Fp-shell Nuclei Research Articles

A method for semi-empirical calculation of isospin mixing probability in nuclei

Abstract A method has been proposed for an isolated two-level model calculation of isospin mixing probability in nuclei. It uses minimum experimental inputs. This method is capable of locating the unperturbed levels of an observed isospin mixed doublet. Isospin mixing probabilities, unperturbed level energies of the observed doublets and isospin mixing matrix elements are calculated for five self-conjugate isotopes of (Mg, P, S, Cl, Ar). Using experimental Gamow-Teller strength data, large isospin mixing probability ( 39.572 − 8.271 + 9.082 %) has been found in the observed doublet (1+, 1+) at 9828.0 and 9965.3 keV, respectively, in 24Mg. This is probably the largest isospin mixing ever found in a nucleus. This is much larger than that ( 20.000 − 9.904 + 9.142 %) has been found recently in non self-conjugate 26Si sd-shell nucleus. This is also larger than that ( 27.5 + 2.5 − 2.5 %) has been measured in non self-conjugate fp-shell nucleus 55Ni. Calculated mixing probability in 24Mg has been used to predict the unknown Gamow-Teller strength for the transitions from the levels of 24Al and 24Alm to the levels in 24Mg. The method is general enough to be applicable to other two-level model applications.

Investigation of Effects of Pairing Correlations on Calculated $$\beta$$-Decay Half-lives of fp-Shell Nuclei

Investigation of Effects of Pairing Correlations on Calculated $$\beta$$-Decay Half-lives of fp-Shell Nuclei

Proof for strong ascendancy of short range effects on inelastic form factors in some open fp-shell nuclei

Short range effects on inelastic C2 form factors in some open fp nuclei ([Formula: see text]Ti, [Formula: see text]Cr, [Formula: see text]Fe and [Formula: see text]Zn) have been researched. The charge densities in these isotopes have been also researched by the one and two particle components of cluster series using single-particle wavefunctions of the harmonic oscillator potential. The short range effects have been inserted into the two particle components of cluster series employing the Jastrow form correlation. The total form factors of isotopes under consideration arise from the model space and core-polarization participations. The one-body density matrix elements (which are needed for computing the model space transition charge density) of valence nucleons for the transition [Formula: see text] have been computed by accomplishment of shell model calculations using the OXBASH code with various interactions. The shape of Tassie model has been adopted for computing the core-polarization transition charge density. The present computations are reliant on b (oscillator parameter) and [Formula: see text] (correlation parameter), where these parameters have been created independently for each individual nucleus by fitting the computed elastic form factors to those of experimental ones. This study provides a proof for strong ascendancy of short range effects on the present calculations, where inclusion of short range correlations on computations seems to be required for performing a notable modification in the computed outcomes which sequentially causes to elucidate the experimental data astoundingly in the momentum transfer under consideration.

Nuclear structure and Gamow-Teller B(GT) Transition Strengths for some Selected fp-shell Nuclei

In the present study, the calculations of the shell model based on large-scale unrestricted fp-model space have been conducted to study the low-lying energy levels and Gamow-Teller B(GT) transition strengths for the transitions (42Ca→42Sc, 42Sc→42Ti, 45Sc→45Ca, 45Ti→45Sc,45V→45Ti) lying in the fp-shell region. The calculations of the yrast levels and Gamow-Teller B(GT) transition strengths were compared with the related measured data. The low-lying energy levels were reasonably reproduced for the studied nuclei. The spin and parity of the unconfirmed energy levels for some studied nuclei have been confirmed. The calculated B(GT) transition strengths of Gamow-Teller (GT) for the selected isotopes lie in the fp-shell region agree very well with the measured data extracted from (3He, t), (3He, t)∗, (t,3He), (t, 3He+γ), and (p, n) reactions.

Neutron-proton pairing in the N=Z radioactive fp-shell nuclei 56Ni and 52Fe probed by pair transfer

The isovector and isoscalar components of neutron-proton pairing are investigated in the N=Z unstable nuclei of the fp-shell through the two-nucleon transfer reaction (p,3He) in inverse kinematics. The combination of particle and gamma-ray detection with radioactive beams of 56Ni and 52Fe, produced by fragmentation at the GANIL/LISE facility, made it possible to carry out this study for the first time in a closed and an open-shell nucleus in the fp-shell. The transfer cross-sections for ground-state to ground-state (J=0+, T=1) and to the first (J=1+, T=0) state were extracted for both cases together with the transfer cross-section ratios σ(0+,T=1)/σ(1+,T=0). They are compared with second-order distorted-wave born approximation (DWBA) calculations. The enhancement of the ground-state to ground-state pair transfer cross-section close to mid-shell, in 52Fe, points towards a superfluid phase in the isovector channel. For the “deuteron-like” transfer, very low cross-sections to the first (J=1+, T=0) state were observed both for 56Ni(p,3He) and 52Fe(p,3He) and are related to a strong hindrance of this channel due to spin-orbit effect. No evidence for an isoscalar deuteron-like condensate is observed.

Stellar Weak Rates and Mass Fractions of 20 Most Important fp-shell Nuclei with A < 65

Stellar Weak Rates and Mass Fractions of 20 Most Important fp-shell Nuclei with A < 65

Calculation of quadrupole deformation parameter β2 from reduced transition probability B(E2) for 0+1 → 2+1 transitions at even-even 62-68Zn isotopes

In this work the excited energy levels, reduced transition probabilities B(E2)↑, intrinsic quadrupole moments, and deformation parameters have been calculated for 62-68Zn isotopes with neutrons number N = 32, 34, 36 and 38. NuSheIIX code has been applied for all energy states of fp-shell nuclei. Shell-model calculations for the zinc isotopes have been carried out with active particles distributed in the lp3/2, 0f5/2, and lp1/2 orbits outside doubly magic closed 56Ni core nucleus. By using f5p model space and f5pvh interaction, the theoretical results have been obtained and compared with the available experimental results. The excited energies values, electric transition probability B(E2), intrinsic quadrupole moment Q0, and deformation parameters β2 have appeared in complete agreement with the experimental values. As well as, the energy levels have been confirmed and determined for the angular momentum and parity of experimental values that have not been well established and determined experimentally. On the other hand, it has been predicted some of the new energy levels and electric transition probabilities for the 62-68Zn isotopes under this study which were previously unknown in experimental information.

Screening Effects on Electron Capture Rates and Type Ia Supernova Nucleosynthesis

Abstract Type Ia supernovae (SNe Ia) are believed to be a thermonuclear explosion of a white dwarf, but the mass of their progenitors is still an open problem. In near-Chandrasekhar-mass (near-M Ch) models of SNe Ia, the central density reaches ≳109 g cm−3. The electron chemical potential becomes higher than the Q-values of electron capture (EC) transitions between fp-shell nuclei, so a portion of the available electrons is captured by iron group elements and thus neutron-rich isotopes are formed. Since EC reaction rates are sensitive to the density, the degree of neutronization is a key to distinguish near- and sub-M Ch models. In order to compare observations and theoretical models, an accurate treatment of EC reactions is necessary. In previous theoretical works, however, effects of electron screening on ECs are ignored. Screening lowers EC rates and thus leads to a higher electron fraction. We implement electron screening on ECs to calculate explosive SN Ia nucleosynthesis in a near-M Ch single degenerate model. It is found that some of neutron-rich nuclear abundances, namely, those of 46,48Ca, 50Ti, 54Cr, 58Fe, 64Ni, and 67,70Zn, decrease when screening effects on ECs are considered. Of these, 50Ti, 54Cr, and 58Fe are particularly interesting because a significant portion of the solar abundance of these nuclei is presumed to originate from SNe Ia. We conclude that implementing the screening effect on ECs in modern SN Ia models is desirable to precisely calculate abundances of neutron-rich nuclides.

The linear term in the isobaric-mass-multiplet equation for fp-shell nuclei in the framework of spectral distribution theory

The one- plus two-body isospin non-conserving nuclear interactions, namely the isovector and isotensor ones, are included in the prediction of the energies of the ground state and the low-lying states of the fp shell nuclei using spectral distribution theory. This in turn is used to calculate the linear term in the isobaric mass-multiplet equation and the predictions are then compared with experimental values after the addition of the Coulomb contribution. The agreement is found to be good as observed for sd shell nuclei earlier. One also sees that in this method the contribution to the linear term comes almost completely from the one-body isovector Hamiltonian, resulting in a huge simplification of the problem.

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.

Investigation of strengths distributions of Gamow-Teller in sd and fp shell nuclei

The strength distribution of Gamow-Teller (GT) in the 26Si→26Al, 28Si→28P, 42Sc→42Ca and 44Sc→44Ca transitions are calculated by using shell model with full basis which assume the valence nucleons in the sd and fp shells. The calculation of the shell model were conducted in the sd and fp shells without any restriction by using USDB and USDA interactions for sd shell and GXFP1A, KB3G and FPD6 effective interactions for fp shell. Our theoretical results of shell model calculations compared with the data obtained from (3He, t) and (p, n) charge-exchange reactions. Calculated B(GT) strength distributions agree well with the recently available experimental data and with the summed transition strengths B(GT).

The Effect of Core Polarization By Means of Tassie and Bohr-mottelson Models for Some FP-Shell Nuclei

The Effect of Core Polarization By Means of Tassie and Bohr-mottelson Models for Some FP-Shell Nuclei

Electron capture cross sections and nuclear partition functions for fp-shell nuclei

We present calculation of electron capture cross sections (ECC), in the limit of zero momentum transfer, using the pn-QRPA model in stellar matter. Towards this aim we make use of our recently introduced recipe for estimation of nuclear partition functions. For low momentum transfer (), the nuclear matrix elements of the operator provide the leading contribution to the total cross section which we estimate using the pn-QRPA model in a multi-shell single-particle space with a schematic interaction. Key fp-shell nuclei (odd-A, even–even and odd–odd) bearing astrophysical importance were selected for the calculation of ECC in stellar environment. These fp-shell nuclei play a crucial role in presupernova evolution of massive stars and core collapse. We further present microscopic calculation of ground and excited states Gamow–Teller strength distributions and stellar electron capture rates on these suite of nuclei. We used two different sets of empirically determined pairing gaps to calculate the ECC and electron capture rates. Results are compared with experimental data and previous computations. Our calculated ECC are systematically smaller at low electron incident energies as compared to the shell-model results.

Electron scattering from some even fp- and N50-shell nuclei with implanting the influence of short-range correlation functions

The influence of two body short range correlations on elastic electron scattering charge form factors, charge densities as well as root mean square charge radii of some fp-shell nuclei (for example, 48Ca, 50Cr, 54Fe, 58Ni, 70Ge and 74Se) and some N50-shell nuclei (for example, 88Sr and 90Zr) is analyzed using the one- and two-body terms in the cluster expansion together with the single particle harmonic oscillator wave functions. The Jastrow-type correlation function is utilized to embed the effect of short range correlations into elastic charge form factors F(q) and charge densities ρ(r). Both F(q) and ρ(r) depend upon the harmonic oscillator parameter b and the correlation parameter β (which initiates from the Jastrow correlation function). Here, the parameters b and β are determined via the fitting to the measured charge form factors. The embedding of short range correlations imitates the measured charge form factors at the high momentum transfers (q ≥ 2 fm−1). It is noticed that the implanting of short range correlations is required for obtaining a remarkable alteration in the computed elastic charge form factors which in turn leads to explain the data of electron scattering astonishingly throughout the entire range of considered momentum transfers.

Study of the Effect of Newly Calculated Phase Space Factor on β-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 prerequisite 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.

Calculations of Quadrupole Deformation Parameters for Nuclei in fp shell

The quadrupole transition rates and intrinsic quadrupole moment are calculated for some fp shell nucleus. For fp-shell nuclei with neutron number N > 20 and with proton number p > 20, fp shell model space and fpd6 effective interaction are used. The calculations are performed using effective charges obtained from Bohr – Mottelson model and also the standard proton and neutron effective charges, and the results are compared with the available experimental values. The results of the quadrupole deformation parameter calculated from transition probabilities are better from the intrinsic quadrupole moment with ST effective charges.

Allowed Gamow-Teller strength distributions and stellar electron capture rates on odd-A nuclei

The allowed Gamow-Teller (GT) strength distributions and associated weak interaction rates on fp-shell nuclei play crucial role in several astrophysical processes, particularly in nucleosynthesis, stellar evolution and supernova explosions. Results from simulation show that the electron capture (EC) rates on medium-heavy nuclei have a significant impact on decreasing the electron-to-baryon ratio of the stellar matter during the late stages of star formation. In this work we present the computation of allowed charge-changing transitions for odd-A fp-shell nuclei by using the deformed pn-QRPA model. The calculated GT strength distributions are compared with previous calculations (including shell and other QRPA models) and measured charge-changing reaction results. The associated EC rates are computed in stellar environment and are compared with previous theoretical results. It is concluded that our results are in good accordance with measured data. For [Formula: see text]Sc, [Formula: see text]V and [Formula: see text]Co, at high stellar temperatures, our calculated EC rates are bigger than the independent-particle and shell model rates. For [Formula: see text]Mn we generally calculate smaller EC rates than previous calculations. The differences with previous calculations may have consequences for supernovae simulators.

The study of nuclear structure for some nuclei

An analytical form of the ground state charge density distributionsfor the low mass fp shell nuclei ( 40  A  56 ) is derived from asimple method based on the use of the single particle wave functionsof the harmonic oscillator potential and the occupation numbers ofthe states, which are determined from the comparison between theoryand experiment.For investigating the inelastic longitudinal electron scattering formfactors, an expression for the transition charge density is studiedwhere the deformation in nuclear collective modes is taken intoconsideration besides the shell model space transition density. Thecore polarization transition density is evaluated by adopting theshape of Tassie model together with the derived form of the groundstate charge density distribution. In this work, we devote ourinvestigation on 0 3 2 3 1 1   transition of Ti 50 , 0 1 2 1 1 1   transitionof Cr 50 and 0 2 2 2 1 1   of Cr 52 nuclei. It is found that the corepolarization effects, which represent the collective modes, areessential for reproducing a remarkable agreement between thecalculated inelastic longitudinal C2 form factors and those ofexperimental data.

Investigation of the γ-decay behavior of 52Cr with the γ3 setup at HIγS

The γ-ray strength function is an important input parameter for the calculation of nucleosynthesis processes. To study the dipole response in more detail, the γ-decay behavior of the fp shell nucleus 52Cr was investigated with the high-efficiency γ3 setup at the High Intensity γ-ray Source facility at TUNL in Durham, USA. The highly intense quasi mono-energetic γ-ray beam allows for excitations selective in multipolarity (J=1 and J=2) and energy. The γ3 setup is a multi-detector array consisting of HPGe and LaBr3 detectors with high efficiency and enables the measurement of γ-γ coincidences. Experimental results of 52Cr will be presented and discussed in this contribution.

Search for weakM1transitions inCa48with inelastic proton scattering

The spinflip M1 resonance in the doubly magic nucleus $^{48}$Ca, dominated by a single transition, serves as a reference case for the quenching of spin-isospin modes in nuclei. The aim of the present work is a search for weak M1 transitions in $^{48}$Ca with a high-resolution (p,p') experiment at 295 MeV and forward angles including 0 degree and a comparison to results from a similar study using backward-angle electron scattering at low momentum transfers in order to estimate their contribution to the total B(M1) strength. M1 cross sections of individual peaks in the spectra are deduced with a multipole decomposition analysis. The corresponding reduced B(M1) transition strengths are extracted following the approach outlined in J. Birkhan et al., Phys. Rev. C 93, 041302(R) (2016). In total, 29 peaks containing a M1 contribution are found in the excitation energy region 7 - 13 MeV. The resulting B(M1) strength distribution compares well to the electron scattering results considering different factors limiting the sensitivity in both experiments and the enhanced importance of mechanisms breaking the proportionality of nuclear cross sections and electromagnetic matrix elements for weak transitions as studied here. The total strength of 1.19(6) $\mu_N^2$ deduced assuming a non-quenched isoscalar part of the (p,p') cross sections agrees with the (e,e') result of 1.21(13) $\mu_N^2$. A binwise analysis above 10 MeV provides an upper limit of 1.62(23) $\mu_N^2$. The present results confirm that weak transitions contribute about 25% to the total B(M1) strength in $^{48}$Ca and the quenching factors of GT and spin-M1 strength are comparable in fp-shell nuclei. Thus, the role of of meson exchange currents seems to be neglible, in contrast to sd-shell nuclei.