Gamma Array Research Articles

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.

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.

Design and Development of a Raman Lidar for Cherenkov Gamma Array Experiments

Future Cherenkov Gamma Ray (CGR) experiments will reach a sensitivity and energy resolution never obtained until now by any other high energy gamma–ray experiment. It is well known that atmospheric conditions contribute particularly in this aspect. Raman lidars can help reduce the systematic uncertainties of the molecular and aerosol components of the atmosphere so these performances can be reached. The motivation for Raman lidars and the design and development of the Montpellier Raman lidar system is described. It provides both multiple elastic and Raman readout channels and custom-made optics design. Preliminary lidar tests and signals show the actual performance of the lidar in consistency with the desired goals.

Neutron inelastic cross section measurements on 54Fe

A 54Fe(n, n'γ) cross section measurement was performed at the Geel Electron LINear Accelerator of EC-JRC, Geel using the Gamma Array for Inelastic Neutron Scattering spectrometer and a 235U fission chamber for flux normalization. The experimental results are presented in comparison with talys 1.9 default and tuned calculations. The tuned calculation, implying modifications of the optical model parameters, improved significantly the description of the experimental values and led to interesting conclusions regarding the interaction of the 54Fe nucleus with neutrons. Since the results of these calculations were already presented extensively in a dedicated paper, the present article focuses on details related to the experimental particularities and data analysis procedure.

High precision neutron inelastic cross sections on16O

This work reports partial results of a (n, nγ) measurement on16O. Theγrays of interest from the inelastic channel were detected using the Gamma Array for Inelastic Neutron Scattering (GAINS) spectrometer at the Geel Electron Linear Accelerator (GELINA) neutron source. A very thick (32.30(4) mm) SiO2target was used. The main goal was to determine the angle-integrated γ-production cross section for the most important transitions. In this work we report the results for the main16O transition and we emphasize a consistency check aiming to ensure data reliability. Our results are compared with theoretical calculations performed using theTALYS1.8 code and with previously reported experimental data.

Neutron inelastic cross section measurements on 58,60Ni

A natural nickel sample was used at the GELINA (Geel Electron LINear Accelerator) neutron source of the European Commission, Joint Research Centre, Geel to measure the neutron inelastic cross sections. The GAINS (Gamma Array for Inelastic Neutron Scattering) spectrometer was employed to detect the emitted γ rays while a 235U fission chamber monitored the neutron flux. We report the preliminary production cross sections corresponding to the first transitions in 58,60Ni in comparison with previously reported data and with TALYS 1.9 calculations performed using the default input parameters.

Investigation of a large change in deformation for the γ -soft nucleus Sm136

Lifetimes of excited states of the ground-state rotational band in the $^{136}\mathrm{Sm}$ nucleus have been measured up to ${I}^{\ensuremath{\pi}}={20}^{+}$ using the Doppler-shift attenuation method. The states were populated in the reaction $^{107}\mathrm{Ag}$($^{32}\mathrm{S}$, $1p2n$) at 145-MeV beam energy and the $\ensuremath{\gamma}$ rays emitted from the excited states were detected using the Indian National Gamma Array at the Tata Institute of Fundamental Research, Mumbai. The extracted transitional quadrupole moments indicate a reduction of collectivity with increasing spin after the band crossing. The results have been compared with the predictions of the cranked shell model as well as the triaxial projected shell-model calculations and indicate that the nucleus evolves from prolate $\ensuremath{\gamma}$-soft to a stable triaxially deformed shape after the first and the second crossing involving $\ensuremath{\pi}{h}_{11/2}^{2}$ and $\ensuremath{\pi}{h}_{11/2}^{2}\ensuremath{\bigotimes}\ensuremath{\nu}{h}_{11/2}^{2}$ configurations, respectively.

Band structures in 169Tm and the structures of Tm isotopes around N = 98

The excited states of 169Tm have been studied via the 169Tm(32S,32S′)169Tm* reaction at the beam energy of 164MeV. The $\gamma$-rays were detected using the Indian National Gamma Array (INGA) setup, composed of 19 Compton-suppressed clover HPGe detectors. A new level scheme of 169Tm with 11 newly placed $ \gamma$-rays has been proposed. A band crossing in the $ \pi [541]1/2^{-}$ band and several interband E1 transitions between this and the $ \pi [411]1/2^{+}$ ground-state band have been observed for the first time in this nucleus. The role of the $N=98$ deformed shell gap has been discussed by comparing the band crossing parameters of the negative parity bands in Tm and other neighboring nuclei. The origin of the interband E1 transitions has been investigated in terms of coupling to octupole degrees of freedom. The shape evolution of the Tm isotopes around $ N=98$ have been studied in the projected and cranked shell model approaches, both of which predict a change in shape from an axial prolate to a triaxial one after band crossing in these nuclei. The new data and the calculations help to understand the unusual structural phenomena reported for the nuclei with $ N=98$.

Evidence of the octupole correlation between the shears bands in 142Eu

Two opposite parity dipole sequences (DB I and DB II) in 142Eu have been investigated using the Indian National Gamma Array (INGA), and heavy-ion induced fusion-evaporation reaction, 116Cd(31P, 5n) at ELab = 148 MeV. The Configurations of these bands are assigned on the basis of tilted axis cranking covariant density functional theory (TAC-CDFT) calculations. The measured B(M1) and B(E2) rates of the DB I and DB II, have been well reproduced within the shears mechanism with the principal axis cranking model (SPAC) calculations leading to the conclusion that these bands have been based on the shears mechanism. The large B(E1) transition rates between DB I and DB II indicate the existence of octupole correlation in 142Eu that has also been corroborated through microscopic multidimensionally-constrained covariant density functional theory (MDC-CDFT) calculations. Thus, the present investigation leads to the novel observation of the presence of octupole correlation between two shears band.

In-beam spectroscopic study of Zn63

Investigation on the excited states of $^{63}\mathrm{Zn}$ was done through in-beam $\ensuremath{\gamma}$-ray spectroscopic techniques using the $^{52}\mathrm{Cr}(^{18}\mathrm{O},\phantom{\rule{4pt}{0ex}}\ensuremath{\alpha}3n)$ fusion-evaporation reaction at a beam energy of 72.5 MeV. Detection of the emitted $\ensuremath{\gamma}$ rays from the excited nuclei was performed in the coincidence mode using 14 Compton suppressed Ge clover detectors of the Indian National Gamma Array. Based on the $\ensuremath{\gamma}\ensuremath{-}\ensuremath{\gamma}$ coincidence data, 13 new transitions have been placed in the level scheme following their coincidence relationship and intensity balance. Spin and parity assignments of the excited levels have been carried out by extracting the directional correlation from oriented states ratio and polarization asymmetry values of the emitted $\ensuremath{\gamma}$ rays. Shell model calculations have been performed in the ${f}_{5/2}p{g}_{9/2}$ model space with a $^{56}\mathrm{Ni}$ core using the jj44bpn interaction, to interpret the observed excited states of the nucleus. A reasonable agreement is found between the experimental findings and the shell-model calculations. In order to understand the evolution of a collective shape built on the $9/{2}^{+}$ (1704 keV) state, we have performed total Routhian surface calculations with a $1{g}_{9/2}$ quasineutron and found reasonable agreement.

Understanding the operation of gamma-ray detectors arranged in the shape of a pyramid

Motivated by the various shapes and configurations of detectors used in modern gamma arrays, for the first time in literature, we have investigated the operation of a composite detector comprising of elements arranged in the shape of a pyramid. A probabilistic formalism is introduced to predict the response of two, three and four level pyramid shaped composite detector in terms of only one probability amplitude. Considering up to four detector hit events, expressions for the addback factor and the peak-to-total ratio have been obtained. The general trend of predicted values of the addback factor, the peak-to-total ratio and the fold distribution match with the available experimental data for other composite detectors. Our novel phenomenological approach could provide a guidance in designing new detectors for gamma-ray spectroscopy.

Evolution of collectivity and evidence of octupole correlations in Br73

High-spin states in the Br73 nucleus have been populated via the Cr50(Si28,αp)Br73 fusion evaporation reaction with a beam energy of 90 MeV. The deexciting gamma rays were detected using the Indian National Gamma Array (INGA). Using the γ-γ coincidence technique, two new bands and eight new interconnecting transitions have been added. The strong interconnecting E1 transitions, between positive and negative parity bands, ensure the existence of octupole correlations at low spin. Line shapes have been observed for 17 transitions, which were analyzed by the Doppler-shift attenuation method to determine the lifetime of excited states of the yrast negative parity band and its signature partner band along with the positive parity band. The deduced transitional quadrupole moments Qt for the ground state band decrease with increasing spin, with their values ranging from 2.88 to 1.00 eb. A similar trend in the quadrupole moment has also been observed for the signature partner as well as for the positive parity band. This decrease in Qt with increasing spin for these bands is interpreted in terms of the cranked Nilsson-Strutinsky model and total Routhian surface calculations, which indicate possible band termination at higher spin.

Observation of rotation about the longest principal axis in Zr89

High-spin states in Zr89 were populated in the Se80(C13,4n) reaction, and γ-ray coincidences were measured using the Indian National Gamma Array. The level scheme of Zr89 has been extended up to spin I=49/2 with the observation of a new dipole band. Directional correlation and polarization asymmetries of the γ rays have been measured to determine spin and parity of the levels. Line shapes of several transitions have been analyzed to determine lifetimes of the levels. Possible configurations of the band have been discussed using the cranked Nilsson-Strutinsky model. The calculations suggest a triaxial shape of the nucleus at high spins, and the band may represent rotation of the nucleus about the longest axis.

Spectroscopy of a tetrahedral doubly magic candidate nucleus

The decay scheme of 160Yb nucleus populated in the 148Sm(16O, 4n)160Yb reaction at 90 MeV has been studied. The gamma-coincidence data have been collected by using Indian National Gamma Array (INGA) composed of twenty Compton-suppressed Clover-germanium detectors. Compared to the prior publications, the level scheme has been significantly modified owing to the observation of triple γ-coincidences and the measurements of γ intensities, Rθ (asymmetry ratio), directional correlation of oriented states and integrated polarisation directional correlation of oriented states ratios. Theoretical results relevant for the exotic shape-coexistence related to the present context are presented and discussed. They illustrate the consequences of the fact that 160Yb is expected to be a doubly-magic (Z = 70 and N = 90) tetrahedral-symmetry nucleus as predicted by other authors. The historical criteria of identification of such a symmetry in nuclei are critically revisited in the context of the actual measurement and interpretation of its results and the precise new criteria are formulated employing recent results of the Fukuoka-Strasbourg collaboration. The experimental results are interpreted and discussed in detail. Limiting values of branching ratios for transitions observed by other authors but unseen in the present work are estimated. The population criteria of the β and γ vibration bands are revisited and discussed. Although no indication of the presence of the stable tetrahedral configuration has been obtained, arguments are given towards the possible presence of the Y32 (‘tetrahedral component’) vibrations around the ground-state equilibrium. With the improvement of the spectroscopic information provided by the new data the way is opened for designed experiment(s) based on the unambiguous identification criteria of tetrahedral symmetry presented in this article.

Spectroscopy of a tetrahedral doubly magic candidate nucleus <sup>160</sup><sub>70</sub>Yb<sub>90</sub>

The decay scheme of $^{160}$Yb nucleus populated in the $^{148}$Sm($^{16}$O, 4n)$^{160}$Yb reaction at 90 MeV has been studied. The gamma-coincidence data have been collected by using Indian National Gamma Array (INGA) composed of twenty Compton-suppressed Clover-Germanium detectors. Compared to the prior publications, the level scheme has been significantly modified owing to the observation of triple $\gamma$-coincidences and the measurements of $\gamma$ intensities, R$_{\theta}$ (asymmetry ratio), DCO (Directional Correlation of Oriented States) and IPDCO (Integrated Polarisation Directional Correlation of Oriented States) ratios.
 
 Theoretical results relevant for the exotic shape-coexistence related to the present context are presented and discussed. They illustrate the consequences of the fact that $^{160}$Yb is expected to be a doubly-magic (Z=70 and N=90) tetrahedral-symmetry nucleus as predicted by other authors. The historical criteria of identification of such a symmetry in nuclei are critically revisited in the context of the actual measurement and interpretation of its results and the precise new criteria are formulated employing recent results of the Fukuoka-Strasbourg collaboration.
 
 The experimental results are interpreted and discussed in detail. Limiting values of branching ratios for transitions observed by other authors but unseen in the present work are estimated. The population criteria of the $\beta$ and $\gamma$ vibration bands are revisited and discussed.
 
 Although no indication of the presence of the stable tetrahedral configuration has been obtained, arguments are given towards the possible presence of the $Y_{32}$ (`tetrahedral component') vibrations around the ground-state equilibrium. With the improvement of the spectroscopic information provided by the new data the way is opened for designed experiment(s) based on the unambiguous identification criteria of tetrahedral symmetry presented in this article.