechanisms and their influence on polarlzatlon measurements. 2. Experimental procedure
R.B. GALLOWAY
detected m comcldence between the gas scmtlllator and a side neutron detector and with random background subtracted. Along with the expected polarized peak due to the neutrons being scattered by the hehum through a mean angle of M 120”, there appears a substantial unpolarized tad. Clearly the mcluslon of tall contrlbutlons m a measurement will introduce error, and consequently the source of the tall and Its extrapolation beneath the peak were thoroughly investigated To check whether the presently reported tad could be due to y-ray events ‘breaking through’ the pulse shape dlscnmmatlon circuitry, test runs were made with no neutron beam, but with y-ray sources placed lmmedlately beside the gas scmtlllator. The gated spectrum so produced was compared with that produced using a neutron beam, a normahzatlon factor bemg provided by the number of events (neutron or y-ray) recorded above a linear threshold of 100 keV electron recoil energy m one of the side hqmd scmtlllators This procedure should grossly overestimate y-ray contamlnation, yet such y-ray breakthrough as was found was confined to the first few channels of the spectrum where It amounted to only a few per cent of the counts recorded m the same channels during the run with the neutron beam. The contrlbutlon from neutrons
2 1. THE POLARIMETER A full description of the polanmeter sketched m fig 2, and Its associated electronic circuitry has been published elsewherell). Briefly, a magnetically analysed deuteron beam of some 50-100 PA was incident upon a water cooled target, the neutron producing material as seen by the deuteron beam being a 340 ,ug cm-’ thick layer of TIED After collimation the neutrons were scattered through a mean angle of z 120” from a 4He gas scmtlllator and the asymmetry of the scattermg m the reaction plane, which allows a measure of the polanzatlon, was recorded by a pair of liquid scmtlllator neutron detectors. All three scmtlllators were mounted on a rotatable cradle to allow for the The data cancellation of spurious asymmetries collected were m the form of hehum recoil energy spectra correspondmg to neutrons detected m comcadence between the gas scmtlllator and a neutron detector Random comcldence spectra were also recorded to allow appropriate subtraction y-ray comcadences were reJected by pulse shape dlscrlmmatlon circuitry
j 2000I 4 v
!
t
F f
s
IOOO-
t
0
Fig. 3 shows a pulse height
from
the hehum
spectrum of the output gas scmtlllator, gated by neutrons
IO
20
30 CHANNEL
2.2. THE HELIUM RECOIL SPECTRUM Fig. 3. Pulse height the tad extrapolation
40
50
60
No
spectrum of gated hehum recods showmg and the peak region selected for analysis.
THE
POLARIZATION
reaching the polarimeter after penetratmg the shreldmg walls was also found to be negligible by recordmg a gated spectrum with solid Inserts placed m the colhmator aperture, normahsation m this case being made by momtormg the target neutron yield. The possibihty that the tail could be due to low energy neutrons arriving from the collimator aperture and being scattered through z 120” by the helium may also be ruled out, since the tall rises sharply m the low energy region which would by this explanation correspond to neutrons recoiling through z 120” with less than the neutron detection threshold energy of the hqmd scmtillators (Z 0 4 MeV). Immediately to the rear of the active volume of the gas scmtillator is a quartz vrewmg window with a stamless steel surround, their dimensions being comparable with a fast neutron mean free path. A possible source of the tail may arise from neutrons scattered both from the helium and thts sohd volume (m either order) before being detected by a hqurd scintillator Experimental support for this hypothesis was obtamed by temporarily re-orientatmg the gas scmtillator so that rt lay transverse to the neutron beam, thus removmg most of the sohd material to the rear of the scmtillatmg volume from direct u-radiation, whereupon the magnitude of the tall was approximately halved In addition, with the scmtrllator still m this onentation, on placmg a replica of the quartz window m contact with the gas scmtillator shell on its side remote from the target, the tail was increased by approximately 50%. As a further test of this hypothesis concernmg the production of the tail, Monte Carlo computations were undertaken with the following simphfymg assumptions. The mcident neutron beam was taken to be parallel and monoenergetic; the differential scattering cross section m the hehum was computed from one set of phase shift angles, i.e. energy loss m any previous scatter m the solid was neglected; the scattermg probabihty m the solid was taken to be a constant Independent of the material, the angle of scatter or the neutron energy; the detection efficiency of the hquid scmtillator was assumed independent of the neutron energy; absorption of the neutrons m the solid, hquid and gas was ignored. On this basis three hehum recoil energy spectra were computed corresponding to. a) neutrons scattered hqurd scintillator;
directly
by the hehum
OF
583
NEUTRONS
steel surround latmg volume) lator;
at the rear of helium gas scmtilbefore reaching a hqmd scmtil-
c) neutrons scattered first by the solid material and then by the helium before reaching a hquid scmtillator These spectra were then given a gaussran smear of 30% fwhm to srmulate the finite resolution of the gas scmtillator, and the results are plotted m fig. 4. Curves 2,3 correspond to the routes (b), (c), outlined above, while curve 1 is a combmation of routes (a), (b) and (c). The magnitude of the tall shown corresponds to a differential cross section of 150 mb sr-i for the scattermg m the solid, which seems a reasonable value at the large backward scattering angles mvolved Due to the approximations assumed m the computation, no great trust can be placed on fig 4 as an exact representation of the experimentally observed tails, however, it does show that double scattering mvolvmg the sohd material to the rear of the gas scmtillator is a plausible source of a malor portion of these tails Lesser contrrbutions to the tail will arise from neutrons scattered by other components of the polarimeter assembly (e. g the scmtrllator and photomultipher containers), and from double scattering m the hehum itself. At first sight it would seem that transverse mountmg of the gas scmtillator with careful neutron colhmatron could be advantageous m reducing the tail. This removes the quartz wmdow and its surround from direct irradiation and thus prevents events bemg recorded due to neutrons first scattered m these solids. However, neutrons scattered first by the hehum and
+++
mto a
b) neutrons scattered first by the hehum and then by the solid material (i.e. the quartz and its stainless
PULSE Fig. 4. Simulated
HEIGHT
pulse height spectrum,
see text.
584
H. DAVIE
AND R. B. GALLOWAY
then by the quartz window or its surround ~111 still be recorded, moreover the mean angle of scatter m the hehum for such events ~111 be greater than that for events recorded folloivmg a slmdar route but with the hehum scintlllator orlented as m the present experiment Consequently the overlap of the peak area by tall events may be increased by transverse mounting, although the magnitude of the tall m the region of low hehum recoil energy, remote from the peak, will be substantially reduced The use of fast timing may also modify the observed tall. If comcldence resolvmg times much shorter than the gas scmtlllator decay time are employed, then low energy hehum recoils may be rejected due to time Jitter effects. The danger m such a modlficatlon lies m it suggestmg an Incorrect extrapolation beneath the peak In the present polarlmeter both the comcldence resolvmg time and the gas scmtdlator decay &me were Z 1o-6 s.
Typically the portion of the peak region selected for the polanzatlon analysis 1s as indicated by the arrows m fig. 3. To correct for the presence of the proposed tall, the asymmetry recorded between such limits reqmred multlphcatlon by a factor which ranged from 1 10 to 1.14 (see table l), the larger factors bemg requn-ed for the poorer resolution spectra If a +50% uncertainty 1s placed on the tall contnbutlons, then the uncertamtles so created m the polarlzatlon results are less than their statlstlcal uncertamtles. The measured asymmetry corrected for the presence of the tall equals the product of the polanzation and the mean analysmg power of the polanmeter. The analysmg power correspondmg to each channel m the simulated hehum spectrum (a) of section 2.2 was computed for a smgle neutron energy, and was found to remam essentially constant throughout this spectrum. Hence the mean analysmg power for the whole peak was found to be an adequate estimate of the analysmg power for that part selected as Indicated m fig. 3 This mean analysmg power was calculated at the mean neutron energy of each of the results using a Monte Carlo technique The phase shift angles of Austin et a1.13) were employed, and a correction term was included m the computation to allow for the spurious asymmetry resultmg from the variation of the ‘H(d, n)3He reaction cross section with reaction angle
2.3. DATA ANALYSIS It 1s Important to obtain an estimate of the extent to which the experimentally observed tall (with near zero asymmetry) extends beneath Its correspondmg peak to allow a relevant correction to the data to be made Further it IS desirable to accept only a portion of the peak around its maxlmum for use m the polarlzatlon computation m order to mmlmlse this tall correction with its accompanymg uncertamty. The solid hne m fig. 3 shows a typical tall extrapolatlon used m the present study, this bemg obtained by the trial of several smoothly varymg extrapolations and by consldermg the asymmetry recorded m groups of channels throughout the peak region This asymmetry, when corrected for the presence of the true tall, ought to remam near constant throughout the peak region, hence its exammatlon helps indicate an approprlate tail extrapolation
3. Results The results obtained are as shown m table 1. In the first column are listed the deuteron energies with thenassociated spreads due to the stopping power of the TI-D target layer Next are listed the asymmetries measured between the limits as indicated m the preVIOUS section accompanied by their standard devlatlons, while column three contams the correction factors required to account for the presence of the zero asymmetry tails. The analysmg powers are contained
TABLE
2H(d,n)3He neutron polarlzatlon Deuteron energy (keV)
Asymmetry m reactlon plane
215 f 60 385f55 500+ 55 620 f 50 730f50 840f45
-0115f0010 -0133fO009 -0127fOOlO -0 119f0.009 -0.135fO 009 -0124fOOll
Tall correctlon factor
1 12 1 12 1 12 1 14 1 10 1 14
1
results for a laboratory
angle of 46”.
Analyzmg power
Neutron polarization
0 89 0 90 0 92 0.93 0 93 0.94
-0.14.5f0.013 -0166f0011 -0.155f0.012 -0.146f0.012 -0.160f0.011 -0 150f0.013
Asymmetry normal to reactlon plane
-0.002f0.010 - 0.006 f 0 009 -0.001 fO.O1O +o.o02fO 010 + 0.007 f 0.009 +0009+0011
THE
POLARIZATION
m the fourth column, and these are followed by the polarization values along with then associated statistical uncertamties The scattering asymmetries recorded m a plane normal to the reaction plane are listed in the final column of the table. These measurements, wluch ought to show zero asymmetry within then statistical uncertainties, were interspersed with each polarization measurement as a check on the operation of the polanmeter. All of the measurements recorded m table 1 were taken at a mean laboratory reaction angle of 46”, the geometrical spread m this angle due to the finite size of the hehum scatterer bemg + 1.75”. Multiple small angle scattering of the deuterons by the titamum of the target layer gives rise to a further spread. Thus followmg Wllhams’4) the r m s angular deviation of the deuterons after having traversed one half of the target thickness was determined to range from 1.5” m the case of the measurement at 875 keV to 4 2” m the case of the measurement at 275 keV The contammation of the direct beam of neutrons emitted by the target with neutrons once scattered m that part of the target assembly viewed by the hehum gas scmtillator was considered, however Monte Carlo computations showed that the error that this would mtroduce m the polarization measurements was considerably less than their statistical uncertamties. 4. Discussion A comparison of the present and past measurements as displayed m fig. 1 shows marked disagreements between the present work and the results of Pasma’), Boersma et a1.6) and Rodmg and Scholermanng). The magnitudes of Pasma’s pioneermg measurements are considerably less than those recorded by all other workers hsted m fig. 1. This IS possibly due to the lack of y-ray reJection cncmtry and the fact that hehum recoil spectra were not recorded. Hence his measurements may have included y-ray events and neutrons scattered m the material surrounding his gas scmullator (the tail events as discussed m section 2.2). The polarizations recorded by Boersma et a16) are also lower m magnitude than the present measurements. They used a y-ray rejection procedure, but did not record their hehum recoil spectra, consequently an unwittmg mclusion of tall events m their measurements could account for their lower magmtudes. The results of Rodmg and Scholermann displayed m fig. 1 were taken at a laboratory reaction angle of 4@“, although their measurements at an angle of 60” do not differ significantly and they predict a maximum
OF NEUTRONS
585
m the polarization magnitude between these angles. Consequently the disagreement between then results and the present ones is significant. The values of analysmg power quoted suggest that they employed scattermg of the neutrons by the hehum m a forward direction, although the angle is not quoted As recorded m a previous paperIl), the data collection efficiency is good at such angles, however, the magnitude of the analysmg power is uncertain to withm M 20%. This may partially account for what seem excessively high polarization magmtudes as compared to the present work, Rodmg and Scholermann did not record hehum recoil spectra during their measurements, however the mclusion of tail events ought to have been reduced by then use of a forward scattermg angle, by the transverse mountmg of their hehum gas scmtillator, and possibly by then use of a time of flight technique to reject y-rays Also, their time of flight spectrum, fig. 3 of ref 9, exhibits a substantial slopmg background under the peak and correction for this may be a further source of error The results of Behof et a1.8), although taken with a thick target and at deuteron energies below 400 keV incident are generally of a similar magnitude to the present results, and show a similar lack of energy dependence Then paper infers that the hehum recoil spectra were recorded, but no spectra are published, nor are details of any corrections required for tail events. The single measurements recorded m fig. 1 due to Levmtov et a1.4) and to Mulder’), both resulting from unique techniques, are m reasonable agreement with the present results. The single result of Smith and Thornton”) is then lowest energy result m a recent sequence of measurements stretching to a deuteron energy of 5 MeV Although little meanmgful comparison may be made with their one result below 1 MeV and the present measurements, a study of their technique shows that tail events may have been included m their measurements, so reducing the magnitude of the polarizations they recorded Instead of gated hehum recoil energy spectra, Smith and Thornton recorded gated time of flight spectra, then flight path being between the target and their liquid hehum scmtillator They indicate that they also performed a trial time of flight spectrum between the liquid hehum scmtillator and a side neutron detector which showed a ‘time correlated background’. This they believe to be the background reported by the Duke group12), however they indicate that its effect on then measurements was negligible Due to then varying routes and energies, the neutrons associated with tail events as described m
586
H.
DAVIE
AND
R.
section 2.2 ~111not exhibit a sharp transrt time between the hehum and a srde detector. Rather they wrll tend to broaden the tumng peak associated with the desrred direct neutrons. Since this trmmg peak ~111 alreadv be broad due to the hehum scmtrllator and side detector dlmensrons being a srgmficant fraction of the timing path, rt 1s difficult to see how such a procedure could sort out tall events. The authors thank Dr D. G. Vass, Mr H. J. Napier, Mr G. Turnbull and Mr D. Green for their valuable assrstance. References 1) L Wolfenstem, Phys. Rev. 75 (1949) 342 2) P. Huber and E. Baumgartner, Helv. Phys Acta 26 (1953) 420; R. Rxamo, Helv. Phys. Acta 26 (1953) 423
B.
GALLOWAY
3) R B Galloway, Nucl. Jnstr. and Meth. 92 (1971) 537; Errata 95 (1971) 393. 4, I I Levmtov, A. V. Mdler, E. Z. Tarumov and V. N. Shamshev, Nucl Phys 3 (1957) 237. 5) P. J Pasma, Nucl Phys. 6 (1958) 141. 6) H J. Boersma, C. C. Jonker, J. G. NlJenhuls and P J. Van Hall, Nucl Phys 46 (1963) 660. 7, J. P F. Mulder, Phys. Letters 23 (1966) 589. 8, A. F. Behof, T. H May and W I. McGarry, Nucl. Phys. A 108 (1968) 250 g, P. Rodmg and H. Scholermann, Nucl. Phys. Al25 (1969) 585. 10) J. R. Smith and S T. Thornton, Can J. Phys. 50 (1972) 783. 11) H. Davle and R. B Galloway, Nucl Instr and Meth 92 (1971) 547. 12) J. R. Sawers, Jr, G. L Morgan, L. A. Schaller and R. L Walter, Phys Rev 168 (1968) 1102, G Spalek, R A. Hardekopf, J. Taylor, Jr, Th Stammbach and R L. Walter, Nucl. Phys A191 (1972) 449. 13) S M Austm, H. H Barschall, R E Shamu, Phys Rev 126 (1962) 1532 14) E. S Wdhams, Rev Mod. Phys 17 (1945) 217.
INSTRUMENTS
AND
THE POLARIZATION
METHODS
108
(1973)
OF NEUTRONS
DEUTERON
FROM
ENERGIES
H DAVIE* Physrcs Department,
581-586;
0
NORTH-HOLLAND
THE 2H(d,n)3He
LESS
THAN
PUBLISHING
REACTION
co.
FOR
1 MeV
and R B GALLOWAY Unrversrty of Edmburgh,
Scotland
Received 11 December 1972 A serx.s of measurements are reported of the polarlzatlon of neutrons emltted at a laboratory reactlon angle of 46” from the zH(d,n)sHe reactlon for deuterons from 0.3 to 0.9 MeV Incident on a thm target Partxcular attention IS given to the collection and analysis of data from the 4He scattering polarlmeter m a way
which detects and corrects for spurious scattermg to give confidence that the polarlzatlon values correctly chart the energy dependence through previously pubhshed discrepant values. A possible dlscrepancles IS discussed
1. Introduction
side counters. As was noted m a preliminary reportll) on the design and operation of the polarlmeter used m the present study, the helmm recoil spectrum associated with neutrons detected m comcldence between the gas scmtlllator and a side detector contamed not only the expected peak, but also a tall Unlike the peak, the tall showed negligible asymmetry effects associated with the polarlzatlon of the incident neutrons. A Duke University group working on various reactions has also reported such a tall, and have seemed uncertain as to its ongm12). In none of the papers referred to m fig 1 was mention made of helmm recoil spectra. Conse-
Hlstoncally the polanzatlon of fast neutrons emitted m a nuclear reaction was first predictedI) and measured’) for the case of the 2H(d,n)3He reaction. Subsequently many measurements of D-D neutron polarlzatlon have been made, as may be seen from the review of Galloway3), and yet there still remain ranges of deuteron energy over which these results show little or no measure of agreement. Fig. 1 shows a plot of results (post 1957) m the energy region with which this report 1s concerned, for laboratory reaction angles around 45”, this being close to the centre of a broad maximum m the polarization angular dlstnbutlon6). With the exception of Behof et al.*) all of the workers whose results are displayed m fig. 1 employed thm targets with stopping powers m the 50-150 keV range. Behof et al.*) used a thick target and reported measurements for incident deuteron energies from 60 to 380 keV, their results showing little or no variation with deuteron energy, and consequently provldmg some justlficatlon for their display along with thm target measurements Fig 1 shows that the dlscrepancles thrown up by the earlier workerss*6) have been compounded by more recent publications’*“). The present study set out to mvestlgate these dlscrepancles by taking accurate polanzatlon measurements and while doing so looking for possible sources of error which might have passed unnoticed in earlier work. The technique employed was similar to that used for all but three4*7,‘0) of the measurements listed m fig 1, namely the neutrons were scattered from a high pressure helium gas scmtlllator and detected m two * Present
address Department of Electromcs and ElectrIcal Engmeermg, Umverslty of Glasgow, Scotland.
581
effects, so as reported here the scatter of orlgm of past
-0 30
1 -0
25
t
t
01
T
I
1
02
04 MEAN
08
06 DEUTERON
ENERGY
IO
(Mb’)
Fig. 1. Polarlzatlon of neutrons emltted at a laboratory angle of about 45” from the 2H(d,n)3He reaction 0 Levmtov et al. (1957)3; 0 Pasma (195Q5) with the analysmg power recomputed usmg the phase shifts of Austm et al ls), n Boersma et al. (1963)‘j), 0 Mulder (1966)7), m Behof et al (1968)*); A Rodmg and Scholermann (1969)g); 0 Smith and Thornton (1972)l”), X present measurements.
582
H. DAVIE
AND
TAZGET
Fig. 2.Sketch
of polarlmeter
m plan.
quently m the present work particular emphasis 1s placed on a thorough mvestlgatlon, both experimentally and by computation, of possible tall producing mechanisms and their influence on polarlzatlon measurements. 2. Experimental procedure
R.B. GALLOWAY
detected m comcldence between the gas scmtlllator and a side neutron detector and with random background subtracted. Along with the expected polarized peak due to the neutrons being scattered by the hehum through a mean angle of M 120”, there appears a substantial unpolarized tad. Clearly the mcluslon of tall contrlbutlons m a measurement will introduce error, and consequently the source of the tall and Its extrapolation beneath the peak were thoroughly investigated To check whether the presently reported tad could be due to y-ray events ‘breaking through’ the pulse shape dlscnmmatlon circuitry, test runs were made with no neutron beam, but with y-ray sources placed lmmedlately beside the gas scmtlllator. The gated spectrum so produced was compared with that produced using a neutron beam, a normahzatlon factor bemg provided by the number of events (neutron or y-ray) recorded above a linear threshold of 100 keV electron recoil energy m one of the side hqmd scmtlllators This procedure should grossly overestimate y-ray contamlnation, yet such y-ray breakthrough as was found was confined to the first few channels of the spectrum where It amounted to only a few per cent of the counts recorded m the same channels during the run with the neutron beam. The contrlbutlon from neutrons
2 1. THE POLARIMETER A full description of the polanmeter sketched m fig 2, and Its associated electronic circuitry has been published elsewherell). Briefly, a magnetically analysed deuteron beam of some 50-100 PA was incident upon a water cooled target, the neutron producing material as seen by the deuteron beam being a 340 ,ug cm-’ thick layer of TIED After collimation the neutrons were scattered through a mean angle of z 120” from a 4He gas scmtlllator and the asymmetry of the scattermg m the reaction plane, which allows a measure of the polanzatlon, was recorded by a pair of liquid scmtlllator neutron detectors. All three scmtlllators were mounted on a rotatable cradle to allow for the The data cancellation of spurious asymmetries collected were m the form of hehum recoil energy spectra correspondmg to neutrons detected m comcadence between the gas scmtlllator and a neutron detector Random comcldence spectra were also recorded to allow appropriate subtraction y-ray comcadences were reJected by pulse shape dlscrlmmatlon circuitry
j 2000I 4 v
!
t
F f
s
IOOO-
t
0
Fig. 3 shows a pulse height
from
the hehum
spectrum of the output gas scmtlllator, gated by neutrons
IO
20
30 CHANNEL
2.2. THE HELIUM RECOIL SPECTRUM Fig. 3. Pulse height the tad extrapolation
40
50
60
No
spectrum of gated hehum recods showmg and the peak region selected for analysis.
THE
POLARIZATION
reaching the polarimeter after penetratmg the shreldmg walls was also found to be negligible by recordmg a gated spectrum with solid Inserts placed m the colhmator aperture, normahsation m this case being made by momtormg the target neutron yield. The possibihty that the tail could be due to low energy neutrons arriving from the collimator aperture and being scattered through z 120” by the helium may also be ruled out, since the tall rises sharply m the low energy region which would by this explanation correspond to neutrons recoiling through z 120” with less than the neutron detection threshold energy of the hqmd scmtillators (Z 0 4 MeV). Immediately to the rear of the active volume of the gas scmtillator is a quartz vrewmg window with a stamless steel surround, their dimensions being comparable with a fast neutron mean free path. A possible source of the tail may arise from neutrons scattered both from the helium and thts sohd volume (m either order) before being detected by a hqurd scintillator Experimental support for this hypothesis was obtamed by temporarily re-orientatmg the gas scmtillator so that rt lay transverse to the neutron beam, thus removmg most of the sohd material to the rear of the scmtillatmg volume from direct u-radiation, whereupon the magnitude of the tall was approximately halved In addition, with the scmtrllator still m this onentation, on placmg a replica of the quartz window m contact with the gas scmtillator shell on its side remote from the target, the tail was increased by approximately 50%. As a further test of this hypothesis concernmg the production of the tail, Monte Carlo computations were undertaken with the following simphfymg assumptions. The mcident neutron beam was taken to be parallel and monoenergetic; the differential scattering cross section m the hehum was computed from one set of phase shift angles, i.e. energy loss m any previous scatter m the solid was neglected; the scattermg probabihty m the solid was taken to be a constant Independent of the material, the angle of scatter or the neutron energy; the detection efficiency of the hquid scmtillator was assumed independent of the neutron energy; absorption of the neutrons m the solid, hquid and gas was ignored. On this basis three hehum recoil energy spectra were computed corresponding to. a) neutrons scattered hqurd scintillator;
directly
by the hehum
OF
583
NEUTRONS
steel surround latmg volume) lator;
at the rear of helium gas scmtilbefore reaching a hqmd scmtil-
c) neutrons scattered first by the solid material and then by the helium before reaching a hquid scmtillator These spectra were then given a gaussran smear of 30% fwhm to srmulate the finite resolution of the gas scmtillator, and the results are plotted m fig. 4. Curves 2,3 correspond to the routes (b), (c), outlined above, while curve 1 is a combmation of routes (a), (b) and (c). The magnitude of the tall shown corresponds to a differential cross section of 150 mb sr-i for the scattermg m the solid, which seems a reasonable value at the large backward scattering angles mvolved Due to the approximations assumed m the computation, no great trust can be placed on fig 4 as an exact representation of the experimentally observed tails, however, it does show that double scattering mvolvmg the sohd material to the rear of the gas scmtillator is a plausible source of a malor portion of these tails Lesser contrrbutions to the tail will arise from neutrons scattered by other components of the polarimeter assembly (e. g the scmtrllator and photomultipher containers), and from double scattering m the hehum itself. At first sight it would seem that transverse mountmg of the gas scmtillator with careful neutron colhmatron could be advantageous m reducing the tail. This removes the quartz wmdow and its surround from direct irradiation and thus prevents events bemg recorded due to neutrons first scattered m these solids. However, neutrons scattered first by the hehum and
+++
mto a
b) neutrons scattered first by the hehum and then by the solid material (i.e. the quartz and its stainless
PULSE Fig. 4. Simulated
HEIGHT
pulse height spectrum,
see text.
584
H. DAVIE
AND R. B. GALLOWAY
then by the quartz window or its surround ~111 still be recorded, moreover the mean angle of scatter m the hehum for such events ~111 be greater than that for events recorded folloivmg a slmdar route but with the hehum scintlllator orlented as m the present experiment Consequently the overlap of the peak area by tall events may be increased by transverse mounting, although the magnitude of the tall m the region of low hehum recoil energy, remote from the peak, will be substantially reduced The use of fast timing may also modify the observed tall. If comcldence resolvmg times much shorter than the gas scmtlllator decay time are employed, then low energy hehum recoils may be rejected due to time Jitter effects. The danger m such a modlficatlon lies m it suggestmg an Incorrect extrapolation beneath the peak In the present polarlmeter both the comcldence resolvmg time and the gas scmtdlator decay &me were Z 1o-6 s.
Typically the portion of the peak region selected for the polanzatlon analysis 1s as indicated by the arrows m fig. 3. To correct for the presence of the proposed tall, the asymmetry recorded between such limits reqmred multlphcatlon by a factor which ranged from 1 10 to 1.14 (see table l), the larger factors bemg requn-ed for the poorer resolution spectra If a +50% uncertainty 1s placed on the tall contnbutlons, then the uncertamtles so created m the polarlzatlon results are less than their statlstlcal uncertamtles. The measured asymmetry corrected for the presence of the tall equals the product of the polanzation and the mean analysmg power of the polanmeter. The analysmg power correspondmg to each channel m the simulated hehum spectrum (a) of section 2.2 was computed for a smgle neutron energy, and was found to remam essentially constant throughout this spectrum. Hence the mean analysmg power for the whole peak was found to be an adequate estimate of the analysmg power for that part selected as Indicated m fig. 3 This mean analysmg power was calculated at the mean neutron energy of each of the results using a Monte Carlo technique The phase shift angles of Austin et a1.13) were employed, and a correction term was included m the computation to allow for the spurious asymmetry resultmg from the variation of the ‘H(d, n)3He reaction cross section with reaction angle
2.3. DATA ANALYSIS It 1s Important to obtain an estimate of the extent to which the experimentally observed tall (with near zero asymmetry) extends beneath Its correspondmg peak to allow a relevant correction to the data to be made Further it IS desirable to accept only a portion of the peak around its maxlmum for use m the polarlzatlon computation m order to mmlmlse this tall correction with its accompanymg uncertamty. The solid hne m fig. 3 shows a typical tall extrapolatlon used m the present study, this bemg obtained by the trial of several smoothly varymg extrapolations and by consldermg the asymmetry recorded m groups of channels throughout the peak region This asymmetry, when corrected for the presence of the true tall, ought to remam near constant throughout the peak region, hence its exammatlon helps indicate an approprlate tail extrapolation
3. Results The results obtained are as shown m table 1. In the first column are listed the deuteron energies with thenassociated spreads due to the stopping power of the TI-D target layer Next are listed the asymmetries measured between the limits as indicated m the preVIOUS section accompanied by their standard devlatlons, while column three contams the correction factors required to account for the presence of the zero asymmetry tails. The analysmg powers are contained
TABLE
2H(d,n)3He neutron polarlzatlon Deuteron energy (keV)
Asymmetry m reactlon plane
215 f 60 385f55 500+ 55 620 f 50 730f50 840f45
-0115f0010 -0133fO009 -0127fOOlO -0 119f0.009 -0.135fO 009 -0124fOOll
Tall correctlon factor
1 12 1 12 1 12 1 14 1 10 1 14
1
results for a laboratory
angle of 46”.
Analyzmg power
Neutron polarization
0 89 0 90 0 92 0.93 0 93 0.94
-0.14.5f0.013 -0166f0011 -0.155f0.012 -0.146f0.012 -0.160f0.011 -0 150f0.013
Asymmetry normal to reactlon plane
-0.002f0.010 - 0.006 f 0 009 -0.001 fO.O1O +o.o02fO 010 + 0.007 f 0.009 +0009+0011
THE
POLARIZATION
m the fourth column, and these are followed by the polarization values along with then associated statistical uncertamties The scattering asymmetries recorded m a plane normal to the reaction plane are listed in the final column of the table. These measurements, wluch ought to show zero asymmetry within then statistical uncertainties, were interspersed with each polarization measurement as a check on the operation of the polanmeter. All of the measurements recorded m table 1 were taken at a mean laboratory reaction angle of 46”, the geometrical spread m this angle due to the finite size of the hehum scatterer bemg + 1.75”. Multiple small angle scattering of the deuterons by the titamum of the target layer gives rise to a further spread. Thus followmg Wllhams’4) the r m s angular deviation of the deuterons after having traversed one half of the target thickness was determined to range from 1.5” m the case of the measurement at 875 keV to 4 2” m the case of the measurement at 275 keV The contammation of the direct beam of neutrons emitted by the target with neutrons once scattered m that part of the target assembly viewed by the hehum gas scmtillator was considered, however Monte Carlo computations showed that the error that this would mtroduce m the polarization measurements was considerably less than their statistical uncertamties. 4. Discussion A comparison of the present and past measurements as displayed m fig. 1 shows marked disagreements between the present work and the results of Pasma’), Boersma et a1.6) and Rodmg and Scholermanng). The magnitudes of Pasma’s pioneermg measurements are considerably less than those recorded by all other workers hsted m fig. 1. This IS possibly due to the lack of y-ray reJection cncmtry and the fact that hehum recoil spectra were not recorded. Hence his measurements may have included y-ray events and neutrons scattered m the material surrounding his gas scmullator (the tail events as discussed m section 2.2). The polarizations recorded by Boersma et a16) are also lower m magnitude than the present measurements. They used a y-ray rejection procedure, but did not record their hehum recoil spectra, consequently an unwittmg mclusion of tall events m their measurements could account for their lower magmtudes. The results of Rodmg and Scholermann displayed m fig. 1 were taken at a laboratory reaction angle of 4@“, although their measurements at an angle of 60” do not differ significantly and they predict a maximum
OF NEUTRONS
585
m the polarization magnitude between these angles. Consequently the disagreement between then results and the present ones is significant. The values of analysmg power quoted suggest that they employed scattermg of the neutrons by the hehum m a forward direction, although the angle is not quoted As recorded m a previous paperIl), the data collection efficiency is good at such angles, however, the magnitude of the analysmg power is uncertain to withm M 20%. This may partially account for what seem excessively high polarization magmtudes as compared to the present work, Rodmg and Scholermann did not record hehum recoil spectra during their measurements, however the mclusion of tail events ought to have been reduced by then use of a forward scattermg angle, by the transverse mountmg of their hehum gas scmtillator, and possibly by then use of a time of flight technique to reject y-rays Also, their time of flight spectrum, fig. 3 of ref 9, exhibits a substantial slopmg background under the peak and correction for this may be a further source of error The results of Behof et a1.8), although taken with a thick target and at deuteron energies below 400 keV incident are generally of a similar magnitude to the present results, and show a similar lack of energy dependence Then paper infers that the hehum recoil spectra were recorded, but no spectra are published, nor are details of any corrections required for tail events. The single measurements recorded m fig. 1 due to Levmtov et a1.4) and to Mulder’), both resulting from unique techniques, are m reasonable agreement with the present results. The single result of Smith and Thornton”) is then lowest energy result m a recent sequence of measurements stretching to a deuteron energy of 5 MeV Although little meanmgful comparison may be made with their one result below 1 MeV and the present measurements, a study of their technique shows that tail events may have been included m their measurements, so reducing the magnitude of the polarizations they recorded Instead of gated hehum recoil energy spectra, Smith and Thornton recorded gated time of flight spectra, then flight path being between the target and their liquid hehum scmtillator They indicate that they also performed a trial time of flight spectrum between the liquid hehum scmtillator and a side neutron detector which showed a ‘time correlated background’. This they believe to be the background reported by the Duke group12), however they indicate that its effect on then measurements was negligible Due to then varying routes and energies, the neutrons associated with tail events as described m
586
H.
DAVIE
AND
R.
section 2.2 ~111not exhibit a sharp transrt time between the hehum and a srde detector. Rather they wrll tend to broaden the tumng peak associated with the desrred direct neutrons. Since this trmmg peak ~111 alreadv be broad due to the hehum scmtrllator and side detector dlmensrons being a srgmficant fraction of the timing path, rt 1s difficult to see how such a procedure could sort out tall events. The authors thank Dr D. G. Vass, Mr H. J. Napier, Mr G. Turnbull and Mr D. Green for their valuable assrstance. References 1) L Wolfenstem, Phys. Rev. 75 (1949) 342 2) P. Huber and E. Baumgartner, Helv. Phys Acta 26 (1953) 420; R. Rxamo, Helv. Phys. Acta 26 (1953) 423
B.
GALLOWAY
3) R B Galloway, Nucl. Jnstr. and Meth. 92 (1971) 537; Errata 95 (1971) 393. 4, I I Levmtov, A. V. Mdler, E. Z. Tarumov and V. N. Shamshev, Nucl Phys 3 (1957) 237. 5) P. J Pasma, Nucl Phys. 6 (1958) 141. 6) H J. Boersma, C. C. Jonker, J. G. NlJenhuls and P J. Van Hall, Nucl Phys 46 (1963) 660. 7, J. P F. Mulder, Phys. Letters 23 (1966) 589. 8, A. F. Behof, T. H May and W I. McGarry, Nucl. Phys. A 108 (1968) 250 g, P. Rodmg and H. Scholermann, Nucl. Phys. Al25 (1969) 585. 10) J. R. Smith and S T. Thornton, Can J. Phys. 50 (1972) 783. 11) H. Davle and R. B Galloway, Nucl Instr and Meth 92 (1971) 547. 12) J. R. Sawers, Jr, G. L Morgan, L. A. Schaller and R. L Walter, Phys Rev 168 (1968) 1102, G Spalek, R A. Hardekopf, J. Taylor, Jr, Th Stammbach and R L. Walter, Nucl. Phys A191 (1972) 449. 13) S M Austm, H. H Barschall, R E Shamu, Phys Rev 126 (1962) 1532 14) E. S Wdhams, Rev Mod. Phys 17 (1945) 217.