R. Pfaff

Evolution of the giant dipole resonance in excited 120Sn and 208Pb nuclei populated by inelastic alpha scattering

Abstract The evolution of the giant dipole resonance (GDR) in 120 Sn and 208 Pb nuclei at excitation energies in the range of 30–130 MeV and 40–110 MeV, respectively, were studied by measuring high energy γ rays from the decay of the resonance. The excited states were populated by inelastic scattering of α particles at beam energies of 40 and 50 MeV/nucleon for 120 Sn and 40 MeV/nucleon for 208 Pb. A systematic increase of the resonance width with increasing excitation energy was observed for both nuclei. The observed width evolution was compared to calculations employing a model that adiabatically couples the collective excitation to the nuclear shape, and to a model based on the collisional damping of nucleons. The adiabatic coupling model described the width evolution in both nuclei well, whereas the collisional damping calculation could describe the width evolution only in 208 Pb. Light-particle inelastic scattering populates low angular momentum states in the target nucleus. The observed width increase is therefore interpreted to be predominantly due to fluctuations in the nuclear shape induced by temperature. This interpretation is consistent with the adiabatic model calculations and with recent angular momentum-gated measurements of the GDR in excited Sn isotopes.

Halflife measurements of the rp-process nuclei 61Ga, 63Ge, and 65As

Abstract Using the A1200 beam analysis device at the National Superconducting Cyclotron Laboratory, we have measured the halflives of several nuclei along the rp-process path near the proton-drip line. Halflife results for 61 Ga, 63 Ge, and 65 As (0.15±0.03 s, 95 −25 +23 ms, and 0.19 −0.07 +0.11 s, respectively) and their implications for the rp-process are presented.

Temperature dependence of the giant dipole resonance width in 208Pb

Abstract The evolution of the giant dipole resonance (GDR) in hot 208Pb nuclei was studied by measuring high energy γ-rays from the decay of the resonance built on excited states. Nuclei in the excitation energy range of 40–110 MeV were populated by inelastic scattering of 40 MeV/nucleon α-particles. The GDR width was observed to increase systematically from 4 MeV at the ground state to ∼8 MeV at the highest excitation energy.

Half-life measurements for Ga-61, Ge-63, and As-65 and their importance in the rp process

The [ital rp] process provides a mechanism whereby energy is generated and proton-rich nuclei are synthesized in various astrophysical environments. Understanding this process requires knowledge of the half-lives, binding energies, and reaction cross sections for the nuclei along its path, which for [ital A][approx gt]60 lies near the proton-drip line. Using the A1200 radioactive-beam facility at the National Superconducting Cyclotron Laboratory, we have performed an experiment to measure the [beta]-decay half-lives of several [ital T][sub [ital z]]=[minus]1/2 nuclei in this mass region by identification and implantation of projectile fragments followed by a rapid beam cutoff and observation of the [beta] decay. Using this technique, the half-lives of [sup 61]Ga, [sup 63]Ge, and [sup 65]As (0.15[plus minus]0.03 s, 95[sub [minus]20][sup +23] ms, and 0.19[sub [minus]0.07][sup +0.11] s, respectively) have been measured and are found to be consistent with expectations from [beta]-decay theory for decay between mirror states with [ital Q][sub [beta]][similar to]9 MeV. In addition, the proton-rich nucleus [sup 66]Se has been observed for the first time. The experimental method used for this measurement and the implications of these results for the [ital rp] process are discussed.

Studies of light nuclei beyond the particle driplines: the two-proton emitter 12O

Abstract The decay dynamics of the ground state two proton emitter 12O, which lies beyond the proton dripline, were investigated following the population of 12O via single-neutron stripping of an 13O projectile. The decay products, two protons and 10C, were detected in coincidence and correlations among these particles were studied. No evidence was seen for 2He emission in the decay. This appears consistent with direct breakup or possibly sequential decay through 11N.

Low-lying structure of Li10 in the reaction B11(7Li,8B)10Li

Momentum spectra have been measured for the reaction [sup 11]B([sup 7]Li,[sup 8]B)[sup 10]Li at 130 MeV and laboratory angles of 5[degree] and 3.5[degree]. There is strong evidence for the existence of a broad state in the unstable nucleus [sup 10]Li, corresponding to a single [ital p]-wave neutron resonance unbound to neutron decay by 538[plus minus]62 keV with wdith [Gamma][sub lab]=358[plus minus]23 keV. There is also weak evidence that the [sup 10]Li ground state may be either an [ital s]- or a [ital p]-wave resonance barely unbound to neutron decay with [ital S][sub [ital n]][ge][minus]100 keV and [Gamma][sub lab][lt]230 keV. The measured [ital Q] values for the reaction leading to these states are [minus]32 908[plus minus]62 keV and greater than [minus]32 471 keV, respectively. A comparison is made with previous measurements and theoretical predictions of the low-lying structure of the neutron-unstable nucleus [sup 10]Li.

Multistep scattering in the 160 MeV208Pb(α, α′) reaction

New experimental data has been obtained for the208Pb(α, α′) reaction induced by 160 MeV alpha particles, for inelastic scattering to forward angles. We use these data to investigate the applicability of the multistep scattering theory of Feshbach, Kerman, and Koonin for describing this reaction. The mechanism we study, following the work of Gadioli et al. [1], is of the incident alpha particle remaining intact throughout the scattering process, exciting nucleon particle-hole pairs through multistep process. We conclude that this mechanism, combined with compound nucleus decay at low emission energies, can account for much of the observed data. However, there are indications that other processes also contribute at energies above the compound nucleus emission regime, and we outline future theoretical analyses that are needed.

Low-lying structure of [sup 10]Li in the reaction [sup 11]B([sup 7]Li,[sup 8]B)[sup 10]Li

Momentum spectra have been measured for the reaction [sup 11]B([sup 7]Li,[sup 8]B)[sup 10]Li at 130 MeV and laboratory angles of 5[degree] and 3.5[degree]. There is strong evidence for the existence of a broad state in the unstable nucleus [sup 10]Li, corresponding to a single [ital p]-wave neutron resonance unbound to neutron decay by 538[plus minus]62 keV with wdith [Gamma][sub lab]=358[plus minus]23 keV. There is also weak evidence that the [sup 10]Li ground state may be either an [ital s]- or a [ital p]-wave resonance barely unbound to neutron decay with [ital S][sub [ital n]][ge][minus]100 keV and [Gamma][sub lab][lt]230 keV. The measured [ital Q] values for the reaction leading to these states are [minus]32 908[plus minus]62 keV and greater than [minus]32 471 keV, respectively. A comparison is made with previous measurements and theoretical predictions of the low-lying structure of the neutron-unstable nucleus [sup 10]Li.

Low-lying structure of Li-10 in the reaction B-11 (Li-7, B-8) Li-10

Momentum spectra have been measured for the reaction [sup 11]B([sup 7]Li,[sup 8]B)[sup 10]Li at 130 MeV and laboratory angles of 5[degree] and 3.5[degree]. There is strong evidence for the existence of a broad state in the unstable nucleus [sup 10]Li, corresponding to a single [ital p]-wave neutron resonance unbound to neutron decay by 538[plus minus]62 keV with wdith [Gamma][sub lab]=358[plus minus]23 keV. There is also weak evidence that the [sup 10]Li ground state may be either an [ital s]- or a [ital p]-wave resonance barely unbound to neutron decay with [ital S][sub [ital n]][ge][minus]100 keV and [Gamma][sub lab][lt]230 keV. The measured [ital Q] values for the reaction leading to these states are [minus]32 908[plus minus]62 keV and greater than [minus]32 471 keV, respectively. A comparison is made with previous measurements and theoretical predictions of the low-lying structure of the neutron-unstable nucleus [sup 10]Li.

Low-lying structure ofLi10in the reactionB11(7Li,8B)10Li

Momentum spectra have been measured for the reaction [sup 11]B([sup 7]Li,[sup 8]B)[sup 10]Li at 130 MeV and laboratory angles of 5[degree] and 3.5[degree]. There is strong evidence for the existence of a broad state in the unstable nucleus [sup 10]Li, corresponding to a single [ital p]-wave neutron resonance unbound to neutron decay by 538[plus minus]62 keV with wdith [Gamma][sub lab]=358[plus minus]23 keV. There is also weak evidence that the [sup 10]Li ground state may be either an [ital s]- or a [ital p]-wave resonance barely unbound to neutron decay with [ital S][sub [ital n]][ge][minus]100 keV and [Gamma][sub lab][lt]230 keV. The measured [ital Q] values for the reaction leading to these states are [minus]32 908[plus minus]62 keV and greater than [minus]32 471 keV, respectively. A comparison is made with previous measurements and theoretical predictions of the low-lying structure of the neutron-unstable nucleus [sup 10]Li.