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Polarization of 3He particles elastically scattered by 12C nuclei
Volume
30B. number
‘7
PHYSICS
POLARIZATION
OF 3He SCATTERED
J. B.A. ENGLAND, Department
LETTERS
24 November
PARTICLES ELASTICALLY BY 12C NUCLEI
R. G. HARRIS> L. H. WATSON, of Physics.
1969
The University
D. H. WORLEDGE
of Bivmingham.
UK
and J. E. EVANS * A.E.R.E..
Harwell.
UK
Received
23 October
1969
The polarization in the elastic scattering of 31.6 MeV 3He particles from has been measured and found to be - 0.014 2 0.044. indicating a spin-orbit for 3He scattered by 12C of not greater than 5 MeV.
In an earlier report [l], preliminary results were given for an experiment in which 29 MeV 3He particles were double-scattered by carbon in an attempt to detect asymmetries at the second scatter caused by spin polarization in the first scattering. This could be the result of a spin-orbit term in the 3He-nucleus interaction and such a term, if large, would be significant in the optical model analysis of 3He particle scattering by nuclei [2]. The first experiments [3], using photographic emulsions as detectors of the second-scattered particles, suffered from unwanted tracks due to (3He,o) and (3He,n) reactions, but suggested an observable degree of polarization. A short investigation with semiconductor counter telescopes as detectors of the asymmetry in the second scatter confirmed this result, for a first and second scattering angle of 35’ (c.m.s.). Theoretical interpretations of these results have been offered [2,4] with the conclusion that the strength of the spin-orbit potential in the usual optical formulation [2] is 6 to 8 MeV, as for nucleons. Since this report considerable experimental effort has been devoted to further investigation of the 3He-12C double-scattering at energies from 29 MeV to 32 MeV, with semi-conductor counter telescopes and particle mass identification [5]. Despite better geometrical accuracy: typically an angular definition at both scatterings of f 0.75’ compared with f 1.4’ in the first measurements, these carbon double-scattering * Deceased. 476
12C at a c.m.s. angle of 31.1’ potential in the optical model
experiments failed both to confirm the original result and to yield any consistent value for the polarization. This was attributed to the rapid angular variation of the 3He-12C elastic scattering cross section [6] which makes the measurement of polarization by the double-scattering technique a much more difficult problem than in the case of proton scattering. After these experiments had been discontinued in favour of an alternative approach described below, a report by Hutson et al. [‘I] appeared in which the polarization of 36 - 42 MeV 3He-partitles scattered by carbon at 30° (c.m.s.) was found to be very small? with a second scattering asymmetry A = - 0.001 + 0.003. The spin-orbit potential derived from this measurement is not greater than 1.7 MeV and the polarization predicted at 29 to 32 MeV with this potential would have been unobservable by the methods of refs. 1 and 3. In an alternative a preach to the problem, the polarization in 3He- lf3C scattering has now been studied with 3He-1H scattering as an analyser. The characteristics of this latter reaction have been determined by Tombrello, by Morrow and Haeberli and by McSherry et al. [8]. When recoil protons are detected at laboratory angles near to 28’, following bombardment by 3He particles of energy about 29 MeV, their differential cross section is found to be slowly varying with both angle and energy and their analysing power for 3He particle polarization is about 25% as seen from Tombrello’s solution I phase shifts (although the experimental measurements by Mc-
Volume 30B. number 7
PHYSICS
Sherry et al. [8] might indicate a lower value of about 15%). This allows a favourable polarimeter to be constructed for analysis of 3He polarization. A beam of up to 40 PA of 32 MeV 3He particles from the Birmingham radial ridge cyclotron was focussed in the exit port of the cyclotron on to a vertical strip of carbon, 5.4 mg cmM2 thick and 12.5 mm wide. This carbon strip was at the object point of a beam transport system of total length 10 m set at a laboratory angle of 25’ (31.1’ c.m.s.) to the incident beam and consisting of one pair of quadrupole magnets which focussed the scattered beam through the shielding wall of the cyclotron vault into a 10’ bending magnet. This was followed by a second set of quadrupole magnets which focussed the beam leaving the exit slit of this bendin magnet into a polythene foil target, 4.8 mg/cm g2 thick and 7.0 mm wide, at the centre of the polarimater chamber. The scattered 3He beam transported to the polythene foil had an energy spread of * 150 keV and an angular spread of f 0.75’. A solid state detector was used to check that the 3He particles incident on the polythene foil had an energy of about 29 MeV. The counter telescopes in the polarimeter were fixed at equal angles of 28’ and subtended angles of f 0.5’ to the polythene target. The mass identification system was set to accept protons. As an additional precaution absorbers were placed in front of the telescopes to stop 3He particles scattered by the polythene from reaching the counters. Backgrounds of protons were observed and to allow for these the polythene foil was replaced by a carbon foil to measure effects from (3He, p) and (3He, 2p) reactions in the carbon. Protons from similar reactions in the first target and with the same magnetic rigidity as the scattered 3He beam were also transported to the second target. An estimate of their scattered intensity from the hydrogen in the polythene relative to the genuine recoil proton intensity following 3He scattering showed that this was at least two orders of magnitude lower than for the genuine intensity. Recoil proton spectra were recorded with a carbon first target and then with a gold first target of such thickness that the energy of the elastically scattered 3He at 25’ was identical to that from the carbon target. The asymmetries are very small, the mean values from several runs are A (carbon - hydrcgen) = 0.011 + 0.011, A(gold- hydrogen) = 0.009 f f 0.004. From the left-right ratios in successive runs with alternate gold and carbon first targets the
LETTERS
24 November 1969
true carbon asymmetry was obtained. From this the polarization for carbon at 31.1’ c.m.s. was found to be PC = +0.014 f 0.044 for a polarimeter analysing power of 0.25. Hutson et al. [7] have analysed their 36 and 42 MeV data in terms of a conventional optical model with spin-orbit terms (in the form proportional to 0. L) with potentials of 0.85, 1.7 and 3.5 MeV. The predicted polarizations are small and fluctuate considerably but are less than 0.05 at 30’ c.m.s. for potentials less than 1.7 MeV at a 3He energy of 36 MeV. In the present work predictions from a similar optical model potential set show that the upper limit of this spinorbit potential at 31.5 MeV is about 5 MeV, in fairly good agreement with Hutson et al. [7], and 91 who deduce also with Patterson and Cramer a value of 2.7 + 0.7 MeV for the 5He-12C spinorbit potential at 22.5 MeV from spin-flip measurements. Calculations by Kunz and Smith quoted by Hutson at al. [7] suggest that the 3Henucleus spin-orbit potential should be very much less than that for the nucleon-nucleus case. It is therefore probable that 3He elastic scattering cross sections are insensitive to spin-orbit effects, except perhaps at large angles where optical model analyses have shown [lo] that the observed cross sections require a non-central term in the optical model to obtain reasonable agreement between theory and experiment.
References
II 2.
3. 4. ii. 6.
7. 8.
9. 10.
W. E. Burcham et al Comgtes Rendus du Conar Int. de Physique nucleaire.. Paris (1964) 877. _ P.E.Hodgson. Adv. in Phys. 17 (1968) 563. J. Catala. A.Garcia and J. M. Perez. Anales de la Real Sociedad Espaniola 6lA (1965) 357. W. E. Frahn and d. Wiechers. Nucl. Phys. 74 (1965) 65. J.B.A. England et al.. University of Birmingham. to be published. H. M. Sen Guuta. E. A. Kine and J. B. A. Enaland. Nucl. Phys. 50 (1964) 549;’ D.J.Baugh, ti.J.B.Pvle. P.M.Ro1phandS.M. Scarrott.vNucl. PhJs..A95 (1967) 115: S. J. \Varsha\v. A. J. Buffa. J. B. Barcngoltz and M.K.Brussel. Nucl. Phgs. A121 (1968) 350. R.L.Hutson et al.. Phys. Letters 278 (1968) 153 T.A.Tombrello. Phgs. Rev. 138 (1965) B40: L. W. Morro\v and W. Haeherli. Nucl, Phgs. Al26 (1969) 225; D.H.McSherry. S.D.Baker. G.R.Plattner and T.B.Clegg. Nucl. Phys. A126 (1969) 233. D. M. Patterson and J. G. Cramer, Phys. Letters 27B (1968) 373. E.F.Gibson et al.. Phys. Rev. 155 (1967) 1194.
477
30B. number
‘7
PHYSICS
POLARIZATION
OF 3He SCATTERED
J. B.A. ENGLAND, Department
LETTERS
24 November
PARTICLES ELASTICALLY BY 12C NUCLEI
R. G. HARRIS> L. H. WATSON, of Physics.
1969
The University
D. H. WORLEDGE
of Bivmingham.
UK
and J. E. EVANS * A.E.R.E..
Harwell.
UK
Received
23 October
1969
The polarization in the elastic scattering of 31.6 MeV 3He particles from has been measured and found to be - 0.014 2 0.044. indicating a spin-orbit for 3He scattered by 12C of not greater than 5 MeV.
In an earlier report [l], preliminary results were given for an experiment in which 29 MeV 3He particles were double-scattered by carbon in an attempt to detect asymmetries at the second scatter caused by spin polarization in the first scattering. This could be the result of a spin-orbit term in the 3He-nucleus interaction and such a term, if large, would be significant in the optical model analysis of 3He particle scattering by nuclei [2]. The first experiments [3], using photographic emulsions as detectors of the second-scattered particles, suffered from unwanted tracks due to (3He,o) and (3He,n) reactions, but suggested an observable degree of polarization. A short investigation with semiconductor counter telescopes as detectors of the asymmetry in the second scatter confirmed this result, for a first and second scattering angle of 35’ (c.m.s.). Theoretical interpretations of these results have been offered [2,4] with the conclusion that the strength of the spin-orbit potential in the usual optical formulation [2] is 6 to 8 MeV, as for nucleons. Since this report considerable experimental effort has been devoted to further investigation of the 3He-12C double-scattering at energies from 29 MeV to 32 MeV, with semi-conductor counter telescopes and particle mass identification [5]. Despite better geometrical accuracy: typically an angular definition at both scatterings of f 0.75’ compared with f 1.4’ in the first measurements, these carbon double-scattering * Deceased. 476
12C at a c.m.s. angle of 31.1’ potential in the optical model
experiments failed both to confirm the original result and to yield any consistent value for the polarization. This was attributed to the rapid angular variation of the 3He-12C elastic scattering cross section [6] which makes the measurement of polarization by the double-scattering technique a much more difficult problem than in the case of proton scattering. After these experiments had been discontinued in favour of an alternative approach described below, a report by Hutson et al. [‘I] appeared in which the polarization of 36 - 42 MeV 3He-partitles scattered by carbon at 30° (c.m.s.) was found to be very small? with a second scattering asymmetry A = - 0.001 + 0.003. The spin-orbit potential derived from this measurement is not greater than 1.7 MeV and the polarization predicted at 29 to 32 MeV with this potential would have been unobservable by the methods of refs. 1 and 3. In an alternative a preach to the problem, the polarization in 3He- lf3C scattering has now been studied with 3He-1H scattering as an analyser. The characteristics of this latter reaction have been determined by Tombrello, by Morrow and Haeberli and by McSherry et al. [8]. When recoil protons are detected at laboratory angles near to 28’, following bombardment by 3He particles of energy about 29 MeV, their differential cross section is found to be slowly varying with both angle and energy and their analysing power for 3He particle polarization is about 25% as seen from Tombrello’s solution I phase shifts (although the experimental measurements by Mc-
Volume 30B. number 7
PHYSICS
Sherry et al. [8] might indicate a lower value of about 15%). This allows a favourable polarimeter to be constructed for analysis of 3He polarization. A beam of up to 40 PA of 32 MeV 3He particles from the Birmingham radial ridge cyclotron was focussed in the exit port of the cyclotron on to a vertical strip of carbon, 5.4 mg cmM2 thick and 12.5 mm wide. This carbon strip was at the object point of a beam transport system of total length 10 m set at a laboratory angle of 25’ (31.1’ c.m.s.) to the incident beam and consisting of one pair of quadrupole magnets which focussed the scattered beam through the shielding wall of the cyclotron vault into a 10’ bending magnet. This was followed by a second set of quadrupole magnets which focussed the beam leaving the exit slit of this bendin magnet into a polythene foil target, 4.8 mg/cm g2 thick and 7.0 mm wide, at the centre of the polarimater chamber. The scattered 3He beam transported to the polythene foil had an energy spread of * 150 keV and an angular spread of f 0.75’. A solid state detector was used to check that the 3He particles incident on the polythene foil had an energy of about 29 MeV. The counter telescopes in the polarimeter were fixed at equal angles of 28’ and subtended angles of f 0.5’ to the polythene target. The mass identification system was set to accept protons. As an additional precaution absorbers were placed in front of the telescopes to stop 3He particles scattered by the polythene from reaching the counters. Backgrounds of protons were observed and to allow for these the polythene foil was replaced by a carbon foil to measure effects from (3He, p) and (3He, 2p) reactions in the carbon. Protons from similar reactions in the first target and with the same magnetic rigidity as the scattered 3He beam were also transported to the second target. An estimate of their scattered intensity from the hydrogen in the polythene relative to the genuine recoil proton intensity following 3He scattering showed that this was at least two orders of magnitude lower than for the genuine intensity. Recoil proton spectra were recorded with a carbon first target and then with a gold first target of such thickness that the energy of the elastically scattered 3He at 25’ was identical to that from the carbon target. The asymmetries are very small, the mean values from several runs are A (carbon - hydrcgen) = 0.011 + 0.011, A(gold- hydrogen) = 0.009 f f 0.004. From the left-right ratios in successive runs with alternate gold and carbon first targets the
LETTERS
24 November 1969
true carbon asymmetry was obtained. From this the polarization for carbon at 31.1’ c.m.s. was found to be PC = +0.014 f 0.044 for a polarimeter analysing power of 0.25. Hutson et al. [7] have analysed their 36 and 42 MeV data in terms of a conventional optical model with spin-orbit terms (in the form proportional to 0. L) with potentials of 0.85, 1.7 and 3.5 MeV. The predicted polarizations are small and fluctuate considerably but are less than 0.05 at 30’ c.m.s. for potentials less than 1.7 MeV at a 3He energy of 36 MeV. In the present work predictions from a similar optical model potential set show that the upper limit of this spinorbit potential at 31.5 MeV is about 5 MeV, in fairly good agreement with Hutson et al. [7], and 91 who deduce also with Patterson and Cramer a value of 2.7 + 0.7 MeV for the 5He-12C spinorbit potential at 22.5 MeV from spin-flip measurements. Calculations by Kunz and Smith quoted by Hutson at al. [7] suggest that the 3Henucleus spin-orbit potential should be very much less than that for the nucleon-nucleus case. It is therefore probable that 3He elastic scattering cross sections are insensitive to spin-orbit effects, except perhaps at large angles where optical model analyses have shown [lo] that the observed cross sections require a non-central term in the optical model to obtain reasonable agreement between theory and experiment.
References
II 2.
3. 4. ii. 6.
7. 8.
9. 10.
W. E. Burcham et al Comgtes Rendus du Conar Int. de Physique nucleaire.. Paris (1964) 877. _ P.E.Hodgson. Adv. in Phys. 17 (1968) 563. J. Catala. A.Garcia and J. M. Perez. Anales de la Real Sociedad Espaniola 6lA (1965) 357. W. E. Frahn and d. Wiechers. Nucl. Phys. 74 (1965) 65. J.B.A. England et al.. University of Birmingham. to be published. H. M. Sen Guuta. E. A. Kine and J. B. A. Enaland. Nucl. Phys. 50 (1964) 549;’ D.J.Baugh, ti.J.B.Pvle. P.M.Ro1phandS.M. Scarrott.vNucl. PhJs..A95 (1967) 115: S. J. \Varsha\v. A. J. Buffa. J. B. Barcngoltz and M.K.Brussel. Nucl. Phgs. A121 (1968) 350. R.L.Hutson et al.. Phys. Letters 278 (1968) 153 T.A.Tombrello. Phgs. Rev. 138 (1965) B40: L. W. Morro\v and W. Haeherli. Nucl, Phgs. Al26 (1969) 225; D.H.McSherry. S.D.Baker. G.R.Plattner and T.B.Clegg. Nucl. Phys. A126 (1969) 233. D. M. Patterson and J. G. Cramer, Phys. Letters 27B (1968) 373. E.F.Gibson et al.. Phys. Rev. 155 (1967) 1194.
477