The GRETINA Spectrometer Cromaz, M
Journal of physics. Conference series,
05/2015, Letnik:
606, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The GRETINA spectrometer is a first generation, gamma-ray tracking spectrometer capable of determining the Compton scattering path of gamma-rays incident on the detector volume. This ability allows ...the Ge detectors to be close packed allowing the detector to be scaled to high efficiencies while maintaining good peak-to-total. GRETINA currently consists of 7 4-detector modules giving approximately 1π solid angle coverage with a calorimetric efficiency of 6.3% and tracked efficiency of 4.7% at 1.3 MeV. The array's sensitivity to the position of the gamma ray's first interaction point enables precision event-by-event Doppler correction which allows one to achieve 1% energy resolution even for sources moving at a large fraction of the speed of light such as those encountered at fragmentation facilities such as NSCL and the future FRIB.
The Gamma-Ray Energy Tracking In-beam Nuclear Array (GRETINA) is a new generation high-resolution γ-ray spectrometer consisting of electrically segmented high-purity germanium crystals. GRETINA is ...capable of reconstructing the energy and position of each γ-ray interaction point inside the crystal with high resolution. This enables γ-ray energy tracking which in turn provides an array with large photopeak efficiency, high resolution and good peak-to-total ratio. GRETINA is used for nuclear structure studies with demanding γ-ray detection requirements and it is suitable for experiments with radioactive-ion beams with high recoil velocities. The GRETINA array has a 1π solid angle coverage and constitutes the first stage towards the full 4π array GRETA. We present in this paper the main parts and the performance of the GRETINA system.
UCGretina, a geant4 simulation of the GRETINA gamma-ray tracking array of highly-segmented high-purity germanium detectors is described. We have developed a model of the array, in particular of the ...Quad Module and the capsules, that gives good agreement between simulated and measured photopeak efficiencies over a broad range of gamma-ray energies and reproduces the shape of the measured Compton continuum. Both of these features are needed in order to accurately extract gamma-ray yields from spectra collected in in-beam gamma-ray spectroscopy measurements with beams traveling at v∕c≳0.3 at the National Superconducting Cyclotron Laboratory and the Facility for Rare Isotope Beams. In the process of developing the model, we determined that millimeter-scale layers of passive germanium surrounding the active volumes of the simulated crystals must be included in order to reproduce measured photopeak efficiencies. We adopted a simple model of effective passive layers and developed heuristic methods of determining passive-layer thicknesses by comparison of simulations and measurements for a single crystal and for the full array. Prospects for future development of the model are discussed.
Intermediate-energy Coulomb excitation measurements are performed on the N ≥ 40 neutron-rich nuclei (66,68)Fe and (64)Cr. The reduced transition matrix elements providing a direct measure of the ...quadrupole collectivity B(E2;2(1)(+) → 0(1)(+)) are determined for the first time in (68)Fe(42) and (64)Cr(40) and confirm a previous recoil distance method lifetime measurement in (66)Fe(40). The results are compared to state-of-the-art large-scale shell-model calculations within the full fpgd neutron orbital model space using the Lenzi-Nowacki-Poves-Sieja effective interaction and confirm the results of the calculations that show these nuclei are well deformed.
The level structure of 158Er has been studied using the Gammasphere spectrometer via the 114Cd(48Ca,4n) reaction at 215 MeV with both thin (self-supporting) and thick (backed) targets. The level ...scheme has been considerably extended with more than 200 new transitions and six new rotational structures, including two strongly coupled high-K bands. Configuration assignments for the new structures are based on their observed alignments, B(M1)/B(E2) ratios of reduced transition probabilities, excitation energies, and comparisons with neighboring nuclei and theoretical calculations. With increasing angular momentum, this nucleus exhibits Coriolis-induced alignments of both neutrons and protons before it then undergoes a rotation-induced transition from near-prolate collective rotation to a noncollective oblate configuration. This transition occurs via the mechanism of band termination around spin 45ℏ in three rotational structures. Two distinct lifetime branches, consistent with the crossing of a collective “fast” rotational structure by an energetically favored “slow” terminating sequence, are confirmed for the positive-parity states, and similar behavior is established in the negative-parity states. Weak-intensity, high-energy transitions are observed to feed into the terminating states. At the highest spins, three collective bands with high dynamic moments of inertia and large quadrupole moments were identified. These bands are interpreted as triaxial strongly deformed structures and mark a return to collectivity at ultrahigh spin.
Two isomers decaying by electromagnetic transitions with half-lives of 4.7(1.1) and 247(73) μs have been discovered in the heavy ^{254}Rf nucleus. The observation of the shorter-lived isomer was made ...possible by a novel application of a digital data acquisition system. The isomers were interpreted as the K^{π}=8^{-}, ν^{2}(7/2^{+}624,9/2^{-}734) two-quasineutron and the K^{π}=16^{+}, 8^{-}ν^{2}(7/2^{+}624,9/2^{-}734)⊗8^{-}π^{2}(7/2^{-}514,9/2^{+}624) four-quasiparticle configurations, respectively. Surprisingly, the lifetime of the two-quasiparticle isomer is more than 4 orders of magnitude shorter than what has been observed for analogous isomers in the lighter N=150 isotones. The four-quasiparticle isomer is longer lived than the ^{254}Rf ground state that decays exclusively by spontaneous fission with a half-life of 23.2(1.1) μs. The absence of sizable fission branches from either of the isomers implies unprecedented fission hindrance relative to the ground state.
Recent results from RIKEN/RIBF on the low-lying level structure of 29F are interpreted within the Particle-Rotor Model. We show that the experimental data can be understood in the Rotation-aligned ...Coupling Scheme, with the 5/2+ ground state as the bandhead of a decoupled band. In this picture, the energy of the observed 1/21+ state correlates strongly with the rotational energy of the core and provides an estimate of the 2+ energy in 28O. Our analysis suggests a moderate deformation, ϵ2∼0.16, and places the 2+ in 28O at ∼ 2.5 MeV.
The structure of 71Zn was investigated by one-neutron transfer and heavy-ion induced complex (deep-inelastic) reactions using the GRETINA-CHICO2 and the Gammasphere setups, respectively. The observed ...inversion between the 9/2+ and 1/2− states is explained in terms of the role of neutron pairing correlations. Non-collective sequences of levels were delineated above the 9/2+ isomeric state. These are interpreted as being associated with a modest oblate deformation in the framework of Monte-Carlo shell-model calculations carried out with the A3DA-m Hamiltonian in the pfg9/2d5/2 valence space. Similarities with the structure of Ni402868 were observed and the shape-coexistence mechanism in the N=40 region of neutron-rich nuclei is discussed in terms of the so-called Type-II shell evolution, with an emphasis on proton–neutron correlations between valence nucleons, especially those involving the shape-driving g9/2 neutron orbital.
PDF
