arXiv subjects

Jhilam Sadhukhan

Publications and source records attributed to Jhilam Sadhukhan.

18 recordsLinked to original sources

arXiv2025

Cross talk between experimental data and simple validation of shell closure in pre-actinides

Two back-to-back experiments, 28Si + 178Hf and 28Si + 186W, were intentionally conducted to validate the role of shell closure in pre-actinides by studying neutron multiplicity in compound nucleus (CN) 206Rn and 214Ra. In the first experiment, Dubey et al. [Phys. Rev. C 112, L011602 (2025)], we established the influence of the neutron shell closure. In the present work, the CN 214Ra was deliberately selected to investigate the dependence of the total neutron multiplicity (Mtotal) on the proton number (Z), while keeping the neutron number constant at N = 126 in the pre-actinide region. The objective of the study is two-fold : (i) to examine the effect of proton shell closure when moving away from Z = 82, and (ii) to correlate the present results on proton shell closure with our previous finding on neutron shell closure. We have also used the previous reported data for N = 126 isotones 210Po, 212Rn, and 213Fr to establish the validation of shell closure. A systematic increase in Mtotal with increasing Z was observed from Z = 82 to Z = 88. Furthermore, comparison of the present results on proton shell closure with our earlier neutron shell closure observation, reveals that the cross-correlation between neutron and proton shell closure shows a systematic increase in Mtotal as one moves away from 208Pb, whether along isotonic or isotopic chains.

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arXiv2025

Moments of inertia of rare-earth nuclei and the nuclear time-odd mean fields within exact solutions of the adiabatic theory

We systematically analyse the nuclear moments of inertia determined within the Skyrme and Gogny density functional theories. The time-odd mean fields generated by collective rotation are self-consistently determined by a novel exact iterative solution of the adiabatic time-dependent Hartree-Fock-Bogoliubov (ATDHFB) equations. Although details of the results depend on the functional used, the calculated moments of inertia are in good overall agreement with the experimental data, with no adjustable parameters. To show the essential importance of the time-odd mean fields, we compared the ATDHFB moments of inertia with those obtained from the Inglis-Belyaev formula. For Skyrme density functionals, we find strong correlations between the effective mass and the impact of the time-odd mean fields on the rotational and vibrational collective inertia.

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arXiv2025

To examine the variation in dissipation near the shell closure using neutron multiplicity as a probe

The pre and post-scission neutron multiplicities have been determined for the fission of the compound nucleus (CN) 206Rn, induced by the reaction 28Si+178Hf within the excitation energy interval of 61.0-90.0 MeV. We intentionally formed CN 206 Rn, which is below the shell closure CN, to examine the variation in N/Z with total neutron multiplicity, as data for other CNs of 208,210,212,214,216Rn have already been published in the literature. We identified a new trend in the N/Z ratio, where the total neutron multiplicity initially decreases as we approach the shell closure of the compound nucleus and then starts to increase as we move away from the shell closure. Furthermore, we have observed that below the neutron shell closure, the dissipation in compound nuclei (CN) escalates with rising excitation energy, remains stable at the shell closure CN, and thereafter diminishes with increasing excitation energy above the shell closure CN.

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arXiv2024

Iterative solutions of the ATDHFB equations to determine the nuclear collective inertia

An iterative adiabatic time-dependent Hartree-Fock-Bogoliubov (ATDHFB) method is developed within the framework of Skyrme density functional theory. The ATDHFB equation is solved iteratively to avoid explicitly calculating the stability matrix. The contribution of the time-odd mean fields to the ATDHF(B) moment of inertia is incorporated self-consistently, and the results are verified by comparing them with the dynamical cranking predictions. The inertia mass tensor is calculated with the density-derivative term evaluated by numerical differentiation.

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arXiv2024

Multimodal fission from self-consistent calculations

When multiple fission modes coexist in a given nucleus, distinct fragment yield distributions appear. Multimodal fission has been observed in a number of fissioning nuclei spanning the nuclear chart, and this phenomenon is expected to affect the nuclear abundances synthesized during the rapid neutron-capture process (r-process). In this study, we generalize the previously proposed hybrid model for fission-fragment yield distributions to predict competing fission modes and estimate the resulting yield distributions. Our framework allows for a comprehensive large-scale calculation of fission fragment yields suited for r-process nuclear network studies. Nuclear density functional theory is employed to obtain the potential energy and collective inertia tensor on a multidimensional collective space defined by mass multipole moments. Fission pathways and their relative probabilities are determined using the nudged elastic band method. Based on this information, mass and charge fission yields are predicted using the recently developed hybrid model. Fission properties of fermium isotopes are calculated in the axial quadrupole-octupole collective space for three energy density functionals (EDFs). Disagreement between the EDFs appears when multiple fission modes are present. Within our framework, the UNEDF1$_{\textrm{HFB}}$ EDF agrees best with experimental data. Calculations in the axial quadrupole-octupole-hexadecapole collective space improve the agreement with the experiment for SkM$^{*}$. We also discuss the sensitivity of fission predictions on the choice of EDF for several superheavy nuclei. Fission fragment yield predictions for nuclei with multiple fission modes are sensitive to the underlying EDF. For large-scale calculations in which a minimal number of collective coordinates is considered, UNEDF1$_{\textrm{HFB}}$ provides the best description of experimental data.

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arXiv2022

Nudged elastic band approach to nuclear fission pathways

The nuclear fission process is a dramatic example of the large-amplitude collective motion in which the nucleus undergoes a series of shape changes before splitting into distinct fragments. This motion can be represented by a pathway in the many-dimensional space of collective coordinates. The collective action along the fission pathway determines the spontaneous fission half-lives as well as mass and charge distributions of fission fragments. We study the performance and precision of various methods to determine the minimum action and minimum-energy fission trajectories in the collective space. We apply the nudged elastic band method (NEB), grid-based methods, and Euler Lagrange approach to the collective action minimization in two and three dimensional collective spaces. The performance of various approaches to the fission pathway problem is assessed by studying the collective motion along both analytic energy surfaces and realistic potential energy surfaces obtained with the Hartree-Fock-Bogoliubov theory. The uniqueness and stability of the solutions is studied. The NEB method is capable of efficient determination of the exit points on the outer turning surface that characterize the most probable fission pathway and constitute the key input for fission studies. This method can also be used to accurately compute the critical points (i.e., local minima and saddle points) on the potential energy surface of the fissioning nucleus that determine the static fission path. The NEB method is the tool of choice for finding the least-action and minimum energy fission trajectories. It will be particularly useful in large-scale fission calculation of superheavy nuclei and neutron-rich fissioning nuclei contributing to the astrophysical r-process recycling.

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arXiv2022

Theoretical description of fission yields: towards a fast and efficient global model

Background: A quantitative microscopic understanding of the fission-fragment yield distributions represents a major challenge for nuclear theory as it involves the intricate competition between large-amplitude nuclear collective motion and single-particle nucleonic motion. Purpose: A recently proposed approach to global modeling of fission fragment distributions is extended to account for odd-even staggering in charge yields and for neutron evaporation. Method: Fission trajectories are obtained within the density functional theory framework, allowing for a microscopic determination of the most probable fission prefragment configurations. Mass and charge yields distributions are constructed by means of a statistical approach rooted in a microcanonical ensemble. Result: We show that the proposed hybrid model can reproduce experimental mass and charge fragment yields, including the odd-even staggering, for a wide range of fissioning nuclei. Experimental isotopic yields can be described within a simple neutron evaporation scheme. We also explore fission fragment distributions of exotic neutron-rich and superheavy systems, and compare our predictions with other state-of-the art global calculations. Conclusion: Our study suggests that the microscopic rearrangement of nucleons into fission fragments occurs well before the scission, and that the subsequent dynamics is mainly driven by the thermal excitations and bulk features of the nuclear binding. The proposed simple hybrid approach is well suited for large-scale calculations involving hundreds of fissioning nuclei.

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arXiv2020

Future of Nuclear Fission Theory

There has been much recent interest in nuclear fission, due in part to a new appreciation of its relevance to astrophysics, stability of superheavy elements, and fundamental theory of neutrino interactions. At the same time, there have been important developments on a conceptual and computational level for the theory. The promising new theoretical avenues were the subject of a workshop held at the University of York in October 2019; this report summarises its findings and recommendations.

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arXiv2020

Efficient method for estimation of fission fragment yields of r-process nuclei

$\textbf{Background}$ More than half of all the elements heavier than iron are made by the rapid neutron capture process (or r process). For very neutron-rich astrophysical conditions, such at those found in the tidal ejecta of neutron stars, nuclear fission determines the r-process endpoint, and the fission fragment yields shape the final abundances of $110\le A \le 170$ nuclei. The knowledge of fission fragment yields of hundreds of nuclei inhabiting very neutron-rich regions of the nuclear landscape is thus crucial for the modeling of heavy-element nucleosynthesis. $\textbf{Purpose}$ In this study, we propose a model for the fast calculation of fission fragment yields based on the concept of shell-stabilized prefragments defined with help of the nucleonic localization functions. $\textbf{Methods}$ To generate realistic potential energy surfaces and nucleonic localizations, we apply Skyrme Density Functional Theory. The distribution of the neck nucleons among the two prefragments is obtained by means of a statistical model. $\textbf{Results}$ We benchmark the method by studying the fission yields of $^{178}$Pt, $^{240}$Pu, $^{254}$Cf, and $^{254,256,258}$Fm and show that it satisfactorily explains the experimental data. We then make predictions for $^{254}$Pu and $^{290}$Fm as two representative cases of fissioning nuclei that are expected to significantly contribute during the r-process nucleosynthesis occurring in neutron star mergers. $\textbf{Conclusions}$ The proposed framework provides an efficient alternative to microscopic approaches based on the evolution of the system in a space of collective coordinates all the way to scission. It can be used to carry out global calculations of fission fragment distributions across the r-process region.

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arXiv2019

Cluster radioactivity of $^{294}_{118}$Og$_{176}$

According to theory, cluster radioactivity becomes an important decay mode in superheavy nuclei. In this work, we predict that the strongly-asymmetric fission, or cluster emission, is in fact the dominant fission channel for $^{294}_{118}$Og$_{176}$, which is currently the heaviest synthetic isotope known. Our theoretical approach incorporates important features of fission dynamics, including quantum tunneling and stochastic dynamics up to scission. We show that, despite appreciable differences in static fission properties such as fission barriers and spontaneous fission lifetimes, the prediction of cluster radioactivity in $^{294}_{118}$Og$_{176}$ is robust with respect to the details of calculations, including the choice of energy density functional, collective inertia, and the strength of the dissipation term.

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arXiv2018

A dynamical model calculation to reconcile the nuclear fission lifetime from different measurement techniques

The pre-scission particle multiplicities suggest a lifetime of 10 20 s for the nuclear fission to occur which is in contrast to the fission lifetime 10 18 s as predicted by atomic probe. This long standing ambiguity, arising due to the orders of magnitude differences among the fission lifetime measured from the nuclear and atomic probes, has been addressed within a dynamical model which includes the contributions from the nuclear shell effects. We show that, at lower excitation energies, these two probes decouples as the fissioning system survives for a long time without any particle evaporation. We also consider a wide range of reactions to study the impact of the excitation energy of compound nucleus on the fission dynamics in general. Our model predicts the average fission life time of superheavy nucleus 302 120, to be more than 10 18 s which is in reasonable agreement with the recent experiments.

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arXiv2017

Formation and distribution of fragments in the spontaneous fission of 240Pu

The goal of this paper is to better understand the structure of fission fragment distributions by investigating the competition between the static structure of the collective manifold and stochastic dynamics. In particular, we study the characteristics of the tails of yield distributions, which correspond to very asymmetric fission. We use the stochastic Langevin framework to simulate the nuclear evolution after the system tunnels through the multi-dimensional potential barrier. For a representative sample of different initial configurations along the outer turning-point line, we define effective fission paths by computing a large number of Langevin trajectories. We extract the relative contribution of each such path to the fragment distribution. We then use nucleon localization functions along effective fission pathways to analyze the characteristics of prefragments at pre-scission configurations. We find that non-Newtonian Langevin trajectories, strongly impacted by the random force, produce the tails of the fission fragment distribution. The prefragments deduced from nucleon localizations are formed early and change little as the nucleus evolves towards scission. On the other hand, the system contains many nucleons that are not localized in the prefragments, even near the scission point. Such nucleons are rapidly distributed at scission to form the final fragments. Our study shows that only theoretical models of fission that account for some form of dissipative/stochastic dynamics can give an accurate description of the structure of fragment distributions. In particular, it should be nearly impossible to predict the tails of these distributions within the standard formulation of time-dependent density functional theory. At the same time, the large number of non-localized nucleons during fission suggests that adiabatic approaches are ill-suited to describe fission fragment properties.

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arXiv2016

Fission fragment mass distributions in reactions populating 200Pb

The fission fragment mass distributions have been measured in the reactions 16O + 184W and 19F+ 181Ta populating the same compound nucleus 200Pb? at similar excitation energies. It is found that the widths of the mass distribution increases monotonically with excitation energy, indicating the absence of quasi-fission for both reactions. This is contrary to two recent claims of the presence of quasi-fission in the above mentioned reactions.

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arXiv2015

Microscopic modeling of mass and charge distributions in the spontaneous fission of 240Pu

In this letter, we outline a methodology to calculate microscopically mass and charge distributions of spontaneous fission yields. We combine the multi-dimensional minimization of collective action for fission with stochastic Langevin dynamics to track the relevant fission paths from the ground-state configuration up to scission. The nuclear potential energy and collective inertia governing the tunneling motion are obtained with nuclear density functional theory in the collective space of shape deformations and pairing. We obtain a quantitative agreement with experimental data and find that both the charge and mass distributions in the spontaneous fission of 240Pu are sensitive both to the dissipation in collective motion and to adiabatic characteristics.

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arXiv2015

No influence of a N=126 Neutron Shell Closure in Fission Fragment Mass Distributions

Mass distributions of the fragments in the fission of $^{206}$Po and the N=126 neutron shell closed nucleus $^{210}$Po have been measured. No significant deviation of mass distributions has been found between $^{206}$Po and $^{210}$Po, indicating the absence of shell correction at the saddle point in both the nuclei, contrary to the reported angular anisotropy and pre-scission neutron multiplicity results. This new result provides benchmark data to test the new fission dynamical models to study the effect of shell correction on the potential energy surface at saddle point.

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arXiv2015

Direct Evidence of Washing out of Nuclear Shell Effects

Constraining excitation energy at which nuclear shell effect washes out has important implications on the production of super heavy elements and many other fields of nuclear physics research. We report the fission fragment mass distribution in alpha induced reaction on an actinide target for wide excitation range in close energy interval and show direct evidence that nuclear shell effect washes out at excitation energy ~40 MeV. Calculation shows that second peak of the fission barrier also vanishes around similar excitation energy.

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arXiv2014

Pairing-induced speedup of nuclear spontaneous fission

Collective inertia is strongly influenced at the level crossing at which quantum system changes diabatically its microscopic configuration. Pairing correlations tend to make the large-amplitude nuclear collective motion more adiabatic by reducing the effect of those configuration changes. Competition between pairing and level crossing is thus expected to have a profound impact on spontaneous fission lifetimes. To elucidate the role of nucleonic pairing on spontaneous fission, we study the dynamic fission trajectories of $^{264}$Fm and $^{240}$Pu using the state-of-the-art self-consistent framework. We employ the superfluid nuclear density functional theory with the Skyrme energy density functional SkM$^*$ and a density-dependent pairing interaction. Along with shape variables, proton and neutron pairing correlations are taken as collective coordinates. The collective inertia tensor is calculated within the nonperturbative cranking approximation. The fission paths are obtained by using the least action principle in a four-dimensional collective space of shape and pairing coordinates. Pairing correlations are enhanced along the minimum-action fission path. For the symmetric fission of $^{264}$Fm, where the effect of triaxiality on the fission barrier is large, the geometry of fission pathway in the space of shape degrees of freedom is weakly impacted by pairing. This is not the case for $^{240}$Pu where pairing fluctuations restore the axial symmetry of the dynamic fission trajectory. The minimum-action fission path is strongly impacted by nucleonic pairing. In some cases, the dynamical coupling between shape and pairing degrees of freedom can lead to a dramatic departure from the static picture. Consequently, in the dynamical description of nuclear fission, particle-particle correlations should be considered on the same footing as those associated with shape degrees of freedom.

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arXiv2013

Spontaneous fission lifetimes from the minimization of self-consistent collective action

The spontaneous fission lifetime of 264Fm has been studied within nuclear density functional theory by minimizing the collective action integral for fission in a two-dimensional quadrupole collective space representing elongation and triaxiality. The collective potential and inertia tensor are obtained self-consistently using the Skyrme energy density functional and density-dependent pairing interaction. The resulting spontaneous fission lifetimes are compared with the static result obtained with the minimum-energy pathway. We show that fission pathways strongly depend on assumptions underlying collective inertia. With the non-perturbative mass parameters, the dynamic fission pathway becomes strongly triaxial and it approaches the static fission valley. On the other hand, when the standard perturbative cranking inertia tensor is used, axial symmetry is restored along the path to fission; an effect that is an artifact of the approximation used.

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