Gamow-Teller Strength Distributions Research Articles

Re-Examination of the Effect of Pairing Gaps on Gamow–Teller Strength Distributions and β-Decay Rates

β-decay is one of the key factors for understanding the r-process and evolution of massive stars. The Gamow–Teller (GT) transitions drive the β-decay process. We employ the proton–neutron quasiparticle random phase approximation (pn-QRPA) model to calculate terrestrial and stellar β-decay rates for 50 top-ranked nuclei possessing astrophysical significance according to a recent survey. The model parameters of the pn-QRPA model affect the predicted results of β-decay. The current study investigates the effect of nucleon–nucleon pairing gaps on charge-changing transitions and the associated β decay rates. Three different values of pairing gaps, namely TF, 3TF, and 5TF, were used in our investigation. It was concluded that both GT strength distributions and half-lives are sensitive to pairing gap values. The 3TF pairing gap scheme, in our chosen nuclear model, resulted in the best prediction with around 80% of the calculated half-lives within a factor 10 of the measured ones. The 3TF pairing scheme also led to the calculation of the biggest β-decay rates in stellar matter.

Re-analysis of the Gamow-Teller distributions for [formula omitted] nuclei, 24Mg, 28Si, and 32S

The Gamow-Teller (GT) strength distributions of sd-shell N=Z nuclei (24Mg, 28Si, and 32S) are investigated within the framework of proton–neutron quasi-particle random phase approximation (pn-QRPA) using a deformed basis. The nuclear properties of these special nuclei were investigated by the Relativistic Mean Field (RMF) model. The RMF framework with density-dependent interactions (DD-PC1 and DD-ME2) was employed to compute ground-state deformation parameters (β2) using potential energy curves. The β2 values computed from the RMF and the finite range droplet model (FRDM) were employed as a free parameter in the pn-QRPA calculation to investigate the resulting GT strength distributions. The present model-based analyses compare well with the observed data.

INVESTIGATION OF GAMOW-TELLER STRENGTHS FOR A=34 AND A=37 NUCLEI

In this work, the Gamow-Teller transition strength distributions in sd-shell region were investigated by using shell model. The calculations of B(GT) and their accumulated sum ∑B(GT) for the nuclei 34Ar→34Cl, 34S→34Cl, 37Cl→37Ar, and 37Ca→37K were performed using the shell model by utilizing the USDB and USDA effective interactions in the sd-shell. The transitions for the nuclei are studied. The results for the B(GT) and ∑B(GT) are discussed and compared to the most recent observed data. The measured GT distributions and the cumulative transition intensities B(GT) are in a fair amount of agreement with the theoretical calculations. We have established a qualitative agreement for the individual transformations, and the measured cumulative transformation strengths nearly match the observed ones.

Stellar weak-interaction rates for rp-process waiting-point nuclei from projected shell model

We propose a projected shell model (PSM) for description of stellar weak-interaction rates between even-even and odd-odd nuclei with extended configuration space where up to six-quasiparticle (qp) configurations are included, and the stellar weak-interaction rates for eight rp-process waiting-point (WP) nuclei, 64Ge, 68Se, 72Kr, 76Sr, 80Zr, 84Mo, 88Ru and 92Pd, are calculated and analyzed for the first time within the model. Higher-order qp configurations are found to affect the underlying Gamow-Teller strength distributions and the corresponding stellar weak-interaction rates. Under rp-process environments with high temperatures and densities, on one hand, thermal population of excited states of parent nuclei tends to decrease the stellar β+ decay rates. On the other hand, the possibility of electron capture (EC) tends to provide increasing contribution to the rates with temperature and density. The effective half-lives of WP nuclei under the rp-process peak condition are predicted to be reduced as compared with the terrestrial case, especially for 64Ge and 68Se.

Re-examination of β-decay in Hg, Pb and Po Isotopes

This study re-examines the effect of nuclear deformation on the calculated Gamow-Teller (GT) strength distributions of neutron-deficient (178−192Hg, 185−194Pb and 196−206Po) nuclei. The nuclear ground state properties and shape parameters were calculated using the Relativistic Mean Field model. Three different density-dependent interactions (D3C, DD-ME2 and DD-PC1) were used in the calculation. Estimated shape parameters were later used within the framework of deformed proton-neutron quasi-random phase approximations model, with a separable interaction, to calculate the GT strength distributions, half-lives and branching ratios for these neutron–deficient isotopes. It was concluded that half-lives and GT strength distributions vary considerably with change in shape parameter.

β^{+} Gamow-Teller Strengths from Unstable ^{14}O via the (d,^{2}He) Reaction in Inverse Kinematics.

For the first time, the (d,^{2}He) reaction was successfully used in inverse kinematics to extract the Gamow-Teller transition strength in the β^{+} direction from an unstable nucleus. The new technique was made possible by the use of an active-target time-projection chamber and a magnetic spectrometer, and opens a path to addressing a range of scientific challenges, including in astrophysics and neutrino physics. In this Letter, the nucleus studied was ^{14}O, and the Gamow-Teller transition strength to ^{14}N was extracted up to an excitation energy of 22MeV. The data were compared to shell-model and state-of-the-art coupled-cluster calculations. Shell-model calculations reproduce the measured Gamow-Teller strength distribution up to about 15MeV reasonably well, after the application of a phenomenological quenching factor. In a significant step forward to better understand this quenching, the coupled-cluster calculation reproduces the full strength distribution well without such quenching, owing to the large model space, the inclusion of strong correlations, and the coupling of the weak interaction to two nucleons through two-body currents.

Investigation of β-decay properties of neutron-rich Cerium isotopes

Reliable and precise knowledge of the β-decay properties of neutron-rich nuclei is important for a better understanding of the r-process. We report the computation of β-decay properties of neutron-rich Cerium isotopes calculated within the proton-neutron quasiparticle random phase approximation (pn-QRPA) approach. A total of 34 isotopes of Ce in the mass range 120 ≤ A ≤ 157 were considered in our calculation. Pairing gaps are recognized amongst the key parameters in the pn-QRPA model to compute Gamow-Teller (GT) transitions. We employed two different values of the pairing gaps obtained from two different empirical formulae in our computation. The GT strength distributions changed considerably with a change in the pairing gap values. This in turn resulted in contrasting centroid and total strength values of the GT distributions and led to differences in calculated half-lives using the two schemes. The traditional pairing gaps resulted in significant fragmentation of GT strength. However, the pairing gaps, calculated employing the formula based on separation energies of neutron and proton, led to computed half-lives in better agreement with the measured data.

Gamow-Teller transitions in magic nuclei calculated by the charge-exchange subtracted second random-phase approximation

The Gamow-Teller (GT) transitions in four magic nuclei, $^{48}\mathrm{Ca}, ^{90}\mathrm{Zr}, ^{132}\mathrm{Sn}$, and $^{208}\mathrm{Pb}$, are studied using a self-consistent Hartree-Fock (HF) plus charge-exchange subtracted second random-phase approximation (SSRPA) model with several Skyrme energy density functions (EDFs). These calculations show that SSRPA improves systematically the description of main GT strength distributions in terms of the excitation energy and the peak height. The quenching factors are evaluated to be 13--20% of the Ikeda sum rule for $^{48}\mathrm{Ca}, ^{90}\mathrm{Zr}$, and $^{132}\mathrm{Sn}$, due to the couplings to two-particle--two-hole (2p-2h) configurations. Also examined are the effects of tensor interactions on the excitation energies and the quenching factors of GT strength distributions.

Investigation of Gamow–Teller strength of 186Hg within deformed pn-QRPA

Recently, the total absorption gamma spectroscopy technique was used to determine the Gamow–Teller (GT) distribution of β-decay of 186Hg. It was concluded that the best description of the measured data was obtained with dominantly prolate components for both parent 186Hg and daughter 186Au. Motivated by the recent findings, we investigate the effect of nuclear deformation on the energy distribution of the GT strength of the decay of 186Hg into 186Au within the framework of proton–neutron quasiparticle random phase approximation (pn-QRPA) based on the deformed Nilsson potential. To do the needful, we first calculate the energy levels and shape prediction of 186Hg within the interacting boson model. The computed GT strength distribution satisfied the model-independent Ikeda sum rule 100% (99.98%) for the prolate (oblate) case. Based on the strength distributions, the deformed pn-QRPA model with separable interaction prefers a prolate shape for the ground state of 186Hg and supports the shape coexistence for this nucleus.

Tensor force effect on pairing correlations for the Gamow–Teller transition in 42Ca, 46Ti, and 18O

Abstract We investigate the tensor force (TF) effect on the Gamow–Teller (GT) transition strength distributions in 42Ca, 46Ti, and 18O, which are known to have strong low-energy GT states, the so-called low-energy super GT (LeSGT) transition, peculiar to nuclei retaining a neutron number N = Z + 2. The TF is explicitly taken into account in the pairing channels of the residual interaction on top of the mean field described by a deformed Woods–Saxon potential. The pairing matrix elements (PMEs) comprising isoscalar and isovector parts, which consistently describe both the ground and the GT excited states, are calculated by a Brückner G-matrix based on the charge-dependent Bonn potential. By switching the TF on and off in the PMEs, we deduce meaningful correlations between the TF and the GT strength distributions. It is found that an attractive TF affects not only the ground state but also plays a crucial role in shifting the main GT peak to the low excitation-energy region leading to the LeSGT.

Investigation of pairing correlations on computed Gamow–Teller strength distributions and associated [formula omitted]-decay half-lives

We investigate the effect of pairing correlations on the computed Gamow–Teller (GT) strength distributions and corresponding β-decay half-lives. The calculations are performed for a total of 47 sd-shell nuclei, for 20 < A < 30, employing the pn-QRPA model. Our calculations use three different values of pairing gaps computed using three different empirical formulae. The GT strength distribution and centroid values change considerably with a change in the pairing gap values. This in turn leads to differences in computed half-lives. The pairing gaps computed using the mass-dependent formula result in the calculated half-lives in better agreement with the measured data.

β-decay half-lives of neutron-rich N=82,81 isotones by shell-model calculations

We perform large-scale shell-model calculations and successfully describe the low-lying spectra and half-lives of neutron-rich N = 82 and N = 81 isotones with Z = 42 − 49 in a unified way. Shell-model study shows that their Gamow-Teller strength distribution has a peak in the low-excitation energies, which significantly contributes to the half-lives. This peak is dominated by the ν0g7/2 → π0g9/2 transition and enhanced on the proton deficient side of these isotones due to the Pauli blocking effect and the change of the effective single-particle energy of the ν0g7/2 orbit.

Total absorption gamma-ray spectroscopy study of the β-decay of 186Hg

The Gamow-Teller strength distribution of the decay of 186Hg into 186Au has been determined for the first time using the total absorption gamma spectroscopy technique and has been compared with theoretical QRPA calculations using the SLy4 Skyrme force. The measured Gamow-Teller strength distribution and the half-life are described by mixing oblate and prolate configurations independently in the parent and daughter nuclei. In this theoretical framework the best description of the experimental beta strength is obtained with dominantly prolate components for both parent 186Hg and daughter 186Au. The approach also allowed us to determine an upper limit of the oblate component in the parent state. The complexity of the analysis required the development of a new approach in the analysis of the X-ray gated total absorption spectrum.

Investigation of Weak Rates for Odd-A Nuclei for Presupernova Simulations

Calculating weak decay rates under stellar conditions for studying presupernova evolution of massive stars is a challenging task. Here, we show the importance of odd-A nuclei for presupernova simulations. In order to calculate the required nuclear matrix elements, we apply the pn-QRPA model in a deformed basis. Nuclear deformation, thought to play an integral role in calculation of associated weak decay rates, is taken into account in our model. We calculate Gamow–Teller (GT) strength distributions, $$\beta ^-$$ emission and positron capture rates for selected odd-A nuclei. Our model does not employ the Brink–Axel hypothesis as used in previous calculations of weak decay rates, and we perform a state-by-state microscopic calculation of GT strength distributions from all parent excited states. Our calculated $$\beta ^-$$ decay rates are in good agreement with large-scale shell model calculations.

Gamow–Teller transitions of neutron-rich N = 82,81 nuclei by shell-model calculations

Abstract $\beta$ -decay half-lives of neutron-rich nuclei around $N=82$ are key data to understand the $r$-process nucleosynthesis. We performed large-scale shell-model calculations in this region using a newly constructed shell-model Hamiltonian, and successfully described the low-lying spectra and half-lives of neutron-rich $N=82$ and $N=81$ isotones with $Z=42\text{--}49$ in a unified way. We found that their Gamow–Teller strength distributions have a peak in the low excitation energies, which significantly contributes to the half-lives. This peak, dominated by $\nu 0g_{7/2} \to \pi 0g_{9/2}$ transitions, is enhanced on the proton-deficient side because the Pauli-blocking effect caused by occupying the valence proton $0g_{9/2}$ orbit is weakened.

Detailed study of the decay of $$^{32}$$Ar

In an experiment performed at the SPIRAL1 facility of GANIL, the $$\beta $$ decay of $$^{32}$$ Ar has been studied by means of the “Silicon Cube” device associated with germanium clover detectors from the EXOGAM array. Beta-delayed protons and $$\gamma $$ rays have been observed and allowed the determination of all relevant decay branches. The Gamow–Teller strength distribution is compared to shell-model calculations and excellent agreement is found. The Fermi strength is inline with expectations. A quasi-complete decay scheme of $$^{32}$$ Ar is established.

Realistic shell-model calculations for astrophysically relevant Gamow-Teller distributions

Electron-capture reaction rates on medium-heavy nuclei are a key ingredient to describe the evolution of core-collapse and thermonuclear supernovae. To estimate these rates it is necessary to know the Gamow-Teller strength distributions involved. In this paper we report some preliminary results of the calculations of Gamow-Teller strength distributions for pf-shell nuclei performed in the framework of the realistic shell model.

Nuclear structure properties of even-even chromium isotopes and the effect of deformation on calculated electron capture cross sections

In this study, we investigate the role of the nuclear deformation on the calculated electron capture cross section (ECC) of even-even chromium (Cr) isotopes. We first determined the nuclear structure properties of these nuclei within the interacting boson model-1 (IBM-1). The energy spectra and E2 transition probabilities were calculated by fitting the parameters in the model formalism. The analysis of the potential energy surface were also performed to predict the geometric shape of the Cr nuclei by plotting their contour plot in the plane of (β, γ) deformation parameters. Later, we calculated the ECC within the proton-neutron quasiparticle random phase approximation (pn-QRPA) model. In particular we studied how the calculated ECC changed with different values of nuclear deformation parameter. The calculated Gamow-Teller (GT) strength distributions were widely spread among the daughter states. The total GT strength decreased with increasing value of the β parameter. The computed ECC values, however, increased with increase in the β value of the Cr isotopes.

Effect of deformation on gamow-teller strength and electron capture cross-section for chromium isotopes

In this work, we explore the role of deformation parameter (β) on the calculated Gamow-Teller (GT) strength distributions and electron capture cross-sections (ECC) for 46,48,50Cr isotopes within the formalism of the proton neutron-quasi-particle random phase approximation (pn-QRPA). Three different β parameters were used in the present study. Two of them were calculated by using the interacting boson model (IBM) and the macroscopic-microscopic (Mac-mic) models. The third one is the experimental β values obtained by employing its relation with the experimental B(E2)↑ values. The GT strength distributions were widely dispersed among all the daughter states of the given isotopes. They were found to have an inverse relation with the β parameter i.e decreasing with increasing the β value. The ECC was computed as a function of the β parameter and the results suggest that the calculated ECC decreased with decreasing value of the β for the selected cases.

Gamow-teller strength and electron capture cross-section calculation by pn-QRPA for selected fp-shell nuclei

The allowed Gamow-Teller (GT) strengths and associated weak interaction rates on fp-shell nuclides are the most familiar processes of spin-isospin (στ) type. These rates play a crucial role in several astrophysical processes, particularly in nuclear synthesis and supernova-explosions. As per simulation consequences, the electron capture cross-section on medium-heavy nuclei has a key impact on decreasing the ratio of electron-to-baryon of the stellar matter during the late stages of stars formation. Stellar model based on the theoretical approaches should be tested against the available measured data. In the current work we present calculated Gamow-Teller strength distributions by pn-QRPA model for selected fp-shell nuclei (42Ti, 46Cr, 50Fe and 54Ni) and compare our results with available measured data. The Gamow-Teller strength distributions are well fragmented over the energy range 0-12 MeV and have a good comparison with experimental data. We calculate the electron capture cross-section for selected nuclei at temperature 0.5 MeV, 1.0 MeV and 1.5 MeV that shows the temperature dependence of calculated electron capture cross-section for astrophysical applications.