I. M. Hibbert

The GREAT triggerless total data readout method

Recoil decay tagging (RDT) is a very powerful method for the spectroscopy of exotic nuclei. RDT is a delayed coincidence technique between detectors usually at the target position and at the focal plane of a spectrometer. Such measurements are often limited by dead time. This paper describes a novel triggerless data acquisition method, which is being developed for the Gamma Recoil Electron Alpha Tagging (GREAT) spectrometer, that overcomes this limitation by virtually eliminating dead time. Our solution is a total data readout (TDR) method where all channels run independently and are associated in software to reconstruct events. The TDR method allows all the data from both target position and focal plane to be collected with practically no dead-time losses. Each data word is associated with a timestamp generated from a global 100-MHz clock. Events are then reconstructed in real time in the event builder using temporal and spatial associations defined by the physics of the experiment.

The EUROBALL neutron wall - design and performance tests of neutron detectors

The mechanical design of the EUROBALL neutron wall and neutron detectors, and their performance measured with a Cm-246,Cm-248 fission source are described. The array consists of 15 pseudohexaconical detector units subdivided into three, 149 mm high, hermetically separated segments and a smaller central pentagonal unit subdivided into five segments. The detectors are filled with Bicron BC501A liquid scintillator. Each section of the hexaconical detectors is viewed by a 130 mm diameter Philips XP4512PA photomultiplier while the sections of pentagonal detectors are viewed by Philips XP4312B PMTs. The tests of n-gamma discrimination performed by zero-crossing and time-of-flight methods show a full separation of gamma- and neutron events down to 50 keV recoil electron energy. These tests demonstrate the excellent timing properties of the detectors and an average time resolution of 1.56 ns. The factors determining the efficiency of neutron detectors are discussed. The total efficiency for the full array for a symmetric fusion-evaporation reaction is predicted to be 0.30

Magnetic rotation in 106Sn and 108Sn

Abstract The nuclei 106Sn and 108Sn have been populated using the 54Fe(58Ni,α2p) and 54Fe(58Ni,4p) reactions, respectively, at a beam energy of 243 MeV and the gamma rays have been detected using the Gammasphere array. Two “rotation-like” structures consisting of magnetic dipole transitions have been observed in each of the nuclei. The bands can be interpreted, using the Tilted Axis Cranking model, as examples of magnetic rotation. The calculations show excellent agreement with the data for 108Sn. However, for the lighter isotope, 106Sn, the model is unable to reproduce the experimental B(M1)/B(E2) ratios. Reasons for this discrepancy are discussed.

M1 transitions between superdeformed states in 195Tl: the fingerprint of the i132 proton intruder orbital

Abstract Dipole transitions linking the superdeformed Yrast signature-partner bands in 195Tl have been observed using the EUROGAM 1 spectrometer. From the measurements of the photon decay branching ratios, taken together with the average quadrupole moment measured in neighbouring superdeformed nuclei, we find that the two signature partner superdeformed bands involve the excitation of the 81 st proton into the i 13 2 Ω= 5 2 intruder orbital

Measurement of transition quadrupole moments of high-spin states in the N=74 isotones 133Pr, 132Ce and 131La

Abstract The Doppler-Shift Attenuation Method has been used to extract transition quadrupole moments of high-spin bands in the N=74 isotones 133 Pr, 132 Ce and 131 La, produced in the 37 Cl + 100 Mo reaction. The results appear to be configuration dependent and, for 133 Pr and 132 Ce, the involvement of Ω=1/2 νh 9/2 and νf 7/2 intruder orbitals appears to enhance the collectivity at high spin ( I>25 ℏ ).

Comparative quadrupole moments of superdeformed bands in 193−196Pb

Abstract Lifetimes have been measured for states in five superdeformed yrast bands in 193−196 Pb using the Gammasphere spectrometer array. Centroid shifts and lineshapes of transitions showing attenuated Doppler shifts have been analyzed. Transition quadrupole moments are deduced for the five bands including, for the first time, bands of two odd-A Pb isotopes. The quadrupole moments are found to be remarkably similar for the yrast bands in 194 Pb (band 1), 195 Pb (bands 1 and 2) and 196 Pb (band 1) (Q t ≈20 b). However, for 193 Pb a smaller value, Q t =17.3(7) b, is obtained. This can be explained in terms of the occupation of the neutron N = 7 intruder orbitals.

Neutron orbitals above the N = 74 shell gap at large deformation: spectroscopy in the superdeformed minimum of 133Ce

Abstract High-spin states in 133 Ce were populated via the 116 Cd( 22 Ne,5n) reaction at 120 MeV. Analysis of these data has revealed three new superdeformed bands in 133 Ce, two of which have transitions which are identical to those observed in the yrast bands of 132 Ce and 136 Nd. These bands have been interpreted as signature partners of the [530] 1 2 − orbital coupled to the 132 Ce yrast superdeformed core. The third band is believed to be built upon a π5 4 ν6 3 configuration at high rotational frequency and to show the effects resulting from an alignment of a pair of f 7 2 neutrons at ℏgω ⋍ 0.7 MeV. It is tentatively suggested that the sharp rise in the dynamic moment of inertia of this band at low rotational frequency may result from an admixture of the π5 4 ν6 1 configuration.

Identification of excited states in 117 Cs : Systematics of the ν ( h 1 1 / 2 ) 2 alignment

Excited states have been identified in the very neutron-deficient {sub 55}{sup 117}Cs{sub 62} nucleus. High-spin spectroscopy has been performed using the Gammasphere array, and the assignment of gamma-ray transitions to {sup 117}Cs has been made in a separate experiment in which gamma rays were detected in coincidence with x rays and with recoiling evaporation residues. A previously observed sequence of five gamma rays has been extended by 11 transitions, to high spin, and has been identified as the yrast {pi}(h{sub 11/2})[550]1/2{sup -} band of {sup 117}Cs. Two additional bands have been observed and are tentatively assigned to be based on protons in the [404]9/2{sup +} and [422]3/2{sup +} orbitals. Alignments of pairs of h{sub 11/2} neutrons and protons are observed in all of the bands. The alignments are compared to cranked Woods-Saxon calculations, and are discussed with respect to the effects of a neutron-proton interaction. Of particular interest are the features of the {nu}(h{sub 11/2}){sup 2} alignment in the {pi}[550]1/2{sup -} band and of the {pi}(h{sub 11/2}){sup 2} alignment in the [422]3/2{sup +} band. The frequencies of these alignments can be qualitatively explained only if a neutron-proton interaction is taken into account.

Differential quadrupole moment measurements of the 1/2{sup +}[660] (i{sub 13/2}) neutron intruder band in {sup 133}Nd and {sup 135}Nd

Quadrupole moment measurements of the 1/2{sup +}[660] (i{sub 13/2}) bands in {sup 133}Nd (N=73) and {sup 135}Nd (N=75) were performed using the Doppler-shift attenuation method. These results, coupled with the previously measured Q{sub 0} for the same configuration in {sup 137}Nd (N=77), clearly demonstrate a trend of decreasing quadrupole deformation with increasing neutron number. The larger quadrupole moment in {sup 133}Nd compared with that in {sup 135}Nd and {sup 137}Nd offers evidence for the role played by the large shell gap at N=72 for {beta}{sub 2}{approximately}0.35{endash}0.40 in stabilizing the shape at enhanced deformation. The comparison of results from gating below and above the level of interest provides information on the time scale of the sidefeeding contributions to highly deformed structures in the A{approximately}130 region. {copyright} {ital 1999} {ital The American Physical Society}

The role of shell gaps and deformation-driving orbitals in superdeformed

Four superdeformed bands have been assigned to following a high-statistics -ray study using the EUROGAM II spectrometer. The bands are interpreted in terms of and signature-partner neutron configurations. One of the bands exhibits a sharp backbend when the orbital crosses the orbital at MeV/ . Below this crossing, the band is predicted to remain superdeformed despite the absence of N = 6 neutron intruders. This suggests that these intruders are not a necessary ingredient of superdeformation in this mass region but that the underlying shell gaps at Z = 58 and N = 72 are important.