Detailed nuclear structure studies far from stability

State-of-the-art spectroscopy of nuclei far from stability has achieved an extraordinary level of sophistication and detail in the last ten years. In principle, if a state can be populated, it can be characterized by its energy, spin, parity, and major decay paths. Sometimes its lifetime can be measured. In practice, one is confronted with enormous complexity. To convert raw spectroscopic data into nuclear structure data involves a complex process of disentangling gamma rays and conversion electrons into decay schemes. Specifically, coincidence techniques, especially coincidence intensities, play a crucial role in this process. Recent examples and methods from work done at UNISOR are presented.     

On-line nuclear orientation study of184Au

The¹⁸⁴Au→¹⁸⁴Pt decay, studied on-line with the UNISOR facility at HHIRF, is discussed. Gamma-ray and conversion-electron spectroscopy of¹⁸⁴Pt as well as on-line nuclear orientation measurements of¹⁸⁴Au were done. A new low-lying level scheme of¹⁸⁴Pt is proposed. Two coexisting bands with different deformations and their respective γ-vibrational bands are established. Internal conversion coefficients for interband transitions between states with the same spin are extracted from the spectroscopy measurements. The relative E0 contents of the transitions are determined by combining internal conversion coefficients with E2/M1 mixing ratios deduced from gamma-ray anisotropies measured from oriented nuclei.     

Ground state shaoe and crossing of near spherical and deformed bands in 182Hg

The energy levels of ¹⁸²Hg have been identified for the first time through comparison of in-beam studies of the reactions ^156,154Gd(³²S, 4n)^184,182Hg and γ-X-ray coincidences. Levels up to 12⁺ in ¹⁸²Hg were established from γ-γ coincidences and singles measurements. The data establish that the ground state shape is near spherical, and that the ground band is crossed by a well-deformed band at 4⁺. In contrast to IBA model predictions that the deformed band will rise in energy in ¹⁸²Hg compared to ¹⁸⁴Hg, the energies of the deformed levels in ¹⁸²Hg continue to drop.     

Fine structure in proton emission from 145Tm discovered with digital signal processing

Fine structure in proton emission from the 3.1(3) mus activity of 145Tm was discovered by using a novel technique of digital processing of overlapping recoil implantation and decay signals. Proton transitions to the ground state of 144Er and to its first excited 2(+) state at 0.33(1) MeV with a branching ratio I(p)(2(+))=9.6+/-1.5% were observed. The structure of the 145Tm wave function and the emission process were analyzed by using particle-core vibration coupling models.     

Shape coexistence and intruder states: theZ=82 region

The phenomena associated with shape coexistence and intruder states are illustrated with examples from the Pt, Au, Hg, Tλ, Pb and Bi isotopes which lie far from beta stability midway between closed neutron shells 82 and 126. The type of spectroscopic measurements which have already been developed to investigate these structures as well as those types of measurements needed to extend and quantify the analysis, such as on-line nuclear orientation, are discussed.     

Precision branching ratio measurement for the superallowed beta+ emitter 62Ga and isospin-symmetry-breaking corrections in A>or=62 nuclei

A high-precision branching ratio measurement for the superallowed beta+ decay of 62Ga was performed at the Isotope Separator and Accelerator radioactive ion beam facility. Nineteen gamma rays emitted following beta+ decay of 62Ga were identified, establishing the dominant superallowed branching ratio to be (99.861+/-0.011)%. Combined with recent half-life and Q-value measurements, this branching ratio yields a superallowed ft value of 3075.6+/-1.4 s for 62Ga decay. These results demonstrate the feasibility of high-precision superallowed branching ratio measurements in the A>or=62 mass region and provide the first stringent tests of the large isospin-symmetry-breaking effects predicted for these decays.     

Very converted transitions and particle-core coupling in neutron-deficient odd-mass au isotopes

Levels in^{185, 187}Au have been studied by radioactive decay of isotopically separated^{185, 187}Hg. A number of low-energy very-converted transitions are observed in the decay. It is shown that the enhanced conversion in these cases comes from an E0 component rather than from an anomaly in theM1 conversion process. A systematic pattern of bands interconnected with these very converted transitions is presented as evidence for a new type of particle-core coupling in^{185, 187}Au.     

The α -decay properties of 186Bi

The α decay of ¹⁸⁶Bi was investigated via the ⁹⁷Mo(⁹²Mo, p2n) reaction and the use of a fragment mass analyzer with a double-sided Si strip detector. In contrast to an earlier work wherein only one α transition was reported, we observed two transitions arising from two isomers in ¹⁸⁶Bi with energies and half-lives of: 7158(20) keV, T_1/2 = 15.0(17) ms; and 7261(20) keV, T_1/2 = 9.8(13) ms.     

The strength of electric monopole transitions and the decay out of superdeformed bands

The strength of electric monopole (E0) transitions is related to differences in mean-square charge radii and differences in quadrupole deformation in nuclei. Estimates are made of E0 decay versus M1, E1, and E2 decay out of superdeformed bands. It is suggested that E0 decay may dominate, providing a (partial) answer to the superdeformed band “drainingr problem.     

Investigations of very neutron-deficient isotopes below100Sn in40Ca-induced reactions

Very neutron-deficient isotopes of rhodium, palladium, silver, cadmium and indium in theN=50 region were studied at the GSI on-line mass separator following bombardment of^{58, 60, 62}Ni and^{63, 65}Cu targets with 4.0 MeV/u ions of⁴⁰Ca. The known properties of the β-delayed proton precursor^95m Pd were confirmed, but a search for direct proton decay of^95m Ag was not successful. Beta-delayed proton activities observed at mass-numbers 94, 96, 97 and 100 were assigned to the precursors⁹⁴Rh(66±14 s),⁹⁶Ag(5.1±0.4s),⁹⁷Cd(3 _?2 ⁺⁴ s) and¹⁰⁰In. In addition,⁹⁴Pd(9.0±0.5 s) was identified from β-γ-X studies. Individual decay schemes of^95m96Pd and^{96, 98}Ag are reported.