Polarization in C12(pp) scattering

2.B:2.L [

Nuclear Physscs 15 (1960) 646-- 652; (~) North-Holland Pub~,sh,ng Co., Amsterdam Not to be reproduced by photopnnt or n~crofilm without written pernasston from the pubhsher

POLARIZATION IN Ct2(pp) SCATTERING J

E EVANS and M A. G R A C E *

Atom)c Energy Research Estabhshment, Harwell, Berks. Received 30 December 1959 Measurements have been made of polaxlzatlon m Cl*(pp) scattering at 60 ° (lab) m the energy range 2.3--4 3 MeV. The results axe slgndmantly ddferent from those derived from a phase-slnft analysm and thin can be explained b y the extreme sensxtlvlty of the polaxxzatmn to small changes m the phase shifts The experimental values then mchcate a correction for the d-wave s p h t t m g

Abstract:

1. Introduction A solid target such as carbon has several advantages over helium when used a s an analyser to measure polarization of protons, or as an agency to produce a polarized beam of protons. These advantages are: 1. The greater density of scattering nuclei makes for better geometry; 2. No containing windows are needed; 3. Less energy is lost in the recoil. In the important energy range 2 5 MeV to 5 MeV very little experimental work has been done on the polarization, although its value has been predicted 1) from phase shift data deduced from scattering experiments 2). If carbon is to be used in this energy range, these phase shift predictions should be tested directly. Furthermore, because polarization can be very sensitive 1) to changes in some of the phase shifts of the proton partial waves, its measurement can reveal small inaccuracies in the phase shift analysis of the elastic scattering. We have determined this polarization in proton elastic scattering from carbon using the left-right asymmetry in helium as a polarization analyser. Because of the discrepancy between the experimental results and the theoretical predictions a measurement was also made using carbon both as polarizer and analyser to provide a check on the helium analysis.

2. Experimental Equipment The arrangement for the experiment is shown in fig. 1. The proton beam from the A.E.R.E. Van de Graaff generator entered the first scattering chamber through a x , , gold stop and was scattered at 60 ° (lab.) by a natural carbon target (2mg/cm~). The scattered beam was then scattered again in helium at t Clarendon Laboratory, Oxford. 646

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Fig. 2 The pulse he]ght spectrum of protons after double scattering from carbon and hehum. The ]mtlal energy was 3 5 MeV

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J. E. EVANS AND M A GRACE

90 ° (lab.) and the protons were detected by scintillation counters fitted with thin (0.0025)" CsI(T1) crystals covered with thin aluminium foil. It is convenient if the carbon foil is self-supporting and uniformly thick. The target used was made by spraying Alcohol-Dag t on a glass slide and floating off in water. The result was a very flexible foil which chemical analysis showed to be 94 % carbon. About 1% consisted of heavier elements, principally iron and the remaining 5 % was probably organic bonding agent which was likely to be reduced quickly to carbon under the heating due to the beam. No evidence for hydrogen in the foil could be seen in the spectrum of scattered protons. The tantalum support for the carbon foil also held a gold foil which could be moved up to replace the carbon when desired. The quartz plate shown could be moved in to intercept the beam and it was marked to show any deviation from the required angle of 60 ° between the beam and the polarimeter axis. The helium used in the polarimeter was 99.8 % pure. It was contained by a Melanex *t window 0.0003" thick which was sealed with Araldlte tit over the 1,, diameter entrance aperture. The pulse height spectrum in the detectors (fig. 2) showed a background without structure which decreased quickly with increasing pulse height and which was attributed to r-radiation. This arose largely from the carbon foil. Negligible background originated in the gold beam stopper. This background was minimized by the lead shielding and b y reducing the thickness of the CsI(T1) crystals to little more than the range of the detected protons. This reduction was achieved on a milling machine, the crystal being held by vacuum. Fine holes were drilled through the top of a brass drum over an area slightly less than the crystal size and the drum was clamped to the milling machine table. A first cut was taken over the surface, the crystal laid on and the drum evacuated. The machining was then carried out until the crystal was 0.0025" thick. Light guides were found to be necessary with the thin crystals in order to distribute the light over the photo-cathode. The pulses were amplified and recorded in two 100 channel pulse height analysers, each monitored by a scaler in parallel. The counting rate for the protons was of the order of 100 counts per hour with a beam current of 2/~A. The hehum polarlmeter could be rotated about the axis of the scattered beam In order to reveal any intrinsic asymmetry in the apparatus. The whole was carefully aligned to ensure that the intersection of the beam and the foil lay on the polarlmeter axis and remained there during a rotation of 180 °. * Alcohol-Dag t? Melanex -m the USA is *t* A r a l d l t e - -

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POLARIZATION IN c l l ( p p )

SCATTERING

649

Fig. 3 shows the carbon polarimeter which was used to check the results obtained with helium. The geometry of the polarimeters was the same except for the change in scattering angle and the fact that the scattering volume was much smaller for the carbon foil.

Fig 3. The carbon polammeter

Two carbon analysing foils were used. One was made from Alcohol-Dag and the other from pure natural carbon. This latter was built up from 40 layers of vacuum-deposited foil by Dr. G. Dearnaley and Mr. M. Nobes. No difference was found between the results obtained with the two foils.

3. Analysis of Results In principle the ratio (R/L) of the counting rates in the right-hand and lefthand detectors gives the product of the polarizations at the first and second scatterings, through the formula R

1 + P1 P,

L

1 - P1P~"

The experimental ratios have to be corrected for asymmetries in the apparatus, and these were investigated using the gold foil at the first scattering. At the energies used, the scattering from gold is not expected to be polarized because of the large Rutherford contribution. This has been confirmed experimentally at

650

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E. EVANS AND M A GRACE

10 MeV s) and, in fact, the ratio R/L with gold was never found to be significantly different from unity. To compensate for any small asymmetry, however, the ratio R[L for the experiment was taken as the mean of two equal runs, the polarimeter being rotated through 180 ° between runs. The ratio was not found to have changed significantly after the rotation. Because of the finite geometry of the experiment a spread in differential scattering cross section a(E, 0) and polarization P(E, 0) was introduced. It is easy to show that if the first scattering takes place with perfect geometry, then R

1 + Pa Pz

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1 -- P1 Pz"

where P~ is a mean polarization defined by

P2 -

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Here ¢(0) is a geometrical factor associated with the apparatus. Numerical integration showed that PIle was not significantly different from PHe at the mean scattering angle of 90 °. As shown in fig. 1 the geometry of the first scattering is very much better than that of the second, and the quantities Px and P2 were accordingly always taken to correspond to the mean scattering angle.

The appropriate values for the helium polarization have been calculated by Dr. M. J. Scott from the results of a phase-shift analysis *) and agreement obtained experimentally at three points s). Our results for Cla(pp) polarization at 60 ° (lab.) are shown in fig. 4. The sign of the polarizatmn is defined b y the vector n = Kout×Km/lKoutXKln]. The horizontal bars on the experimental points indicate the energy spread arising from target thickness while the vertical bars represent the errors from counting statistics. It is seen that the experimental points lie close to a smooth curve which is markedly higher than the predicted 1) polarization at all energies greater than about 2 5 MeV. The polarization at 60 ° (c.m.) has been measured up to 2.5 MeV by Sorokin and Taranov e) and all three results are in reasonable agreement in the region of overlap. The greatest discrepancy occurs at about 4 MeV where our result is more than twice the predicted value. Accordingly the double scattering from carbon was carried out an this neighbourhood. The result was a product of two values for the carbon polarization at 60 ° (lab.), at different energies because of the energy loss in the scatterings. This product was measured as 0.13+0.022. The results using the helium analyser give 0.107+0.015 but use of the polarization predicted by the phase shift analysis leads to a value of only 0.02. It is clear that the carbon and helium analyser results are self consistent but both axe in disagreement with the phase shift analysis.

POLARIZATION IN clS(pp) SCATTERING

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5. Cll(pp) p o l a n z a b o n a t 4 MeV and 60 ° (c m ) as calculated from t h e p h a s e shifts of Reich et al s) wzth t h e d - w a v e s p h t t m g vaned

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A N D Mo" A

GRACE

A possible explanation for the discrepancy lies in the extreme sensitivity of the polarization to small changes in the two d wave phase shifts, ~2+ and ~2-. The presence of such effects in the off-resonance region was noted by Phillips and Miller 1). Values for these two phase shifts were derived by Reich e¢ al. 2) from measurement of the differential scattering cross section at 54044 ' and 125°16 ', these angles being zeros of the second-order Legendre polynomial, chosen to simplify the analysis. We find that if the derived values are separated b y a further 6°, the polarization at 4 MeV agrees with our results (see fig. 5). The change in cross section produced by the new phase shifts is + 3 ~ at 54 ° 44' and + 5 ~o at 125°16 ', and this is of the order of accuracy quoted for the cross section measurement ~). The polarization at 4 MeV is much less sensitive to changes in the s-wave and p-wave phase shifts. We are indebted to Dr. E. B. Paul for suggesting this work and to other members of the Van de Graaff group for helpful discussions. The A.E.R.E. workshops carried out the milling of the CsI(T1) crystals. References 1) G. C Phflhps and P. D. Miller, Proceedings of the International Conference on Nuclear Physics, Paris July 1958 (Dunod, 1959) p. 522 2) C. W. Reich, G C. Phflhps and J. L. Russell, J r , Phys. Rev. 104 (1956) 143 3) L Rosen and J E. Brolley, Jr., Paper 668, Second Umted Natlonal Internatlonal Conference on the Peaceful Uses of Atomic Energy, Geneva 1958 4) C L Cntchfleld and D C Dodder, Phys. Rev 76 (1949) 602 5) M J Scott, Phys. Rev. U 0 (1958) 1398 6) P. V. Sorokm and A. I Taranov, Doklady U l (1956) 82