Polarization in the elastic scattering of 9.5 MeV and 21.6 MeV protons by deuterons

2.B:2.C I

Nuclear Physics A122 (1968) 689---696; (~) North-Holland Publishiny Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

P O L A R I Z A T I O N IN THE ELASTIC SCATTERING OF 9.5 MeV A N D 21.6 MeV P R O T O N S BY D E U T E R O N S J. H. P. C. M E G A W and A. R. J O H N S T O N

Department of Pure and Applied Physics, Queen's University, Belfast, N.Ireland W. R. G I B S O N

Physics Department, Queen Mary College, London and F. G. K I N G S T O N

Physics Department, Westfield Colleye, London Received 13 September 1968

Abstract:

Polarization in the elastic scattering of p r o t o n s by deuterons has been measured at 21.6 MeV in the angular range 93°-160 ° c.m. and at 9.5 MeV in the range 20°-160 ° c.m. The polarized p r o t o n beam o f the R H E L linear accelerator was used in conjunction with deuterated polyethylene targets and a double-focussing magnetic spectrometer. The 21.6 MeV results extend earlier measurements at 22 MeV to larger angles but indicate a higher angle o f cross-over f r o m positive to negative polarization and a smaller m a x i m u m value o f negative polarization than before. The backward positive m a x i m u m is in good agreement with the n-d data. At 9.5 MeV, the present results are in good agreement with a previous measurement at 10.0 MeV and extend the angular range to 20 ° c.m. Although a dip to negative polarization in this region has been reported at 11.0 MeV, no evidence for this is found at 9.5 MeV.

E

N U C L E A R R E A C T I O N S ~H(p, p)2H, E = 9.5, 21.6 MeV; measured P(O). Enriched target.

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1. Introduction

In recent years, there has been a considerable number of measarements of the differential cross section and polarization for the elastic scattering of protons from deuterons with proton energies up to 50 MeV. The experimental results and their theoretical interpretation have recently been reviewed by Noyes 1). For proton energies greater than 20 MeV, the main features of the angular distributions of the polarization have been well established. The polarization is positive at small angles, changes to negative values at 70 ° to 80 ° c.m., has its largest negative value about 110 ° and changes rapidly back to positive values in the region 120 ° to 130 °. At backward angles, the 30 MeV [ref. 2)], 35 MeV [ref. 3)1, 40 MeV [ref. 4)1 and 50 MeV [ref. 5)1 data have maximum values of about + 0.20, which is approximately the value found in neutrondeuteron scattering 6) at 22.7 MeV. The polarization measurements of Conzett et aL [ref. 7)] of proton-deuteron scattering at 22 MeV do not extend beyond 130°; therefore comparison of the polarization positive peak at backward angles in neutron and pro689

690

J. tt. P. C. MEGAWet al.

ton scattering at the same energy was not possible. This paper reports the extension of the 22 MeV data to larger angles. In the proton energy range 4-20 MeV, measurements have been reported by Clegg and Haeberli 8) and McKee et al. 9,1 o). The latter reported negative polarizations at small forward angles. Clegg and Haeberli 8) have measured the polarization at 4, 6, 8, 10 and 12 MeV in the angular range 30°-160 ° but find that the polarization is always positive having a broad maximum at backward angles and levelling to between 0.02 and 0.04 at forward angles. As an additional contribution to the information in this energy range, the polarization in proton-deuteron elastic scattering at 9.5 MeV has been measured. 2. E x p e r i m e n t a l a p p a r a t u s and m e t h o d

The experiment was carried out using the polarized beam of the proton linear accelerator at the Rutherford High Energy Laboratory. The beam current was 16 pA and polarization 0.50. The protons were transported from the accelerator to the scattering chamber by a conventional quadrupole and bending magnet system. To obtain a 22 MeV proton energy, the 30 MeV beam of the accelerator was degraded and collimated at the entrance to the scattering chamber, since attempts first to degrade the 30 MeV beam to 22 MeV and then m o m e n t u m analyse it resulted in too low an intensity for reasonable counting rates. An aluminium degrader immediately preceded a tantalum collimator with a 0.3 cm diam. circular aperture situated 35 cm upstream from the target which was at the centre of the scattering chamber. A second collimator of mild steel with a 0.5 cm diam. circular aperture was situated 7 cm upstream from the target. The 30 MeV beam energy before entering the degrader was measured 11) and the energy of the beam after the degrader calculated to be 22.2 MeV. A 10 MeV beam was obtained by using only the first accelerating tank ot the linear accelerator. Collimation at the scattering chamber was not employed but, instead the beam was tocussed on the target, where the beam spot size at the centre of the scattering chamber was defined by 0.09 cm wide and 0.75 cm high tantalum slits, before the last quadrupole doublet of the beam line. The beam energy was determined using a thin polyethylene target by observing the angle at which protons elastically scattered from hydrogen had the same energy as protons inelastically scattered from carbon leaving the carbon in its first excited state. The energy was measured to an accuracy of _ 50 keV to be 9.96 MeV. At 22 MeV, the beam polarization was measured before degrading using the normal carbon target sampling polarimeter 12) for the 30 MeV beam. This polarimeter depended on the scattering of 15.7 MeV protons from carbon and could therefore not be used for the 10 MeV experiment. At 10 MeV, a beryllium target was also mounted on the rotating wheel of the sampling polarimeter and enabled, by a simple switching process, the analysing power of the beryllium target for an incident 10 MeV beam to be calibrated against the carbon target for the 30 MeV beam during stable

2H(p, p)~H

691

periods of the polarized ion source of the accelerator. The ratio of the analysing power of the carbon target and beryllium was 2.085+0.031. The absolute analysing power [ref. 13)] of the carbon polarimeter was - 0.626__ 0.007. The targets were made by heating and pressing deuterated polyethylene into thin discs of thickness chosen to give a compromise between energy resolution and counting rates in the experiment. At 22 MeV, a 50 mg • c m - 2 target normal to the incident beam was used giving a mean scattering energy of 21.6 MeV. The spread in energy was due mainly to the degrader and was approximately +0.6 MeV. At 10 MeV, a 15 mg • cm -2 target was used with the target rotated through 40 ° from the normal to the incident beam. The total energy loss in the target was 920 keV and gave a mean scattering energy of 9.5 MeV. The scattering chamber was 60 cm in diam., and particles scattered in the horizontal plane from the target were m o m e n t u m analysed in a magnetic spectrometer. The acceptance angles of the spectrometer were __0.5 ° in the horizontal plane and + 3.0 ° in the vertical plane. Particles were detected in a 13.5 x 2.5 x 0.3 cm 3 block of NE102A scintillator mounted at the focal plane and in the vacuum system of the spectrometer, where the length of the scintillator block over which particles were detected could be varied by two remotely controlled adjustable jaws. At angles greater than 95 ° c.m., elastic scattering events were identified in both experiments by detecting the recoiling deuterons; their high magnetic rigidity facilitated separation from other particles in the spectrometer. At angles less than 95 ° c.m., the elastically scattered protons were detected. The effective detector length used was a m a x i m u m at all angles except the most forward angles at 10 MeV where it was reduced to eliminate elastically scattered protons from carbon. No measurements were attempted below 93 ° c.m. at 22 MeV, but at I0 MeV, measurements were performed down to 20 ° c.m., i.e. the limit determined by the separation of protons from carbon and deuterium. Background measurements were performed at all angles with equivalent C H 2 targets; detection of scattering from the hydrogen in this case was kinematically impossible. Polarization measurements were performed in the usual manner by scattering for short periods with proton spin alternately "up" and "down". Since only one spectrometer was available, it was necessary to monitor the position and intensity of the beam accurately during successive spin up and spin down runs. The horizontal position of the beam was monitored by a split ionization chamber mounted at the back of the scattering chamber and sensitive to beam spot shifts of 10 #m. Such a shift occurring between a run with spin up and a run with spin down would cause in the worst case a 0.0017 ~ change of solid angle, which corresponds to an error in the final value of polarization P of 0.0017 and a change of scattering angle of 0.001 °. The fastest rate of change of counting rate with lab scattering angle was for deuteron detection where it was 10 ~ per degree; a change of 0.001 ° corresponded to a 0.01 change of counting rate and an error in P of 0.01. For forward scattered protons, tbe rate of change of counting rate with lab angle was less; for the angular range of our

692

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m e a s u r e m e n t s in p a r t i c u l a r for the f o r w a r d angle m e a s u r e m e n t s at 10 MeV, it was 1.5 % p e r degree, c o r r e s p o n d s to a n e r r o r in P o f 0.0015. The a b o v e assumes a systematic b e a m s p o t shift of 10 # m between runs with spin up a n d spin down, b u t n o evidence for such a shift was shown b y the split i o n i z a t i o n c h a m b e r . The b e a m intensity was satisfactorily m o n i t o r e d b y an o r d i n a r y i o n i z a t i o n c h a m b e r b e h i n d the split ionization chamber. 3. Results

T h e present values at 21.6 M e V are given in table 1 a n d are c o m p a r e d in fig. 1 with the results o f C o n z e t t e t al. for p r o t o n - d e u t e r o n scattering a t 22 M e V a n d with t h o s e TABLE 1 Proton polarizations P in p-d elastic scattering at 21.6 MeV c.m. angle (deg)

P

93

--0.036

110

--0.102

120 130 140 150 160

--0.051 0.199 0.193 0.098 0.035

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0"20

20

i

40

10 6

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I 100

1

I

I

120

140

! 160

C.M. SCATTERINGANGLE DEGREES

Fig. 1. Comparison of measurements of polarization in proton-deuteron and neutron-deuteron elastic scattering. ~ - present results, p-d 21.6 MeV; ~d. - Conzett et al., p-d 22 MeV; ~x - Malanify et. al., n-d 22.7 MeV.

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Fig. 2. Variation with energy o f c.m. cross-over angle from positive to negative polarization in p-el scattering. ~. 19.1 MeV, [ref. g)]; ~-~- 21.6 MeV, present results; ~ - 22 M e V [ref. 7)]; • . 29 MeV, [ref. 2)]; V - 35 MeV, [ref. a)]; [] . 40 MeV, [ref. 4)]; • . 50 M e V [ref. x4)].

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Fig. 3. Comparison of measurements of polarization in proton detlteron elastic scattering - present results at 9.5 MeV, ±~ - Clegg and Haeberli at 10.0 MeV.

694

s.u.P.c. MEGAWet al.

of Malanify et al. 6) for neutron-deuteron scattering at 22.7 MeV. Because of the large beam energy spread due to the degrader, background corrections for the present measurements were large, e.g. 2 3 ~ at 93 °, 14~o between 100 ° and 130 ° and 6 ~o at 140 ° but only 1 ~ at the largest scattering angles. Errors shown are statistical but do not include the error due to the calibration of the polarimeter. The m a x i m u m positive polarization at backward angles is less than that obtained by Conzett et al. but is in better agreement with the polarization in neutron-deuteron elastic scattering measured by Malanify et al. The m a x i m u m negative polarization at 110 ° is less than that obtained by Conzett et al., or in neutron-deuteron scattering. Further, the present results would suggest that the cross-over point from positive to negative polarization occurs in the region of 84 ° rather than 72 ° as suggested by the results of Conzett et al. F r o m the results at 19.1 MeV [ref. 9)], 22 MeV [ref. 6)], 30 MeV [ref. 2)], 35 MeV [ref. a)], 40 MeV [ref. 4)] and 50 MeV [ref. 14)], this cross-over point appears to be a monotonically decreasing function of energy apart from the result of Conzett et al. at 22 MeV. The new value given in this paper is in better agreement with the other data as shown in fig. 2. When the present data are plotted on a contour diagram of polarization as a function of energy and scattering angle, they yield smoother contours than the previous results of Conzett et al. The experimental results at 9.5 MeV are given in table 2 and are compared in fig. 3 with the results of Clegg and Haeberli at 10.0 MeV. The background correction was 1 ~ or less at all angles except the most forward where it was 4 ~ . The errors shown are statistical but do not include the error due to the calibration of the polarimeter. Within the limits of the experimental errors, there is a good agreement with the results of Clegg and Haeberli especially at backward angles. At forward angles, the present TABLE 2

Proton polarization P in p-d elastic scattering at 9.5 MeV c.m. angle (deg)

P

AP

20 25 30 40 45 50 60 80 90 100 110 120 130 140 150 160

0.032 0.044 0.033 0.030 0.044 0.038 0.048 0.040 0.055 0.093 0.118 0.154 0.126 0.084 0.049 0.031

-4-0.010 4-0.007 4-0.006 4-0.007 ±0.007 4-0.007 4-0.007 4-0.009 4-0.013 4-0.011 4-0.011 4-0.010 4-0.010 4-0.011 4-0.007 4-0.006

2H(p, p)~H

695

results extend the data to 20 ° c.m. The polarization still appears to be + 0.03, and there is no indication of the rapid swing to negative values reported by McKee et al. at 11.0 MeV and 11.6 MeV. Since the effective thickness of the target in the present experiment was large compared with that of Clegg and Haeberli and that of McKee et al. there is the possibility that, if the dip to negative polarization is extremely energy sensitive, it might be obscured by the energy spread in the scattering. McKee et al. have justified the negative polarization using the simple Rodberg theory 15) relating the zeros of polarization and the maxima and minima in the cross-section data. Although it is doubtful 16) that this theory is applicable at small forward angles, it has been used to predict polarizations for the present experiment by differentiating the theoretical cross-section curves of Van Oers and Brockman 17) and Arvieux 18) and averaging over the beam energy spread. Normalization to the experimental polarization data was made at 90 ° c.m., and at 20 ° c.m. a small negative polarization of - 0.005 was predicted from the data of Van Oers and a dip to + 0.005 was predicted from the Arvieux data. These values are not confirmed by our experimental data. It must be emphasized that the validity of the theory is extremely doubtful in this case, and that the normalisation at 90 ° predicted a negative polarization at backward angles which completely disagrees with the experimental data. The present experiment shows that the theory cannot necessarily be used to justify negative polarization at forward angles. Due to the fixed energy of linear accelerators, the experiment could not be carried out at 11.0 MeV, and it can only be stated that, in agreement with Clegg and Haeberli, there is no indication of even a trend towards negative polarizations at forward angles at 9.5 MeV. Purrington and Gammel 19) have calculated the polarization in neutron deuteron scattering at 9.0 MeV using the Born approximation with tensor interaction included. The predicted polarization is less than those measured in the present experiment especially at forward angles. We wish to acknowledge the facilities provided by the Rutherford High Energy Laboratory, the assistance of S. A. Harbison, G. T. A. Squier and N. M. Stewart during the experimental runs and the continuing interest of Professor W. D. Allen and Dr. C. J. Batty. Two of us (J.H.P.C.M. and F . G . K . ) wish to acknowledge the financial support of the Ministry of Education for Northern Ireland.

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696 7) 8) 9) 10) 11) 12) 13) 14)

15) 16) 17) 18) 19)

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