2. Recent HRIBF Research  Dynamic
Polarization in the TwoBody Breakup of ^{17}F
(J.F. Liang, spokesperson)
Coulomb dissociation is a useful technique for studying radiative
capture reactions in nuclear astrophysics when direct measurements are
difficult or impossible, such as those involving shortlived radioactive
nuclei [1]. It is important to understand the breakup processes in
order to correctly extract relevant information. For loosely bound
protonrich nuclei, first order perturbation theory calculations are
not reliable and the inclusion of higher order effects is required. Of
particular importance is the dynamical polarization effect where the
valence proton is displaced behind the core nucleus and shielded from
the target. This is similar to the tail of a comet pointing away from
the sun when it flies close to the sun. This effect is proportional
to the cube of the target charge, Z^{3}_{T}.
It manifests in reducing the
breakup probability as compared to first order perturbation theory and
the reduction in breakup probability is expected to be smaller for a
target of lower Z [2].
We have measured the breakup of ^{17}F by bombarding ^{58}Ni
and ^{208}Pb
targets. The objective is to compare the breakup of ^{17}F into
oxygen and proton with respect to firstorderperturbation theory
for the two targets. The radioactive ^{17}F
was produced at
the HRIBF by an ISOL method and accelerated to an energy of 10
MeV per nucleon. The reaction products were measured by a stack of
three largearea Sistrip detectors. The front and middle detectors
form an ΔEE telescope for identifying the breakup oxygen. The breakup
proton has sufficiently high energy to pass through the first two
detectors and is detected in the third detector. This enables us to
measure angular distributions of oxygen ions in singles, and oxygen and
proton in coincidence.
Figure 21 presents the measured data (red circles) and first order
perturbation theory predictions (blue curves). The angular distribution of the
oxygen ions in coincidence with protons for the ^{17}F
on ^{58}Ni reaction
is reproduced by firstorder perturbation theory
with E1 and E2 excitations included. For the
^{208}Pb target, the angular distribution of oxygen and proton in
coincidence shows a larger reduction of breakup probability with
respect to firstorderperturbation theory. Comparisons with dynamic
calculations that take into account the Z^{3}_{T}
correction to study the
dynamical polarization in the breakup of ^{17}F
are underway.
Figure 21: Angular distributions of the oxygen ions in coincidence with
protons from the
breakup of ^{17}F by bombarding ^{58}Ni (left panel)
and ^{208}Pb
(right panel) at the energy of 10 MeV per nucleon. The
experimental data are
closed circles and the solid curves are predictions by firstorder perturbation theory
including E1 and E2 excitations.
[1] G. Baur, C. A. Bertulani, H. Rebel, Nucl. Phys. A458, 188 (1986).
[2] H. Esbensen, G. F. Bertsch, Nucl. Phys. A706, 383 (2002).
