J. A. Cizewski

GAMMASPHERE + FMA: A journey beyond the proton drip-line

The majority of experiments performed during the 2-year long stay of GAMMAS-PHERE at the Argonne National Laboratory aimed to study proton-rich nuclei far from the line of stability at and beyond the proton drip-line. A high reaction channel selectivity was required to assign in-beam {gamma}-ray transitions to weakly populated exotic nuclei in the presence of background from strong reaction channels. In many of the experiments this was achieved by using the Argonne fragment mass analyzer to separate heavy-ion fusion-evaporation reaction products from scattered beam and disperse them according to their mass-over-charge-state ratio. For medium mass and heavy a and proton emitters the Recoil-Decay Tagging method was implemented. In-beam {gamma}-ray transitions were observed in several proton emitters between Z=50 and Z=82. Among others, rotational bands were assigned to {sup 141}Ho and {sup 131}Eu. A quadruple deformation of {beta}=0.25(4) was deduced for the ground state in {sup 141}Ho from the extracted dynamic moment of inertia. Based on observed band crossings and signature splittings the 7/2{sup {minus}} [523] and 1/2{sup +}[411] configurations were proposed for the ground state and the isomeric state, respectively. Comparison with particle-rotor calculations indicates, however, that {sup 141}Ho may have significant hexadecapole deformation and could be triaxial.

Level scheme of 194Ir

Abstract Levels of 194 Ir were studied using neutron capture and ( d , p ) reaction spectroscopy. A pair spectrometer was used to measure the high-energy γ-ray spectrum from thermal-neutron capture in an enriched 193 Ir target over the energy range 4640–6100 keV. From the same reaction, low-energy γ-radiation was studied using curved-crystal spectrometers, and conversion electrons were observed with magnetic spectrometers. Prompt and delayed γγ-coincidences were measured using semiconductor and scintillation detectors. Averaged resonance capture measurements were performed with 2 keV and 24 keV neutrons for primary transitions leading to excitation energies from 0 to 580 keV. Using 22 MeV deuterons, the 193 Ir( d , p ) high resolution spectra were observed with a magnetic spectrograph. The deduced nuclear level scheme of 194 Ir includes 38 levels connected by 184 transitions. Unambiguous spins and parities were determined for 25 levels. The rotor-plus-particle model was used for the interpretation of the level scheme assuming a strong mixing for Nilsson configurations having identical parities and K quantum numbers. IBFFM model calculations were performed and the obtained results were compared with the experimental level scheme.

Limits of the energy-spin phase space beyond the proton drip line: entry distributions of Pt and Au isobars

Abstract Entry distributions in angular momentum and excitation energy have been measured for several very proton-rich isotopes of Pt and Au. This is the first systematic study of the energy-spin phase space for nuclei near and beyond the proton drip line. Comparisons are made between the distributions associated with proton-unbound Au nuclei and more stable Pt isobars. In 173 Au the first evidence is seen for the limits of excitation energy and angular momentum which a nucleus beyond the proton drip line can sustain.

High-spin states in

The 16O+58Ni reaction was used to study yrast and non-yrast excitations in 71As, 72Se, and 72Br. High-spin yrast and negative-parity non-yrast bands were observed in 72Se. The f7/2 proton extruder orbital was identified in 71As. The odd-even staggering in the πg9/2νg9/2 decoupled band in 72Br is compared with similar structures in heavier Br isotopes.

Neutron capture on 193Ir and σ<σ max states in 195Ir

Multiple neutron-capture measurements on 193Ir have populated numerous low-spin positive-parity states in 195Ir. These states form a complete set of 1/2, 3/2, 5/2 states with sigma =13/2, 11/2 and tau =1/2, 3/2, 5/2 expected in a U(6/4) contains/implies Spin(6) boson-fermion symmetry.