Polarisation in neutron-deuteron elastic scattering below 1.0 MeV

Nuclear Physics 33 (1962) 477--481 ; ~) North-Holland Publishing Co., Amsterdam Not

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POLARISATION IN NEUTRON-DEUTERON ELASTIC SCATTERING B E L O W 1.0 MeV A. T. G. F E R G U S O N and R. E. WHITE

Nuclear Physics Division, A.E.R.E., Harwell, Didcot, Berks. Received 11 October 1961 A b s ~ e t : The polarisation o f neutrons scattered through 80°4-15°(c.m.) by deuterons has been measured from 0.5 to 1.0 MeV. The observed polarisations are positive and between 5 % and 15 ~o and are in good agreement with predictions for this energy region.

1. Introduction

The three-body problem has received considerable attention in the past through studies of n-d and p-d elastic scattering. Predictions have now appeared, based on the analysis of these data, for the polarisation to be expected in n-d scattering up to a few MeV 1). (A sign error in these calculations has been reported recently 2) so the polarisations in figs. 5 and 6 of ref. 1) should be positive). These show a maximum of about 12 % in the polarisation for c.m. scattering angles of 90 ° to 120° at 1.0 MeV. At other energies the polarisation should drop rapidly (fig. 1). i

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neutron energy (MeV)

Fig. 1. Predicted polarisations for neutrons scattered by deuterons through 90°(c.m.).

A number of measurements near 3 MeV 3) confirmed the small polarisation predicted there. Measurements at 1.0 MeV by Darden et al. 4) using the Levintov technique yielded values of 9.2_5 % and 7+_5 % for e.m. angles of 70 ° and 110° in reasonable 477

478

A. T. G. FERGUSON AND R. E. WHITE

agreement with the predictions. This paper reports further measurements below 1.0 MeV made using a different method from that of Darden et al. 2. Experimental The apparatus, shown schematically in fig. 2, was designed originally for polarisation

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S Fig. 2. Schematic diagram of the apparatus. measurements with high mass scatterers and has been describea in its basic form previously 5). Neutrons are produced by bombarding a 90 keV thick LiaN target with protons and those neutrons emitted at 50 ° lab. are collimated onto a scatterer 2 cm x 3 cm x 7.5 cm. Neutrons scattered at 55 ° are detected in stilbene crystals using pulse shape discrimination to reduce gamma-ray background. The normal R/L ratio, then yields the polarisation induced in the scattering knowing the incident neutron polarisation 5). The main uncertainty in such measurements arises from small asymmetries inherent in the system which, while readily detected, are difficult to eliminate. We overcome this by using as collimator the pole faces of an electromagnet designed to produce a field transverse to the incident neutron direction. The ratio R/L is measured first with zero

479

N E U T R O N - D E U T E R O N ELASTIC S C A T T E R I N G

field and then with a field applied such that the incident neutron s1~'-s precess through 180 ° in the field thereby inverting the incident beam polarisation and interchanging the roles o f the two detectors 6). By combining these measurements to yield A

(R/L)field off

RoffLon

(R/L)fie,d on

~

(R) 2 L off

we eliminate geometrical asymmetries and any dependence on detector efficiencies. Fringing fields were found to be troublesome due to shifting of the proton beam and to small changes in the behaviour o f the photomultipliers with the field on. By housing the photomultipliers in iron shields and by shielding the proton beam with iron for a distance o f about 91.4 cm from the target, and to within 1.3 cm of it (fig. 3), these effects were eliminated.

Fig. 3. Beam collimation near the target. The inner cylinder is made from 0.64 cm thick iron with a tantalum end plate. The outer brass can carries the target and rotates. The magnet has been calibrated using a MoO4 scatterer at energies where the oxygen polarisation is large so that A will be large. Fig. 4 shows the variation o f A 1.8

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CARBON

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Fig. 4. Variation of A with magnetic field for 0.4 MeV neutrons scattered by MoOs.

with magnet current for 400 keV neutrons. The dotted curve is the expected variation normalised to the measured points at a current corresponding to a field o f 12 kG.

480

A. T. G . F E R G U S O N A N D R . E. W H I T E

Here the neutrons spins precess through 180 ° and A is maximum. Carbon is used to check for residual asymmetries since scattering from carbon produces no polarisation below about 800 keV so that A should be unity for all fields. A typical measurement for carbon is shown. The chief difficulty in the n-d experiment was finding a suitable scatterer. We used LiD and calculated the deuterium scattering by making separate measurements on a geometrically similar Li scatterer. These, combined with background measurements, yielded both n-d and n-Li polarisations. Running times of several hours were needed at each energy and to avoid long term drift effects these were broken up into short cycles. 3. Results

The polarisation of the LiV(p, n)Be 7 neutrons emitted at 50 ° has been studied by several workers and the values we use have already been published s). The observed n-d polarisations are shown in fig. 5 for a c.m. scattering angle of 80°-I- 15 °. We thus 30

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0.8 1.0 neutron e n e r g y ( M e V )

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Fig. 5. Observed neutron polarisation (Basle convention); full lines this experiment, dotted lines Darden et al. 3, 7); curve, theoretical for 80° c.m. 9). confirm the positive value found previously at 1.0 MeV. Our results do not extend below 0.5 MeV because when measurements were made near 0.4 MeV, large positive polarisations of the order of 40 ~o rather sensitive to neutron energy were found. This is unexpected and in contradiction to a recent measurement at 410 keV 7) which shows a very small polarisation there. We attribute this to oxygen contamination in our sample of LiD since both the positive oxygen polarisation and scattering cross-section are very large near 0.4 MeV s). Assuming that our observed LiD asymmetry at 0.45 MeV results from oxygen and lithium polarisations only, with zero n-d polarisation, we estimate that the sample must contain about 4 ~ by weight of oxygen. A chemical analysis of the sample showed that it contained 4.2 ~o by weight of oxygen. At about 0.5 MeV, however, the oxygen polarisation is negative 8) and considerably smaller and the cross-section is much reduced. In this energy region the effect of this

NEUTRON=DEUTERON ELASTIC SCATI'ERING

481

oxygen c o n t a m i n a t i o n w o u l d be to m a k e the n-d p o l a r i s a t i o n s shown in fig. 5 low, b u t only by a b o u t 2 %. W e have increased the errors o n o u r results s o m e w h a t to allow for this. A t 1.0 MeV, where the oxygen p o l a r i s a t i o n is again large, o u r energy interval included regions o f b o t h positive a n d negative p o l a r i s a t i o n s so the overall effect s h o u l d be small. It now seems certain t h a t the p o l a r i s a t i o n in n-d scattering is positive b e l o w 1.0 M e V a n d in g o o d a g r e e m e n t at 80 ° c.m. with the p r e d i c t i o n s o f ref. i). T h e i m p l i c a t i o n s o f this are discussed elsewhere 2).

References 1) L. M. Delves and D. Brown, Nuclear Physics 11 (1959) 432 2) L. M. Delves, Nuclear Physics 33 (1962) 482 3) M. BruUman, H. J. Gerber, D. Meier and P. Scherrer, Nuclear forces and the few-nucleon problem, Vol. II (Pergamon Press, London, 1960) 4) S. E. Darden, C. A. Kelsey and T. R. Donoghue, Nuclear Physics 16 (1960) 351 5) D. Brown, A. T. G. Ferguson and R. E. White, in Proc. Basel Conf. on Polarisation Phenomena, Helv. Phys. Acta Suppl. No. 6 (1961); Nuclear Physics 2,5 (1961) 604 6) P. Hillman, G. H. Stafford and C. Whitehead, Nuovo Cim. 4 (1956) 67 7) S. E. Darden, private communication 8) S. M. Austin, S. E. Darden, A. Okazaki and Z. Wilhemi, Nuclear Physics 22 (1961) 451 9) L. M. Delves, private communication