Search

In this paper, we provide a formalism for the characterization of tracking arrays with emphasis on the proper corrections required to extract their photopeak efficiencies and peak-to-total ratios. The methods are first applied to Gammasphere, a well characterized 4π array based on the principle of Compton suppression, and subsequently to GRETINA. The tracking efficiencies are then discussed and some guidelines as to what clustering angle to use in the tracking algorithm are presented. It was possible, using GEANT4 simulations, to scale the measured efficiencies up to the expected values for the full 4π implementation of GRETA.

A 985 MeV 178Hf beam was Coulomb excited by a 208Pb target at the ATLAS accelerator of Argonne National Laboratory. Gammasphere and the CHICO particle detector recorded particle-γ coincidence data. The aim was to populate and determine the mechanism of previously observed Coulomb excitation of the Kπ = 6+ (t1/2 = 77 ns), 8- (4 s) and 16+ (31 y) isomer bands. New rotational bands were identified including an aligned band which appears to mix with the ground-state band (GSB) and the γ-vibrational band above ~ 12 ħ of angular momentum. Newly observed γ-decay transitions into the three isomer bands may elucidate the K-mixing which allows Coulomb excitation of these isomer bands, but direct decays from the GSB into the 16+ isomer band have not yet been confirmed. [ABSTRACT FROM AUTHOR]

Abstract: Due to recent advances in detection techniques, excited states in several trans-fermium nuclei were studied in many laboratories worldwide, shedding light on the evolution of nuclear structure between stable nuclei and the predicted island of stability centered around spherical magic numbers. In particular, studies of K-isomers around the Z=100 and N=152 deformed shell closures extended information on the energies of Nilsson orbitals at the Fermi surface. Some of these orbitals originate from spherical states, which are relevant to the magic gaps in super-heavy nuclei. The single-particle energies can be used to test various theoretical predictions and aid in extrapolations towards heavier systems. So far, the Woods-Saxon potential reproduces the data best, while self-consistent approaches miss some of the observed features, indicating a need to modify the underlying effective nucleon-nucleon interactions. [Copyright &y& Elsevier]

Abstract: New, low-lying levels in the odd–odd, nucleus 62Ga have been identified using a sensitive technique, where in-beam γ rays from short-lived nuclei are tagged with β decays following recoil mass identification. A comparison of the results with shell-model and IBM-4 calculations demonstrates good agreement between theory and experiment, with the majority of predicted low-lying, low-spin states now identified. There is a dramatic change in the level density at low excitation energies for the nucleus 62Ga when compared with neighbouring odd–odd Ga isotopes where, in contrast, the low-lying level structure is dominated by configurations with pairing interactions between excess neutrons. This illustrates the distinctively different aspects of nuclear structure exhibited by nuclei with . [Copyright &y& Elsevier]

Deep-inelastic reactions and γ-ray spectroscopy have been used to study excited states in 195Au. A three-quasiparticle isomer with a mean-life of 18.6(3) µs has been assigned at 2461 + Δ keV, with decays into newly identified structures. Possible configurations for the isomer are discussed including a Jπ = 31/2- intrinsic state produced by coupling the 11/2- [505] proton hole to the 10+ state obtained from the 9/2+ [624], 11/2+ [615] two-neutron holes, expected from configuration-constrained potential-energy-surface calculations. Residual interactions are evaluated within a semi-empirical shell model basis. New lifetime information is also obtained for the 21/2+ and 25/2+ states at 1813 and 1980 keV. The relationship between these and other newly identified states and the negative-parity states in the even-even neighbors is discussed. [ABSTRACT FROM AUTHOR]

New high-spin microsecond isomers, with Jπ = (31+) and Jπ = (28-), have been identified in 206Bi at 10 170 and 9233 keV, respectively, using γ-ray coincidence spectroscopy following deep-inelastic reactions with the 76Ge + 208Pb system. Yrast and near-yrast levels populated in the decay of these isomers have been located and interpreted with the help of shell-model calculations. The states identified at energies up to approximately 7 MeV can be described well in terms of one-proton-particle, three-neutron-hole couplings. [ABSTRACT FROM AUTHOR]

High-spin states in the odd-proton thulium isotopes 173Tm and 175Tm have been studied using deep-inelastic reactions and γ-ray spectroscopy. In l73Tm, the low-lying structure has been confirmed and numerous new states have been identified, including a three-quasiparticle Kπ = 19/2- isomer with a lifetime of τ = 360(100) ns at 1906 keV and a five-quasiparticle Kπ = 35/2- isomer with a lifetime of τ = 175(40) ns at 4048 keV. The Kπ = 35/2- state is interpreted as a t-band configuration that shows anomalously fast decays. In 175Tm, the low-lying structure has been reevaluated, a candidate state for the 9/2- [514] orbital has been identified at 1175 keV, and the 7/2-[523] bandhead has been measured to have a lifetime of τ = 460(50) ns. Newly identified high- AT structures in 175Tm include a Kπ = 15/2- isomer with a lifetime of τ = 64(3) ns at 947 keV and a Kπ = 23/2- isomer with a lifetime of τ = 30(20) µs at 1518 keV. The Kπ = 15/2- isomer shows relatively enhanced decays to the 7/2-[523] band that can be explained by chance mixing with the 15/2- member of the 7/2- band. Multiquasiparticle calculations have been performed for 173Tm and l75Tm, the results of which compare well with the experimentally observed high-spin states. [ABSTRACT FROM AUTHOR]

The structure of 58Fe was investigated at Gammasphere using 48Ca(13,14C,xn) fusion-evaporation reactions at a beam energy of 130 MeV. The level scheme has been revised and extended to J ∼ 17ħ and an excitation energy of 16.6 MeV. Regular band structures consisting of low-energy ΔJ = 1ħ transitions have been observed at moderate spin (J ∼ 8ħ-l5ħ) and are candidates for magnetic rotational bands. Self-consistent tilted-axis-cranking calculations within a relativistic mean-field theory were applied to investigate these bands and were found to reproduce the experimental results well. In other parts of the level scheme, quasirotational bands composed of stretched-£'2 transitions have been extended to high spin, and other new bands have been identified. Positive-parity experimental states were compared to predictions of the spherical shell model using the GXPF1 A, KB3G, and FPD6 effective interactions in the fp model space. The projected shell model, with a deformed quasiparticle basis including the neutron νg9/2 orbital, was applied to interpret regular ΔJ = 2ħ band structures that extend beyond the maximum spin available for π[(f7/2)-2] ⨷ ν[(p3/2f5/2P1/2)4] configurations and exhibit features characteristic of rotational alignment. It is clear that the νg9/2 intruder orbital plays a crucial role in describing the quasirotational structures in this nucleus, even starting as low as J ∼ 5h. [ABSTRACT FROM AUTHOR]

The available experimental information on 34P has been greatly increased through the analysis of γ decays in coincidence with protons from the interaction of an 18O beam at 24 MeV with an 18O target. Light charged particles from the reaction were detected with Microball, and multiple γ-ray coincidences with Gammasphere. Many observed γ transitions have been identified and placed in the level scheme. Additionally, for most states, spins have been assigned based on measured γ-ray angular distributions while parities were inferred from lifetimes determined through Doppler-broadened line-shape analysis. Most of the states observed have been interpreted in terms of shell-model calculations using the WBP-a and SDPF-NR interactions having one particle in the 0f7/2 or 1P3/2 orbital. The two calculations agree almost equally well with the data resulting in root-mean-square differences of about 200 keV. However, a few high-lying states observed with long lifetimes challenge current calculations. Two of these may be associated with stretched πf7/2 ⊗ νf7/2 states, but the calculations overpredict their energies by 2-3 MeV. Furthermore, a long-lived 7919-keV state is established for which no explanation is available at present. [ABSTRACT FROM AUTHOR]

Excited states in 67Ni were populated in deep-inelastic reactions of a 64Ni beam at 430 MeV on a thick 238U target. A level scheme built on the previously known 13-μS isomer has been delineated up to an excitation energy of 5.3 MeV and a tentative spin and parity of (21/2-). Shell model calculations have been carried out using two effective interactions in the f5/2pg9/2 model space with a 56Ni core. Satisfactory agreement between experiment and theory is achieved for the measured transition energies and branching ratios. The calculations indicate that the yrast states are associated with rather complex configurations, herewith demonstrating the relative weakness of the N = 40 subshell gap and the importance of multi-particle-hole excitations involving the g9/2 neutron orbital. [ABSTRACT FROM AUTHOR]

The energy levels and γ-ray decay scheme of the positive-parity states in the Tz=1/2 nucleus 67As have been studied by using the 40Ca(36Ar,2αp)67As reaction at a beam energy of 145 MeV. Two new band structures have been identified which can be connected to the previously known levels. The results for these bands are compared with configuration-dependent cranked Nilsson-Strutinsky calculations. The good level of agreement between theory and experiment suggests that these structures can be interpreted in terms of configurations that involve three g9/2 particles and that both possess noncollective terminating states. [ABSTRACT FROM AUTHOR]