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Andreoiu, C.

Publications and source records attributed to Andreoiu, C..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2020

Absence of Low-Energy Shape Coexistence in 80 Ge: The Nonobservation of a Proposed Excited 0 2 + Level at 639 keV

The 80 Ge structure was investigated in a high-statistics β-decay experiment of 80 Ga using the GRIFFIN spectrometer at TRIUMF-ISAC through γ, β-e, e-γ and γ-γ spectroscopy. No evidence was found for the recently reported 0 2 + 639-keV level suggested as evidence for low-energy shape coexistence in 80Ge. Large-scale shell model calculations performed in 78,80,82 Ge place the 0 2 + level in 80 Ge at 2 MeV. The new experimental evidence combined with shell model predictions indicate that low-energy shape coexistence is not present in 80 Ge.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Multiple chiral bands in $^{137}$Nd

Two new bands have been identified in 137 Nd from a high-statistics JUROGAM II gamma-ray spectroscopy experiment. Constrained density functional theory and particle rotor model calculations are used to assign configurations and investigate the band properties, which are well described and understood. It is demonstrated that these two new bands can be interpreted as chiral partners of previously known three-quasiparticle positive- and negative-parity bands. The newly observed chiral doublet bands in 137 Nd represent an important support to the existence of multiple chiral bands in nuclei. Overall, the present results constitute the missing stone in the series of Nd nuclei showing multiple chiral bands, which becomes the most extended sequence of odd-even and even-even nuclei presenting multiple chiral bands in the Segre chart.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Onset of high-spin rotational bands in the $N = Z$ nucleus 62 Ga

The fusion-evaporation reaction 28 Si + 40 Ca at 122 MeV beam energy was used to populate high-spin states in the odd-odd $N = Z$ nucleus 62 Ga. With the combination of the Gammasphere spectrometer and the Microball CsI(Tl) charged-particle detector array the decay scheme of 62 Ga was extended beyond 10 MeV excitation energy. The onset of band structures was observed. These high-spin rotational states are interpreted and classified by means of cranked Nilsson-Strutinsky calculations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Signatures of enhanced octupole correlations at high spin in 136 Nd

We report experimental signatures of moderately enhanced octupole correlations at high spin in 136 Nd are indicated for the first time. The extracted dipole moments of two negative-parity bands are only two times smaller than those of the lanthanide nuclei with $\textit{N}$ ≈ 90 which present well-established octupole correlations. Calculations using the cranked quasiparticle random phase approximation and a model of quadrupole-octupole rotations with octupole vibrations reveal the structure of the bands and the enhanced octupole correlations at high spin in 136 Nd .

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Search for Nova Presolar Grains: $γ$-Ray Spectroscopy of 34 Ar and its Relevance for the Astrophysical 33 Cl($p,γ$) Reaction

The discovery of presolar grains in primitive meteorites has launched a new era of research in the study of stellar nucleosynthesis. However, the accurate classification of presolar grains as being of specific stellar origins is particularly challenging. Recently, it has been suggested that sulfur isotopic abundances may hold the key to definitively identifying presolar grains with being of nova origins and, in this regard, the astrophysical Cl 33 ( p , γ ) Ar 34 reaction is expected to play a decisive role. As such, we have performed a detailed γ -ray spectroscopy study of Ar 34 . Excitation energies have been measured with high precision and spin-parity assignments for resonant states, located above the proton threshold in Ar 34 , have been made for the first time. Uncertainties in the Cl 33 ( p , γ ) reaction have been dramatically reduced and the results indicate that a newly identified ℓ = 0 resonance at E r = 396.9 ( 13 ) keV dominates the entire rate for T = 0.25 – 0.40 GK . Furthermore, nova hydrodynamic simulations based on the present work indicate an ejected S 32 / S 33 abundance ratio distinctive from type-II supernovae and potentially compatible with recent measurements of a presolar grain.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Mass measurements of neutron-rich gallium isotopes refine production of nuclei of the first r -process abundance peak in neutron-star merger calculations

We report mass measurements of neutron-rich Ga isotopes 80–85 Ga with TRIUMF's Ion Trap for Atomic and Nuclear science. The measurements determine the masses of 80–83 Ga in good agreement with previous measurements. Here, the masses of 84 Ga and 85 Ga were measured for the first time. Uncertainties between 25 and 48 keV were reached. The new mass values reduce the nuclear uncertainties associated with the production of A ≈ 84 isotopes by the r-process for astrophysical conditions that might be consistent with a binary neutron star (BNS) merger producing a blue kilonova. Our nucleosynthesis simulations confirm that BNS merger may contribute to the first abundance peak under moderate neutron-rich conditions with electron fractions Y e = 0.35–0.38.

59 ≤ A ≤ 89↗
DOE OSTI2019

Highly deformed bands in Nd nuclei: New results and consistent interpretation within the cranked Nilsson-Strutinsky formalism

Three new highly-deformed (HD) bands are identified in 136 Nd and the highly deformed band of 137 Nd is extended at higher spin by four transitions, revealing a band crossing associated with the occupation of the second $νi_{13/2}$ intruder orbital. Extended cranked Nilsson-Strutinsky calculations are performed for all HD bands observed in 134 Nd, 136 Nd, and 137 Nd, achieving for the first time a consistent interpretation of all HD bands in the Nd nuclei. The new interpretation has significant consequences, like the change of parity of the yrast HD bands of 134 Nd and 136 Nd, and the involvement of two negative-parity neutron intruder orbitals in the configurations of most HD bands. The present experimental results and their theoretical interpretation represent an important step forward in the understanding of the second-minimum excitations in the Nd nuclei.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Direct Observation of Proton Emission in Be 11

The rare β - p + decay was observed in Be 11 by directly measuring the emitted protons and their energy distribution for the first time with the prototype Active Target Time Projection Chamber in an experiment performed at ISAC-TRIUMF. The measured β - p + branching ratio is orders of magnitude larger than any other theoretical model predicted. This can be explained by the presence of a narrow resonance in B 11 above the proton separation energy.

delayed particle emission↗
DOE OSTI2019

Isospin symmetry in B ( E 2 ) values: Coulomb excitation study of Mg 21

The T z = -$\frac{3}{2}$ nucleus 21 Mg has been studied by Coulomb excitation on 196 Pt and 110 + ground state to the first excited $\frac{1}{2}$ + state in 21 Mg was observed for the first time. Coulomb excitation cross-section measurements with both targets and a measurement of the half-life of the $\frac{1}{2}$ + state yield an adopted value of B (E2; $\frac{5}{2}$ + → $\frac{1}{2}$ + ) = 13.3 (4) W.u. A new excited state at 1672(1) keV with tentative $\frac{9}{2}$ + assignment was also identified in 21 Mg . This work demonstrates a large difference in the B (E2; $\frac{5}{2}$ + → $\frac{1}{2}$ + ) value between T = $\frac{3}{2}$, A = 21 mirror nuclei. Lastly, the difference is investigated in the shell-model framework employing both isospin conserving and breaking USD interactions and using modern ab initio nuclear structure calculations, which have recently become applicable in the s d shell.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Collective rotation of an oblate nucleus at very high spin

A sequence of nine almost equidistant quadrupole transitions is observed in 137 Nd. The sequence represents an extremely regular rotational band that extends to a spin of about 75/2 and an excitation energy of ≈ 4.5 MeV above yrast. Here, cranked mean-field calculations of the Nilsson-Strutinsky type suggest an oblate shape for the band. They reproduce the observed I(I + 1) dependence of the rotational energy whereas predicting a pronounced decrease in the deformation, which is the hallmark of antimagnetic rotation.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Observation of excited states in 20 Mg sheds light on nuclear forces and shell evolution

The exotic Borromean nucleus 20 Mg with N = 8, located at the proton drip line provides a unique testing ground for nuclear forces and the evolution of shell structure in the neutron-deficient region. We report on the first observation of proton unbound resonances together with bound states in 20 Mg from the 20 Mg (d, d ') reaction performed at TRIUMF. Phenomenological shell-model calculations offer a reasonable description. However, our experimental results present a challenge for current first-principles nuclear structure approaches and point to the need for improved chiral forces and ab initio calculations. Furthermore, the differential cross section of the first excited state is compared with distorted-wave Born approximation calculations to deduce a neutron quadrupole deformation parameter of β$_n$ = 0.46 ± 0.21 . Lastly, this provides the first indication of a possible weakening of the N = 8 shell closure at the proton drip line.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Mass measurements of neutron-rich indium isotopes toward the N = 82 shell closure

Precise mass measurements of the neutron-rich 125–130 In isotopes have been performed with the TITAN Penning trap mass spectrometer. TITAN's electron beam ion trap was used to charge breed the ions to charge state q = 13+ thus providing the necessary resolving power to measure not only the ground states but also isomeric states at each mass number. Here, the properties of the ground states are investigated through a series of mass differentials, highlighting trends in the indium isotopic chain as compared to its proton-magic neighbor, tin (Z = 50). In addition, the energies of the indium isomers are presented. The (8 - ) level in 128 In is found to be 78 keV lower than previously thought and the (21/2 - ) isomer in 127 In is shown to be lower than the literature value by more than 150 keV.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Precision mass measurements of magnesium isotopes and implications for the validity of the isobaric mass multiplet equation

If the mass excess of neutron-deficient nuclei and their neutron-rich mirror partners are both known, it can be shown that deviations of the isobaric mass multiplet equation (IMME) in the form of a cubic term can be probed. Such a cubic term was probed by using the atomic mass of neutron-rich magnesium isotopes measured using the TITAN Penning trap and the recently measured proton-separation energies of 29 Cl and 30 Ar. The atomic mass of 27 Mg was found to be within 1.6σ of the value stated in the Atomic Mass Evaluation. The atomic masses of 28,29 Mg were measured to be both within 1σ, while being 7 and 33 times more precise, respectively. Using the 29 Mg mass excess and previous measurements of 29 Cl, we uncovered a cubic coefficient of d = 28(7)keV, which is the largest known cubic coefficient of the IMME. This departure, however, could also be caused by experimental data with unknown systematic errors. Hence there is a need to confirm the mass excess of 28 S and the one-neutron separation energy of 29 Cl, which have both come from a single measurement. Lastly, our results were compared with ab initio calculations from the valence-space in-medium similarity renormalization group, resulting in a good agreement.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗