G. M. Crawley

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.

Electron capture strength for Ni 60 , 62 and Ni 58 , 60 , 62 , 64 ( p , n ) Cu 58 , 60 , 62 , 64 reactions at 134.3 MeV

Background: The strength of electron capture for medium mass nuclei has a significant effect on the evolution of supernovae. There is insufficient knowledge of these strengths and very little data for important radioactive nuclei. Purpose: Determine whether it is feasible to obtain EC strength from studies of T{sub o}+1 excitations in (p, n) reactions, and whether this might yield information for radioactive nuclei. Methods: Cross sections for the {sup 58,60,62,64}Ni(p, n){sup 58,60,62,64}Cu reactions were measured over the angular range of 0.3 deg. to 11.6 deg. at 134.3 MeV using the IUCF neutron time-of-flight facility. Results: The T{sub o}+1 excitations in {sup 60,62}Ni were identified by comparison with inelastic proton scattering spectra, their B(GT) were extracted, and the corresponding electron capture rates in supernovae were calculated. Data from the TRIUMF (n, p) experiments at 198 MeV were reanalyzed; the electron capture rates for the reanalyzed data are in moderately good agreement with the higher resolution (p, n) results, but differ in detail. The possibility of future measurements with radioactive nuclei was considered. Conclusions: It may be possible to obtain low-lying electron capture strength for radioactive nuclei by studying (p, n) reactions in inverse kinematics.

Isovector giant resonances in 6He, 12B, 90Y, 120In, and 208Tl observed in the (7Li,7Be) charge-exchange reaction

Abstract The (7Li,7Be) and (7Li,7Beγ) reactions have been studied at bombarding energies of 50 A MeV on targets of 6Li, 12C, 90Zr, 120Sn, 208Pb, and polystyrene. The detection of Doppler-shifted γ-rays from excited 7Be permits the identification of the spinflip and non-spinflip characteristics. Transitions to over 20 discrete states and resonances have been observed. Angular distributions for 6Li and 12C are well described by microscopic one-step distorted-waves calculations with shell-model transition amplitudes. The transitions to the ground states of 6He and 12C exhibit significant Gamow-Teller strength, but ∼7% and ∼17%, respectively, of the calculated cross sections at θ ≈ 0° arise from the tensor interaction. A broad resonance at ∼5.6 MeV in the halo nucleus 6He with Jπ = (2+,1−,0+) seems to correspond to a structure predicted at this excitation energy containing soft modes of excitation with Jπ = 0+and/or 2+. A resonance in 12B at ∼4.5 MeV is described by a superposition of known 2− and 4− states, and another one at ∼7.5 MeV is dominated by a transition to a 2− state with weaker contributions from two 1− states (spin dipole and giant dipole). Four to six resonances each, mostly spin-dipole resonances, were identified in the final nuclei 90Y, 120In, and 208Tl. The non-resonant quasi-free charge-exchange reaction pbound(7Li,7Be)nfree was observed for all targets up to effective excitation energies of 50 MeV. The mechanism is related to that on free protons. The effective number of participating protons was determined.

Isovector giant resonances in 6He, 12B, 90B, 120In, and 208Tl

Abstract The (7Li,7Be) charge-exchange reaction has been studied at E( 7 Li)=350 MeV on targets of 6Li, 12C, 90Zr, 120Sn, and 208Pb. Coincidences with Doppler-shifted γ-rays from the de-excitation in flight of the first-excited state in 7Be were recorded to identify spinflip transitions. Numerous discrete states and giant resonances were observed. Angular distributions were obtained for 6Li and 12C targets. They are well described by microscopic finite-range distorted-waves calculations with theoretical shell-model transition amplitudes and a nucleon-nucleon interaction containing central and tensor components.

Spin-isospin multipole excitations by means of the (6Li,6He) reaction at 100 A.MeV

Abstract The charge exchange reaction (6Li,6 He) has been investigated around 0° at 100 A.MeV on 12 C, 13 C, 58 Ni, 90 Zr and 208Pb targets. Strong selectivity for L = 0, S = 1 states in 12N and 13N nuclei is observed. Due to the finite angular opening of the spectrometer (±1.3°) the known Gamow-Teller resonances (L = 0) in 58 Cu, 90 Nb and 208Bi appear in the residual energy spectra superimposed on much broader structures, located at higher excitation energies, most probably related to a strong excitation of the Spin Dipole (1 ω,L = 1), Spin Quadrupole (2 ω,L = 2) and Spin Isovector Monopole (2 ω,L = 2) resonances. An attempt has been made to reproduce the excitation energy spectra up to 40 MeV, assuming that only L = 0,1 and 2 spin-multipole resonances contribute to the (6Li,6He) cross section on 58 Ni, 90 Zr and 208Pb targets. A very good overall fit of the spectra is obtained with the excitation energy and width parameters very close to the original values. A general trend of the (6Li,6He) reaction at 100 A.MeV is that the GT state (L = 0) contains about 6% of the cross section observed in the investigated angular range (≈5–10 mb sr ), whereas the remaining 94% of the cross section is accounted for by the strong population of the (1 ω,L = 1 and (2 ω,L = 0,2) resonances.

Decay properties of high-lying single-particles modes

Abstract The neutron decay of high-lying single-particle states in 64 Ni, 90 Zr, 120 Sn and 208 Pb excited by means of the (α, 3 He) reaction has been investigated at 120 MeV incident energy using the multidetector EDEN. The characteristics of this reaction are studied using inclusive spectra and angular correlation analysis. The structure located between 11 and 15 MeV in 91 Zr, and between 8 and 12 MeV excitation energy in 209 Pb display large departures from a pure statistical decay. The corresponding non-statistical branching ratios are compared with the results of two theoretical calculations.

Half-life measurements for [sup 61]Ga, [sup 63]Ge, and [sup 65]As and their importance in the [ital 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.

Half-life measurements forGa61,Ge63, andAs65and their importance in therpprocess

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.