arXiv subjects

C. M. Petrache

Publications and source records attributed to C. M. Petrache.

At least 37 records · Page 2Linked to original sources

arXiv2020

Comment on "Erratum: Negative-parity high-spin states and a possible magnetic rotation band in $^{135}_{59}$Pr$_{76}$ [Phys. Rev. C 92, 054325 (2015)]"

In [Ritika Garg et al., Phys. Rev. C 100, 069901(E) (2019)] the experimental results on the polarization asysmetry were revised due to a claimed change of the geometry asymmetry. However, the revised results can not be reproduced as claimed in the erratum by simply changing the geometry asymmetry in extracting the polarization asymmetry, without re-extracting the polarization asymmetry from the original experimental data. It is possible that the quoted errors were significantly underestimated.

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arXiv2020

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 $^{80}$Ge. 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.

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arXiv2020

Robustness of chiral symmetry in atomic nuclei with reflection-asymmetric shapes

The present paper is a comment regarding the robustness of the chirality in presence of the space-reflection asymmetry, which leads to pairs of interleaved positive- and negative-parity bands. The recent results reported in Ref. \cite{2006.12062} which introduced the $chiture$ and $chiplex$ quantum numbers to describe an ideal nuclear system with simultaneous chiral and reflection symmetry breaking, are commented.

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arXiv2020

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. The present results constitute the missing stone in the series of Nd nuclei showing multiple chiral bands, which becomes the most extended sequence of nuclei presenting multiple chiral bands in the Segré chart.

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arXiv2020

Overestimated isomer depletion due to contamination

The recent paper by Chiara et al. provided the first experimental evidence of nuclear excitation by electron capture (NEEC), responding a long-standing theoretical prediction. NEEC was inferred to be the main channel to excite an isomer in Molybdenum-93 to a higher state, leading to a rapid release of full isomer energy (isomer depletion). The deduced large excitation probability $P_{exc}$=0.010(3) for this mechanism implied strong influence on the survival of nuclei in stellar environments. However, the excitation probability is much higher than the estimated NEEC probability $P_{NEEC}$ according to a following theoretical work by approximately 9 orders of magnitude. Nevertheless, the reported $P_{exc}$ is predicted to be due to other unknown mechanism causing isomer depletion, which is expected to open up a new era of the storage and release of nuclear energy. Here we report an analysis of the reported experimental results, showing that the observed isomer depletion is significantly overestimated due to the contamination.

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arXiv2020

Decay Spectroscopy of $^{129}$Cd

Excited states of $^{129}$In populated following the $β$-decay of $^{129}$Cd were experimentally studied with the GRIFFIN spectrometer at the ISAC facility of TRIUMF, Canada. A 480-MeV proton beam was impinged on a uranium carbide target and $^{129}$Cd was extracted using the Ion Guide Laser Ion Source (IG-LIS). $β$- and $γ$-rays following the decay of $^{129}$Cd were detected with the GRIFFIN spectrometer comprising the plastic scintillator SCEPTAR and 16 high-purity germanium (HPGe) clover-type detectors. %, along with the $β$-particles were detected with SCEPTAR. From the $β$-$γ$-$γ$ coincidence analysis, 32 new transitions and 7 new excited states were established, expanding the previously known level scheme of $^{129}$In. The $\log ft$ values deduced from the $β$-feeding intensities suggest that some of the high-lying states were populated by the $ν0 g_{7/2} \rightarrow π0 g_{9/2}$ allowed Gamow-Teller (GT) transition, which indicates that the allowed GT transition is more dominant in the $^{129}$Cd decay than previously reported. Observation of fragmented Gamow-Teller strengths is consistent with theoretical calculations.

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arXiv2020

$β$ Decay of $^{132}$In and Spectroscopy of $^{132}$Sn and $^{131}$Sb with the GRIFFIN Spectrometer

Spectroscopy of doubly magic $^{132}_{50}$Sn$_{82}$ has been performed with the GRIFFIN spectrometer at TRIUMF-ISAC following the $β$ decay of $^{132}_{49}$In$_{83}$. The analysis has allowed for the placement of a total of 70 transitions and 29 excited states in $^{132}$Sn. Detailed spectroscopy has also been performed on $^{131}$Sb, resulting from the $β$ decay of $^{131}$Sn, produced from the $β$-delayed neutron decay of $^{132}$In. Measurement of $γ$-rays in both $^{131}$Sn and $^{131}$Sb has led to the determination of the $β$-delayed neutron emission probability, $P_{n}$, from $^{132}$In. This is the first time the $P_{n}$ has been measured for this nucleus using $γ$ spectroscopy, and the new value of 12.3(4)% is consistent with the most recent $β-n$ counting experiment. Additionally, $γ$-$γ$ angular correlations have been performed in $^{132}$Sn, supporting the spin assignments of several excited states. Novel ab initio calculations are presented which describe several of the excited states, and these are compared to the experimental spectrum.

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arXiv2020

Collective 2p-2h intruder states in $^{118}$Sn studied via $β$-decay of $^{118}$In using GRIFFIN

The low-lying structure of semi-magic $^{118}$Sn has been investigated through the $β$-decay of $^{118}$In ($T_{1/2}=4.45$ min) to study shape coexistence via the reduced transition probabilities of states in the 2p-2h proton intruder band. This high-statistics study was carried out at TRIUMF-ISAC with the GRIFFIN spectrometer. In total, 99 transitions have been placed in the level scheme with 43 being newly observed. Three low-lying $γ$-ray transitions with energies near 285 keV have been resolved from which the 2$^+_{\mathrm{intr.}} \rightarrow 0^+_{\mathrm{intr.}}$ 284.52-keV transition was determined to have half of the previous branching fraction leading to a $B(E2;2^+_2\rightarrow 0^+_2)$ of 21(4) W.u. compared to 39(7) W.u. from the previous measurement. Calculations using $sd$ IBM-2 with mixing have also been made to compare the experimental $B(E2)$ values to the theoretical values and to make comparisons to the $^{114,116}$Sn isotopes previously studied using the same theoretical model.

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arXiv2019

Transverse wobbling in an even-even nucleus

Two new bands built on the two-quasiparticle $π(h_{11/2})^2$ configuration of the even-even nucleus $^{130}$Ba are investigated using constrained triaxial covariant density functional theory combined with quantum particle rotor model calculations. The energy difference between the two bands, as well as the available electromagnetic transition probabilities $B(M1)_{\textrm{out}}/B(E2)_{\textrm{in}}$ and $B(E2)_{\textrm{out}}/B(E2)_{\rm{in}}$, are well reproduced. The analysis of the angular momentum geometry reveals that the higher band represents transverse wobbling motion of a two-quasiparticle configuration. This is the first example of two-quasiparticle wobbling bands in an even-even nucleus.

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arXiv2019

Chirality of $^{135}$Nd reexamined: Evidence for multiple chiral doublet bands

One new pair of positive-parity chiral doublet bands have been identified in the odd-$A$ nucleus $^{135}$Nd which together with the previously reported negative-parity chiral doublet bands constitute a third case of multiple chiral doublet (M$χ$D) bands in the $A\approx130$ mass region. The properties of the M$χ$D bands are well reproduced by constrained covariant density functional theory and particle rotor model calculations. The newly observed M$χ$D bands in $^{135}$Nd represents an important milestone in supporting the existence of M$χ$D in nuclei.

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arXiv2019

Decoherence of collective motion in warm nuclei

Collective states in cold nuclei are represented by a wave function that assigns coherent phases to the participating nucleons. The degree of coherence decreases with excitation energy above the yrast line because of coupling to the increasingly dense background of quasiparticle excitations. The consequences of decoherence are discussed, starting with the well studied case of rotational damping. In addition to superdeformed bands, a highly excited oblate band is presented as a new example of screening from rotational damping. Suppression of pair correlation leads to incoherent thermal M1 radiation, which appears as an exponential spike (LEMAR) at zero energy in the $γ$ strength function of spherical nuclei. In deformed nuclei a Scissors Resonance appears and LEMAR changes to damped magnetic rotation, which is interpreted as partial restoration of coherence.

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arXiv2019

Valence Neutron-Proton Orientation in Atomic Nuclei

It is shown that the renormalized nuclear deformations in different mass regions can be globally scaled by two probability partition factors of Boltzmann-like distribution, which are derived from the competing valence $np$ and like-nucleon interactions. The partition factors are simply related to the probabilities of anti-parallel and fully-aligned orientations of the angular momenta of the neutrons and protons in the valence $np$ pairs, responsible for spherical- and deformed-shape phases, respectively. The partition factors derived from the renormalized deformations are also present in the new scaling law for the energies of the first $2^+$ states. A striking concordance between the distributions of the renormalized deformations and of the newly introduced parameter for the energies of the first $2^+$ states over the extended mass region from Ge to Cf is achieved, giving strong support to the existence of two phases: anti-aligned and fully-aligned subsets of $np$ pairs.

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arXiv2019

Diversity of shapes and rotations in the gamma-soft 130Ba nucleus: first observation of a t-band in the A=130 mass region

Several new bands have been identified in 130Ba, among which there is one with band-head spin 8+. Its properties are in agreement with the Fermi-aligned νh11/2^2 , 7/2+[523] \otimes 9/2-[514] Nilsson configuration. This is the first observation of a two-quasiparticle t-band in the A=130 mass region. The t-band is fed by a dipole band involving two additional h11/2 protons. The odd-spin partners of the proton and neutron S-bands and the ground-state band at high spins are also newly identified. The observed bands are discussed using several theoretical models, which strongly suggest the coexistence of prolate and oblate shapes polarized by rotation aligned two-proton and two-neutron configurations, as well as prolate collective rotations around axes with different orientations. With the new results, 130Ba presents one of the best and most complete sets of collective excitations that a gamma-soft nucleus can manifest at medium and high spins, revealing a diversity of shapes and rotations for the nuclei in the A = 130 mass region.

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arXiv2019

$β$ and $β$-delayed neutron decay of the $N=82$ nucleus $^{131}_{~49}$In$_{82}$

The half-lives of three $β$ decaying states of $^{131}_{~49}$In$_{82}$ have been measured with the GRIFFIN $γ$-ray spectrometer at the TRIUMF-ISAC facility to be $T_{1/2}(1/2^-)=328(15)$~ms, $T_{1/2}(9/2^+)=265(8)$~ms, and $T_{1/2}(21/2^+)=323(55)$~ms, respectively. The first observation of $γ$-rays following the $βn$ decay of $^{131}$In into $^{130}$Sn is reported. The $β$-delayed neutron emission probability is determined to be $P_{1n} = 12(7)\%$ for the $21/2^+$ state and $2.3(3)\%$ from the combined $1/2^-$ and $9/2^+$ states of $^{131}_{~49}$In$_{82}$ observed in this experiment. A significant expansion of the decay scheme of $^{131}$In, including 17 new excited states and 34 new $γ$-ray transitions in $^{131}_{~50}$Sn$_{81}$ is also reported. This leads to large changes in the deduced $β$ branching ratios to some of the low-lying states of $^{131}$Sn compared to previous work with implications for the strength of the first-forbidden $β$ transitions in the vicinity of doubly-magic $^{132}_{~50}$Sn$_{82}$.

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arXiv2018

Multiple chiral doublets in four-$j$ shells particle rotor model: five possible chiral doublets in $^{136}_{~60}$Nd$_{76}$

A particle rotor model, which couples nucleons in four single-$j$ shells to a triaxial rotor core, is developed to investigate the five pairs of nearly degenerate doublet bands recently reported in the even-even nucleus $^{136}$Nd. The experimental energy spectra and available $B(M1)/B(E2)$ values are successfully reproduced. The angular momentum geometries of the valence nucleons and the core support the chiral rotation interpretations not only for the previously reported chiral doublet, but also for the other four candidates. Hence, $^{136}$Nd is the first even-even candidate nucleus in which the multiple chiral doublets exist. Five pairs of chiral doublet bands in a single nucleus is also a new record in the study of nuclear chirality.

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arXiv2016

In-beam spectroscopy of medium- and high-spin states in $^{133}$Ce

Medium and high-spin states in $^{133}$Ce were investigated using the $^{116}$Cd($^{22}$Ne, $5n$) reaction and the Gammasphere array. The level scheme was extended up to an excitation energy of $\sim22.8$ MeV and spin 93/2 . Eleven bands of quadrupole transitions and two new dipole bands are identified. The connections to low-lying states of the previously known, high-spin triaxial bands were firmly established, thus fixing the excitation energy and, in many cases, the spin parity of the levels. Based on comparisons with cranked Nilsson-Strutinsky calculations and tilted axis cranking covariant density functional theory, it is shown that all observed bands are characterized by pronounced triaxiality. Competing multiquasiparticle configurations are found to contribute to a rich variety of collective phenomena in this nucleus.

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arXiv2016

Transverse wobbling motion in $^{134}$Ce and $^{136}$Nd

The existence of one-phonon and possible two-phonon transverse wobbling bands is proposed for the first time in two even-even nuclei, $^{134}$Ce and $^{136}$Nd. The predominant $E2$ character of the $ΔI = 1$ transitions connecting the one-phonon wobbling band in $^{134}$Ce to the two-quasiparticle yrast band supports the wobbling interpretation. The extracted wobbling frequencies decrease with increasing spin, indicating the transverse character of the wobbling motion, with the angular momenta of the two quasiparticles aligned perpendicular to the axis of collective rotation. A candidate for two-phonon wobbling motion is also proposed in $^{136}$Nd. The wobbling frequencies calculated in the harmonic frozen approximation are in good agreement with the experimental ones for both the$^{134}$Ce and $^{136}$Nd nuclei.

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arXiv2016

Possible chiral symmetry in $^{138}$Nd

The pheomenological Generalized Coherent State Model Hamiltonian is amended with a many body term describing a set of nucleons moving in a shell model mean-field and interacting among themselves with paring, as well as with a particle-core interaction involving a quadrupole-quadrupole and a hexadecapole-hexdecapole force and a spin-spin interaction. The model Hamiltonian is treated in a restricted space consisting of the core projected states associated to the bands ground, $β, γ,\widetildeγ, 1^+$ and $\widetilde{1^+}$ and two proton aligned quasiparticles coupled to the states of the ground band. The chirally transformed particle-core states are also included. The Hamiltonian contains two terms which are not invariant to the chiral transformations relating the right handed trihedral $({\bf J_F}, {\bf J_p}, {\bf J_n})$ and the left handed ones $(-{\bf J_F}, {\bf J_p}, {\bf J_n})$, $({\bf J_F}, -{\bf J_p}, {\bf J_n})$, $({\bf J_F}, {\bf J_p}, -{\bf J_n})$ where ${\bf J_F}, {\bf J_p}, {\bf J_n}$ are the angular momenta carried by fermions, proton and neutron bosons, respectively. The energies defined with the particle-core states form four chiral bands, two of them being degenerate. Electromagnetic properties of the chiral bands are investigated. Results are compared with the experimental data on $^{138}$Nd.

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