Excited-state spectroscopy from the first experiment at the Facility for Rare Isotope Beams (FRIB) is reported. A 24(2)-μs isomer was observed with the FRIB Decay Station initiator (FDSi) through a ...cascade of 224- and 401-keV γ rays in coincidence with ^{32}Na nuclei. This is the only known microsecond isomer (1 μs≤T_{1/2}<1 ms) in the region. This nucleus is at the heart of the N=20 island of shape inversion and is at the crossroads of the spherical shell-model, deformed shell-model, and ab initio theories. It can be represented as the coupling of a proton hole and neutron particle to ^{32}Mg, ^{32}Mg+π^{-1}+ν^{+1}. This odd-odd coupling and isomer formation provides a sensitive measure of the underlying shape degrees of freedom of ^{32}Mg, where the onset of spherical-to-deformed shape inversion begins with a low-lying deformed 2^{+} state at 885 keV and a low-lying shape-coexisting 0_{2}^{+} state at 1058 keV. We suggest two possible explanations for the 625-keV isomer in ^{32}Na: a 6^{-} spherical shape isomer that decays by E2 or a 0^{+} deformed spin isomer that decays by M2. The present results and calculations are most consistent with the latter, indicating that the low-lying states are dominated by deformation.
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.
Effects of magnetic fields on HPGe tracking detectors Lee, I. Y.; Macchiavelli, A. O.
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
01/2021, Letnik:
992
Journal Article
Recenzirano
Odprti dostop
Here, we present a study of magnetic fields effects on the position resolution and energy response of hyper-pure germanium detectors. Our results provide realistic estimates of the potential impact ...on the resolving power of tracking-arrays from (fringe) magnetic fields present when operating together with large spectrometers. By solving the equations of motion for the electron and holes in the presence of both electric and magnetic fields, we analyzed the drift trajectories of the charge carriers to determine the deviations in the positions at the end point of the trajectories, as well as changes in drift lengths affecting the energy resolution and peak shift due to trapping. Our results show that the major effect is in the deviation of the transverse (to the electric field direction) position and suggest that, if no corrective action is taken in the pulse-shape and tracking data analysis procedures, a field strength ≳ 0.1 T will start to impact the intrinsic position resolution of 2 mm (RMS). At fields above ~1 T, the degradation of the energy response becomes observable.
With new experimental information on nuclei far from stability being available, a systematic investigation of excitation energies and electromagnetic properties along the N=10,11,12 isotones and ...Z=10,11,12 isotopes is presented. The experimental data are discussed in the context of the appearance and disappearance of shell closures at N=Z=8,14,16,20, and compared to an effective-interaction approach applied to neutrons and protons in d5/22,3,4 configurations. In spite of its simplicity the model is able to explain the observed properties.
This article aims at covering various low energy nuclear physics themes that can benefit from taking advantage of active targets and time projection chambers. They are naturally oriented towards the ...study of short-lived radioactive nuclei, for which high efficiency and thick targets are necessary to boost the luminosity of the experiments due to the weak intensity of the available beams. The use of active targets is particularly crucial when the recoil energy of the kinematically important particle is small and looses too much energy or does not emerge from a solid target.
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.
Coriolis Coupling in the Continuum Macchiavelli, A. O.; Casten, R. F.; Clark, R. M. ...
Journal of physics. Conference series,
09/2020, Letnik:
1610, Številka:
1
Journal Article
Recenzirano
Odprti dostop
In 1956 A. Kerman explained the observed energy levels of the lowest K = 1/2 and K = 3/2 bands in 183W as resulting from rotational perturbations due to the effects of the Coriolis force. Following ...from that seminal paper, in this work we consider Kerman's problem when one of the Nilsson single-particle levels involved is a resonant state. This is the simplest model of Coriolis mixing that allows for analytical expressions derived in first-order perturbation theory, without losing the main physical ingredients of the continuum effects. We present results for the perturbed solution of the ground state band energies as a function of the state width, which suggest a general behavior of both increased moment of inertia and staggering.
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