REACTIONS INDUCED BY POLARIZED ELECTRONS ON POLARIZED 3He AT THE MIT-BATES ACCELERATOR

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REACTIONS INDUCED BY POLARIZED ELECTRONS ON POLARIZED 3He AT THE

MIT-BATES ACCELERATOR

R. Milner

To cite this version:

R. Milner. REACTIONS INDUCED BY POLARIZED ELECTRONS ON POLARIZED 3He AT

THE MIT-BATES ACCELERATOR. Journal de Physique Colloques, 1990, 51 (C6), pp.C6-479-C6-

482. �10.1051/jphyscol:1990656�. �jpa-00230926�

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REACTIONS INDUCED BY POLARIZED ELECTRONS ON POLARIZED 3 ~ AT THE MIT-BATES e ACCELERATOR

R.G. MILNER

Bates Linear Accelerator Center, Laboratory for Nuclear Science and Department of Physics, Massachusetts Institute of Technology, Cambridge M A 02139, U.S.A.

The asymmetry in spin-dependent quasielastic electron scattering of longitudinally polarized electrons from a polarized 3He gas target has been measured. This measurement represents the first demonstration of a new method for studying electromagnetic nuclear structure: the scattering of polarized electrons from a polarized nuclear target.

1. SPIN-DEPENDENT ELECTRON SCATTERING FROM 3HE

For many years one of the most important ways to investigate nuclear structure and dynamics has been the use of elastic and inelastic electron scattering from nuclei. In such studies, one probes the electromagnetic structure of the nucleus, which may be characterized by matrix elements of the electromagnetic current operators. Particularly for few-nucleon systems, this allows a relatively direct comparison of experimental data with the results of detailed theoretical calculations. For the case of inclusive electron scattering from unpolarized nuclei, it is only possible to extract two response functio~~s from the differential scattering cross-section; these correspond to the longitudinal and transverse components of the nuclear current. This separation, which is made by varying the kinematics of the incident and scattered electrons while keeping the energy and three-momentum transfer fixed, is the usual Rosenblutll decomposition for inclusive electron scattering1. In general, the longitudinal response involves the sum of the squares of the charge multipole matrix elements, while the transverse response contains the sun1 of the squares of the electric and magnetic multipole matrix elements which are permitted by conservation of angular moinentum and parity and invariance under time reversal. Since a complete understanding of the electromagnetic structure of the nucleus requires a knowledge of all of the individual matrix elements, and since each response function contains a number of different matrix elements, it is apparent that in gencral it is not possible to extract all of the individual terms from such experiments. This limitation can be overcome by the use of polarization degrees of freedom in inclusive electron scattering2. With polarized beam and target it is possible to perform a generalized version of the Rosenbluth decomposition of the cross-section into additional response functions. In this way an optimal amount of nuclear structure information may be obtained from inclusive spin-dependent electron scattering. As a result, new experimental programs utilizing polarized beams and targets are being planned for all major electron accelerator laboratories (SLAC, MIT-Bates, CEBAF, Mainz, Bonn, DESY HERA, and Novosibirsk).

3He is a particularly interesting nucleus with which to carry out the generalized Rosenbluth separation.

It is anticipated that spin-dependent quasielastic scattering from 3He should be sensitive to the neutron electromagnetic form-factors. Since the nucleus in its ground state is predomina.ntly a spatially symmetric S-state, its two protons are mainly in a spin singlet state. Further, the three body system has the important advantage that realistic calculations can be carried out. In fact, a Faddeev calculation by Blankleider and Woloshyn3 has demonstrated that in the vicinity of the quasielastic pealc spin-dependent effects should be determined primarily by scattering from the neutron. The differential cross-section for inclusive spin- dependent scattering may be written as2

The asymmetry, A, is the ratio of spin-dependent to unpolarized cross-sections

and the experimental asymmetry, Aexp, is related to A by

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990656

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COLLOQUE DE PHYSIQUE

where p, and p~ are the electron and target polarizations respectively. The direction of the target polarization is specified by the angles @* and $*, as defined in Figure 1. Further, for 3He, a spin-+ target, the asymmetry is given by2

{COS @*vT, RTj

+

²^sin^6'* cos (b*uTLl FT(Q')RTL~) A = -

{ , ~ L R L

+

^VTRT}

where the v~ are kinematic factors (see reference 2). R,,(QP,w) and RT(Q2,w) are the longitudinal and transverse response functions contributing to the uppolarized cross-section and are functions of the square of the momentum transfer Q2 and the electron energy loss W. RTJ (Q2, W) and RTL, (Q2, W) are two new response functions that do not contribute to the unpolarized inclusive cross section.

F i g u r e 1. Kinematics for electron scattering from polarized targets. Here U, is along the direction of nlomentum transfer q. The vector uy is normal to the electron scattering pIane and U, = uy X U, lies in the scattering plane. The target polarization direction is then specified by the angles

**( @ * , p )**

in this coordinate system.

By orientation of the target spin at B* = 0" (along q) and at @* = 90' (normal to q), one can select the resulting asymmetry t o be proportional to the transverse and transverse-longitudinal interference terms, respectively. Assuming the impulse approximation (ignoring final-state interactions and meson-exchange currents) the calculations of Blankleider and Woloshyn indicate that RT, is predominantly proportional t o the square of the neutron magnetic form-factor

~b~

and RTLt is very sensitive to the product GGGE.

Experimental confirmation of this picture m l d give important motivation for future experiments while deviations from the simple picture would constrain our understanding of the three-body system. The dependence of RTLt on the neutron electric form factor is particularly interesting since this form-factor is poorly determined experimentally.

2. EXPERIMENTAL DATA

Data was taken in March and April 1990 on two optically pumped 3He gas targets, one using spin exchange optical pumping4 and another using metastability exchange optical pumping. Data taken with the spin exchange target (experiment 88-10) are at present being analysed. Results have been published for the metastability exchange target5 and this experiment is discussed here.

The experiment 88-02 was carried out at the MIT-Bates Linear Accelerator Center and measured the asymmetry in the inclusive quasielastic scattering of longitudinally polarized electrons from a polarized 3He gas target. The energy of the polarized electron beam was 574 MeV. This value was chosen for the experiment because the g - 2 precession of an electron extracted from the source after acceleration and transport into the experimental hall is 2 r radians. Thus, electrons with longitudinal polarization extracted

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description of the target will be reported elsewhere8; a brief description is presented here. The polarized 3He target, shown in Figure 2, employed an infrared laser to polarize 3He atoms in a pyres cell by metast,ability exchange optical pumping. The pumping cell was connected by a transfer tube to a copper ta.rget cell, which was cooled to 15 E<. A coa.ting of

NZ

was placed on the target cell to increase the wall relaxation time of the polarized atoms. The direction of the target polarization was determined by a set of Helmholtz coils, which provided a holding field of 30 gauss. The pressure in the target was 2 torr. The polarization of the pumping cell was continuously measured by detection of the circular polarization of the 667 nm line in the discharge. The polarization of the atoms in the target cell was inferred from the known polarization in the pumping cell and the time constants of the coupled system.

Electrons scattered from the polarized 3He target were detected at quasielastic kinematics in the One Hundred Inch Proton Spectrometer (OHIPS). The spectrometer was configured at an angle of 51' and a central momentum of 462 MeV/c for an average four-momentum transfer Q2 of 0.2 ( G ~ V / C ) ~ . Collimators were used to limit the target thickness viewed by the spectrometer to 10 cm. With no gas in the target the rate in the spectrometer was always less than 10% of the rate with the target full. Since the polarized 3He target atoms are present as pure atomic species the only unpolarized material contributing to the observed yield was the target cell itself and so was accounted for as an 8% reduction in the experimentally measured asymmetry. The target thickness viewed by the OHIPS spectrometer was 1.2 X 10'' atoms/cm2. The unpolarized 3He quasielastic cross-section was found to be in good agreement with previous da.ta and served as a continuous check on beam halo and target-wall scattering.

The scattering asymmetry was illeasured for three different configurations of taxget direction and the results are given in Table 1. A total of 1478 pA ohours of charge was collected. Combining all the data yields for the asymmetry A = -3.49 f 1.23 % rt 0.54%. The calculation of Blankleider and woloshG3 has been averaged over the spectrometer acceptance at these kinematics to yield the prediction A = -4.3 f 0.2

GLASS PUMPING r C E L L

--

_POLARIZED

ELECTRCN BEAM

ALUM l NUM SCATTERING CHAMBER

Figure 2 . Polarized 3He gas target.

%. A similar calculatioil based on Faddeev wave functions from Friar2ives the prediction A = -4.5 f 0.2

%. Thus, the results of the experiment are well described by the conventional model.

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COLLOQUE DE PHYSIQUE

Table 1. Results of asymmetry measurements.

Charge O* $* A

pA.hours degrees degrees %

310 0.9 180 -3.5 h 2.4

342 7.9 180 -2.1 2.7

626 172.1 0 t 4 . 2 rt- 1.7

14'76 (combined) -3.49 f 1.23 f 0.54

Theory (Ref. 3) Theory (Ref. 8)

In summary, the first generalised Rosenbluth separation using polarized beam and target has been carried out on the 3He nucleus. These results demonstrate a powerful new technique in electromagnetic nuclear physics. In addition, the measured asymmetry supports the picture of the polarized 3He nucleus as an effective polarized neutron. It should be noted that the experiment becomes feasible because of the new polarized gas target. The low holding field of 30 gauss permits rapid orientation of the target spin without affecting the trajectories of incoming and scattered particles; the absence of unpolarized material in the target eliminates dilution of the asymmetry and the necessity for significant corrections. These results provide strong motivation to proceed with futher experiments on spin-dependent electron scattering from 3He.

Experiment 88-02 was a collaboration of Caltech and MIT. The polarized target was developed at Caltech. The author's research was supported in part by the U.S. Departmentof Energy, Nuclear Physics Division, under contract W-31-109-ENG-38 and in part by the MIT Sloan Fund.

REFERENCES

M.N. Rosenbluth, Phys. Rev. 79 (1950), 615

T.W. Donnelly and A.S. Raskin, Annals of Physics, 169 (1986), 247 B. Blankleider and R.M. Woloshyn, Phys. Rev. C29 (1984), 538 T.E. Chupp et al., Phys. Rev. 36C, (1987) 2244

C.E. Woodward et al., Phys. Rev. Lett., 65 (1990), 698 E.J. Beise et al. (to be published)

R.G. Milner, R.D. McKeown, and C.E. Woodward, Nucl. Instr. and Meth. A274 (1989) 56 C.E. Woodward et al. (to be published)

J . Friar (private communication)