Gregory Lane

Three-dimensional position sensitivity in two-dimensionally segmented HP-Ge detectors

Abstract Measured- and simulated-pulse shapes in electrically segmented coaxial Ge detectors have been investigated. Three-dimensional position sensitivities have been determined experimentally and theoretically in a 36-fold segmented Ge detector. By using the two-dimensional segmentation in conjunction with pulse-shape analysis, a position sensitivity of better than 1 mm can be obtained in three dimensions at an energy of 374 keV. This is achieved by analyzing the shape of net charge signals of segments containing interactions and of transient image charge signals of neighboring segments. The ability to locate interactions in three-dimensions is one of the crucial properties in the proposed γ-ray energy tracking array (GRETA). The concept of γ-ray tracking will not only increase the efficiency in detecting γ radiation but also enables the localization and characterization of unknown γ-ray sources with much higher accuracy than is possible with current instruments.

Performance of the GRETA Prototype Detectors

Abstract A working, two-dimensionally segmented Ge detector is one of the crucial elements in the development of GRETA – a next-generation 4π germanium detector array that uses three-dimensional positions and energies to of individual interactions of γ rays in the detector to reconstruct the full energies and direction vectors of the individual γ rays by employing tracking algorithms. The three-dimensional position and the energy of interactions will be determined by using a two-dimensionally segmented Ge detector along with pulse-shape analysis of the signals. The current prototype is a 36-fold segmented HP-Ge detector in a closed-ended coaxial geometry. Preamplifiers with a compact design, low noise, and very good response properties have been built and implemented. An integrated noise level of about 5 keV has been measured for the segment channels. The average energy resolution of this detector was measured to be 1.14 and 1.93 keV at 60 and 1332 keV, respectively. Using pulse-shape analysis, a three-dimensional position sensitivity of 0.2 to 0.5 mm (R.M.S) has been obtained at 374 keV, dependent on the position and the direction. The results represent a major step towards the feasibility of a γ-ray tracking detector.

Non-yrast states and shape co-existence in light Pt isotopes

Abstract Low-lying states in the even-even light platinum isotopes 176Pt, 178Pt, 180Pt and 182Pt have been populated using β+ /EC decay from parent gold nuclei, created in (HI,xn) reactions. State energies, spins and parities and γ-ray branching ratios were determined using γ-ray and electron spectroscopy. Whereas non-yrast states were observed in 178Pt, 180Pt and 182Pt, none were seen in 176Pt. The excitation energies of the observed states are analysed in terms of a band-mixing model, yielding the moments of inertia of the unperturbed bands. Branching ratios and ground-state-band quadrupole moments are calculated and compared with experimental values. The results indicate that the two lowest-lying 0+ states in each of the light Pt isotopes are formed from the mixing of two intrinsic states of different deformation, and other low-lying states can be described as admixtures of rotational states built on these intrinsic states, and on γ-vibrational states.

Backbending in 180W: a t-band crossing

Abstract A Fermi-aligned ( i 13 2 ) 2 rotational band, with K≈8, has been observed in an even-even nucleus, 180W. Its structure corresponds to a t-band in the tilted-cranking representation. It crosses the ground-state band at I = 16 h , giving rise to backbending in the yrast sequence. Contrary to the usual interpretation, backbending in this case is not caused by the s-band. Comparison is made with other possible t-band crossings in the A ≈ 180 region.

Collective T=0 pairing in N=Z nuclei? Pairing vibrations around 56Ni revisited.

Abstract We present a new analysis of the pairing vibrations around 56 Ni, with emphasis on odd-odd nuclei. This analysis of the experimental excitation energies is based on the subtraction of average properties that include the full symmetry energy together with the volume, surface, and Coulomb terms. The results clearly indicate a collective behavior of the isovector pairing vibrations and do not support any appreciable collectivity in the isoscalar channel.We present a new analysis of the pairing vibrations around 56Ni, with emphasis on odd-odd nuclei. This analysis of the experimental excitation energies is based on the subtraction of average properties that include the full symmetry energy together with volume, surface and Coulomb terms. The results clearly indicate a collective behavior of the isovector pairing vibrations and do not support any appreciable collectivity in the isoscalar channel.

Is there np pairing in N=Z nuclei?

The binding energies of even-even and odd-odd N=Z nuclei are compared. After correcting for the symmetry energy we find that the lowest T=1 state in odd-odd N=Z nuclei is as bound as the ground state in the neighboring even-even nucleus, thus providing evidence for isovector np pairing. However, T=0 states in odd-odd N=Z nuclei are several MeV less bound than the even-even ground states. We associate this difference with a pair gap and conclude that there is no evidence for an isoscalar pairing condensate in N=Z nuclei.

Non-yrast states and shape co-existence in 172Os

Abstract Previous studies of 172 Os noted an anomaly in the behaviour of the moment of inertia of the yrast band at low spin. A phenomenological model of shape coexistence based on interacting rotational bands was proposed to explain this anomaly and this model predicted low-lying non-yrast states. In order to test these predictions, the β-decay of 172 Ir has been used to populate 172 Os. Excited states have been observed and classified into positive-parity “quasi-β” and “quasi-γ” bands and a negative-parity band. The energies of the quasi-β band states are seen to be in general agreement with the predictions of the phenomenological model and the model is refined to take into account the new data. The bands involved are determined to have significantly different moments of inertia.

Shape coexistence in 185Tl and 187Tl — investigation of the deformed minima

Abstract High spin gamma-ray spectroscopy of 185 Tl and 187 Tl has been performed with the reactions 154 Gd( 35 Cl, 4n) and 159 Tb( 32 S, 4n). Positive γ-ray identification with the thallium isotopes was made via X-ray coincidences, and supported by mass selected γ-ray spectra, the latter obtained with the reactions 154 Gd( 36 Ar, p4n) and 155 Gd( 36 Ar, p3n). Rotational bands associated with both prolate and oblate shape were observed. The bandheads of the proposed oblate 13 2 + [606] states were found to be isomeric, with meanlives of 12 ± 2 ns in 185 Tl and 1.0 ± 0.2 ns in 187 Tl. Prolate deformed i 13 2 bands were observed in both nuclei, while in 187 Tl, bands due to h 9 2 and f 7 2 protons coupled to the prolate shape are also assigned. An h 9 2 band is tentatively assigned in 185 Tl. The observation of these rotation-aligned bands at low excitation energy implies that the development of prolate deformed minima in the odd nuclei is not necessarily blocked by occupation of a single deformation-driving orbital. Equilibrium deformation calculations for intrinsic states in a range of thallium nuclei are presented. Experimental trends with mass number are reproduced, but absolute excitation energies, and energy differences between the prolate and oblate states are not, continuing the persistent discrepancy between theory and experiment in the mercury region. Theoretical calculations of intruder orbital occupation probabilities show a correlation between prolate deformation and h 9 2 and f 7 2 proton pair population, in particular of the 1 2 − [541] orbital from the h 9 2 proton shell. They also show that blocking of the 1 2 − [541] orbital significantly suppresses the prolate deformation. Implications for the structure of the prolate deformed mercury and thallium isotopes are considered, leading to the conclusion that the prolate mercury core nuclei consist of a mixture of low-Ω proton intruder excitations.

Spectroscopy and shell model interpretation of high-spin states in the N = 126 nucleus 214Ra

Abstract Excited states in the N = 126 nucleus 214 Ra have been studied using γ-ray and electron spectroscopy following reactions of 12 C and 13 C on 206 Pb targets. Levels were identified to spins of ∼ 25 ħ and excitation energies of ∼ 7.8 MeV. Lifetimes and magnetic moments were measured for several levels, including a spin (25 − ) core-excited isomer at 6577.0 keV with τ = 184 ± 5 ns and g = 0.66 ± 0.01. The level scheme, lifetime and magnetic moment data are compared with, and discussed in terms of, empirical shell-model calculations and multiparticle octupole-coupled shell-model calculations. In general, the experimental data are well described by the empirical shell model.

Octupole coupling and proton-neutron interactions in 214Fr

Abstract Excited states in the odd-odd nucleus 214 Fr have been studied using γ-ray and electron spectroscopy following 208 Pb( 11 B, 5n) and 205 Tl( 13 C, 4n) reactions. Levels were identified to spins of around 36 ħ and excitation energies of ∼ 8.6 MeV. A number of isomeric states have been measured and g -factors obtained using the TDPAD method. At low spin, semi-empirical shell-model calculations appear to provide a good description of the states observed. An understanding of the structure of higher spin states in terms of the probable yrast configurations requires the addition of two units of spin at an energy of around 2 MeV. States with spins around 30 ħ are formed by core-excited configurations, with double core-excitation suggested for the highest states observed. The properties of many of the isomeric states can be understood in terms of multiparticle octupole coupling, with the properties of these states well reproduced by multiparticle octupole-coupled shell-model calculations.