J Simpson

Application of a sectored Ge(Li) detector as a Compton polarimeter

Abstract The application of a sectored Ge(Li) detector as a Compton polarimeter has been investigated. Its sensitivity and efficiency to γ -ray linear polarisation has been determined in the energy range 0.3–4.4 MeV. The detector has been used to measure the linear polarisation of γ -ray transitions in 158 Er populated by the reaction 146 Nd( 16 O, 4n) 158 Er at an 16 O bombarding energy of 84 MeV.

Near-yrast discrete line γ-ray spectroscopy of 159Er and 160Er to I⩾ or ≈40h̄

High-spin states in 159Er and 160Er have been populated using the 116Cd+48Ca reaction at a 48Ca bombarding energy of 210 MeV. Gamma rays were detected in an array (TESSA2) of six escape suppressed germanium spectrometers and a 50 element BGO multiplicity and summed energy detector. Decay scheme and angular correlation data for these nuclei were deduced from the gamma - gamma -BGO coincidence data. In 159Er the yrast band has been identified up to Ipi =89/2+ and three side bands have been observed to 69/2-(77/2-), 43/2+ and 51/2-(59/2-). In 160Er the lowest energy positive-parity band has been observed up to 38+(40+) and the two lowest energy negative-parity side bands up to 40-(44-) and 39-(41-). The ground state level sequence has been observed up to (28+). The spin and parity of the low-lying states (I<20h(cross)) in the bands observed in 160Er were established by measuring gamma -ray angular distributions and internal conversion coefficients following the 148Nd(16O, 4n) reaction at a beam energy of 80 MeV. The band structures can be understood in terms of aligning quasiparticles using the cranked shell model. The alignment of the first pair of h11/2 protons is observed in the three lowest energy bands in each nucleus. The increase in crossing frequency of this alignment with neutron number is further evidence for the increase in deformation for the light Er isotopes. Evidence is presented for the competition of favoured single-particle configurations with the collective structures at the highest spins.

The response of an isobutane filled ion chamber to heavy ions

A gridded ion chamber with its anode split to record both total energy and rate of energy loss has been used with isobutane gas to detect ions of seventeen elements (17⩽Z1⩽92) in the velocity range 2<(137v/c)<6. The rate of energy loss and the ionisation collection efficiency were found to be independent of pressure over the range 15–50 Torr. The effective charge of the ion moving in the gas is observed to oscillate with Z1. This variation and its effect on the resolution of the rate of energy loss signal are discussed. The effective charge of the moving ion in the ion chamber window (solid polypropylene) does not oscillate with Z1 but probably depends on the ion charge before entering the window.

Experimental investigation of the structure of 124Ce

Gamma-ray transitions have been observed for the first time in 124Ce using the Daresbury Recoil Separator. The excitation energy of the first excited state, 142 keV, implies a deformation epsilon 2 approximately=0.31. This value confirms the trend to higher deformation for the more neutron-deficient cerium isotopes, but it is larger than that predicted by recent calculations.

Rotational band structures in 154Gd

The N=90 rare earth nucleus 154Gd has been studied in the spin regime of I<or=26h(cross) by using the reaction 150Nd(9Be, 5n)154Gd at 55 MeV. Gamma-gamma coincidence and angular correlation data were obtained using the TESSA2 array. Two positive parity sequences have been extended to 26+ and four negative parity structures have each been observed up to spins of I=22-24h(cross). The rotational structures observed in 154Gd are interpreted within the framework of the cranked shell model (CSM) in terms of quasiparticle excitations. The characteristics of the rotational band sequences and the band crossings that occur are compared with those in neighbouring nuclei and to the predictions of this model. The first, second and third i13/2 neutron alignments in 154Gd and the systematics of the neutron alignments in the negative parity rotational bands in the even-even N=90 nuclei are discussed. Evidence for quasiproton excitations at the highest spins is discussed.

A level scheme for 220Ra

The nucleus 220Ra has been populated by the reaction 18O+208Pb at a beam energy of 83 MeV. The positive-parity members of the yrast band have been identified to Jpi =12+ and the negative-parity members from Jpi =5- to Jpi =11-. The alternating positive- and negative-parity levels are strongly coupled by E1 transitions. The sequence of levels observed and the large ratio B(E1)/B(E2) suggest that 220Ra has a stable octupole deformation.

High-spin gamma -ray spectroscopy of 122Xe

High-spin states were populated in 54122Xe68 using the fusion evaporation reaction 96Zr(30Si,4n) at a beam energy of 135 MeV. The level scheme of 122Xe has been extended up to spin approximately 30h(cross). The rotational bands and band crossings in 122Xe are assigned within the framework of the cranked shell model. Three bands are found to lose their rotational character or to branch above spin approximately 20h(cross). This is interpreted as the collective rotation of a prolate nucleus being replaced by a regime of non-collective single-particle states at high spin. Single-particle configurations are suggested for the high-spin non-collective states using modified oscillator and Woods-Saxon potentials. The development of the nuclear shape is analysed employing potential energy and total Routhian surface calculations.

High-spin structure of 121Xe: triaxiality and band termination

High-spin states of the odd-neutron 121Xe nucleus have been studied with Eurogam using the 96Zr(30Si,5n)121Xe fusion-evaporation reaction. The level scheme has been extended up to 67/2h(cross) and an excitation energy of approximately 14 MeV. Several new rotational bands have been observed and the previously known bands extended. Two of the bands lose their regular character at high spins, which may be interpreted as a transition from collective behaviour to a regime of noncollective oblate states. The deduced high-spin structure is compared with Woods-Saxon TRS cranking and CSM calculations. Configurations of the bands have been suggested on the basis of the measured routhians, aligned angular momenta and B(M1)/B(E2) ratios. The yrast nu h(cross)11/2 band is interpreted as having a triaxial shape. Neutron (h(cross)11/2)2 alignments have been found at the frequencies predicted by the TRS and CSM calculations. The B(M1)/B(E2) ratios for one of the bands show deviation from the predictions of the geometrical model. Enhanced E1 transitions have been observed between bands built on nu d5/2 and nu h(cross)11/2 orbitals.

Studies of octupole structures in the isotopes 220,222Rn populated via α decay.

The nuclei 220,222Rn have been studied, using the alpha -radioactive sources 226Ra and 228Th, via the technique of alpha - gamma angular correlation. This has enabled Jpi assignments to be made to the previously known low-lying levels in 220,222Rn and alpha hindrance factors to be interpreted. The systematics of alpha hindrance factors for these Z=86 nuclei are examined and compared with those of Z=88, 90 nuclei. The subsequent alpha decays to 216,218Po were observed and 2+ assignments confirmed for the first excited states in these nuclei.

Shape coexistence in 138Sm and evidence for the rotational alignment of a pair of N=6 neutrons

The level scheme of 138Sm has been extended up to Ipi =(32+) using gamma -ray coincidence data collected with the Eurogam array. The 106Cd(35Cl,3p)138Sm fusion-evaporation reaction was used at a bombarding energy of 150 MeV. Several new bands have been observed and the known bands extended to higher spin. Band crossings and shape coexistence are discussed with the aid of Woods-Saxon cranking calculations (CMS and TRS). Evidence is presented for a neutron (i132/)2 pair alignment which is predicted to drive the nucleus to a prolate shape with enhanced quadrupole deformation beta 2=0.32. A strongly coupled band was also established and is interpreted as a collectively rotating oblate structure.