arXiv subjects

G. Mukherjee

Publications and source records attributed to G. Mukherjee.

At least 19 recordsLinked to original sources

arXiv2026

Single Particle Excitations, Band Structures and Octupole Correlation in $^{65}$Zn

The excitation scheme of the $^{65}$Zn ($Z = 30, N = 35$) nucleus has been probed following its population in the $^{63}$Cu($\alpha$,pn) reaction at E$_{beam}$ = 30 MeV and using an array of Compton suppressed HPGe clovers as the detection system. This work has identified several new transitions of the nucleus and have modified the placements of some of the previously known ones. The multipolarities and the electric/ magnetic nature of the observed $\gamma$-ray rays have been measured, using the conventional methodologies. The spin-parity assignments for the levels have consequently been made; some of the spin-parities are new while others are either validation of the existing values or are modified results based on the present analysis. The experimental level scheme exhibits collective as well as single particle structures. The measured level energies have been compared with those calculated in the framework of the large basis shell model using a model space of $p_{3/2}, f_{5/2}, p_{1/2}, g_{9/2}$ orbitals and two different interactions. The collective excitations of the nucleus were probed through the properties of its band structures and through the calculations of the Total Routhian Surface (TRS) for the associated deformations/ shapes. The results of this study brings out the essential features of evolving structural characteristics and developing collectivity with increasing number of nucleons outside a doubly-magic core and with their occupancy of deformation driving high-$j$ orbitals.

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arXiv2026

Level structure of light neutron-rich La isotopes beyond the N=82 shell closure

The high spin excited states of Lanthanum isotopes $^{140-143}$La, above the $N=82$ closed shell, have been populated in fission reactions. The prompt $\gamma$-ray transitions were measured using two complementary methods; a) in coincidence with the isotopically identified fragments produced in the fission of the $^{238}$U+$^{9}$Be system using the VAMOS++ and the AGATA spectrometers and b) high statistics three-fold $\gamma-\gamma-\gamma$ and four-fold $\gamma-\gamma-\gamma-\gamma$ coincidence data from the spontaneous fission of $^{252}$Cf using the Gammasphere. This work reports the first identification of a pair of parity doublet structures in $^{143}$La and the new high spin level structure in $^{140-142}$La from prompt $\gamma$-ray spectroscopy. The level structures are interpreted in terms of the systematics of neighbouring odd-$Z$ nuclei above $Z=50$ shell closure and large-scale shell model calculations. The present results indicate the presence of stable octupole deformation, in $^{143}$La. The excitation energy pattern and their comparison with neighbouring isotones, moving away from the N=82 closed shell, point towards a transition from single particle structures to an alternating parity rotational band structure in the La isotopic chain.

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arXiv2024

Measurement of enhanced electric dipole transition strengths at high spin in $^{100}$Ru: Possible observation of octupole deformation

The majority of atomic nuclei have deformed shapes and nearly all these shapes are symmetric with respect to reflection. There are only a few reflection asymmetric pear-shaped nuclei that have been found in actinide and lanthanide regions, which have static octupole deformation. These nuclei possess an intrinsic electric dipole moment due to the shift between the center of charge and the center of mass. This manifests in the enhancement of the electric dipole transition rates. In this article, we report on the measurement of the lifetimes of the high spin levels of the two alternate parity bands in $^{100}$Ru through the Doppler Shift Attenuation Method. The estimated electric dipole transition rates have been compared with the calculated transition rates using the triaxial projected shell model without octupole deformation, and are found to be an order of magnitude enhanced. Thus, the observation of seven inter-leaved electric dipole transitions with enhanced rates establish $^{100}$Ru as possibly the first octupole deformed nucleus reported in the A $\approx$ 100 mass region.

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arXiv2024

Existance of octupole correlation in 116Sn

The negative parity states in 116Sn have been investigated in terms of octupole correlation. The same is probed by using the Indian National Gamma Array (INGA) facility at Variable Energy Cyclotron Centre, Kolkata using the reaction, 114Cd({\alpha},2n) 116Sn at 34 MeV energy. Three new {\gamma}-transitions relevant to the present investigation are reported and the spin-parities of the associated levels are assigned based on the DCO-ratios and polarisation measurements. The interband transitions between the positive parity ground band and the negative parity band are newly observed and the corresponding extracted ratio of transition probability, B(E1)/B(E2) indicated the existence of octupole correlation in these nuclei. The enhanced transition rates for E1 and E3 transitions between these opposite parity bands and corroboration of soft octupole deformation ultimately aid the onset of octupole excitation in this isotope.

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arXiv2024

First (calibration) experiment using proton beam from FRENA at SINP

This work presents the first calibration experiment of a 3 MV Tandetron accelerator, FRENA, performed in May 2022. The $^7$Li(p,n) reaction threshold was measured to calibrate the terminal voltage measuring device. A LiF target of thickness 175 $\mu$g/cm$^2$ was used in the experiment. The measured threshold was 1872$\pm$2.7 keV, indicating 6$-$10 keV energy shift.

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arXiv2023

Search for the origin of wobbling motion in the $ A \approx 130 $ region: The case of $^{131}$Xe

In-beam $ \gamma $-ray spectroscopy of $^{131}$Xe has been carried out to study the structure of the intruder $ \nu h_{11/2} $ band. Excited states were populated via an $ \alpha $-induced fusion-evaporation reaction at E$ _{\alpha} = 38 $ MeV. Inspection of $ \gamma \gamma $-coincidence data resulted in the identification of a new rotational sequence. Based on the systematics of excitation energy, assigned spin-parity, decay pattern, and the electromagnetic character of the inter-band $ \Delta I = 1 $ $ \gamma $-transitions, this sequence is proposed as the unfavoured signature partner of the $ \nu h_{11/2} $ band. The structure of this band is further illuminated in the light of the triaxial particle rotor model (TPRM). The possibility of wobbling excitation in $ N = 77 $ Xe-Ba-Ce isotones has been explored in a systematic manner.

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arXiv2023

Measurement of the Hoyle State Radiative Transition Width

The radiative decay of the Hoyle state is the doorway to the production of heavier elements in stellar environment. Here we report, an exclusive measurement of electric quadruple (E$_2$) transitions of the Hoyle state to the ground state of $^{12}$C through the $^{12}$C(p, p$^\prime$$\gamma$$\gamma$)$^{12}$C reaction. Triple coincidence measurement yields a value of radiative branching ratio $\Gamma_{rad}$/$\Gamma$ = 4.01 (30) $\times$ 10$^{-4}$. The result has been corroborated by an independent experiment based on the complete kinematical measurement $via.$ $^{12}$C(p, p$^\prime$)$^{12}$C reaction ($\Gamma_{rad}$/$\Gamma$ = 4.04 (30) $\times$ 10$^{-4}$). Using our results together with the currently adopted values of $\Gamma_{\pi}$(E$_0$)/$\Gamma$ and $\Gamma_{\pi}$($E_0$), the radiative width of the Hoyle state is found to be 3.75 (40) $\times$ 10$^{-3}$ eV. We emphasize here that our result is not in agreement with 34 $\%$ increase in the radiative decay width of the Hoyle state measured recently but consistent with the currently adopted value.

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arXiv2023

Particle Identification at VAMOS++ with Machine Learning Techniques

Multi-nucleon transfer reaction between 136Xe beam and 198Pt target was performed using the VAMOS++ spectrometer at GANIL to study the structure of n-rich nuclei around N=126. Unambiguous charge state identification was obtained by combining two supervised machine learning methods, deep neural network (DNN) and positional correction using a gradient-boosting decision tree (GBDT). The new method reduced the complexity of the kinetic energy calibration and outperformed the conventional method, improving the charge state resolution by 8%

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arXiv2023

Rotation of pear-shaped $^{100}$Ru nucleus

Atomic nuclei in general can have deformed shapes and nearly all these shapes are symmetric with respect to reflection. Only a few Actinide nuclei have stable reflection asymmetric pear shapes in their ground state and exhibit characteristic rotational bands. In this article, we report on the observation of two alternate parity rotational bands in 100Ru, which are connected by seven interleaved electric dipole transitions and their rates are found to be enhanced. In addition, the moments of inertia associated with these two opposite parity rotational bands have been found to be similar. These experimental observations indicate the rotation of a stable pear-shaped 100Ru nucleus, which is the first such observation outside the Actinide mass region. This shape is built on an excited configuration and originates from the rotational alignment of the angular momenta of a pair of neutrons. This unique observation establishes an alternate mechanism by which an atomic nucleus can assume a pear shape.

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arXiv2023

Coexistence of single particle and collective excitation in $^{61}$Ni

The high spin states in 61 Ni have been studied using the fusion evaporation reaction, Ti( $^{14}$C,3n) $^{61}$Ni at an incident beam energy of 40 MeV. A Compton suppressed multi-HPGe detector setup, consisting of six Clover detectors and three single crystal HPGe detectors was used to detect the de-exciting $\gamma$ rays from the excited states. The level scheme has been extended up to an excitation energy of 12.8 MeV and a tentative J$_{\pi}$ = 35/2$^+$ . The low-lying negative parity levels are found to be generated by single particle excitation within the f p shell and also excitations to the g$_{9/2}$ orbitals as explained well with shell model calculations using the GXPF1Br+V M U (modified) interaction. Two rotational structure of regular E2 sequences with small to moderate axial deformation have been established at higher excitation energy. Most interestingly, two sequences of M1 transitions are reported for the first time and described as magnetic rotational bands. The shears mechanism for both the bands can be described satisfactorily by the geometrical model. The shell model calculation involving the cross shell excitation beyond the fp shell well reproduce the M1 and E2 sequences. The shell model predicted B(M1) values for the magnetic rotational band B1 show the decreasing trend with spin as expected with closing of the shears.

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arXiv2022

Improving Fission-product Decay Data for Reactor Applications: Part I -- Decay Heat

Effort has been expended to assess the relative merits of undertaking further decay-data measurements of the main fission-product contributors to the decay heat of neutron-irradiated fissile fuel and related actinides by means of Total Absorption Gamma-ray Spectroscopy (TAGS/TAS) and Discrete Gamma-ray Spectroscopy (DGS). This review has been carried out following similar work performed under the auspices of OECD/WPEC-Subgroup 25 (2005-2007) and the International Atomic Energy Agency (2010, 2014), and various highly relevant TAGS measurements completed as a consequence of such assessments. We present our recommendations for new decay-data evaluations, along with possible requirements for total absorption and discrete high-resolution gamma-ray spectroscopy studies that cover approximately 120 fission products and various isomeric states.

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arXiv2022

A fast-rotating pear-shaped nucleus

The spectroscopic studies have identified few Actinide and Lanthanide nuclei of the periodic table, which can assume the pear shape. The low frequency collective rotation of these nuclei has been established by determining the band structure of the excited levels and their gamma decay rates. In this article, we report the rotation of a pear-shaped nucleus in 100 Ru, which rotates nearly three times faster than the previously known cases. The three novel consequences of this fast rotation are: the realization of a pear-shape in an excited state, its moment of inertia becoming a constant of motion and its shape evolution. These inferences have been arrived at by comparing the characteristics of the rotational band of 100 Ru with three of the best-known examples of a rotating pear-shaped nucleus and through the theoretical interpretation of the data. The observation of a pear-shaped nucleus in the lighter mass region opens up the possibility of a systematic study of the effects of fast rotation on this shape.

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arXiv2022

Shape evolution in the rapidly rotating $^{140}$Gd nucleus

Ground state band of $^{140}$Gd has been investigated following their population in the $^{112}$Sn($^{35}$Cl,~$α$p2n)$^{140}$Gd reaction at 195 MeV of beam energy using a large array of Compton suppressed HPGe clovers as the detection setup. Apart from other spectroscopic measurements, level lifetimes of the states have been extracted using the Doppler Shift Attenuation Method. Extracted quadrupole moment along with the pairing independent cranked Nilsson-Strutinsky model calculations for the quadrupole band reveal that the nucleus preferably attains triaxiality with $γ$ = -30$^\circ$. The calculation though shows a slight possibility of rotation around the longest possible principal axis at high spin $\sim$ 30$\hbar$ which is beyond the scope of the present experiment.

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arXiv2022

Experimental study of the $^{40,48}$Ca+ $^{40,48}$Ca reactions at 35 MeV/nucleon

In this article we investigate $^{40,48}$Ca+$^{40,48}$Ca peripheral and semi-peripheral reactions at 35 MeV/nucleon. Data were obtained using the unique coupling of the VAMOS high acceptance spectrometer and the INDRA charged particle multidetector.The spectrometer allowed high resolution measurement of charge, mass and velocity of the cold projectile-like fragment (PLF), while the INDRA detector recorded coincident charged particles with nearly $4π$ acceptance.The measured isotopic composition of the PLF identified in VAMOS and the average light charged particle (LCP) multiplicities are promising observables to study the isospin diffusion.The detection of the PLF in coincidence with LCP allows the reconstruction of the mass, charge and excitation energy of the associated initial quasi-projectile nuclei (QP), as well as the extraction of apparent temperatures.We investigate the suitability of the isoscaling method with the PLF and the experimental reconstructed QP.The extracted $α$ and $Δ$ isoscaling parameters present a dependence on the considered system combination that could justify their use as a surrogate for isospin asymmetry in isospin transport studies.The reconstruction of the QP allows to observe an evolution of the $α/Δ$ with the size of the QP, the latter being consistent with a strong surface contribution to the symmetry energy term in finite nuclei. This leads to the conclusion that the reconstruction of the primary source is mandatory for the study of the symmetry energy term based on the isoscaling method for such reactions.

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arXiv2021

Preparation of Isotopically enriched $^{112,116,120,124}$Sn targets at VECC

Resistive heating and mechanical rolling methods have been employed to prepare isotopically enriched thin target foils of 116Sn (~380 μg/cm2), 124Sn(~400 μg/cm2) and thicker foils of 112Sn (1.7 mg/cm2),120Sn (1.6 mg/cm2),respectively. Preparation of enriched targets with small amount of material, selection of releasing agent for thin targets and separation of deposited material insolvent were among the several challenges while fabrication of the thin targets. Uniformity of the targets has been measured using 241Am α-source. NaCl has been used as releasing agent in preparation of the thin targets. These targets have been successfully used in nuclear physics experiments at VECC.

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arXiv2021

Development of a Reference Database for Beta-Delayed Neutron Emission

Beta-delayed neutron emission is important for nuclear structure and astrophysics as well as for reactor applications. Significant advances in nuclear experimental techniques in the past two decades have led to a wealth of new measurements that remain to be incorporated in the databases. We report on a coordinated effort to compile and evaluate all the available beta-delayed neutron emission data. The different measurement techniques have been assessed and the data have been compared with semi-microscopic and microscopic-macroscopic models. The new microscopic database has been tested against aggregate total delayed neutron yields, time-dependent group parameters in 6-and 8-group re-presentation, and aggregate delayed neutron spectra. New recommendations of macroscopic delayed-neutron data for fissile materials of interest to applications are also presented. The new Reference Database for Beta-Delayed Neutron Emission Data is available online at: http://www-nds.iaea.org/beta-delayed-neutron/database.html.

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arXiv2020

Nuclear level density and thermal properties of $^{115}$Sn from neutron evaporation

The nuclear level density of $^{115}$Sn has been measured in an excitation energy range of $\sim $2 - 9 MeV using the experimental neutron evaporation spectra from the $^{115}$In($p,n$)$^{115}$Sn reaction. The experimental level densities were compared with the microscopic Hartree-Fock BCS (HFBCS), Hartree-Fock-Bogoliubov plus combinatorial (HFB+C), and an exact pairing plus independent particle model (EP+IPM) calculations. It is observed that the EP+IPM provides the most accurate description of the experimental data. The thermal properties (entropy and temperature) of $^{115}$Sn have been investigated from the measured level densities. The experimental temperature profile as well as the calculated heat capacity show distinct signatures of a transition from the strongly-paired nucleonic phase to the weakly paired one in this nucleus.

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arXiv2020

Evidence for the reduction of nuclear level density away from the $β$-stability line

The isospin dependence of nuclear level density has been investigated by analyzing the spectra of evaporated neutrons from excited $^{116}$Sn and $^{116}$Te nuclei. These nuclei are populated via $p$ + $^{115}$In and $^{4}$He + $^{112}$Sn reactions in the excitation energy range of 18 - 26 MeV. Because of low excitation energy, the neutron spectra are predominantly contributed by the first-chance decay leading to the $β$-stable $^{115}$Sn and neutron-deficient $^{115}$Te as residues for the two cases. Theoretical analysis of the experimental spectra have been performed within the Hauser-Feshbach formalism by employing different models of the level density parameter. It is observed that the data could only be explained by the level density parameter that decreases monotonically when the proton number deviates from the $β$-stable value. This is also confirmed by performing a microscopic shell-model calculation with the Wood-Saxon mean field. The results have strong implication on the estimation of the level density of unstable nuclei, and calculation of astrophysical reaction rates relevant to $r$- and $rp$-processes.

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