2. Recent HRIBF Research - Dynamic
Polarization in the Two-Body Breakup of 17F
(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 short-lived radioactive nuclei . It is important to understand the breakup processes in order to correctly extract relevant information. For loosely bound proton-rich 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, Z3T. 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 .
We have measured the breakup of 17F by bombarding 58Ni and 208Pb targets. The objective is to compare the breakup of 17F into oxygen and proton with respect to first-order-perturbation theory for the two targets. The radioactive 17F 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 large-area Si-strip detectors. The front and middle detectors form an ΔE-E 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 2-1 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 17F on 58Ni reaction is reproduced by first-order perturbation theory with E1 and E2 excitations included. For the 208Pb target, the angular distribution of oxygen and proton in coincidence shows a larger reduction of breakup probability with respect to first-order-perturbation theory. Comparisons with dynamic calculations that take into account the Z3T correction to study the dynamical polarization in the breakup of 17F are underway.
Figure 2-1: Angular distributions of the oxygen ions in coincidence with protons from the breakup of 17F by bombarding 58Ni (left panel) and 208Pb (right panel) at the energy of 10 MeV per nucleon. The experimental data are closed circles and the solid curves are predictions by first-order perturbation theory including E1 and E2 excitations.
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