Indian National Gamma Array Research Articles

Indications of octupole correlation in 84Sr

Excited states in the 84Sr nucleus were investigated via the 76Ge(12C,4n)84Sr reaction at a beam energy of 58 MeV. The de-excited γ-rays were detected using the Indian National Gamma Array (INGA) spectrometer at Inter-University Accelerator Center, New Delhi. Directional Correlation from Oriented (DCO) states ratio and the polarization asymmetry (Δ) measurements were done to confirm the spin-parity of the low-lying states. Eight new γ-ray transitions were placed in the level scheme of 84Sr. The systematic behaviour of energy staggering S(I) of the γ-band (Band 1 and Band 2) was compared with the γ-bands in the mass A ≈ 80 region and the nuclei of other mass regions with similar behaviour (odd-I down). The E1 character is confirmed for strong γ-ray transitions connecting Band 3 to the Yrast band. Comparison of new results such as B(E1)/B(E2) ratio, frequency ratio ω −/ω + and energy displacement ΔE in 84Sr with those of 72Se, 150Sm, 152Gd, 220Ra and 224Th nuclei suggests the presence of octupole correlations in 84Sr.

Evidence for prolate-oblate shape coexistence in the odd- ABr383573 nucleus

The excited states in $^{73}\mathrm{Br}$ nucleus have been investigated through the fusion evaporation reaction $^{50}\mathrm{Cr}(^{28}\mathrm{Si}$, $\ensuremath{\alpha}p)^{73}\mathrm{Br}$ at a beam energy of 90 MeV using the Indian National Gamma Array. The $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence technique has been used to add eight new $\ensuremath{\gamma}$-ray transitions in the level scheme. The mixing ratio of $\mathrm{\ensuremath{\Delta}}I=0$ (mixed with $E2$ and $M1$) transitions have been determined using angular distribution and ${R}_{\mathrm{DCO}}$-polarization measurement. The half-life of the ${9/2}^{+}$ isomeric state has been measured to be ${\ensuremath{\tau}}_{1/2}=52(2)$ ns from the variation in the intensity of delayed $\ensuremath{\gamma}$-ray transition as a function of coincidence time window. The two state mixing model calculations were performed to obtain the mixing amplitude, and mixing interaction of two different configurations of $^{73}\mathrm{Br}$. The calculated mixing amplitudes along with the deformations of two different configurations provide the monopole transition strength ${\ensuremath{\rho}}^{2}(E0)$ for Se, Br, and Kr isotopes in a semiempirical approach. These results support a prolate-oblate shape coexistence in the odd-$A\phantom{\rule{4pt}{0ex}}^{73}\mathrm{Br}$ nucleus. The observed structural properties have been discussed in terms of projected shell model calculations.

Experimental evidences of shape co-existence in 154Ho

The excited states of odd-odd 154Ho nucleus have been studied by the 141Pr(16O, 3n)154Ho reaction at Elab= 82 MeV utilizing the Indian National Gamma Array (INGA) setup at BARC-TIFR Pelletron facility, Tata Institute of Fundamental Research, Mumbai. Data from γ−γ coincidence, directional correlation (DCO), and polarization measurements have been analyzed to assign and confirm the spins and parities of the levels. The reported level scheme of 154Ho has been improved by including new levels and transitions. Mixing ratios for the mixed transitions extracted from DCO ratios and parity assignments from polarisation data are reported for the first time. The low-lying states in H154o, which are mostly collective, characterized by regularity in the excitation spectra, are proposed to be arising from neutron excitation. At higher excitation, there is a transition from collective to single-particle character, accompanied by the appearance of irregular and complex structure caused by proton excitation. The newly identified E1 gamma transitions connecting the low-lying positive and negative parity bands indicate octupole correlation in 154Ho. These new connections also support the phenomenological analysis to interpret the observed experimental features from a different perspective.

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.

Different manifestations of triaxial shapes of the positive and negative parity bands in Os187

The excited states of $^{187}\mathrm{Os}$ have been studied via $^{186}\mathrm{W}(^{4}\mathrm{He},3n)^{187}\mathrm{Os}$ reaction at a beam energy of 36 MeV. The $\ensuremath{\gamma}$ rays were detected using the Indian National Gamma Array at the Variable Energy Cyclotron Centre having seven Compton-suppressed clover high-purity germanium (HPGe) detectors and one low-energy photon spectrometer (LEPS) detector with a digital data acquisition system. The level scheme of $^{187}\mathrm{Os}$ has been extended substantially up to $\ensuremath{\approx}$3.86 MeV of excitation energy and $37/2\phantom{\rule{0.16em}{0ex}}\ensuremath{\hbar}$ of spin with the placement of more than 90 new $\ensuremath{\gamma}$ rays. All known bands have been extended and new band structures have been identified. The results show evidence of triaxial shapes for different configurations of $^{187}\mathrm{Os}$ and different manifestations of nonaxial shape have been observed in the same nucleus. A comparison of the observed band crossing frequency in $^{187}\mathrm{Os}$ with neighboring nuclei gives evidence of a deformed shell gap at $N=110$. The experimental results are well explained using total Routhian surface calculations.

Alignment effects in the medium-spin level structure of Se78

The excited states of $^{78}\mathrm{Se}$ were populated using the $^{76}\mathrm{Ge}(\ensuremath{\alpha},2n)$ fusion evaporation reaction. The deexcited gamma rays were detected using the Indian National Gamma Array (INGA) spectrometer comprising clover and low-energy photon spectrometer (LEPS) detectors. The ground state band of $^{78}\mathrm{Se}$ has been extended up to ${E}_{x}$ = 8.5 MeV and $J$ = $16\ensuremath{\hbar}$; this excitation regime is very close to the $\ensuremath{\pi}(1{g}_{9/2}^{2})$ crossing frequency. Several newly observed in-band and intraband transitions belonging to the other bands of $^{78}\mathrm{Se}$ have also been placed in the level scheme based on their coincidence relationships. From the newly identified $E1$ transitions, an enhancement in the $B(E1)$/$B(E2)$ values with spin has been observed which indicates the onset of octupole correlations in the band structure built on the ${3}^{\ensuremath{-}}$, 2508-keV state. Persistence of triaxial shape up to the highest observed spin of the ground state band has been observed and the feature of successive $1{g}_{9/2}^{2}$ quasiparticle alignments due to protons and neutrons has been unveiled. The consequences arising due to the occurrence of the neutron $1{g}_{9/2}$ quasiparticle alignments in the other observed correlated band structures are also presented. The results are interpreted in terms of total Routhian surface (TRS) calculations and the level structure systematic of the neighboring nuclei.

Shape coexistence and octupole correlations in Se72

In the present paper, we report the results from the study of excited states in the $^{72}\mathrm{Se}$ nucleus using the $^{50}\mathrm{Cr}(^{28}\mathrm{Si}, \ensuremath{\alpha}2p)^{72}\mathrm{Se}$ fusion evaporation reaction at a beam energy of 90 MeV. The deexciting $\ensuremath{\gamma}$ rays were detected using the Indian National Gamma Array (INGA). A total of ten new $\ensuremath{\gamma}$-ray transitions have been identified using the $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence technique. The ${K}^{\ensuremath{\pi}}={0}_{2}^{+}$ band based on an isomeric state has been extended up to a (${10}^{+}$) state at 5.473 MeV excitation energy, and four new interconnecting transitions have been placed between this band and yrast band. Further, the enhanced interconnecting $E1$ transitions between positive- and negative-parity bands suggest the existence of octupole correlations in this nucleus. The characteristics of the observed bands in the experiment have been interpreted in terms of the total Routhian surface (TRS) calculations.

Three-phonon multiplets in 116Sn

Low-lying states in the even-even 116Sn nucleus have been studied by populating them using the 114Cd(α, 2n)116Sn reaction at the beam energy of 34 MeV. γγ coincidences, directional correlation ratio, and polarization asymmetry measurements were performed using the Indian National Gamma Array (INGA) with a digital data acquisition system. Three-phonon multiplets in the 116Sn nucleus have been investigated. The experimental energy levels are analyzed using theoretical approaches of the interacting boson model (IBM) in the U(5) limit and large-scale shell-model calculations using two different model spaces.

Collective and noncollective states in Zn66

Excited states in the $^{66}\mathrm{Zn}$ nucleus were populated via a $^{56}\mathrm{Fe}(^{12}\mathrm{C},2p\ensuremath{\gamma}$) fusion-evaporation reaction at a beam energy of $\ensuremath{\approx}62$ MeV. The deexciting $\ensuremath{\gamma}$ rays were detected using the Indian National Gamma Array (INGA). The level scheme of the $^{66}\mathrm{Zn}$ nucleus has been updated by placing several new $\ensuremath{\gamma}$ rays as well as by assigning the spin and parity of various excited states from the present spectroscopic results. The microscopic structure of the observed states have been investigated in the light of large shell-model calculations. The shape of this nucleus in the low-spin regime has been studied under the framework of total Routhian surface (TRS) calculations. The lifetime of first ${3}^{\ensuremath{-}}$ state at 2826 keV is experimentally measured using the Doppler-shift attenuation method and the deduced $B(E1)$ value indicates the presence of octupole collectivity in this nucleus.

Evidence of antimagnetic rotational motion in Pd103

Lifetime measurements have been carried out for the levels of the negative parity yrast sequence in $^{103}\mathrm{Pd}$ nucleus using the Doppler shift attenuation method. The levels were populated via $^{94}\mathrm{Zr}(^{13}\mathrm{C}, 4n\ensuremath{\gamma})^{103}\mathrm{Pd}$ fusion-evaporation reaction at a beam energy of 55 MeV. De-exciting $\ensuremath{\gamma}$ rays were detected by utilizing the Indian National Gamma Array. The extracted transition probabilities and other auxiliary observations indicate that the sequence may be resulting from the antimagnetic rotational (AMR) motion of valence nucleons. The key characteristic feature of the AMR motion is the steady decrease of the $B(E2)$ transition probability with spin, which is seen in the present measured transitions for $^{103}\mathrm{Pd}$. The experimental results are compared with the theoretical predictions of tilted axis cranked approach based on the covariant density functional theory. It is noted that the properties of the AMR band structure for $^{103}\mathrm{Pd}$ predicted in this model analysis are in good agreement with the present experimental findings. Further, semi-classical particle-rotor model has been employed to substantiate the AMR interpretation of the observed band structure in $^{103}\mathrm{Pd}$ and it is shown that results are similar to the band structures observed in the neighboring isotopes, which have also been considered as candidates for AMR motion.

Evolution of collectivity and shape transition in Zn66

Excited states in $^{66}\mathrm{Zn}$ were investigated through the in-beam $\ensuremath{\gamma}$-ray spectroscopic techniques using the $^{52}\mathrm{Cr}(^{18}\mathrm{O}, 2p2n$) fusion-evaporation reaction at a beam energy of 72.5 MeV. The $\ensuremath{\gamma}$-rays emitted by the de-exciting nuclei were recorded in coincidence mode using the 14 Compton suppressed Ge clover detectors of the Indian National Gamma Array. With 14 new transitions being identified, the level scheme of $^{66}\mathrm{Zn}$ has been extended up to the excitation energy $\ensuremath{\approx}12.3\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$ and spin $\ensuremath{\approx}17\phantom{\rule{0.28em}{0ex}}\ensuremath{\hbar}$. A rotational band, associated with the two quasineutrons from the $1{g}_{9/2}$ orbital, has been found to exhibit a band crossing with the ground-state band at a spin of $6\ensuremath{\hbar}$. The evolution of the collectivity and shape transition in this nucleus have been discussed in the framework of the total Routhian surface calculations and in comparison with the neighboring $^{68,70}\mathrm{Ge}$ nuclei.

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.

Lifetime measurements in 104Pd

High spin states in 104Pd were populated via 94Zr(13C, 3nγ) fusion-evaporation reaction at a beam energy of 55 MeV. The Indian National Gamma Array was utilized to detect the de-exciting γ-rays. Lifetimes of states in the negative parity bands have been determined using the Doppler shift attenuation method. The present lifetime results suggest a moderate quadrupole deformation (β 2 ≈ 0.2) for the negative parity bands. The Woods–Saxon total Routhian surface calculations have also been carried out which predicted a similar deformation for these bands. In addition, lifetimes of states in the positive parity band have also been determined. Present results found in agreement with the earlier reported values of lifetimes and hence agreed with the antimagnetic rotational character of the band as interpreted earlier.

Evidence of antimagnetic rotation in Pd100

Lifetimes of states in a negative-parity band involving the $\ensuremath{\nu}{h}_{11/2}$ orbital in $^{100}\mathrm{Pd}$ have been determined through Doppler shift attenuation method (DSAM) measurements performed using Indian National Gamma Array (INGA) spectrometer. The measured quadrupole transition rates, $B(E2)$, exhibit a decreasing behavior with increasing angular momentum and with large observed ${\ensuremath{\mathfrak{I}}}^{(2)}/B(E2)$ values. The experimental observations and semiclassical particle-rotor model calculations favor the antimagnetic rotation (AMR) phenomenon based on the $\ensuremath{\pi}{({g}_{9/2})}^{\ensuremath{-}4}\ensuremath{\bigotimes}\ensuremath{\nu}[{h}_{11/2}\phantom{\rule{0.16em}{0ex}}{({g}_{7/2})}^{3}]$ configuration for the negative-parity band in the $^{100}\mathrm{Pd}$ isotope.

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.

Quasi- γ band in Te114

The low-lying non-yrast states in $^{114}$Te have been investigated using the Indian National Gamma Array through the fusion-evaporation reaction $^{112}$Sn($^{4}$He, 2n) at a beam energy of 37 MeV. Eight new $\gamma$-transitions have been placed in the level scheme to establish the quasi-$\gamma$ band in this nucleus. Spin and parity of several excited states have been assigned from the present spectroscopy measurements. The comparison of experimental results on the observed bands with the Interacting Boson Model (IBM) and Triaxial Projected Shell Model (TPSM) confirming the existence of the quasi-$\gamma$ band structure in the $^{114}$Te nucleus.

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.

Revealing multiple band structures in Xe131 from α -induced reactions

The excited states in the transitional nucleus $^{131}\mathrm{Xe}$ have been populated by using $\ensuremath{\alpha}$-induced fusion-evaporation reaction and the de-exciting $\ensuremath{\gamma}$ rays were detected with the Compton suppressed clover detector setup of the Indian National Gamma Array coupled to digital data acquisition system. The existing level structure of $^{131}\mathrm{Xe}$ has been significantly extended with the observation and placement of 72 new $\ensuremath{\gamma}$-ray transitions. The use of light-ion ($\ensuremath{\alpha}$) beam helped to identify several new band structures in $^{131}\mathrm{Xe}$ with different quasiparticle (qp) configurations. The multipolarities of the observed $\ensuremath{\gamma}$ rays have been determined on the basis of the directional correlation from oriented states ratio and polarization asymmetry measurements. The yrast negative-parity band has been confirmed up to ${35/2}^{\ensuremath{-}}$ spin and the highly nonyrast signature partner of this band has been identified for the first time. The positive-parity band, based on the ${3/2}^{+}$ ground state, has been extended up to ${23/2}^{+}$ with the observation of a signature inversion, which signifies a pair of particle alignment around the spin of 15/2 $\ensuremath{\hbar}$. A dipole band, consisting of $M1$ transitions has been identified and assigned a 5-qp configuration. A new band structure built on a 3-qp ${23/2}^{+}$ state has been observed with a large signature splitting. A comparison in the isotopic and isotonic chains reveals the transitional nature of the $N=77$ nuclei. Total Routhian surface calculations have been performed to understand the structure of $^{131}\mathrm{Xe}$ associated with different configurations.

Lifetime measurement and shell model description of negative parity states up to band-termination in 49V

The yrast and a few of the yrare negative-parity states in the 49V nucleus have been investigated employing the 27Al(28Si, α2p)49V reaction. Most of the levels and the deexciting γ rays, reported earlier in 49V, have been observed in this measurement using the Indian National Gamma Array (INGA) spectrometer. Mean lifetimes up to the band-terminating state have been extracted following Doppler Shift Attenuation Method (DSAM), and reduced transition probabilities have been deduced. Large Scale Shell Model (LSSM) calculations in the full fp shell employing various effective interactions with no restrictions imposed on the valence orbitals have been performed. The results that have been obtained from calculations are in fair agreement with the experimentally obtained values.

Exploring the structure of Xe isotopes in A ~ 130 region: Single particle and collective excitations

High and medium spin structures of 130,131Xe have been studied using α-induced fusion-evaporation reaction and the Indian National Gamma Array (INGA) coupled with a digital data acquisition system. Various new band structures and near yrast levels of 131Xe have been established. The multipolarities of the observed transitions have been assigned on the basis of the DCO ratios and the polarization asymmetry measurements. Band structures based on 1-quasi-particle (qp), 3-qp configurations have been observed. A new Magnetic Rotational (MR) band based on 5-qp configuration has also been established in 131Xe. The MR band has been interpreted in terms of shears mechanism with principal axis cranking (SPAC) calculations. Shell Model calculations are carried out to describe the non yrast states of 131Xe above the 11/2− isomer. New excited states have also been identified in 130Xe, produced in the same reaction.