arXiv subjects

Sabin Stoica

Publications and source records attributed to Sabin Stoica.

18 recordsLinked to original sources

arXiv2026

Precise determination of electron-capture $Q$ value of $^{113}$Sn decay related to electron neutrino mass measurements

A high-precision measurement of the electron-capture (EC) decay $Q$ value for the ground-state-to-ground-state (gs-to-gs) transition of $^{113}$Sn to $^{113}$In has been performed using the JYFLTRAP double Penning trap mass spectrometer. Employing the phase-imaging ion-cyclotron-resonance technique, the isomeric state of $^{113}$Sn at 77.389(19) keV was resolved, and the cyclotron frequency ratio measured between the isomer $^{113m}$Sn and the daughter nucleus $^{113}$In. This yielded an isomer-to-ground-state $Q$ value of 1116.64(19) keV and gs-to-gs $Q$ value of 1039.25(19) keV. The atomic mass excess of $^{113}$Sn was determined as $-$88327.87(27) keV/c$^2$, in excellent agreement with the Atomic Mass Evaluation 2020 (AME2020) but with a sixfold precision improvement. Using nuclear energy-level data for $^{113}$In, we identified two low $Q$-value transitions of the ground state of $^{113}$Sn to excited states of $^{113}$In at 1024.280(50) keV ($Q_{EC}^* = 14.97(20)$ keV, second forbidden non-unique) and 1029.650(50) keV ($Q_{EC}^* = 9.60(20)$ keV, allowed). The allowed transition exhibits small energy differences ($Δ_{L1} = 5.58(20)$ keV, $Δ_{L2} = 5.87(20)$ keV) from L1 and L2 shell binding energies, enhancing endpoint events. Partial half-lives and energy-release spectra were calculated using the self-consistent Dirac-Hartree-Fock-Slater (DHFS) method (including exchange, overlap, shake-up, and shake-off corrections) together with the nuclear shell model, show enhanced endpoint sensitivity for the allowed transition to the state at 1029.650 keV. Including subthreshold atomic states in the spectral function enhances the EC rate near the zero-neutrino-momentum region by a factor of five, enabling new approaches for low $Q$-value EC reactions in neutrino-mass studies.

nucl-ex↗

arXiv2025

High-precision direct decay energy measurements of the electron-capture decay of $^{97}$Tc

A direct measurement of the ground-state-to-ground-state electron-capture decay $Q$ ($Q_{\rm EC}$) value of $^{97}$Tc has been conducted employing the high resolving power phase-imaging ion-cyclotron-resonance technique with the double Penning trap mass spectrometer JYFLTRAP. The resulting $Q_{\rm EC}$ value for $^{97}$Tc is 324.82(21) keV, exhibiting a precision approximately 19 times higher than the value adopted in the newest Atomic Mass Evaluation (AME2020) and differing by 1.2$σ$. Furthermore, by combining this refined $Q$ value with nuclear energy-level data for the decay-daughter $^{97}$Mo, a potential ultra-low Q-value transition, possibly of allowed type, $^{97}$Tc (9/2$^{+}$, ground state) $\rightarrow$ $^{97}$Mo$^{*}$ (320(1) keV), was evaluated for future long-term neutrino-mass determination experiments. The ground-state-to-excited-state electron-capture decay $Q$ value ($Q^{*}_{\rm EC}$) of this transition was determined to be 4.8(10) keV, confirming it to be energetically allowed with a confidence level of exceeding 4$σ$. The captures of electrons occupying the L and higher shells for this transition are energetically allowed, giving a value of 2.0(10) keV for the closest distance of $Q^{*}_{\rm EC}$ to the allowed binding energy of the L1 shell. To predict partial half-lives and energy-release distributions for this transition, the atomic self-consistent many-electron Dirac--Hartree--Fock--Slater method and the nuclear shell model have been employed. Dominant correction terms such as exchange and overlap corrections, as well as shake-up and shake-off effects, were included in the final results. Moreover, the normalized distribution of released energy in the electron-capture decay of $^{97}$Tc to excited states of $^{97}$Mo, is compared with that of $^{163}$Ho, which is being used for electron-neutrino-mass determination.

nucl-ex↗

arXiv2025

$β^-$-decay half-lives of even-even nuclei using the recently introduced phase space recipe

We present the beta decay half-lives calculation for selected even even nuclei that decay through electron emission. The kinematical portion of the half-life calculation was performed using a recently introduced technique for computation of phase space factors (PSFs). The dynamical portion of our calculation was performed within the proton neutron quasiparticle random phase approximation (pn QRPA) model. Six nuclei (20O, 24Ne, 34Si, 54Ti, 62Fe, and 98Zr) were selected for the present calculation. We compare the calculated PSFs for these cases against the traditionally used recipe. In our new approach, the Dirac equation was numerically solved employing a Coulomb potential. This potential was adopted from a more realistic proton distribution of the daughter nucleus. Thus, the finite size of the nucleus and the diffuse nuclear surface corrections are taken into account. Moreover, a screened Coulomb potential was constructed to account for the effect of atomic screening. The power series technique was used for the numerical solution. The calculated values of half-lives, employing the recently developed method for computation of PSFs, were in good agreement with the experimental data.

nucl-th↗

arXiv2024

High-precision measurements of the atomic mass and electron-capture decay $Q$ value of $^{95}$Tc

A direct measurement of the ground-state-to-ground-state electron-capture decay $Q$ value of $^{95}$Tc has been performed utilizing the double Penning trap mass spectrometer JYFLTRAP. The $Q$ value was determined to be 1695.92(13) keV by taking advantage of the high resolving power of the phase-imaging ion-cyclotron-resonance technique to resolve the low-lying isomeric state of $^{95}$Tc (excitation energy of 38.910(40) keV) from the ground state. The mass excess of $^{95}$Tc was measured to be $-$86015.95(18) keV/c$^2$, exhibiting a precision of about 28 times higher and in agreement with the value from the newest Atomic Mass Evaluation (AME2020). Combined with the nuclear energy-level data for the decay-daughter $^{95}$Mo, two potential ultra-low $Q$-value transitions are identified for future long-term neutrino-mass determination experiments. The atomic self-consistent many-electron Dirac--Hartree--Fock--Slater method and the nuclear shell model have been used to predict the partial half-lives and energy-release distributions for the two transitions. The dominant correction terms related to those processes are considered, including the exchange and overlap corrections, and the shake-up and shake-off effects. The normalized distribution of the released energy in the electron-capture decay of $^{95}$Tc to excited states of $^{95}$Mo is compared to that of $^{163}$Ho currently being used for electron-neutrino-mass determination.

nucl-ex↗

arXiv2024

Theoretical analysis and predictions for the double electron capture of $^{124}$Xe

We provide a complete theoretical description of the two-neutrino electron capture in $^{124}$Xe, improving both the nuclear and the atomic structure calculations. We improve the general formalism through the use of the Taylor expansion method, leading to higher order terms in the decay rate of the process. The nuclear part is treated with pn-QRPA and interacting shell model (ISM) methods. The nuclear matrix elements (NMEs) are calculated with the pn-QRPA method with spin restoration by fixing the input parameters so that the experimental decay rate is reproduced, resulting in values significantly lower than in previous calculations. The validity of the pn-QRPA NMEs is tested by showing their values to be comparable with the ones for double-beta decay with emission of two electrons of $^{128,130}$Te, which have similar pairing features. Within the ISM, we reproduce the total experimental half-life within a factor of two and predict the capture fraction to the KK channel of about 74\%. We also predict the capture fractions to other decay channels and show that for the cumulative decay to the $\rm{KL_{1}}$-$\rm{KO_{1}}$ channels, a capture fraction of about 24\% could be observed experimentally. On the atomic side, calculations are improved by accounting for the Pauli blocking of the decay of innermost nucleon states and by considering all $s$-wave electrons available for capture, expanding beyond the K and L$_1$ orbitals considered in previous studies. We also provide improved atomic relaxation energies of the final atomic states of $^{124}$Te, which may be used as input for background modeling in liquid Xenon experiments.

nucl-th↗

arXiv2023

Predicting the neutrinoless double-beta decay matrix element of $^{136}$Xe using a statistical approach

Calculation of the nuclear matrix elements (NMEs) for double-beta decay is of paramount importance for guiding experiments and for analyzing and interpreting the experimental data, especially for the search of the neutrinoless double beta decay mode ($0νββ$). However, there are currently still large differences between the NME values calculated by different methods, hence a quantification of their uncertainties is very much required. In this paper we propose a statistical analysis of $0νββ$ NME for the $^{136}Xe$ isotope, based on the interacting shell model, but using three independent effective Hamiltonians, emphasizing the range of the NMEs' most probable values and its correlations with observables that can be obtained from the existing nuclear data. Consequently, we propose a common probability distribution function for the $0νββ$ NME, which has a range of (1.55 - 2.65) at 90\% confidence level, with a mean value of 1.99 and a standard deviation of 0.37.

nucl-th↗

arXiv2020

Lorentz violation effects in two neutrino double-beta decay

Observable effects for the Lorentz invariance violation (LIV) at a low energy scale can also be investigated in double beta decay (DBD). For example, by comparing the theoretical predictions with a precise analysis of the summed energy spectra of electrons in $2νββ$ decay, one can constrain the $\mathring{a}_{of}^{(3)}$ coefficient that governs the time-like component of the Lorentz invariance violating operator that appears in the Standard Model extension theory. In this work, we perform calculations of the phase space factors and summed energy spectra of electrons as well as of their deviations due to LIV necessary in such experimental investigations. The Fermi functions needed in the calculation are built up with exact electron wave functions obtained by numerically solving the Dirac equation in a realistic Coulomb-type potential with the inclusion of the finite nuclear size and screening effects. We compared our results with those used in previous LIV investigations that were obtained with approximate (analytical) Fermi functions and found differences of up to $30\%$ for heavier nuclei. Our work includes eight experimentally interesting nuclei. Next, we estimate and discuss the uncertainties of our calculations associated with uncertainties in Q-values measurements and the differences raised from the inclusion of the kinematic terms in the formalism. Finally, we provide the ratio between the standard phase space factors and their LIV deviations and the energies where the LIV effects are expected to be maximal. We expect our study to be useful in the current LIV investigations in $2νββ$ decay and to lead to improved constraints on the $\mathring{a}_{of}^{(3)}$ coefficient.

nucl-th↗

arXiv2019

Study of the effect of newly calculated phase space factor on beta-decay half-lives

We present results for $β$-decay half-lives based on a new recipe for calculation of phase space factors recently introduced. Our study includes $fp$-shell and heavier nuclei of experimental and astrophysical interests. The investigation of the kinematics of some $β$-decay half-lives is presented, and new phase space factor values are compared with those obtained with previous theoretical approximations. Accurate calculation of nuclear matrix elements is a pre-requisite for reliable computation of $β$-decay half-lives and is not the subject of this paper. This paper explores if improvements in calculating the $β$-decay half-lives can be obtained when using a given set of nuclear matrix elements and employing the new values of the phase space factors. Although the largest uncertainty in half-lives computations come from the nuclear matrix elements, introduction of the new values of the phase space factors may improve the comparison with experiment. The new half-lives are systematically larger than previous calculations and may have interesting consequences for calculation of stellar rates.

nucl-th↗

arXiv2019

Computation of products of phase space factors and nuclear matrix elements for the Double Beta Decay

The nuclear matrix elements (NME) and phase space factors (PSF) entering the half-life formulas of the double-beta decay (DBD) process are two key quantities whose accurate computation still represents a challenge. In this paper we propose a new approach of calculating them, namely to compute directly their product as an unique formula. This procedure allows a more coherent treatment of the nuclear approximations and input parameters appearing in both quantities and avoids possible confusion in interpreting the DBD data due to different individual expressions adopted for PSF and NME (and consequently their reporting in different units) by different authors. Our calculations are performed for both two neutrino ($2νββ$) and neutrinoless ($0νββ$) decay modes, and for five nuclei of most experimental interest. Further, using the most recent experimental limits for $0νββ$ decay half-lives, we provide new constraints on the light mass neutrino parameter. Finally, by separating in the half-lives formulas the factor representing the axial-vector constant to the forth, we advance suggestions on how to reduce the errors introduced in calculation by the uncertain value of this constant by exploiting the DBD data from different isotopes and/or decay modes.

nucl-th↗

arXiv2016

New phase space calculations for beta-decay half-lives

We revisit the computation of the phase space factors (PSF) involved in the positron decay and electron capture (EC) processes for a large number of nuclei of experimental interest. To obtain the electron/positron wave functions needed in computation, we develop a code for solving accurately the Dirac equation with a nuclear potential derived from a realistic proton density distribution in the nucleus. The finite nuclear size (FNS) and screening effects are included through recipes which differ from those used in previous calculations. Comparing our results with former calculations employing approximate methods but computed with the same Q-values, we find a close agreement for positron decays, while for the EC process there are relevant differences. For the EC process we also find that the screening effect has a notable influence on the computed PSF values specially for light nuclei. Further, we re-computed the same PSF values but using the most recent Q-values reported in literature. In several cases these new Q-values differ significantly from the older ones, which results in large differences in the PSF values as compared with previous results. These new PSF values proposed here, can contribute to a more reliable calculation of the beta decay rates, which are key quantities in the study of nuclei far from the stability line, as well as to better understanding of the stellar evolution.

nucl-th↗

arXiv2016

First-forbidden transitions and stellar $β$-decay rates of Zn and Ge isotopes

First-forbidden (FF) charge-changing transitions become relatively important for nuclei as their proton number increases. This is because the strength of allowed Gamow-Teller (GT) transitions decreases with increasing Z. The FF transitions play an important role in reducing the half-lives as against those calculated from taking the GT transitions alone into account. In this paper we calculate allowed GT as well as $0^{+} \rightarrow 0^{-}$ and $0^{+} \rightarrow 2^{-}$ transitions for neutron-rich Zn and Ge isotopes. Two different pn-QRPA models were used with a schematic separable interaction to calculate GT and FF transitions. Half-lives calculated after inclusion of FF transitions were in excellent agreement with the experimental data. Our calculations were also compared to previous QRPA calculations and were found to be in better agreement with measured data. Stellar $β$-decay rates were calculated for these nuclei including allowed GT and unique FF transitions for astrophysical applications. $^{86,88}$Ge has a sizeable contribution to the total stellar rate from unique FF transitions.

nucl-th↗

arXiv2014

Unique first-forbidden $β$-decay rates for neutron-rich nickel isotopes in stellar environment

In astrophysical environments, allowed Gamow-Teller (GT) transitions are important, particularly for $β$-decay rates in presupernova evolution of massive stars, since they contribute to the fine-tuning of the lepton-to-baryon content of the stellar matter prior to and during the collapse of a heavy star. In environments where GT transitions are unfavored, first-forbidden transitions become important especially in medium heavy and heavy nuclei. Particularly in case of neutron-rich nuclei, first-forbidden transitions are favored primarily due to the phase-space amplification for these transitions. In this work the total $β$-decay half-lives and the unique first-forbidden(U1F) $β$-decay rates for a number of neutron-rich nickel isotopes, $^{72-78}$Ni, are calculated using the proton-neutron quasi-particle random phase approximation (pn-QRPA) theory in stellar environment for the first time. For the calculation of the $β$-decay half-lives both allowed and unique first-forbidden transitions were considered. Comparison of the total half-lives is made with measurements and other theoretical calculations where it was found that the pn-QRPA results are in better agreement with experiments and at the same time are suggestive of inclusion of rank 0 and rank 1 operators in first-forbidden rates for still better results.

nucl-th↗

arXiv2014

Constraints on light neutrino parameters derived from the study of neutrinoless double beta decay

The study of the neutrinoless double beta ($0 ββ$) decay mode can provide us with important information on the neutrino properties, particularly on the electron neutrino absolute mass. In this work we revise the present constraints on the neutrino mass parameters derived from the $0 ββ$ decay analysis of the experimentally interesting nuclei. We use the latest results for the phase space factors (PSFs) and nuclear matrix elements (NMEs), as well as for the experimental lifetimes limits. For the PSFs we use values computed with an improved method reported very recently. For the NMEs we use values chosen from literature on a case-by-case basis, taking advantage of the consensus reached by the community on several nuclear ingredients used in their calculation. Thus, we try to restrict the range of spread of the NME values calculated with different methods and, hence, to reduce the uncertainty in deriving limits for the Majorana neutrino mass parameter. Our results may be useful to have an up-date image on the present neutrino mass sensitivities associated with $0 ββ$ measurements for different isotopes and to better estimate the range of values of the neutrino masses that can be explored in the future double beta decay (DBD) experiments.

nucl-th↗

arXiv2013

Study of Nuclear Effects in the Computation of the 0νββ Decay Matrix Elements

We analyse the effects that different nuclear structure approximations associated with the short range correlations (SRC), finite nucleon size (FNS), higher order terms in the nucleon currents (HOC) and with some nuclear input parameters, have on the values of the nuclear matrix elements (NMEs) for the neutrinoless double beta (0νββ) decay. The calculations are performed with a new Shell Model(ShM) code which allows a fast computation of the two-body matrix elements of the transition operators. The treatment of SRC, FNS and HOC and include the use of quenched or unquenched values for the axial vector coupling constant produce the most important effects on the NMEs values. Equivalent effects of some of these approximations are also possible, which may lead (accidentally) to close final results. We found that the cummulative effect of all these nuclear ingredients on the calculated nuclear matrix elements NMEs is significant. Since the NMEs values are often obtained with different approximations and/or with different input parameters, a convergent view point on their inclusion/neglection and an uniformization of the calculations are needed, in order to enter in an era of precision concerning the computation of the NMEs for double beta deacay.

nucl-th↗

arXiv2013

New Calculations for Phase Space Factors Involved in Double Beta Decay

We present new results for the phase space factors involved in double beta decay for beta-beta- transitions to ground states and excited 0+1 states, for isotopes of experimental interest. The Coulomb distortion of the electron wave functions is treated by solving numerically the Dirac equation with inclusion of the finite nuclear size and electron screening effects, and using a Coulomb potential derived from a realistic proton density distribution in the daughter nucleus. Our results are compared with other results from literature, obtained in different approximations, and possible causes that can give differences are discussed.

nucl-th↗

arXiv2012

Fast, Efficient Calculations of the Two-Body Matrix Elements of the Transition Operators for Neutrinoless Double Beta Decay

To extract information about the neutrino properties from the study of neutrinoless double-beta (0νββ) decay one needs a precise computation of the nuclear matrix elements (NMEs) associated with this process. Approaches based on the Shell Model (ShM) are among the nuclear structure methods used for their computation. ShM better incorporates the nucleon correlations, but have to face the problem of the large model spaces and computational resources. The goal is to develop a new, fast algorithm and the associated computing code for efficient calculation of the two-body matrix elements (TBMEs) of the 0νββ decay transition operator, which are necessary to calculate the NMEs. This would allow us to extend the ShM calculations for double-beta decays to larger model spaces, of about 9-10 major harmonic oscillator shells. The improvement of our code consists in a faster calculation of the radial matrix elements. Their computation normally requires the numerical evaluation of two-dimensional integrals: one over the coordinate space and the other over the momentum space. By rearranging the expressions of the radial matrix elements, the integration over the coordinate space can be performed analytically, thus the computation reduces to sum up a small number of integrals over momentum. Our results for the NMEs are in a good agreement with similar results from literature, while we find a significant reduction of the computation time for TBMEs, by a factor of about 30, as compared with our previous code that uses two-dimensional integrals.

nucl-th↗

arXiv2011

A mobile detector for measurements of the atmospheric muon flux in underground sites

Muons comprise an important contribution of the natural radiation dose in air (approx. 30 nSv/h of a total dose rate of 65-130 nSv/h), as well as in underground sites even when the flux and relative contribution are significantly reduced. The flux of the muons observed in underground can be used as an estimator for the depth in mwe (meter water equivalent) of the underground site. The water equivalent depth is an important information to devise physics experiments feasible for a specific site. A mobile detector for performing measurements of the muon's flux was developed in IFIN-HH, Bucharest. Consisting of 2 scintillator plates (approx. 0.9 m2) which measure in coincidence, the detector is installed on a van which facilitates measurements at different locations at surface or underground. The detector was used to determine muon fluxes at different sites in Romania. In particular, data were taken and the values of meter water equivalents were assessed for several locations from the salt mine from Slanic Prahova, Romania. The measurements have been performed in 2 different galleries of the Slanic mine at different depths. In order to test the stability of the method, also measure- ments of the muon flux at surface at different elevations were performed. The results were compared with predictions of Monte-Carlo simulations using the CORSIKA and MUSIC codes.

astro-ph.IM↗

arXiv2009

Shell Model Analysis of the Neutrinoless Double Beta Decay of $^{48}$Ca

The neutrinoless double beta ($0νββ$) decay process could provide crucial information to determine the absolute scale of the neutrino masses, and it is the only one that can establish whether neutrino is a Dirac or a Majorana particle. A key ingredient for extracting the absolute neutrino masses from $0νββ$ decay experiments is a precise knowledge of the nuclear matrix elements (NME) describing the half-life of this process. We developed a new shell model approach for computing the $0νββ$ decay NME, and we used it to analyze the $0νββ$ mode of $^{48}$Ca. The dependence of the NME on the short range correlations parameters, on the average energy of the intermediate states, on the finite-size cutoff parameters, and on the effective interaction used for the many-body calculations is discussed.

nucl-th↗