R. A. E. Austin

Radioactive beam experiments with large gamma-ray detector arrays

High-resolution γ-ray spectroscopy is one of the most powerful and versatile experimental techniques in low-energy nuclear physics research. With the continuing development of hyper-pure germanium (HPGe) detector technology, including multi-crystal detectors, contact segmentation, and digital signal processing techniques, large γ-ray detector arrays will continue to play a major role in the experimental programs at existing and future radioactive ion beam facilities. This paper provides an overview of recent progress in, and future plans for, the development of large γ-ray spectrometers at such facilities, including the recent commissioning of the 8π spectrometer at ISAC-I and the proposed TRIUMF-ISAC gamma-ray escape suppressed spectrometer array for the ISAC-II facility.

Halo neutrons and the {beta} decay of {sup 11}Li

The {beta} decay of {sup 11}Li has been investigated at TRIUMF-ISAC using a high-efficiency array of Compton suppressed HPGe detectors. From a line-shape analysis of the Doppler-broadened peaks observed in the {sup 10}Be {gamma} spectrum, both the half-lives of states in {sup 10}Be and the energies of the {beta}-delayed neutrons feeding those states were obtained. Furthermore, it was possible to determine the excitation energies of the parent states in {sup 11}Be with uncertainties comparable to those obtained from neutron spectroscopy experiments. These data suggest that the {beta} decay to the 8.81 MeV state in {sup 11}Be occurs in the {sup 9}Li core and that one neutron comprising the halo of {sup 11}Li survives in a halolike configuration after the {beta}-delayed neutron emission from this level.

Evidence for shape coexistence at medium spin in 76Rb

Four previously known rotational bands in Rb-76 have been extended to moderate spins using the Gammasphere and Microball gamma ray and charged particle detector arrays and the Ca-40(Ca-40,3pn) reaction at a beam energy of 165 MeV. The properties of two of the negative-parity bands can only readily be interpreted in terms of the highly successful Cranked Nilsson-Strutinsky model calculations if they have the same configuration in terms of the number of g(9/2) particles, but they result from different nuclear shapes (one near-oblate and the other near-prolate). These data appear to constitute a unique example of shape coexisting structures at medium spins

DETERMINATION OF OPTIMUM GATING FOLD, SHAPE AND WIDTH FOR ANALYSIS OF HIGH-FOLD GAMMA-RAY COINCIDENCE DATA

Abstract An investigation to find the optimum gate width and shape for the analysis of high-fold gamma-ray coincidence data has been undertaken. Results are presented which determine the effectiveness of using ellipsoidal rather than cuboidal gates. We also demonstrate that only when optimum gating fold, shapes and widths are adopted, is it possible to utilize the full sensitivity of the new generation of gamma-ray spectrometers such as Gammasphere and Euroball.

Halo neutrons and the β-decay of 11Li

The β-decay of 11Li has been studied at ISAC/TRIUMF using the 8pi spectrometer, an array of 20 Compton-suppressed high-purity germanium detectors. Most of the 11Li β-decay strength is observed to proceed through unbound states in 11Be, which subsequently decay by one-neutron emission to 10Be. This results in the observation of a γ-spectrum dominated by the decay of the excited states in 10Be. These transitions exhibit characteristic Doppler broadened lineshapes, due to the the recoiling effect induced by the neutron emission. A Monte-Carlo simulation was developed to analyze the complex shape of these γ-lines. Both the half-lives of states in 10Be and the energies of the β-delayed neutrons feeding those states were obtained. It was also possible to determine the excitation energies of the parent states in 11Be. The present contribution was the subject of a publication in a scientific journal (F. Sarazin et al., Phys. Rev. C 70, 031302(R)(2004) )shortly before the conference. It was judged not appropriate to submit for peerreviewing a contribution with nearly the same content. The reader is therefore invited to read the original publication.

High-Precision Half-Life Measurement for the Superallowed {beta}{sup +} Emitter {sup 26}Al{sup m}

A high-precision half-life measurement for the superallowed β+ emitter 26Al(m) was performed at the TRIUMF-ISAC radioactive ion beam facility yielding T 1/2 6346.54 ± 0.46(stat) ± 0.60 (syst) ms, consistent with, but 2.5 times more precise than, the previous world average. The 26Al(m) half-life and ft value, 3037.53(61) s, are now the most precisely determined for any superallowed β decay. Combined with recent theoretical corrections for isospin-symmetry-breaking and radiative effects, the corrected Ft value for (26)Al(m), 3073.0(12) s, sets a new benchmark for the high-precision superallowed Fermi β-decay studies used to test the conserved vector current hypothesis and determine the V(ud) element of the Cabibbo-Kobayashi-Maskawa quark mixing matrix.

Internal {gamma} Decay and the Superallowed Branching Ratio for the {beta}{sup +} Emitter {sup 38}K{sup m}

The branching ratio for the superallowed beta(+) decay of (38)K(m) was measured at TRIUMFs ISAC radioactive ion beam facility. The M3 internal transition between the isomer and the ground state of (38)K(m) was observed with a branching ratio of 330(43) ppm. A search for the nonanalogue beta-decay branch to the first excited 0(+) state in (38)Ar was also performed and yielded an upper limit of < or =12 ppm at 90% C.L. These measurements lead to a revised superallowed branching ratio for (38)K(m) of 99.967(4)%, and increase the (38)K(m) ft value by its entire quoted uncertainty to ft=3052.1(10) s. Implications for tests of the nuclear-structure dependent corrections in superallowed beta decays and the extraction of the Cabibbo-Kobayashi-Maskawa matrix element V(ud) are discussed.

High-precision half-life determination for the superallowed {beta}{sup +} emitter {sup 62}Ga

The half-life of the superallowed {beta}{sup +} emitter {sup 62}Ga has been measured at TRIUMFs Isotope Separator and Accelerator facility using a fast-tape-transport system and 4{pi} continuous-flow gas proportional counter to detect the positrons from the decay of {sup 62}Ga to the daughter {sup 62}Zn. The result, T{sub 1/2}=116.100{+-}0.025 ms, represents the most precise measurement to date (0.022%) for any superallowed {beta}-decay half-life. When combined with six previous measurements of the {sup 62}Ga half-life, a new world average of T{sub 1/2}=116.121{+-}0.021 ms is obtained. This new half-life measurement results in a 20% improvement in the precision of the {sup 62}Ga superallowed ft value while reducing its mean by 0.9{sigma} to ft=3074.3(12) s. The impact of this half-life measurement on precision tests of the CVC hypothesis and isospin symmetry breaking corrections for A{>=}62 superallowed decays is discussed.

{beta} decay of {sup 32}Na

The {beta}-decay of {sup 32}Na has been studied using {beta}-{gamma} coincidences. New transitions and levels are tentatively placed in the level scheme of {sup 32}Mg from an analysis of {gamma}-{gamma} and {beta}-{gamma}-{gamma} coincidences. The observation of the indirect feeding of the 2321 keV state in {sup 32}Mg removes some restrictions previously placed on the spin assignment for this state. No evidence of a state at 2117 keV in {sup 32}Mg is found. Previously unobserved weak transitions up to 5.4 MeV were recorded but could not be placed in the decay scheme of {sup 32}Na.

Search for the low-lying ({pi}1g{sub (9/2)}){sup 4} 6{sub 2}{sup +} state in {sup 94}Ru

Excitations in {sup 94}Ru were populated through the {beta}{sup +}/{epsilon} decay of {sup 94}Rh following the fusion evaporation reaction {sup 58}Ni({sup 40}Ca,3pn){sup 94}Rh. Recoiling nuclei were implanted on the Yale moving tape collector at the Wright Nuclear Structure Laboratory, and delayed {gamma}-rays were observed via an array of four Compton suppressed HPGe clover detectors. Nine new {gamma}-transitions and five new levels were added to the level scheme of ({pi}1g{sub (9/2)}){sub J{sup {pi}}=6{sup +}}{sup 4} level, in close agreement with prediction.