Analyzing powers in
4
He„
6
Li
,
6
Li…
4
He
P. V. Green, K. W. Kemper, P. L. Kerr, K. Mohajeri, E. G. Myers, and D. Robson
Department of Physics, Florida State University, Tallahassee, Florida 32306
K. Rusek
Soltan Institute for Nuclear Studies, Warsaw, Poland
I. J. Thompson
Department of Physics, University of Surrey, Guildford, Surrey, United Kingdom
Received 30 January 1996
Elastic cross sections and a complete set of analyzing powers ( iT
11
, T
20
, T
21
, and T
22
) were measured for
the first time for the
4
He(
6
Li
,
6
Li system at E
c.m.
=11.1 MeV. Elastic cross-section data for the same
reaction at 15 MeV are also reported. The angular range covered is from 10° to 160° c.m., and all analyzing
powers reported have large magnitudes except for T
21
. Additionally, an analyzing power excitation function is
reported for the same reaction between E
c.m.
=10.3 and 11.9 MeV. Results of both optical model and optical
model plus exchange processes including transfers from the 2 S and 1 D components of the
6
Li ground state
wave function are presented. The analysis shows the presence of an explicit spin orbit potential, a very small
and negative spectroscopic amplitude of the 1 D component b
D
), and an angular momentum-dependent
absorption. S0556-28139600306-8
PACS numbers: 24.70.+s, 25.70.Bc
I. INTRODUCTION
This work reports elastic cross-section and analyzing
power AP data for
4
He(
6
Li
,
6
Li at E
c.m.
=11.1 MeV and
elastic cross-section data at E
c.m.
=15 MeV. The nuclear in-
teraction under study here has been of interest for some time
as a beam polarization monitor 1 and several workers have
reported sporadic data points for vector and second-rank
AP’s which have been useful in establishing reproducible
polarization monitoring in various laboratories around the
world 2,3. Aside from this utilitarian function, however,
and because of the very large binding energy roughly 20
MeV of the spinless target particle, the
6
Li + interac-
tion itself allows the study of fundamental properties associ-
ated with the spin degrees of freedom of the
6
Li nucleus
such as the origin of its extremely large AP’s 2 and relative
2 S to 1 D cluster structure of its ground state 4–6.
The theoretical analysis of the elastic scattering data re-
ported here confines itself to the nuclear optical model and
optical model plus exchange reaction. This investigation
thus focuses on the physical processes associated with the
spin degrees of freedom of the elastic
6
Li + interaction.
II. EXPERIMENTAL PROCEDURE
There are three measurements reported in the present
work and each is described separately below. The first is a
complete set of analyzing powers ( iT
11
, T
20
, T
21
, and
T
22
) for
6
Li
+ at E
c.m.
=11.1 MeV, and the second is an
excitation function for the
6
Li
+ vector analyzing power
iT
11
in the range E
c.m.
=10.3–11.9 MeV. The third set of
measurements are of the cross sections for
6
Li + at
E
c.m.
=11.1 and 15 MeV.
The FSU optically pumped polarized lithium ion source
OPPLIS7 produced the polarized
6
Li beam. After laser
optical pumping, subsequent magnetic substate selection if
any, ionization, and charge exchange in a cesium charge
exchange cell, the spin quantization axis of the polarized
6
Li
-
is oriented by the magnetic field of a Wien filter and
the beam preaccelerated to 85 kV and injected into the FSU
Super FN Tandem accelerator. The scattering chambers used
in the measurements consist of an evacuated 85 cm scatter-
ing chamber followed by a 45.7 cm 6.5 cm chamber filled
with helium gas slightly below atmospheric pressure. All po-
larization data reported here were taken in the helium-filled
chamber. The actual
6
Li beam energy momentum selected
by the 90° magnet was 31.8 MeV since the
6
Li loses 0.78
MeV passing through the 2.26 mg/cm
2
Havar foil window
between the 85 cm scattering chamber and the helium-gas-
filled polarimeter, and an additional 3.15 MeV traversing the
26.7 cm of helium gas between the Havar entrance window
and the center of the chamber. The absolute beam energy at
the center of the polarimeter is only accurate to 1% over
this series of runs due to the need to change the Havar foil,
carbon buildup on the foil, and changes in the gas pressure
monitoring system. Two E -E silicon surface barrier tele-
scopes, one on each side of the beam axis, were arranged to
rotate about the center of the chamber, and 1 mm 1 cm
and 2 mm 1 cm tantalum collimators determined the
beam-target interaction volume. The angular calibration of
the detectors was established by normalizing the relative
yields to an absolute angular cross section distribution for
+
6
Li at E
c.m.
=11.1 MeV reported by Bingham et al. 8.
The lithium beam polarizations produced by the OPPLIS
have been found over several years of operation to be quite
consistent and are measured at the source by means of laser
induced fluorescence LIF. As reported by Mendez et al.
3, vector and second-rank on-target beam polarizations
PHYSICAL REVIEW C JUNE 1996 VOLUME 53, NUMBER 6
53 0556-2813/96/536/28628/$10.00 2862 © 1996 The American Physical Society