P Hungerford

The neutron-rich barium isotopes

The nuclei 142Ba, 144Ba and 146Ba (Z=56) have been studied using the technique of gamma-gamma angular correlations. The activities were produced using the on-line fission product mass separator OSTIS at the Institut Laue-Langevin. Spins have been assigned to most of the low-lying levels in these isotopes and multipole mixing ratios have been deduced. On the basis of coincidence data new information has been included in the decay schemes. The onset of nuclear deformation has been identified in these nuclei and found to occur between N=86 and N=88, which is two neutrons earlier than might be expected by comparison with the well known transitional nuclei in the Z=60 and 64 region. The current data are used to test the ability of the simplest form of the interacting boson model to describe this new transitional region.

Levels and transitions in 142,144Ce populated following the βdecay of 142,144La

The levels populated in 142,144Ce and the gamma -ray transitions between these levels have been studied by the gamma gamma directional correlation method following the beta decay of 142,144La. The lanthanum activities were produced by the decay of the corresponding caesium fission products. These were selected by the on-line mass separator OSTIS and the measurements were made in an off-line position. Coincidences were recorded in the event-by-event mode by a pair of Ge(Li) detectors and the correlations were measured at six angles. The coincidence data were analysed for a selection of gates to produce extensive decay schemes and transition intensities were also determined. The correlation data provide firm spin assignments to most levels and yielded multipole mixing ratios for many transitions in each isotope. Parity assignments to levels were made on the basis of dominant multipolarities. The experimental results provide extensive data on these transitional nuclei.

Levels and transitions in 140Ba populated following the β decay of 140Cs

Levels in 140Ba were populated following the beta decay of 140Cs (T12/=64 s) which was produced by the online fission product mass separator OSTIS at the Institut Laue-Langevin. Gamma data were recorded in an event-by-event mode and the coincidence results enable a decay scheme with 52 levels up to 5.8 MeV to be established. Directional correlation measurements were made on 14 direct cascades and 11 skip cascades, all in coincidence with the 21+ to 01+ ground state transition. Definite spin assignments were made for 14 levels and tentative assignments for 5 levels, with multipole mixing ratios being extracted where applicable. In addition, approximate log ft values were calculated for all the beta -fed levels and these, together with the relevant modes of decay, were used as a bias for tentative assignments for all but two of the levels not studied using the correlation technique. The experimental data for the neutron-rich barium isotopes 140-146Ba have been fitted using IBM-2 and the 2+ level at 1994 keV in 140Ba is identified as a mixed-symmetry state.

The onset of deformation in the neutron-rich Ba isotopes

The isotopes 142,144Ba, produced by the fission-product mass separator OSTIS at the Institut Laue-Langevin, has been investigated via on-line gamma - gamma angular correlation measurements. The spin assignments deduced allow the effects of increasing neutron number in the neutron-rich Ba isotopes to be studied, and clearly identify the onset of deformation in this region.

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.

The level structure of 96Y from the decay of 96Sr

Abstract The 1s β − decay of 96 Sr was studied with γ-γ coincidences, internal conversion coefficient measurements and γγ(θ) angular correlations. The 96 Sr activity was obtained as a result of β − decay of 96 Rb which was produced by the OSTIS mass separator. The α k internal conversion coefficients were determined for 5 of the most intense transitions between levels in 96 Y. Angular correlations were deduced from gammas coincident with the 122.3 keV and 530.0 keV transitions in 96 Y. A level scheme of 96 Y is proposed and definite spin-parity assignments are made for 5 levels. The scheme is discussed within the context of level systematics of odd-odd Y isotopes.

Gamma-gamma directional correlation measurements in 162,164Dy following thermal neutron capture

An on-line angular correlation facility to measure directional correlation coefficients of gamma-ray cascades following thermal neutron capture is described. The apparatus has been used to measure gamma-gamma directional correlations in 162,164Dy and multipolarities of sixteen transitions in 162Dy and nine in 164Dy were determined. In addition, new coincidence relationships were found in both isotopes. The transitions between the gamma and ground bands have been considered theoretically using the symmetric rotor model of Bohr and Mottelson, (1975).

A test for T violation by a directional correlation of cascade gamma rays emitted by polarised 49Ti

The authors have carried out a high-precision test for a time-violating asymmetry in the directional correlation pattern of the 0.342-1.381 MeV gamma gamma cascade in 49Ti. Polarised 49Ti was prepared following the capture of polarised neutrons. The correlation at fixed angles of 45 degrees and 135 degrees was compared in a multidetector configuration for alternate directions of the neutron (titanium-49) spin. A precision of three parts in 105 in the asymmetry was achieved and indicates that the phase difference between the E2 and M1 components in the 342 keV transition is 180.52+or-0.40 and is consistent with T invariance. The overall precision of the system is of the order of a few parts in 105 and is the greatest so far achieved in this type of measurement.

Gamma-Gamma Angular Correlation Measurements on 142Ba

Gamma-gamma angular correlations on 142Ba have been performed. The activity was produced using the on-line mass-separator “OSTIS” at the Institut Laue-Langevin. Spins have been assigned to the first 9 levels and multipole mixing ratios have been extracted. On the basis of the coincidence data two new levels have been tentatively proposed.

An On-Line Angular Correlation System at the Institut Laue-Langevin ; Multipole Mixing in the Gamma to Ground Bands of 164Dy

An on-line angular correlation facility to measure directional correlation coefficients of gamma-ray cascades following thermal neutron capture is described. Systems tests using favourable transition in 114Cd are mentioned and provisional results obtained for the mixing ratios of transitions between the gamma and ground state bands in 164Dy are quoted and commented on.