NUCLEAR REACTION STUDIES USING POLARIZED 3He TARGETS AND BEAMS

G.C. PHILLIPS*

Rice University, Houston

I INTRODUCTION

This paper reports work at Rice University using polarized 3ne targets and beams. The 3He is polarized by the optical Piµtlping method described by G.K. Walters in t~e preceding paper 1J. Some of this work was reported last year 2) but for the sake of com­ pleteness, all of the nuclear studies made at Rice University u­ sing polarized 3He will be reviewed.

The study of nuclear reactions with the target and/or the beam polarized are of basic importance because of the strong spin de­ pendence of nuclear forces ; indeed, until a nuclear process in­ volving particles with spins has been studied so as to ascertain the spin dependence, the processes are basically unknown. Thus, muchly studied reactions such as elastic and inelastic proton scattering from nuclei, and deuteron-nucleon "stripping" reac­ tions, are really not yet completely studied for just this rea­ son.,

Two examples serve to emphasize this point. When protons are scat­ tered from 3He, the unstable nucleus 4Li is formed as an interme­ diate, compound-nuclear system, and the experimental determination of the elastic scattering phase shifts for energies up to about 10 MeV gives information about the spins and parities of T = 1 sta­ tes of the mass-4 system. If only elastic scattering with unpolari­ zed beams and targets is available, along with spin measurements of the scattered proton, it is basically impossible to deduce a u­ nique set of phase shifts. However, if the spins are also measured, 216 G.C. PHILLIPS

the degenerate solutions can be rejected, and a unique determina­ tion of phase shifts, resonance parameters, etc, becomes possible.

A second example of typical nuclear experiments that badly need spin determinations are (d,p) or (d,n) "stripping" reac.tions. Such processes typically produce strongly forward-peaked angular distri­ butions of nucleons and polarization of the nucleons and are des­ cribed in terms of an optical model potential that must contain spin dependent terms. If only angular distributions and nucleon spin determinations are available, it is impossible to deduce a uni que spin dependence of the optical potential.

Both of the above problems have been studied at Rice using polari­ zed targets and will be discussed later.

II EXPERIMENTAL REQUIREMENTS OP POLARIZED 3He TARGETS

The preceding paper by Professor Walters has discussed the techni­ ques for producing 3He gas with useful nuclear polarization. Since some of these requirements are rather in conflict with the require­ ments for producing a polarized 3He target useful in nuclear measu­ rements, it is important to discuss these conflicts in detail.

To make a polarized 3He gas target useful for nuclear experiments, it is necessary to admit a beam and perhaps to allow charged parti­ cles to emerge from the gas. This necessitates employing foils well sealed to the vessel with the resulting possibilities of damage to the electrical discharge properties of the cell, of introducing ma­ gnetic inhomogeneities and/or surfaces that serve to relax the po­ larization, and of introducing more possibilities for leaks and for outgassing. Indeed, the beam itself may introduce contaminants, cau· se leaks, and certainly increases outgassing.

To be useful for a nuclear physics experiment, a polarized target must be capable of being sealed off, attached to an accelerator or reactor, and operate with useful polarization for several hours or days. Thus, if detectors are placed internal to the target volume, they must not outgas and must not be significantly damaged or af­ fected by the electrical discharge and light produced in the gas. Finally, it is necessary that the nuclear instrumentation not in­ troduce magnetic gradients.

The most versatile 3He target designed at Rice to date is shown in figure 1. This target attempts to eliminate all of the difficulties 3 REACTIONS WITH He TARGETS AND BEAMS 217

Reflector-

He 4 lamp High Voltage RF

To He 4 Reservoir

Plane Polorizer Quart Wove Plate E==::::::i Gloss Window

RF

CROSS SECTION OF POLARIZED He 3 SCATTERING CHAMBER

Fig. 1 A versatile 3He polarized target arrangement. See text.

discussed above. To facilitate outgassing, the 3He gas is contai­ ned in a precision ground glass cylinder through which the beam is passed via thin foils while reaction product charged particles can emerge through thin windows to either side in an angular range of 30° to 150°. The foils are sealed to the glass with indium "0"-rings and pressed against the glass by a precision retaining piece. This vessel is contained inside an evacuated vessel in which two silicon solid-state detectors and telescopes can be rotated. This feature allows very thin foils to be employed, and thus low-range particles detected, since the differential pressure across the foils can be very small ; the design also eliminate leakage into the 3He gas. Circularly polarized pumping light is admitted through the glass cover of the outer chamber and the flat top and bottom of the inner glass 3He cell. Construction of this target system is now nearing completion. The cell and associated optical and electronic compo- 218 G.C. PHILLIPS nents will be portable so that the target may be transported to other laboratories for use.

III SPECIFIC STUDIES USING POLARIZED 3He TARGETS

Four nuclear processes have been studied to date using these tar­ gets ; three of these have been reported elsewhere 1) but will be summarized below. New data or. proton-3He scattering will be pre­ sented.

This reaction was the first use of a p9larized 3He gas target prepared by opt~cal pumping and demonstrated the usefulness of the technique 3J. Figure 2 shows a schematic diagram of the appa­ ratus which was employed with the Rice tandem Van de Graaff and a-particle beams of about 7 MeV energy. Earlier experiments had shown that a strong resonance for 3He-a elastic scattering at a­ bout 7 MeV had Jn = 7/2- and allowed the extraction of unique phase shift fits near resonance which iP turn predicted near 100 per cent 3He polarization either side ~f resonance at certain an­ gles. These predictions were checked by the apparatus of figure 2.

Fig. 2 Schematic diagram of the apparatus used- to study the elastic scattering of alpha particles from polarized 3He, See text and refe­ rence 3. REACTIONS WITH 3He TARGETS AND BEAMS 219

The expected left-right assymetry is A = P Pt, where Pn is the nu­ clear polarization of the 3He caiculated f¥om the phase shifts and Pt is the target polarization measured optically 1J. The measured value of A= (N 1 - ·NR)/(N1 + NR), determined from the left or right counting rates N1 , NR confirmed the predictions and demonstrated the usefulness of the method.

Several weaknesses of the design of this first target cell were apparent. These included the fact that the electrical discharge was very nonuniform in the partially metal cell, and that the cell which contained detectors outgassed and did not have a very long useful life, and a rather low polarization (about 8 per cent).

B 3 He(d,p) 4He

The study of this reaction with a polarized 3He target has provi­ ded the first indication that optical model theories of "stripping" processes must include tensor interactions.

These measurements were carried out using a glass sphere 3He cell with 1/4 mm walls. The beam passed through two 1/3 mil. Al foils. The rather energetic protons could penetrate the glass and were detected in two thick Li-drifted silicon detectors operated in the air behind collimators 4J. Typical angular distributions of A(e) are shown in figure 3. Also shown are P(e), the proton spin polarization -0f the reaction when an unpolarized target is used 5).

6 MeV

~ •t.0 o t.O r"T-",.....-.....,.....,,...... ,-,....,...-,,,.-.-,-,.-,-, "' 10 MeV ·•<•> •E ..~ ol----~~+--->-~---i

Fig. 3 3He(d,p)4He angular distributions of the left-right asymmetry, A(e) (see reference 4) using a polarized 3He target, and the proton spin polarization, P(e~ (see reference ;) using an unpolarized target. 220 G.C. PHILLIPS

Note that at forward angles, where the cross-section is largest, that A(e) ~ - P(e). Now ordinary optical model potentials that as­ sume central plus spin-orbit forces, predict that A(9) = - 1/3P(e), independent of the ra~ge and strengths of the potentials, a result deduced by Tanifugi 6). Professor Ian Duck at Rice further demons­ trated that the only way that departure from A(9) = - 1/3P(e) can be obtained is for there to be large spin-flip amplitude~ corres­ ponding to strong tensor forces 7). It is clear that oth~r polari­ zed target and beam studies need to be carried out for other "stripping" reactions to further define the spin dependence of op­ tical potentials.

This experiment was undertaken at the Materials Testing Reactor in collaboration with Dr Robert Spencer by using a beam of polarized neutrons obtained by Bragg scattering from a saturated Co-Fe sin­ gle crystal. The large thermal capture cross-section (N 50~0 barns) was suspected to be due to a (virtual) resonance state in He and thus should be detected for either the Jn = o+ or 1+ configurations of the two spinors depending on the spin of the 4He Qtate involved. The measurements confirmed Brookhaven measurements 8) and showed that the capture is dominated by the singlet state and confirms that the 4He first excited state (at near zero p-3H energy) is a o+ state.

The fundamental problem of the T = 1 excited states of the mass-4 nuclei can be studied by this elastic scattering process. The ex­ perimental determinations of cross-section and proton polarization using an)unpolarized target have been phase shift analyzed by Tom­ brello 9 and more recently by Morrow and Haeberli 10).

The Rice measurements of the elastic scattering A(e) using a pola­ rized 3He target have been carried out using a glass target cell with Al beam entrance 'and exit windows and with foils at 45° and 90° for left-right scattered proton detection which is shown in figure 4 11 J. The data were taken in the bombarding energy range of 4 to 11 MeV. The 3He polarization was about 10 per cent and was measured optically.

Measurements of the asymmetry are shown in figure 5. These measu­ rements may be compared to the calculation of Tombrello whose pha­ se shifts predict asymmetries of 40 to 50 per cent at 90°. Since smaller asymmetries are observed, it is clear that the Tombrello 3 REACTIONS WITH He TARGETS AND BEAMS 221

Fig. 4 Apparatus for the study of elastic proton scattering from a polarized 3He target. Protons of 4 to 11 MeV bombard the cell and are scattered to detectors at 45° and 90°.

-0.1 >- 3He (p,p) 3He c- Rough data analysis

I I I I 4 5 6 7 8 9 10 11 Elab

Fig, 5 3He(p,p) 3 He left-right asymmetry, A(e) (see reference 11), for e = 45° and 90° in the proton energy range 4 to 11 MeV,

16 222 G.C. PHILLIPS

phase shifts are in error, although the size or nature of the dis­ crepancy is as yet unknown.

Preliminary calculations by Morrow and Haeberli 10) yield two dege· nerate sets of phase shifts to fit earlier cross-section and pola­ rization data. There is hope that one of these sets may describe the present asymmetry results and suggests that a unique determina· tion of the p-wave, T = 1 states of 4Li may be obtainable upon in­ clusion of all data in the fitting.

IV POLARIZED 3He BEAM

An apparatus designed to produce 3He+ polarized beams has been constructed. The polarized ion source is similar to a conventional R.F. ion source except that polarized pumping light is supplied to the discharge and great care is taken to provide for outgassing th1 ion-source and associated plumbing and to supply a flow of high pu· riti 3He gas. This apparatus has successfully produced several µA of 3He+ ions over periods of many hours while simultaneous optical pumping of th~ discharge produced up to 20 per cent polarization of the gas 1 2).

The amount of polarization of the 3He+ ions is, of course, related to the gas polarization, but in a complex and presently unknown way$ Many factors should lower the polarization : for example, io­ nization of the atoms, collisions of ions with surfaces or impuri­ ties, and passage through magnetic gradients. One effect may res­ tore some polarization to the 3He+ ions : charge exchange colli­ sions with polarized 3He atoms. Thus, it is necessary to devise a method to measure the beam polarization.

The 2n(3ne,p)4He reaction has been chosen to measure the beam po­ larization, and an apparatus named a Demtan (Tandem backwards) has been constructed. The Demtan consists of the polarized ion source attached to an acceleration tube and elevated to+ 150 kV above ground, with vacuum pumps at ground, and the deuterium target floating at - 150 kV off ground at the end of a second accelera­ tion tube. In this way, the deuterium target can be bombarded with 300 keV 3ne+ ions. The apparatus is shown in figures 6 and 7.

The beam polarization will be measured by means of measurement of the spin polarization of the 2H(3He,p) protons. This will be ac­ complished by means of left-right scattering of the energetic pro­ tons from 4He gas. A polarimeter has been constructed and tested that employs 4He gas at 35 atmospheres with the protons being REACTIONS WITH 3He TARGETS AND BEAMS 223

Rice University Dern-tan Accelerator

j;:orizing Light -- Oplicol-Electrost:- Source Pumping Focussing 1 Cell Lenses ,~,,·· \ \ Accelerating Columns I :· t I ., =-erI I Extraction Electrode r-f for Weak 4 Discharge He Polarimeter IPositive Table + 150 kV

Mo9netic Field

Fig. 6 Schematic of the Demtan accelerator to be used to test the 3He+ ion beam polarization, See text.

Fig, 7 Photograph of the Demtan. 224 G.C. PHILLIPS

scattered along a 19 cm path into collimated counters at about 60° to the proton direction, see figure 8. The protons will have ener­ gies of 6.5 to 11.1 MeV in the 4He gas, and the p-4He scattering asymmetry at those energies and angles is nearly 100 per cent for completely polarized protons. The beam polarization Pb is related to the proton polarization by PP = 2/3 Pb for protons emitted per­ pendicular to the beam polarization.

pressure vessel ( Aluminum)

inches ~1--~~--1T Polarimeter cm 012345678

Fig. 8 Schematic of the proton-polarimeter. See text.

This apparatus is all built and tested 13) and the test of 3He+ beam polarization will be carried out shortly.

V CONCLUSIONS AND ACKNOWLEDGMENTS

The use of polarized 3He targets has materially aided studies of nuclear-structure and nuclear reactions. It has provided the first proof of the need for inclusion of tensor forces in optical models of stripping, has helped determine the spin and parity of the firsi excited state of the a-particle, and appears to be capable of deci­ ding the proper phase shifts for p + 3He scattering and determinine the spins and parities of the T = 1 states of 4Li. The extension oi these measurements to lower and higher energies, to other projecti­ les, and especially to use with polarized beams should be very fru: ful in providing additional new nuclear structure information. REACTIONS WITH 3He TARGETS AND BEAMS 225

I am indebted to many colleagues for help in preparing this paper. The work spans the last five years. For some of the earlier nu­ clear physics measurements I am indebted to Professor Pat Windham of North Texas State University and Dr Robert Spencer of Phillips Petroleum Company and to Dr Elmer Carter. In all the recent work, Professor Stephen D. Baker has taken significant responsibility and leadership.

The continuing close collaboration at Rice of the Nuclear Physics group with Professor Walters' Atomic Physics group has made the work possible and is also gratefully acknowledged, as is the clo­ se collaboration with members of the Nuclear Theory group, Profes­ sor Duck, and Dr Tanifugi.

Note and References

* On leave as NSF Senior Postdoctoral Fellow Brookhaven National Laboratory.

1) G.K. Walters, these Proceedings, p. 201. 2) G.C. Phillips, Proceedings of the 2nd International Symposium on Polarization Phenomena of Nucleons, Karlsruhe, September 6-11, 1965 (P. Huber and H. Schopper, Eds., Birkhauser Verlag, Basel and Stuttgart, 1966), p. 113-125. 3) G.C. Phillips, R.R. Perry, P.M. Windham, G.K. Walters and L.D. Schearer, Jr, Phys. Rev. Letters, 1962, ~, 522. 4) s.D. Baker, G. Ray, G.C. Phillips and G.K. Walters, Phys. Rev. Letters, 1965, .12., 115. 5) R.I. Brown and W. Haeberli, Phys. Rev., 1963, 12Q, 1163. 6) M. Tanifugi, Phys. Rev. Letters, 1965, .12, 113. 7) I. Duck, Nucl. Phys., 1966, 80, 617. 8) L. Passel and R.I. Schermer, Bull. Am. Phys. Soc., 1965, .1.Q, 587. 9) T.A. Tombrello, Phys. Rev., 1965, 1Ul..§., 40. 0) L.W. Morrow and W. Haeberli, Private communication. 1) S.D. Baker, D.H. McSherry, D.O. Findley and G.C. Phillips, Bull. Am. Phys. Soc. (N.Y. Meeting, January, 1967). 2) N.D. Stockwell, E.B. Carter, G.C. Phillips and G.K. Walters, Bull. Am. Phys. Soc., 1965, .1.Q, 156. 3) D.O. Findley, M.A. Thesis, Rice University, 1967 (unpublished).

Work partially supported by the u.s. Atomic Energy Commission.