G. G. Colvin

M1 transitions from the gamma band and F-spin mixing in 168Er

Abstract The M1 strength in a number of γ → g transitions in 168Er have been determined from high-precision L-subshell conversion electron intensity measurements. The results are combined with earlier data for transitions within the γ-band and compared with the predictions of an IBA-2 calculation. It is shown that a satisfactory description of both E2 and M1 properties can be obtained with the appropriate degree of F-spin symmetry breaking in the hamiltonian, which in turn relates to the degree of neutron-proton symmetry in the collective wavefunction.

Level structure of 191Os: Coexisting oblate and prolate configurations

Abstract An extensive level scheme for the transitional nucleus 191 Os has been constructed on the basis of (n, γ), (n, e − ) and (d, t) reaction data. γ-rays following thermal capture and average resonance capture (ARC) were detected with pair spectrometers. Secondary γ-ray spectra were obtained with the GAMS bent crystal spectrometers. The high precision of the resultant energies was the critical factor in developing a level scheme on the basis of Ritz combinations. γγ coincidences were obtained from both primary and secondary transitions. Conversion electrons, detected with the BILL spectrometer, gave multipolarity information. The (d, t) reaction measured with a Q3D magnetic spectrometer gave additional information on the hole states in 191 Os. A discussion of the resulting level scheme briefly reviews the fragmentation of negative-parity strength but focuses mostly on a rare example of an anti-aligned decoupled set of coexisting positive-parity states. For these, triaxial particle-plus-rotor calculations are carried out. Reasonable agreement is obtained and the calculations also explain the unusually strong population in (n, γ) of this set of mostly medium spin states.

Neutron capture studies of 189Os

Abstract An extensive study of the level structure of 189 Os has been carried out using the (n, γ) and (n, e − ) reactions. The use of the average resonance capture technique ensures that the complete set of 1 2 − , 3 2 − states has been established up to 1500 keV in excitation energy and secondary γ-rays have been measured in singles and coincidence to build up the detailed level scheme. The results are compared with existing Nilsson model calculations and with the results of algebraic models. None of the approaches are able to provide a satisfactory description of the negative-parity states.

E0 transitions in 106Pd studied by neutron capture

The reactions 105Pd(n, gamma ) and 105Pd(n, e-) have been used to study E0 transitions in the nucleus 106Pd. From these results, E0 strengths and X(E0/E2) ratios have been determined. In general, 106Pd is well described by the IBA-2 model for both excitation energies and electromagnetic transition properties. A detailed comparison of these results with the calculations shows good agreement for transitions between the first five 0+ states.

Low-lying positive parity states in 195Ir following double neutron capture

Abstract Multiple neutron capture reactions have been studied on 193 Ir targets and gamma and electron spectroscopy performed. Double neutron capture events were separated from all others and a level scheme of 195 Ir deduced. Gamma-ray energies and multipolarities are tabulated. A neutron capture cross-section for the reaction 194 Ir ( n , γ) of 1500±290 b has been determined and the neutron binding energy of 195 Ir measured to be 7231.92 (6) keV. The results are discussed in terms of perturbed spin(6) symmetry and U ( 6 4 ) supersymmetry. A σ σ max family of levels has clearly been identified.

The level structure of 76Se and 78Se and the systematics of selenium isotopes within the framework of the DDM

The possibility of shape coexistence within the selenium isotopes and the general features of their structures have been examined within the framework of the DDM, IBM-1 and IBM-2. New data on 76Se and 78Se were obtained from gamma -ray and IC electron transitions. Levels and transitions in 76Se were studied following the beta decay of oriented 76As which had been dissolved in iron and cooled to 10.4 mK. The measured anisotropies of gamma transitions provided the multipole mixing ratios of 18 transitions, and previously proposed substantial L=0 mixing in three first-forbidden beta transitions was confirmed. Gamma-ray directional correlations and the IC electron spectrum of transitions in 78Se were measured following thermal neutron capture by 77Se. The correlation experiments gave the spins of 18 levels and the multipole mixing ratios of the more intense gamma -ray transitions. The good resolution electron spectrum allowed E0 transitions to be identified and also allowed the evaluation of IC coefficients and of X(E0:E2) ratios for Delta I=0 transitions. The 2+ levels at 1.778 MeV (76Se) and 1.996 MeV (78Se) appear to have the features characteristic of the lowest mixed-symmetry state in a vibrational-like nucleus. The level energies, B(E2) ratios, multipole mixing ratios and X values are compared with the results obtained from the DDM and IBM. The comparisons between theoretical and experimental data are extended to even-even selenium isotopes in the range A=72 to 82.

Excited 0+ states and E0 transitions in 200Hg

Measurements of the internal conversion electron spectrum provide additional information about the nature of the excited 0+ states in 200Hg. The states were excited following thermal neutron capture and levels at 1029.4, 1515.0, 1856.8 and 2116.9 keV were identified as 0+. The last one is a new spin assignment and the authors find no evidence for a previously reported 0+ level at 2697.1 keV. Altogether 16 E0 transitions were observed. An IBA-2 analysis provides in general an adequate description of the 0+ levels and their E0 decay modes.

An unusualM1 electromagnetic transition connecting octupole and quadrupole vibrations in179Hf

A detailed level scheme for179Hf has been constructed on the basis of extensive (n, γ), (d, p) and (d, t) data. The low lying levels are grouped into 15 rotational bands of which 14 are classified by Nilsson quantum numbers or vibrational configurations. One of the most interesting results concerns a dominant transition matrix element which connects an octupole vibration built on the 1/2[510] Nilsson orbit with a quadrupole vibration built on the 9/2+ [624] Nilsson ground state. A simple interpretation of this unusual decay mode is offered in terms of microscopic wave functions for the vibrational states. This interpretation leads to reasonable estimates forE1 hindrance factors from the octupole vibration to lower lying quasi-particle levels.

The experimental structure of199Au and the interacting boson-fermion model

Gamma rays of199Au obtained after double neutron capture in197Au were measured at the ILL high flux reactor. A level scheme up to 1770 keV excitation energy is established. The result is compared with IBFM and Boson-Fermion-Symmetry calculations.

Spectroscopy of the ODD-ODD nucleus 76As and its supersymmetric description

Abstract Gamma rays and conversion electrons following thermal-neutron capture in 75 As have been measured using the Ge pair spectrometer PN4, the bent crystal spectrometers GAMS and the β -spectrometer BILL at the ILL. Coincident γ -rays were recorded at the CEN/SCK Mol, and average resonance capture at E n = 2 and 24 keV has been studied at the HFBR of BNL. The neutron binding energy in 76 As was determined to be 7328.498 ± 0.075 keV. The resulting level scheme for 76 As greatly improves the knowledge on this odd-odd nucleus. The low-lying positive-parity states are discussed in the U(5) limit of the U v (6/12)⊗U π (6/12) supersymmetry using the same hamiltonian to describe the quartet 76 Se- 77 Se- 75 As- 76 As having the same total number of bosons plus fermions.