Daeg S. Brenner

Retardation of B(E2; 01+ → 21+) rates in 90-96Sr and strong subshell closure effects in the A ~ 100 region

Abstract Lifetimes of low-lying levels in 90, 92, 94, 96Sr were measured using a recently developed βγγ fast timing method. The deduced B(E2; 01+ → 21+) rates which fill the N = 52–58 gap in the known B(E2) strengths for 78–100Sr, are exceptionally low, ≈8 W.u., and provide evidence for a strong Z = 38 subshell effect extending from N = 50 to N = 58. Furthermore, these rates establish a close similarity between 88–96Sr and 90–98Zr nuclei, which form a region of the lowest B(E2) values (for nuclei with A > 56) second only to 204–210Pb. The SrB(E2) values are compared with predictions based on various global, regional and local systematics as well as the results of laser spectroscopic measurements. The discrepancy with the latter is explained as due to octupole vibrational collectivity. Unique to the Q0 systematics in the Sr region is a sudden shape change from spherical to deformed at N = 60. This transformation is so remarkably large and abrupt for Sr nuclei that it suggests a “phase change” in Sr collectivity.

A Four-detector System For γ - γ Angular Correlation Studies

Abstract A multiple detector system for γ - γ coincidence and angular correlation measurements is described. The system consists of four large coaxial Ge detectors set at fixed positions, enabling the measurement of γ - γ coincidences between any pair. Thus, six distinct angles are measured simultaneously. The performance of the system was studied with a 152 Eu source. An application for γ - γ correlation studies in 146 Ce is described. Systematic errors, if present, are shown to be less than 0.03 and 0.05 for A 2 and A 4 , respectively. The extension of this system to more than four detectors is straightforward.

The level scheme of 183Re and systematics of odd-A re isotopes: Interplay of ε2 and ε4 deformations in the W-Re region

Abstract The levels of 183Re populated in the decay of 13 h 183gOs and 9.9 h 183mOs have been studied using singles Ge(Li) detectors, a Compton-suppression spectrometer and a Ge(Li) γ-coincidence spectrometer. A detailed decay scheme has been developed which allows previously proposed Nilsson band assignments to be confirmed, and disagreements concerning assignments for the highly decoupled 1 2 [541] band to be resolved. New Nilsson assignments are proposed as follows: 5 2 + 5 2 [642], 1040.7 keV ; 1 2 − 1 2 [541], 0 + , 1414.7 keV ; 3 2 − 1 2 [541], 0 + , 1563.0 keV ; 9 2 − 9 2 [514], 0 + , 1711.7 keV . It is shown that the systematic behavior of the proton orbits throughout the odd-A Re isotopes can be understood in terms of the relative changes in e2 and e4 deformations, deduced from earlier studies of odd neutron nuclei in this region. However, while this analysis confirms the proposed peaking of the e4 deformation at N = 108, a determination of the e4 value appropriate to 183Re indicates that the absolute magnitudes of the hexadecapole deformations for odd-proton nuclei in this region are about a factor of two smaller than previously proposed for odd-neutron nuclei.

Simplified shell model calculations for valence mirror nuclei

Abstract By exploiting the valence mirror nucleus concept for the singly magic N = 82 and Sn partners, simplified shell model calculations are carried out. Special attention is paid to multiplets of negative-parity (primarily seniority ν = 2) states. The remarkable empirical similarity of corresponding nuclei in these two regions, as well as certain critical differences as a function of valence nucleon number, result from a combination of the valence mirror aspect, the single-particle energies, the effects of pn residual interactions and the Pauli Principle.

Are There X(5) Nuclei In The A∼80 and A∼100 Regions?

Iachello has introduced a new class of symmetries based on solutions to differential equations to model phase transition and critical point behavior in nuclei. For the shape transition region between a spherical vibrator and an axial rotor the dynamical symmetry is denoted X(5). A search of existing data for the A∼80 and A∼100 regions provides tantalizing hints of nuclei with X(5) character in both regions.

Measurements of Se-70, Se-71, Br-72, and Br-73

The path and termination point for the rapid proton-capture (rp) process above {sup 56}Ni is uncertain due to a lack of knowledge of nuclear properties, especially masses, near the proton drip line. To address this need we have begun a program to measure masses along the rp-process path in the A{approx}60--80 region using {beta}-{gamma} coincidence spectroscopy. Q{sub EC} values derived from {beta}{sup +} spectrum end points were used to calculate mass excess values for {sup 70}Se, {sup 71}Se, {sup 72}Br, and {sup 73}Br. The results are compared to predictions of mass models and to estimates based on systematic trends in this region.

Mass Measurements at the Wright Nuclear Structure Laboratory

A program to measure masses along the astrophysical rp-process path in the A ~ 60–80 region is underway at the Wright Nuclear Structure Laboratory, Yale University. The classic technique of end-point determinations for β + spectra measured in coincidence with daughter γ-rays is used to determine Q EC which, in turn, is used to calculate the mass. Several innovations have been incorporated to increase the sensitivity and selectivity of the method. Results of recent experiments are reported.

Half-life for the rp-Process Waiting Point Nuclide 80Zr Using Delayed Gamma Tagging

A 4.1(5) s half-life has been measured for 80Zr at the Holifield Heavy Ion Research Facility of Oak Ridge National Laboratory. Using the inverse fusion reaction 24Mg(68Ni,2n)80Zr, nuclides of interest were produced and separated by the Recoil Mass Spectrometer prior to implantation onto the tape of a Moving Tape Collector. The 80Zr half-life was then determined by observing delayed gamma rays depopulating the 4 μs isomer at 312 keV in the daughter 80Y, a technique referred to as Delayed Gamma Tagging, or DGT.

Mass measurements along the rp-process path

The path and termination point for the t-p-process above {sup 56}Ni is uncertain due to a lack of knowledge of nuclear properties, especially masses, near the proton drip line. To address this need we have begun a program to measure masses of nuclei along the r-p-process pathway in the A{approximately}60-80 region using {beta}-{gamma} coincidence spectroscopy. Results for {sup 71}Se are presented and a preliminary experiment for {sup 72}Br is described.

E2 and M1 Strengths and Strong Sub‐Shell Closure Effects in Neutron‐Rich A∼100 Nuclei

E2 strengths of several A∼100 nuclei were deduced from ps level‐lifetime measurements at the fission‐product separator TRISTAN. The exceptionally low B(E2) values for 90,92,94,96Sr reveal a close similarity between spherical Sr and Zr nuclei. For Sr and Y nuclei with N≥60, B(M1) and B(E2) values indicate that the deformation saturates just at its onset. A dramatic change in the Sr collectivity occurs at N=60, where the B(E2) strength abruptly increases by a factor of ∼15, suggesting a ‘‘phase change’’ in the collectivity.