W D Hamilton

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 124Te following thermal-neutron capture by 123Te and the beta decay of oriented 124Sb

Levels and gamma -ray transitions in 124Te have been studied using the techniques of directional correlation and nuclear orientation. Directional correlation measurements were performed on the gamma -rays observed following thermal-neutron capture by 123Te and the selection of several coincidence gates has allowed a comprehensive level scheme to be produced. The spins of 15 levels have been proposed from the directional correlation results and multiple mixing ratios have been extracted for 8 of the most intense transitions. Directional distribution measurements have been performed on the gamma radiation observed following the beta decay of oriented 124Sb. The orientation was obtained by diffusing the antimony into iron and cooling to 6.5 mK. The observed anisotropies in the gamma -radiation patterns have allowed 3 spin assignments to be made and 14 multipole mixing ratios to be extracted. The levels in 124Te are discussed in terms of the simple vibrational phonon and interacting boson models. It is shown that 124Te is a new example of O(6) nuclear symmetry within the boson model.

A shared mixed-symmetry state in 56Fe

Abstract Analysis of the γ-decay data suggests that the lowest mixed-symmetry 2 + state in 56 Fe is shared almost equally between the two 2 + levels at 2.658 MeV and 2.960 MeV.

An application of the dynamic deformation model to the tellurium isotopes

The recent developments of the dynamic deformation model (DDM) make it readily applicable to a wide range of nuclei. The authors report a study of the even-mass tellurium isotopes from N=68 to the closed neutron shell at N=82. Within this region there is experimental evidence for nuclei with the characteristics of vibrational, rotational or gamma -unstable level sequences. They show that the model is well able to account for these features as typified by level energies, electric quadrupole moments and gamma -ray transition probabilities across this region when the only parameter which changes is the neutron number. For comparison the experimental data were also fitted to IBM-2 and the results from these fits are in general in good agreement with those from the DDM.

The low-lying levels of 147Pm

Unique spin-parity assignments have been made for levels of 147Pm populated following the beta - decay of 147Nd. The magnitude and sign of the multipole mixing ratios of most E2+M1 transitions have also been unambiguously determined. The measurements used the complementary techniques of gamma -ray directional distribution from an oriented source and gamma - gamma directional correlation using a liquid source. Source orientation was achieved through the magnetic hyperfine interaction when the neodymium activity was dissolved in ferromagnetic gadolinium metal and subsequently cooled to millidegree temperatures. Cooling was obtained with a 3He:4He dilution refrigerator to which an adiabatic stage was coupled. The level scheme and measured multipole mixing ratios are compared with the predictions of an intermediate coupling calculation.

E0 transitions in the γ-unstable nucleus 124Te

Measurements of the internal conversion electron spectrum and the identification of E0 transitions provide additional evidence for the gamma -soft description of 124Te. 0+ levels at 1657.9, 1833.3 and 2309.5 keV were identified and altogether thirteen E0 transitions were observed. The dynamic deformation model (DDM) predicts that 124Te is a well deformed gamma -unstable nucleus and gives excellent agreement with experimental results. The E0 transitions provide evidence which helps in the assignment of n and tau labels to the 02+ and 03+ levels. The alternative but equivalent O(6) description within IBM-2 is also generally satisfactory. The authors find evidence indicating that the 2+ level at 2.09 MeV has the characteristic features of a mixed symmetry state in an O(6)-like nucleus.

A search for mixed-symmetry states in the mass A approximately 50 region

Mixed-symmetry 2+ states are identified in vibrational-like nuclei lying between the Z=20 and 28 closed shells. The M1 strengths of 2+-2+ transitions are obtained from level lifetimes and gamma -ray multipole mixing ratios. The latter were measured using the directional distribution of gamma -rays from an oriented source (56Fe) and gamma gamma directional correlations on cascades which follow neutron capture (48Ti, 54Cr and 58Fe). There is strong evidence for the sharing the characteristics of the mixed-symmetry 2+ level with neighbouring 2+ levels. The identification of a 4+-4+ transition with a large B(M1) value in 56Fe indicates the need for g-bosons in the IBA-2 which is used as a framework for discussing the data.

A new example of O(6) nuclear symmetry: 52124Te72

The establishment of several excited 0+ states in 124Te provides a sensitive test which shows that the collective levels in this nucleus can be described by the O(6) limiting symmetry of the interacting boson model. Unlike other O(6) nuclei the spin-dependent L.L term in the Hamiltonian is negative and causes the highest spin levels in each multiplet to lie at lowest energy. It is suggested that the even tellurium isotopes may provide an opportunity to study the Su(5) to O(6) transition.

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.

Nuclear orientation of156Tb in gadolinium matrix

The anisotropy of the angular distribution of gamma-rays from the decay of156Tb, oriented in a gadolinium matrix at low temperatures, has been measured at the angles of 0 and π/2 with respect to the applied magnetic field direction in the range of temperatures from 14·6 to 68·4 mK. The temperature dependence of anisotropy was measured for the first time. The parameters of hyperfine magnetic dipole and electric quadrupole splittings have been determined and the values of the magnetic dipole moment ¦Μ156¦=(9·6±1·3)×10−27 J/T and the electric quadrupole momentQ156=(2·9±0·9)×10−28 m2 of the156Tb ground state have been calculated. Multipole mixing ratios andB(E2) branching ratios of many gamma-ray transitions occurring in156Gd have been found and the results have been discussed in terms of the rotational-vibration and pairing-plus-quadrupole models.