P. E. Garrett

Search for nuclei exhibiting the U(5) dynamical symmetry

Abstract The level energy spectra of a set of 26 nuclei presenting a vibrational structure were analyzed using a simple U(5) prescription in the IBM-1 framework. Twelve of these nuclei are relatively well fitted and present level patterns in accordance to the model predictions. They are thus confirmed or possible U(5) nuclei. The analysis suggests that they present extensive multiphonon structures. This implies the survival of such structures at higher energies. The experimental B(E2) values between multiphonon states are shown to agree with the finite N dependence predicted by the IBM. It is also observed that the 21+ energies are quite generally about 15% lower than expected from the global nuclear properties.

TIGRESS: TRIUMF-ISAC gamma-ray escape-suppressed spectrometer

The TRIUMF-ISAC gamma-ray escape-suppressed spectrometer (TIGRESS) is a new γ-ray detector array being developed for use at TRIUMFs Isotope Separator and Accelerator (ISAC) radioactive ion beam facility. TIGRESS will comprise 12 32-fold segmented clover-type HPGe detectors coupled with 20-fold segmented modular Compton suppression shields and custom digital signal processing electronics. This paper provides an overview of the TIGRESS project and progress in its development to date.

On the robustness of surface vibrational modes: case studies in the Cd region

Recent studies of the stable Cd isotopes, using inelastic neutron scattering, radioactive decay and nucleon-transfer reactions, have resulted in the need to fundamentally reassess the concept of low-energy vibrational modes in these isotopes. We present a synopsis of the nuclear physics that has led to this situation, followed by the present status of spectroscopy for and against vibrational modes in nuclei neighbouring the Cd isotopes. We continue with some details of the spectroscopic techniques used and examples of the data obtained which lead to the above statements. We provide further perspective with some details of octupole degrees of freedom. We conclude with an outlook of where spectroscopic data are needed to further assess the presence or absence of low-energy vibrations in nuclei; and we provide some reflections on where future theoretical investigations should be directed.

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.

The nature of 0+ excitations in 166Er

Abstract Lifetimes of excited 0 + states in 166 Er have been measured with the (n,n′γ) reaction. The first and second excited 0 + states have no significant collective enhancements of their decays, and their properties are suggestive of pair-type excitations rather than those of a β vibration or phonon excitation built on the γ vibration. The third excited 0 + state at 1934 keV has an enhanced decay to the ground state band with B ( E 2; 0 4 + → 2 gsb + ) = 8.8 ± 0.9 W.u., consistent with that expected for a β vibration.

Study of 124Te by the 122Sn(α,2nγ) reaction and by the decay of 124I

Abstract In-beam γ-ray spectroscopy and the decay of 124 I were used to study the nuclear structure of 124 Te. On the basis of γγ-coincidences, a level scheme was constructed with over 100 levels and 170 transitions. The use of excitation functions and angular distributions for the (α, 2n) reaction, and the log( ft ) values for the decay, allowed spin assignments for a large number of the levels to be made. The resulting level scheme was interpreted in the framework of the interacting boson model and the particle-core coupling model.

On the nature of “three-phonon” excitations in 112Cd☆

Abstract The lifetimes of proposed 3-phonon states in 112 Cd have been measured using the Doppler shift attenuation method (DSAM) following the (n,n′γ) reaction. The experimental results allow the determination of relevant B ( E 2) values which are compared to different theoretical descriptions. Collective enhancements favouring the interpretation of collectivity in the phonon model are observed for 3 + and 4 + members of the 3-phonon quintuplet. However, for the proposed 2 + member, this simple model fails most likely due to the subtle interplay between intruder, mixed-symmetry, and phonon states, and the decay pattern is strongly perturbed. The best overall agreement is obtained with IBM-2 calculations.

High-resolution γ-ray spectroscopy: a versatile tool for nuclear β-decay studies at TRIUMF-ISAC

High-resolution γ-ray spectroscopy is essential to fully exploit the unique, high-quality beams available at the next generation of radioactive ion beam facilities such as the TRIUMF isotope separator and accelerator (ISAC). The 8π spectrometer, which consists of 20 Compton-suppressed HPGe detectors, has recently been reconfigured for a vigorous research programme in weak interaction and nuclear structure physics. With the addition of a variety of ancillary detectors it has become the worlds most powerful device dedicated to β-decay studies. This paper provides a brief overview of the apparatus and highlights from recent experiments.

Nuclear structure of 166Tm from (α,3n) γ-ray and conversion electron measurements

Abstract Excited levels of the strongly-deformed odd-odd nucleus 166 Tm, populated by the 165 Ho(α,3nγ) reaction, were investigated using standard in-beam γ-ray spectroscopy, including measurements of a few conversion electron lines, and by high-resolution curved-crystal spectroscopy. A level scheme consisting of 178 transitions among 6 well-developed and 4 partial rotational bands is proposed. Level spins and parities were determined and the results were interpreted in the framework of the Nilsson model by comparison with predictions. The Gallagher-Moszkowski splitting for the { π 1 2 + [411] ± ν 5 2 + [642] } configuration was determined to be 245(20) keV. An anomalous signature splitting was observed for the {π 7 2 + [404] + ν 5 2 + [642]}K π = 6 + configuration.

Nuclear structure of 192 Ir studied with direct transfer reactions

The nuclear structure of 192Ir was investigated via the 193Ir(d, t) and 191Ir(d, p) reactions, using beams of 18 and 16 MeV deuterons, respectively. The reaction products were analyzed with a magnetic spectrograph, and detected with photographic plates. The energy resolutions (FWHM) of the detected particles were typically ≅ 5.7 keV for the (d, t) and ≅ 8.9 keV for the (d, p) reactions. Spectra were recorded at a large number of angles (⩾ 15). The angular distributions of cross sections were fitted with theoretical distributions from DWBA calculations, and the spectroscopic strengths were obtained. For the (d, t) reaction, the fits to the angular distributions were performed in many cases with the sum of two l-values. Level energies, parities, spectroscopic strengths, and possible spin values were determined up to approximately 1 MeV excitation energy. Spectra were also recorded at a few select angles for the 193Ir(3He, α)192Ir and 195Pt(p, α)192Ir reactions using beams of 25.5 MeV 3He and 18 MeV protons, respectively. In order to obtain more information regarding the spins of levels in 192Ir, multidimensional scaling programs were applied to known γ-ray intensities. The method appeared to be successful, and spins were suggested for many low-lying levels. The experimental results were compared with the IBFFM and the Nilsson model. It was found that the models can only approximately reproduce the structure of 192Ir.