Large-basis Shell-model Calculations Research Articles

Single-particle configurations of the excited states of Po203

Excited states of the $^{203}$Po ($Z = 84, N = 119$) have been investigated after populating them through $^{194}$Pt($^{13}$C,4n) fusion-evaporation reaction at E$_{beam}$ = 74 MeV and using a large array of Compton suppressed HPGe clover detectors as the detection setup for the emitted $\gamma$-rays. Standard techniques of $\gamma$-ray spectroscopy have been applied towards establishing the level structure of the nucleus. Twenty new $\gamma$-ray transitions have been identified therein, through $\gamma-\gamma$ coincidence measurements, and spin-parity assignments of several states have been determined or confirmed, following the angular correlation and linear polarization measurements on the observed $\gamma$-rays. The excited states have been interpreted in the framework of large basis shell model calculations, while comparing their calculated and experimental energies. They have been principally ascribed to proton population in the $h_{9/2}$ and $i_{13/2}$ orbitals outside the $Z = 82$ closure and neutron occupation of the $f_{5/2}$, $p_{3/2}$ and $i_{13/2}$ orbitals in the $N = 126$ shell.

Structural evolution and K mixing in V49

Lifetimes of a few negative parity levels and multipole mixing ratios ($\ensuremath{\delta}$) for a few dipole transitions in $^{49}\mathrm{V}$, populated through the $^{48}\mathrm{Ti}(^{4}\mathrm{He},2np)^{49}\mathrm{V}$ reaction with a 48 MeV $^{4}\mathrm{He}$ beam, were measured. The Indian National Gamma Array (INGA) facility was used to detect $\ensuremath{\gamma}$ rays at three different angles. Large basis shell model calculations were performed to understand the microscopic origin of these levels and to interpret the observations. $K$ mixing between different $K$ bands in $^{49}\mathrm{V}$ were established from one-nucleon-transfer spectroscopic factor calculations and total Routhian surface (TRS) calculations.

Emerging collectivity in neutron-hole transitions near doubly magic 208Pb

Excited-state lifetimes were measured by direct fast-timing methods in three N=125 isotones — 209Po, 211Rn, and 213Ra — near doubly magic 208Pb. These nuclei have a single neutron hole and successively add pairs of protons relative to 208Pb. The first-excited state to ground-state transition, 5/21−→1/21−, has almost identical energy in each isotone and can be associated with the single neutron-hole transition νf5/2−1→νp1/2−1. The extent to which the protons act as spectators is assessed based on the measured transition rates, which show a systematic increase along the isotone chain, and by comparisons with large-basis shell-model calculations. The shell model accounts for some of the increased transition strength but consistently underestimates the experimental values. It also fails to explain the near-constant transition energies. These results suggest emerging collectivity beyond the shell-model valence space and show that the near-constant transition energies are not a consequence of a pure neutron-hole transition, but rather the outcome of complex nucleon-nucleon correlations that increase quadrupole collectivity.

Interplay between single particle and collective excitation in 49V

High spin states of 49V populated through 48Ti(4He, 2np)49V reaction with 48 MeV 4He beam, have been studied using the Indian National Gamma Array (INGA) facility. The relative intensities, RDCO and polarization measurements have been carried out for a few transitions in 49V. The level lifetimes have been extracted for a few negative parity yrast levels using Doppler shift attenuation method. Large basis shell model calculations have been performed to understand the microscopic structure of these levels.

Spectroscopic study of K38 above the 31.67μs isomer

High-spin states of $^{38}\mathrm{K}$ above the $31.67\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{s}$ (${\ensuremath{\tau}}_{m}$) isomer, populated through the $^{12}\mathrm{C}(^{28}\mathrm{Si},\phantom{\rule{0.16em}{0ex}}np)^{38}\mathrm{K}$ reaction with a 110 MeV $^{28}\mathrm{Si}$ beam, have been studied by using the Indian National Gamma Array (INGA) facility. Two new levels and four new transitions have been added to the existing level scheme. The spins and parities of most of the levels above the isomer have been assigned, modified, and confirmed from ${R}_{\mathrm{DCO}}$, ${R}_{\mathrm{ADO}}$, and linear polarization measurements. The multipole mixing ratios ($\ensuremath{\delta}$) for a few transitions have been measured. Large-basis shell-model calculations have been performed to understand the microscopic origin of these levels. In our calculations, different particle restrictions in $sd$- and $pf$-shell orbitals were used to reproduce the experimental level energies. Two-nucleon transfer spectroscopic factors have also been calculated for the levels above the isomer to support the new spin and parity assignments. Prediction of collective excitation at high excitation energy in $^{38}\mathrm{K}$ is also discussed.

High spin states of Ar37

High spin states of $^{37}\mathrm{Ar}$, populated through the $^{27}\mathrm{Al}(^{12}\mathrm{C},np)^{37}\mathrm{Ar}$ reaction with a 40 MeV $^{12}\mathrm{C}$ beam, were studied using the Indian National Gamma Array (INGA) facility. The existing level scheme has been extended up to 10.5 MeV by adding some new levels and transitions. The spins and parities of the new levels were assigned from ${R}_{\mathrm{DCO}}$, ${R}_{\mathrm{ADO}}$, and linear polarization measurements. The spins and parities of the existing levels also were modified or confirmed in the present experiment. The multipole mixing ratios ($\ensuremath{\delta}$) for most of the transitions were measured and compared with the earlier measurements wherever available. Large basis shell model calculations with different particle restrictions in $sd$ and $pf$ orbitals were performed to understand the microscopic origin of these levels. A simple two-level mixing calculation was also performed to extract the amount of multiparticle multihole configuration mixing for a few levels.

Single particle configurations in Ni61

Excited states of the $^{61}\mathrm{Ni}$ ($Z=28,N=33$) nucleus have been probed using heavy-ion-induced fusion evaporation reaction and an array of Compton suppressed germanium (clover) detectors as detection system for the emitted $\ensuremath{\gamma}$ rays. Seventeen new transitions have been identified and placement of six transitions have been modified with respect to the previous measurements, following which the level scheme of the nucleus has been extended up to an excitation energy ${E}_{x}\ensuremath{\approx}7$ MeV and spin $\ensuremath{\approx}10\ensuremath{\hbar}$. Higher excitations involving the ${g}_{9/2}$ orbital in the $fpg$ model space have been established. The experimental results on the level structure of the nucleus have been interpreted in the light of large basis shell model calculations that lead to an understanding of the single particle configurations underlying the level structure of the nucleus. The comparison can be suggestive of further refinements in the shell model interactions for better overlap of the theoretical results with the experimental data.

Lifetimes and transition probabilities for the low-lying states in I131 and Xe132

Lifetimes are measured for the low-lying states of $^{131}\mathrm{I}$ and $^{132}\mathrm{Xe}$ nuclei populated from the decay of radio-chemically separated Te fission fragments. The VENTURE array comprised of eight fast ${\mathrm{CeBr}}_{3}$ detectors is used for the lifetime measurement with $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ fast timing technique. Large basis shell model calculations have been performed with the ``nushellx'' code to interpret the low-lying level structure and to calculate level lifetimes in $^{131}\mathrm{I}$ and $^{132}\mathrm{Xe}$. The measured lifetimes and absolute transition probabilities are discussed in the light of systematics with the neighboring nuclei. Single particle excitation and loss of collectivity is observed with increasing neutron number up to $N=82$, validating the double shell closure at $^{132}\mathrm{Sn}$. Enhanced $B(E1)$ and $B(E3)$ strengths are found for the 1646-keV, 11/${2}^{\ensuremath{-}}$ level in $^{131}\mathrm{I}$.

Cluster states at low excitation energy in S34

Large basis shell model calculations with various particle truncations have been carried out to identify the $^{30}\mathrm{Si}+\ensuremath{\alpha}$ cluster states at lower excitation energy ($<12$ MeV) in the non-$\ensuremath{\alpha}$-conjugate nucleus $^{34}\mathrm{S}$. The states of interest in $^{34}\mathrm{S}$ have been calculated and correlated to the cluster SD states in $^{35}\mathrm{Cl}$ and $^{36}\mathrm{Ar}$ in terms of one and two nucleon transfer spectroscopic factors, respectively, obtained from large basis shell model calculations.

Single-particle excitations in the level structure of Cu64

Excited states of the $^{64}\mathrm{Cu}\phantom{\rule{4pt}{0ex}}(Z=29,N=35)$ nucleus have been probed using heavy-ion-induced fusion evaporation reaction and an array of Compton-suppressed Clovers as detection system for the emitted $\ensuremath{\gamma}$ rays. More than 50 new transitions have been identified and the level scheme of the nucleus has been established up to an excitation energy ${E}_{x}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}\phantom{\rule{4pt}{0ex}}6$ MeV and spin $\ensuremath{\sim}10\ensuremath{\hbar}$. The experimental results have been compared with those from large-basis shell-model calculations that facilitated an understanding of the single-particle configurations underlying the level structure of the nucleus.

High-spin states in the semimagic nucleusY89and neutron-core excitations in theN=50isotones

The semimagic nucleus $^{89}\mathrm{Y}$ has been investigated using the $^{82}\mathrm{Se}(^{11}\mathrm{B},4n)$ reaction at beam energies of 48 and 52 MeV. More than 24 new transitions have been identified, leading to a considerable extension of the level structures of $^{89}\mathrm{Y}$. The experimental results are compared with the large-basis shell model calculations. They show that cross-shell neutron excitations play a pivotal role in high-spin level structures of $^{89}\mathrm{Y}$. The systematic features of neutron-core excitations in the $N=50$ isotones are also discussed.

Spectroscopy and shell model calculations in Si isotopes

The $^{29}\mathrm{Si}$ nucleus has been studied using heavy-ion-induced fusion-evaporation reactions and, for the first time, using a large array of high-resolution $\ensuremath{\gamma}$-ray detectors. High-spin states of the nucleus are populated using $^{18}\mathrm{O}$($^{16}\mathrm{O},\ensuremath{\alpha}n$) and $^{18}\mathrm{O}$($^{13}\mathrm{C},2n$) reactions at ${E}_{\mathrm{lab}}=30$--34 MeV. Previously reported levels are confirmed and a new level is identified in the present study. Spin-parity assignments are carried out based on the anisotropy and polarization measurements of the observed $\ensuremath{\gamma}$ transitions. Level lifetimes are measured using the Doppler shift attenuation method, with modified analysis techniques for the thick molecular target (${\mathrm{Ta}}_{2}{\mathrm{O}}_{5}$) used in the present setup. The lifetime of the lowest negative-parity state at ${E}_{x}=3624$ keV is substantially modified from the previously reported value. Large-basis shell model calculations are carried out for the nucleus using updated interactions and the results corroborate the experimental findings. The calculations are also carried out for the neighboring $^{28,30}\mathrm{Si}$ isotopes. In the case of the $^{28}\mathrm{Si}$ nucleus, the calculations adequately reproduce most of the deformed structures, as represented by the quadrupole moments extracted therefrom. In the $^{30}\mathrm{Si}$ nucleus, the negative-parity states are reproduced for the first time without any ad hoc lowering of the single-particle energies. It can be generally stated that the shell model calculations adequately describe the experimental observations in these nuclei.

Lifetime measurements and the high-spin structure of 36Cl

High-spin states in 36Cl were populated through the 24Mg(14N,2p)36Cl reaction at E(14N)=31 MeV. Lifetimes have been determined for fifteen states by applying the Doppler shift attenuation method. The results indicated the onset of collectivity in the high spin negative parity states. Large basis shell model calculations have been performed to understand the underlying structure of these states.

Level lifetimes inP32obtained using the Doppler-shift attenuation method with thick molecular targets

Level lifetimes in the $^{32}\mathrm{P}$ nucleus have been determined using the Doppler-shift attenuation method (DSAM). Conventional DSAM measurements employ a thin target on a thick, high-$Z$ backing. An extension of the technique to thick molecular targets is presented. The necessary modifications in the standard analysis procedures, pertaining to the incorporation of stopping power estimations for molecular media and evolution of residue cross section in a thick target, have been implemented. Further, x-ray powder diffraction (XRD) and scanning electron microscopy (SEM) have also been carried out to probe the structural composition of the target. The lifetime results have been validated with respect to the previous measurements and upper limits on lifetimes of seven levels have been determined for the first time. Large-basis shell model calculations have been carried out and compared with the experimental measurements.

High-resolution two-proton stripping to 2p-1h 7/2- states via the 59Co(3He,n $\gamma$ )61Cu reaction

The challenge of achieving high resolution in binary reactions involving an outgoing high-energy neutron is solved by detecting the \( \gamma\)-ray decay of populated excited states in an array of escape-suppressed HPGe detectors in coincidence with fast neutrons. The selectivity of the arrangement is of the order of 1 in 1000 and is demonstrated by L = 0 two-proton stripping to 7/2- 2p-1h levels using the 59Co(3He,\(n \gamma\))61Cu reaction at Elab = 22.5 MeV. The observed relative two-proton stripping strengths are compared with large-basis shell-model calculations.

Proton Spectroscopic Factors Deduced from Helium-3 Global Phenomenological and Microscopic Optical Model Potentials

Global phenomenological GDP08 and microscopic helium-3 optical model potentials have been recently derived. We evaluate these two potential sets by comparing the elastic scattering data of 25 MeV 3He on 16O, 18O, 19F, 23Na, 24Mg, 25Mg, 26Mg, 27Al, 28Si, 30Si, 31P, 32S, 34S, 35Cl, 37Cl, and 39K isotopes. Using the deuteron angular distributions calculated with the distorted wave Born approximation model, we extract the ground-state proton spectroscopic factors from (3He, d) reactions on the same set of nuclei. The extracted proton spectroscopic factors are compared with the large-basis shell-model calculations.

Collective excitations inS33

High spin states of $^{33}\mathrm{S}$ populated through ${}^{27}\mathrm{Al}{(}^{12}\mathrm{C},\ensuremath{\alpha}\mathrm{pn})^{33}\mathrm{S}$ reaction at E$(^{12}\mathrm{C})\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}40$ MeV have been studied using the Indian National Gamma Array (INGA) facility. The level scheme was extended and modified utilizing data from the $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence, directional correlation, and linear polarization measurements. Three levels of the negative parity yrast sequence were found to be connected by strong E2 transitions. The lifetimes of these states determined by the Doppler shift attenuation method have been utilized to study the evolution of collectivity with spin. Large basis shell model calculations have been performed to understand the microscopic origin of these levels.

High spin spectroscopy in34Cl

High spin states of ${}^{34}\text{Cl}$ populated through ${}^{27}\text{Al}$(${}^{12}\text{C}$,$\ensuremath{\alpha}n$)${}^{34}\text{Cl}$ reaction at $E{(}^{12}\text{C})=40$ MeV, have been studied using the Indian National Gamma Array facility. The level scheme has been extended up to 10.6 MeV utilizing the results of intensity, directional correlation, and linear polarization measurements. Lifetimes of a few excited states have been estimated for the first time using the Doppler shift attenuation method. Large-basis shell-model calculations within the $sd\text{\ensuremath{-}}pf$ space have been done to understand the microscopic origin of the excited states. Involvement of $pf$ orbitals have been found to be essential to reproduce the negative-parity as well as high spin positive-parity states. Onset of collectivity manifested through short half-lives and large $B(E2)$ values have been reproduced well in the calculations.

Electromagnetic properties of the21+state in134Te: Influence of core excitation on single-particle orbits beyond132Sn

The $g$ factor and $B(E2)$ of the first excited ${2}^{+}$ state have been measured following Coulomb excitation of the neutron-rich semimagic nuclide ${}^{134}$Te (two protons outside ${}^{132}$Sn) produced as a radioactive beam. The precision achieved matches related $g$-factor measurements on stable beams and distinguishes between alternative models. The $B(E2)$ measurement exposes quadrupole strength in the ${2}_{1}^{+}$ state beyond that predicted by current large-basis shell-model calculations. This additional quadrupole strength can be attributed to coupling between the two valence protons and excitations of the ${}^{132}$Sn core. However, the wave functions of the low-excitation positive-parity states in ${}^{134}$Te up to ${6}_{1}^{+}$ remain dominated by the $\ensuremath{\pi}{({g}_{7/2})}^{2}$ configuration.

Neutron spectroscopic factors ofAr34andAr46from(p,d)transfer reactions

Single-neutron-transfer measurements using ($p$,$d$) reactions have been performed at 33 MeV per nucleon with proton-rich $^{34}\mathrm{Ar}$ and neutron-rich $^{46}\mathrm{Ar}$ beams in inverse kinematics. The extracted spectroscopic factors are compared to the large-basis shell-model calculations. Relatively weak quenching of the spectroscopic factors is observed between $^{34}\mathrm{Ar}$ and $^{46}\mathrm{Ar}$. The experimental results suggest that neutron correlations have a weak dependence on the asymmetry of the nucleus over this isotopic region. The present results are consistent with the systematics established from extensive studies of spectroscopic factors and dispersive optical-model analyses of $^{40\ensuremath{-}49}\mathrm{Ca}$ isotopes. They are, however, inconsistent with the trends obtained in knockout-reaction measurements.