Heavy Mass Region Research Articles

Exploring fission and quasi-fission in 31Al + 197Au: probing energies near and beyond Coulomb barrier

An idea to understand the fragmentation of a formed nuclear entity (i.e. compound nucleus), induced via heavy-ion reactions, is an interesting task especially at incident energies near, above and far away from the Coulomb barrier. In the present work, we are studying the decay dynamics of a formed nuclear entity i.e. 228U* compound nucleus (CN), via the radioactive beam of Aluminium-31 (31Al) striking on the stable Gold-197 (197Au) target nucleus, for incident energies E c.m. = 106.2–163.2 MeV. A comprehensive knowledge regarding the probable decaying fragments from 228U* has been accumulated using the collective clusterization approach of the Dynamical Cluster-decay Model (DCM) in terms of mass and charge dispersion, which built up on the Quantum Mechanical Fragmentation Theory (QMFT). Through both of these dispersion cases, we have identified the decaying fragments in the fission and heavy-mass regions, at different scale of incident energies. Further, the fission cross-sections ( σfisTheo ) have been obtained for 31Al + 197Au → 228U* reaction at the above said E c.m. (MeV) and compared with the experimental data. This study has been carried out using two different sets of Skyrme force parameters (SIII and SSk), which are employed within the semiclassical approach of Skyrme Energy Density Formalism. Unlike for energies below and near the barrier, a significant difference has been observed between calculated ( σfisTheo ) and experimental ( σfisExpt ) cross-sections for above barrier energies, with independent choice of Skyrme forces. Such discrepancy noticed has been addressed with the presence of quasi-fission (QF) event. Similar phenomena has also been seen for 19F-induced reaction forming the same CN (228U*).

Sequential decay analysis of $^{235}$U(n$^{th}$,f) reaction using fragmentation approach

Abstract Numerous experimental and theoretical observations conclude that the probability of the three fragment emission (ternary fission) or the binary fission increases when one proceeds towards the heavy mass region of nuclear periodic table. The collinear cluster tripartition (CCT) channel of $^{235}$U(n$^{th}$,f) reaction is studied and it was observed that the CCT may be a sequential process or a simultaneous emission phenomena. Till now, different approaches are introduced to study the CCT process as a simultaneous process or sequential process, but the decay dynamics of these modes is not fully explored. It will be of interest to identify the three fragments of the sequential process and to explore their related dynamics using some excitation energy dependent approach. Hence, in present work, an attempt is made to study the sequential decay mechanism of $^{235}$U(n$^{th}$,f) reaction using quantum mechanical fragmentation theory (QMFT). The decay mechanism is considered in two steps, where initially the nucleus splits into an asymmetric channel. In the second step, the heavy fragment obtained in the first step divides into two fragments. Stage I analysis is done by calculating the fragmentation potential and the preformation probability for the spherical and deformed choice of the decaying fragments. The most probable fragment combination of stage I are identified in view the dips in the fragmentation structure, and the corresponding maxima's of the preformation probability ($P_0$). The excitation energy of the decay channel is calculated using an iteration process. The obtained excitation energy of the identified heavy fragments is further used for the fragmentation analysis, and the subsequent binary fragments of the sequential process are obtained. The identified three fragments of the sequential process are in agreement with the experimental observation and are found nearby the neutron or proton shell closure. Finally, the kinetic energy of the observed fragments is calculated and the middle fragment of the CCT mechanism is identified.

Atomic mass determination of uranium-238

The atomic mass of uranium-238 has been determined to be 238.050787618(15) u, improving the literature uncertainty by two orders of magnitude. It is obtained from a measurement of the mass ratio of U47+238 and Xe26+132 ions with an uncertainty of 3.5×10−12. The measurement was carried out with the Penning-trap mass spectrometer and was accompanied by a calculation of the binding energies EU and EXe of the 47 and 26 missing electrons of the two highly charged ions, respectively. These binding energies were determined using an multiconfiguration Dirac-Hartree-Fock method to be EU=39927(10) eV and EXe=8971.2(21) eV. The new mass value will serve as a reference for high-precision mass measurements in the heavy mass region of the nuclear chart up to transuranium nuclides. Published by the American Physical Society 2024

Study of various ground state decay mechanisms of Actinide nuclei* *Financial support from the Department of Science and Technology (DST), New Delhi, India in the form of a research project grant (CRG/2021/001144) is gratefully acknowledged.

The special property of the actinide mass region is that nuclei belonging to this group are radioactive and undergo different ground state processes, such as alpha decay, cluster radioactivity (CR), heavy particle radioactivity (HPR), and spontaneous fission (SF). In this study, the probable radioactive decay modes of the heavy mass region (Z = 89−102) are studied within the framework of the preformed cluster model (PCM). In the PCM, the radioactive decay modes are explored in terms of the preformation probability () and penetration probability (P) at the turning point , where the penetration of fragments is initiated [R = R (touching state) + (neck length parameter)]. First, the alpha decay half-lives are calculated for light and heavy nuclei, and points are obtained by optimizing the neck length parameter. These points are further employed to fix the Q-value dependent turning point ((Q)). Then, using the ((Q)) relation, the decay half-lives are computed, and the calculated results are compared with the available theoretical and experimental data. The isotopical trend of and P is studied with respect to the mass number of the parent and daughter nucleus, respectively. The CR and HPR channels are also explored, and a comparison of calculated data is conducted with the available literature. Comparative analysis of the fragmentation potential and preformation probability is carried out for alpha decay and SF. The mass distribution of the nuclei is studied as a function of fragment mass (A ) by considering the spherical and hot-compact deformation of the decaying fragments. Finally, the most probable fission fragments are identified using the fragmentation structure, and the corresponding fission fragment total kinetic energy of the identified fragments is calculated and compared with available data, wherever applicable.

Simple model for a study of Λ hypernuclei in medium and heavy mass regions

We propose a simple and versatile model for Λ hypernuclei in medium- and heavy-mass regions to obtain their low-lying energy levels and electromagnetic transitions. In this model, we treat the core nucleus as a simple indivisible union of nucleons, and its properties are considered based on the experimental data. The interaction between nucleons in the core and a Λ particle is assumed to be weak and is treated perturbatively. We study medium- and heavy-Λ hypernuclei, i.e., V, V, Y, La, and Pb. To check the applicability of the model to a lighter hypernucleus, we apply it to F, where γ-ray transitions have been experimentally observed. The present study demonstrates that the low-lying excited states of the core nucleus below 2 MeV, in addition to the ground state of the core nucleus have to be considered in the calculation of spectra and electromagnetic transitions in the low-lying states of Λ hypernuclei. The proposed model helps us theoretically study specific medium- and heavy-hypernuclei where large-scale shell-model studies are intractable to carry out.

Nucleon-pair truncation of the shell model for medium-heavy nuclei

Background: Establishing computationally tractable models of atomic nuclei is a long-time goal of nuclear structure physics. A flexible framework which easily includes excited states and many-body correlations is the configuration-interaction shell model (SM), but the exponential growth of the basis means one needs an efficient truncation scheme, ideally one that includes both deformation and pairing correlations.Purpose: We propose an efficient truncation scheme of the SM: starting from a pair condensate variationally defined by Hartree-Fock single-particle states and the particle-number conserved Bardeen-Cooper-Schrieffer (NBCS) approximation, we carry out projection of states with good angular momentum.Methods: After generating Hartree-Fock single-particle states with Kramers degeneracy in a SM space, we optimize the pair amplitudes in the NBCS by minimizing the energy, and then use linear algebra projection (LAP) of states with good angular momentum. Both NBCS and LAP are computationally fast.Results: Our calculations yield good agreement with full configuration-interaction SM calculations for low-lying states of transitional and rotational nuclei with axially symmetric and triaxial deformation in medium- and heavy-mass regions: $^{44,46,48}\mathrm{Ti}$, $^{48,50}\mathrm{Cr}$, $^{52}\mathrm{Fe}$, $^{60,62,64}\mathrm{Zn}$, $^{66,68}\mathrm{Ge}$, $^{68}\mathrm{Se}$, and $^{108,110}\mathrm{Xe}$. We predict low-lying states of $^{112--114}\mathrm{Ba}$ and $^{116--120}\mathrm{Ce}$, nuclei difficult to reach by large-scale SM calculations.Conclusions: Both pair correlation and the configuration mixing between different intrinsic states play a key role in reproducing collectivity and shape coexistence, demonstrating the utility of this truncation scheme of the SM to study transitional and deformed nuclei.

Nuclear landscape in a mapped collective Hamiltonian from covariant density functional theory

The nuclear landscape is investigated within the triaxial relativistic Hartree-Bogoliubov theory with the PC-PK1 density functional, and the beyond-mean-field dynamical correlation energies are taken into account by a microscopically mapped five-dimensional collective Hamiltonian without additional free parameters. The effects of triaxial deformation and dynamical correlations on the nuclear landscape are analyzed. The present results provide a better description of the experimental binding energies of even-even nuclei, in particular for medium and heavy mass regions, in comparison with previous global calculations with the state-of-the-art covariant density functionals DD-PC1 and TMA. The inclusion of the dynamical correlation energies plays an important role in the PC-PK1 results. It is emphasized that the nuclear landscape is considerably extended by the PC-PK1 functional in comparison with the previous results obtained with the covariant density functionals DD-PC1 and TMA, which may be due to the different isovector properties in the density functionals.

Alpha decay half-lives of superheavy nuclei 104 ≤ Z ≤ 125

In order to describe scattering, fusion, fission and ground state masses, Krappe and collaborators developed unified nuclear potential, by generalizing liquid drop model. They have incorporated phenomenological parameters accounting for the attractive force between two separated fragments. One of the phenomenological parameters involved in this model is the range of folded Yukawa function, which accounts for surface diffuseness of the potential and short range attractive interaction. The role of range of folding function of Yukawa-plus-exponential potential is analyzed for alpha decay of heavy and superheavy nuclei. Significant effect of this function is noted in preformation probability which improves the accuracy of half-lives of alpha decay. Half-lives for alpha decay are better obtained for two values of the range of folding function 0.54 and 0.8[Formula: see text]fm for heavy and superheavy mass regions, respectively. The study confirms the associated shell structure [Formula: see text] in heavy nuclei and [Formula: see text] and [Formula: see text] in superheavy nuclei. The calculations are extended to predict the half-lives of superheavy nuclei with [Formula: see text] and [Formula: see text] which are not yet synthesized experimentally.

Systematics of the semimicroscopic proton-nucleus optical potential at low energies relevant for nuclear astrophysics

Astrophysical models studying the origin of the p-nuclei require knowledge of the reaction rates of photodisintegrations and capture reactions. Since experimental data at astrophysically relevant energies are limited, reaction rate calculations rely on Hauser-Feshbach (HF) theory predictions. The HF theory requires nuclear physics input such as masses, level densities, $\gamma$-ray strength functions and proton-nucleus optical potentials (pOMP). The scope of this work is to improve a global semi-microscopic pOMP at energies relevant to the p-process. This is achieved by adjusting the normalization parameters of the OMP to all available proton-capture cross sections measured at low energies. By establishing the systematic behaviour of these parameters, one expects to enhance the predictive power of the pOMP when expanding to mass regions where no data exists. The HF calculations were obtained using TALYS code. The normalization parameters for the real and imaginary central potentials ($\lambda_V$ and $\lambda_W$) were adjusted to fit the proton data in the energy range where the cross-section are independent of the other nuclear inputs. Results show that the $\lambda_V$ parameter has a strong mass dependence that can be described by a second-degree polynomial function for A $\leq$ 100 and an exponential increase for 100 < A < 162. Though variations of the $\lambda_W$ have a smaller effect on the calculations, a global increase by 50$\%$ improves the results for certain nuclei without affecting the rest of the cases. The resulting adjustment functions were obtained by fitting all suitable proton data and can be used with reasonable confidence to generate the global semi-microscopic pOMP for nuclei in the medium to heavy mass region. For better statistics, more low-energy (p,$\gamma$) cross section data are needed for heavier nuclei with mass A $>$ 100.

Experimental studies of the two-step scheme with an intense radioactive Sn132 beam for next-generation production of very neutron-rich nuclei

The study of nuclei in the yet-unexplored, very neutron-rich, mid- to heavy-mass region of the nuclear chart, comprising perhaps half of all nuclei predicted to exist, is leading to new and/or upgraded facilities, as well as to research into the underlying production processes. In the present study, the usefulness of the two-step scheme, an alternate method to produce very neutron-rich nuclei, by a combination of an isotope-separation online (ISOL) system as a first step, and in-beam fragmentation of re-accelerated radioactive isotopes (RIs) as a second step, was investigated with a $^{132}\mathrm{Sn}$ beam. Very neutron-rich RIs around the neutron-rich neutron number $N=82$ region were produced from the 278-MeV/nucleon $^{132}\mathrm{Sn}$ beam impinging on a 5.97-mm Be target, and their production cross sections were measured. Yields were then estimated for the two-step scheme with the $^{132}\mathrm{Sn}$ beam relative to the ones by a one-step scheme, in-flight fission of a $^{238}\mathrm{U}$ beam, for 1-MW proton and $^{238}\mathrm{U}$ beams at respective RI-beam facilities. This comparison suggests that the two-step scheme with the $^{132}\mathrm{Sn}$ beam provides yields $&gt;40$-times higher than those with the one-step scheme for the very neutron-rich $N=82$ region. Moreover, by using various RI beams over the nuclear chart from ISOL, certain kinds of very neutron-rich RIs around the supernova $r$-process path can be produced with greater yields than by the one-step approach.

TDHF and a Macroscopic Aspect of Low-Energy Nuclear Reactions

Time-dependent Hartree--Fock (TDHF) method has been applied to various low-energy nuclear reactions, such as fusion, fission, and multinucleon transfer reactions. In this Mini Review, we summarize recent attempts to bridge a microscopic nuclear reaction theory, TDHF, and a macroscopic aspect of nuclear reactions through nucleus--nucleus potentials and energy dissipation from macroscopic degrees of freedom to microscopic ones obtained from TDHF in various colliding systems from light to heavy mass regions.

Analysis of n-evaporation from light mass nuclei formed via p-induced reactions

This work deals with the decay analysis of three compound nuclei [Formula: see text], [Formula: see text] and [Formula: see text] formed in proton-induced reactions [Formula: see text], [Formula: see text] and [Formula: see text] at incident beam energies of 1–5 MeV using the Dynamical Cluster-decay Model (DCM). The motive is to explore the decay of compound systems formed via light charged particles as projectiles. The experimentally available data of n-evaporation for the aforementioned systems are addressed by optimizing the neck-length parameter [Formula: see text], using spherical fragmentation approach. The comparative analysis of the decay structure of the chosen systems is carried out at a common incident beam energy [Formula: see text] MeV. The effect of angular momentum [Formula: see text] and quadrupole [Formula: see text]-deformations is explored in reference to the decay structure/fragmentation of compound systems. In addition to this, the sensitivity of DCM-based cross-sections toward level density parameter (LDP) [Formula: see text] is also analyzed. The relative role of mass-dependent level density parameter [Formula: see text] is also investigated for compound systems belonging to light and heavy mass region. Lastly, a theoretical systematics is explored where the proton beam in the reaction [Formula: see text] is replaced by a neutron beam forming the compound system [Formula: see text], having the same [Formula: see text], but [Formula: see text] one less than that of the compound system formed in the reaction using proton beam, and its effect on the decay characteristics such as preformation probability, penetration probability and barrier height is analyzed.

Systematic analysis on spectral statistics of odd-A nuclei

Level statistics for given spin and parity below 10MeV within A≤71, 71≤A≤151, and 151≤A≤221 mass regions are systematically analyzed. We have taken 5316 level spacings from 266 odd-A nuclei in the light mass region, 6897 level spacings from 478 ones in the medium mass region, and 2768 level spacings from 143 ones in the heavy mass region for the statistics. In the analysis, the chaoticity parameter in the Berry–Robnik distribution is determined based on the method of maximum likelihood estimation. The results show a mass number A-dependence in the level statistics. The larger the mass number, the relatively stronger in chaoticity. In addition, with the level statistics in using data sequences classified according to the order of magnitude of half-lives, it is shown that the level energies to be more chaotic for longer lived light and heavy mass odd-A nuclei, and intermediate between regular and chaotic in medium mass odd-A nuclei. The level statistical analysis in concerning E2 transition strengths of E2 or mixed E2+M1 transitions in odd-A nuclei is also made, which shows chaos in level energies likely occurring in those with weaker E2 transition strengths. Therefore, it can be concluded that odd-A nuclei are, more often than not, chaotic. Chaos is likely emergent in level energies of stable light mass nuclei with weaker E2 transition strengths, while it is definitely emergent in level energies of heavy mass nuclei, and even stronger in comparatively stable ones and those with weaker E2 transition strengths.

Effect of a realistic three-body force on the spectra of medium-mass hypernuclei

We adopt the Hartree–Fock method in the proton–neutron-Λ formalism and the nucleon-Λ Tamm–Dancoff approximation to study the energy spectra of medium-mass hypernuclei. The formalism is developed for a potential derived from effective field theories which includes explicitly the 3-body NNN forces plus the YN LO potential. The energy spectra of selected medium-mass hypernuclei are presented and their properties discussed. The present calculation is the first step of a project devoted to ab initio studies of hypernuclei in medium and heavy mass regions. This may provide a guide for a better understanding of the YN interactions at momentum scales not accessible in few-body hypernuclei.

New shell-model investigation of the 208Pb mass region

In the present contribution, a new investigation of the heavy mass region beyond 208Pb closed core is performed within the shell-model framework. Making use of a new effective interaction H208 derived from CD-Bonn nucleon–nucleon potential through the procedure, the low-lying state energies, the electromagnetic transitions, and the binding energies of Z = 82 isotopes and N = 126 isotones are calculated, providing a complete description of their wave functions components. Our calculations are widely discussed and compared to KHBW theoretical results and to the experimental data.

The S3 symmetric model with a dark scalar

We study the S3 symmetric extension of the Standard Model in which all the irreducible representations of the permutation group are occupied by SU(2) scalar doublets, one of which is taken as inert and can lead to dark matter candidates. We perform a scan over parameter space probing points against physical constraints ranging from unitarity tests to experimental Higgs searches limits. We find that the latter constraints severely restrict the parameter space of the model. For acceptable points we compute the value of the relic density of the dark scalar candidates and find that it has a region for low dark matter masses which complies with the Higgs searches bounds and lies within the experimental Planck limit. For masses ≳80 GeV the value of the relic density is below the Planck bound, and it reaches values close to it for very heavy masses ∼5 TeV. In this heavy mass region, this opens the interesting possibility of extending the dark sector of the model with additional particles.

Application of Calorimetric Low-Temperature Detectors for the Investigation of Z-Yield Distributions of Fission Fragments

In recent experiments, the new concept of calorimetric low-temperature detectors (CLTDs) was applied for the first time for the investigation of isotopic yields of fission fragments. Fragments from neutron-induced fission were mass-separated by the LOHENGRIN spectrometer at the ILL Grenoble and, after passing silicon nitride membranes used as degraders, detected in a CLTD array. The concept of a CLTD provides a fundamental advantage over conventional ionization-mediated detectors, in particular for heavier particle masses at low energies. Using fissile targets of 235U, 239Pu and 241Pu, nuclear charge separation was studied for selected masses in the region 82 ≤ A ≤ 139. For light fragments, the Z-resolution matches historically best values with conventional techniques, while for heavier masses substantial improvement was attained. We have gained first LOHENGRIN data on the isotopic yields in the range A = 91 to 112 of 241Pu(n,f). Towards mass-symmetry, known Z-yield data were extended to the range A = 110 to 113 for 239Pu(n,f). Extended data sets were cumulated for A = 92 and 96 because of a recent request from studies on the reactor antineutrino spectrum. Furthermore, considerable progress was achieved to extend isotopic yield measurements up to the heavy-mass region, hardly accessible until now.

Material engineering to fabricate rare earth erbium thin films for exploring nuclear energy sources

High vacuum evaporation and cold-rolling techniques to fabricate thin films of the rare earth lanthanide-erbium have been discussed in this communication. Cold rolling has been used for the first time to successfully fabricate films of enriched and highly expensive erbium metal with areal density in the range of 0.5–1.0 mg/cm2. The fabricated films were used as target materials in an advanced nuclear physics experiment. The experiment was designed to investigate isomeric states in the heavy nuclei mass region for exploring physics related to nuclear energy sources. The films fabricated using different techniques varied in thickness as well as purity. Methods to fabricate films with thickness of the order of 0.9 mg/cm2 were different than those of 0.4 mg/cm2 areal density. All the thin films were characterized using multiple advanced techniques to accurately ascertain levels of contamination as well as to determine their exact surface density. Detailed fabrication methods as well as characterization techniques have been discussed.

Isospin Conservation in Neutron Rich Systems of Heavy Nuclei

It is generally believed that isospin would diminish in its importance as we go towards heavy mass region due to isospin mixing caused by the growing Coulomb forces. However, it was realized quite early that isospin could become an important and useful quantum number for all nuclei including heavy nuclei due to neutron richness of the systems [1]. Lane and Soper [2] also showed in a theoretical calculation that isospin indeed remains quite good in heavy mass neutron rich systems. In this paper, we present isospin based calculations [3, 4] for the fission fragment distributions obtained from heavy-ion fusion fission reactions. We discuss in detail the procedure adopted to assign the isospin values and the role of neutron multiplicity data in obtaining the total fission fragment distributions. We show that the observed fragment distributions can be reproduced rather reasonably well by the calculations based on the idea of conservation of isospin. This is a direct experimental evidence of the validity of isospin in heavy nuclei, which arises largely due to the neutron-rich nature of heavy nuclei and their fragments. This result may eventually become useful for the theories of nuclear fission and also in other practical applications.

Application of Calorimetric Low-Temperature Detectors for the Investigation of Z-Yield Distributions of Fission Fragments

In recent experiments, the new concept of calorimetric low-temperature detectors (CLTDs) was applied for the first time for the investigation of isotopic yields of fission fragments. Fragments from neutron-induced fission sources were mass-separated by the LOHENGRIN spectrometer at the ILL Grenoble and, after passing silicon nitride membranes used as degraders, detected in a CLTD array. The new detector concept of a thermal detector provides a fundamental advantage over conventional ionization-mediated detectors, in particular for heavier particle masses at low energies. Using fissile targets of235U,239Pu and241Pu, nuclear-charge separation was studied in the mass region 82 ≤ A ≤ 139. For light fragments, the Z resolution matches historically best values with conventional techniques, while for heavier masses substantial improvement was attained. We have gained first LOHENGRIN data on the isotopic yields in the light-mass group of241Pu. Towards mass-symmetry, known Z-yield data were supplemented in the range A = 110 to 113 for241Pu and239Pu. Extended data sets were cumulated for A = 92 and 96 due to a recent request from studies on the reactor anti-neutrino spectrum. Furthermore, considerable progress was achieved to extend isotopic yield measurements up to the heavy-mass region, hardly accessible until now.