Nucleus Excitation Energy Research Articles

Nucleation of baryons in relativistic hadron-nucleus collisions

We suggest a new theoretical method to describe the baryon clusterization of nuclei in hadron-nucleus reactions. As an example we have explored the nuclei production in π−+C and π−+W collisions at plab=1.7 GeV by using the hybrid approach consisting of the Ultra Relativistic Quantum Dynamics Model (UrQMD) and the Statistical Multifragmentation Model (SMM). The UrQMD describes the production of new baryons, and the propagation toward the subnuclear densities with the fluctuations leading to the formation of excited baryonic clusters. The SMM describes the production of final nuclei and hypernuclei after interaction of baryons inside these clusters. We demonstrate the transverse momenta, rapidity, mass distributions and excitation energies of both primary clusters and final nuclei (including hypernuclei). The results of the UrQMD and UrQMD+SMM model calculations for different clusterization parameters are compared with the available HADES experimental data on baryon production, providing a very promising window for future research on nuclei and hypernuclei formation in these reactions.

Photonuclear experiments: from the bremsstrahlung to the Compton backward scattering photons

The features of the methods for obtaining the information on photonuclear reaction cross sections using the beams of bremsstrahlung and quasimonoenergetic photons from the annihilation in flight of relativistic positrons with which such the absolute majority of such kind data was obtained in the nucleus excitation energy up to ∼40 MeV are discussed. It was shown that significant disagreements between the results of such experiments in cross sections absolute values and shapes are due to both methods definite shortcomings which to a certain extent can be overcome by using the beams of the photons from the processes of Compton backward scattering.

Dependence of average total kinetic energy of fission fragments on the excitation energy of the compound nucleus

The expression of dependence of average total kinetic energy release of fission fragments on the excitation energy of the compound nucleus is obtained in the framework of a simple statistical model. The experimental data for the neutron-induced fission of $^{233,235,238}\mathrm{U}$ and $^{239}\mathrm{Pu}$ measured in wide ranges of the excitation energy of the compound nuclei are well described using the obtained expression. The origin of the energy dependence of the average total kinetic energy is discussed in detail.

Preformation probability of light charged particles emitted equatorially in ternary fission of Cf252

We investigate the preformation probability of the light particle that formed in the preliminary stage of the ternary fission process of the $^{252}\mathrm{Cf}$ nucleus. Considering the equatorial cluster tripartition kinematics, the relative yields of the ternary fission channels with $^{4,6}\mathrm{He}, ^{10}\mathrm{Be}$ and $^{14}\mathrm{C}$ as light third nucleus, are calculated. We use the Wentzel-Kramers-Brillouin approximation to calculate the penetrability of the different nuclei involved in the ternary fission process, based on microscopic Skyrme potentials. The obtained results indicate the optimum total excitation energy of the heavy nuclei produced in the $\ensuremath{\alpha}$ ternary fission channels to be within the range of 3--4 MeV. The estimated $\ensuremath{\alpha}$-preformation probability corresponding to this range is comparable with that estimated for the spontaneous $\ensuremath{\alpha}$ decays of the participating heavy nuclei. While the extracted preformation probability at excitation energy less than its indicated optimum values exhibits unphysical large values, the calculations based on the higher excitation energies indicate extremely small values. In the ternary fission process, the preformation probability of the emitted light particles heavier than the $\ensuremath{\alpha}$ particle exponentially decreases with increasing its mass number. A phenomenological expression is suggested based on the obtained results to estimate the preformation probability of an emitted cluster of a given mass number.

Mirror energy differences in T=1/2f7/2 -shell nuclei within isospin-dependent density functional theory

Background: Small asymmetry between neutrons and protons, caused by the differences in masses and charges of the up and down constituent quarks leads to the isospin symmetry breaking. The isospin non-conservation affects broad range of observables from superallowed Fermi weak interaction to isospin-forbidden electromagnetic rates. Its most profound and cleanest manifestation are systematic shifts in masses and excitation energies of mirror atomic nuclei. Purpose: Recently, we constructed the charge-dependent DFT that includes class II and III local interactions and demonstrated that the model allows for very accurate reproduction of Mirror and Triplet Displacement energies in a very broad range of masses. The aim of this work is to further test the charge-dependent functional by studying Mirror Energy Differences (MEDs) in function of angular momentum $I$. Methods: To compute MEDs we use DFT-rooted no core configuration interaction model. This post mean-field method restores rotational symmetry and takes into account configuration mixing within a space that includes relevant (multi)particle-(multi)hole Slater determinants. Results: We applied the model to $f_{7/2}$-shell mirror pairs of $A=43$, $45$, $47$, and $49$ focusing on MEDs in low-spin part (below band crossing) what allowed us to limit the model space to seniority one and three (one broken pair) configurations. Conclusions: We demonstrate that, for spins $I\leq 15/2$ being subject of the present study, our model reproduces well experimental MEDs which vary strongly in function of $I$ and $A$. The quality of model's predictions is comparable to the nuclear shell-model results by Bentley et al. Phys. Rev. C 92, 024310 (2015).

Fission fragment mass distribution in the S32+Sm144 reaction

Background: An asymmetric fission was reported by Andreyev et al. in \ensuremath{\beta}-delayed fission of $^{180}\mathrm{Tl}$ [Phys. Rev. Lett. 105, 252502 (2010)]. Subsequent theoretical calculations suggested that the asymmetric nature of the mass distribution is not restricted to the $^{180}\mathrm{Hg}$ only but is also expected for many other nuclei in the mass region $A\ensuremath{\approx}180$. Thus, it is important to investigate fission fragment mass distribution for different fissioning systems over a wide range of excitation energy in the mass region $A\ensuremath{\approx}180$.Purpose: Present measurements have been carried out to study the nature of the fission fragment mass distribution in the $^{32}\mathrm{S}+^{144}\mathrm{Sm}\ensuremath{\rightarrow}^{176}\mathrm{Pt}$ reaction in the compound nucleus excitation energy range of 38.7--47.5 MeV and investigate the role of multimodal fission.Method: Mass distributions have been determined from the time of flight (TOF) of the fission fragments, which was measured with respect to the beam pulse. Two multiwire proportional counters were placed at the folding angle to detect the fission fragments. Measured TOF of the fission fragments was used to obtain their velocities, which were further used to obtain the fission fragment mass distribution.Results: The fission fragment mass distributions at all three beam energies were observed to have flattop natures, which could not be fitted well by a one-Gaussian function. A fit using a two-Gaussian function significantly improved the ${\ensuremath{\chi}}^{2}$ values. The ratio of the most probable heavy to light fragment mass (${A}_{\mathrm{H}}/{A}_{\mathrm{L}}$) was observed as $\ensuremath{\approx}99.4/76.6$. A systematic study of the centroid values of asymmetric peaks in the mass distribution for different fissioning systems around mass region $A\ensuremath{\approx}180$ showed that heavier mass peak is centered around ${A}_{\mathrm{H}}\ensuremath{\approx}100$. Further analysis of ${A}_{\mathrm{H}}$ and ${A}_{\mathrm{L}}$ to obtain the corresponding neutron and proton numbers gave heavy fragment neutron number (${N}_{\mathrm{H}}$) around $\ensuremath{\approx}56$ and light fragment proton number (${Z}_{\mathrm{L}}$) in the range of $\ensuremath{\approx}34--36$.Conclusions: Observation of the flattop nature indicated the contributions from multimodal fission having both symmetric and asymmetric fission components, similar to those reported earlier in the mass region $A\ensuremath{\approx}180$, although it did not show a clear dip in symmetry as observed in some of the studies. The observed values of the neutron number in the heavy fragment (${N}_{\mathrm{H}}$) and proton number in the light fragment (${Z}_{\mathrm{L}}$) are consistent with the values recently proposed in Phys. Rev. C 100, 041602(R) (2019) and arXiv:2007.16184] for fissioning systems in the similar mass range. Comparison of the data on the width of the mass distribution from the present paper with those from similar fissioning systems with different ${Z}_{P}{Z}_{T}$ values suggested that the observed width of the mass distribution has a dependence on the neutron-proton configuration along with the entrance channel dynamics.

Excitation energy dependence of prompt fission γ -ray emission from Pu241*

Prompt fission $\ensuremath{\gamma}$ rays (PFGs) resulting from the $^{240}\mathrm{Pu}(d,p\mathrm{f})$ reaction have been measured as a function of fissioning nucleus excitation energy ${E}_{\text{x}}$ at the Oslo Cyclotron Laboratory. We study the average total PFG multiplicity per fission, the average total PFG energy released per fission, and the average PFG energy. No significant changes in these characteristics are observed over the range $5.75<{E}_{\text{x}}<8.25$ MeV. The physical implications of this result are discussed. The experimental results are compared to simulations conducted using the computational fission model FREYA. We find that FREYA reproduces the experimental PFG characteristics within $8%$ deviation across the ${E}_{\text{x}}$ range studied. Previous excitation energy-dependent PFG measurements conducted below the second-chance fission threshold have large uncertainties, but are generally in agreement with our results within a $2\ensuremath{\sigma}$ confidence interval. However, both a published parametrization of the PFG energy dependence and the most recent PFG evaluation included in ENDF/B-VIII.0 were found to poorly describe the PFG excitation-energy dependence observed in this and previous experiments.

New isospin-breaking “USD” Hamiltonians for the sd shell

Two new USD-type Hamiltonians, USDC and USDI, have been developed that directly incorporate Coulomb and other isospin-breaking interactions. Starting from ab initio interactions, linear combinations of two-body matrix elements were constrained by experimental energy levels in sd-shell nuclei. With this method, binding energies and excitation energies of proton-rich nuclei in the shell can be added to the data set used in the fit. USDC is based on the same renormalized $G$ matrix used in the derivation of previous USD-type Hamiltonians, while USDI is derived from in-medium similarity renormalization group (IMSRG) interactions. Both contain an analytic Coulomb interaction with Miller-Spencer short-range correlations and an effective isotensor interaction. Also presented are modifications to these interactions, USDCm and USDIm, that have had the Coulomb interaction constrained to better reproduce the experimental $b$ coefficients of the isobaric mass multiplet equation. These Hamiltonians are used to provide new predictions for the proton-dripline and to examine isospin-level mixing and other properties of $sd$-shell nuclei. An empirical expression for the Thomas-Ehrman shift in loosely bound and unbound proton-rich states is presented, and several such states are examined.

The nuclear level density parameter

The nuclear level density parameter in non Equi-Spacing Model (NON-ESM), Equi-Spacing Model (ESM) and the Backshifted Energy Dependent Fermi Gas model (BSEDFG) was determined for 106 nuclei; the results are tabulated and compared with the experimental works. It was found that there are no recognizable differences between our results and the experimental -values. The calculated level density parameters have been used in computing the state density as a function of the excitation energies for 58Fe and 246Cm nuclei. The results are in a good agreement with the experimental results from earlier published work.

Validation of a new “3D calorimetry” of hot nuclei with the HIPSE event generator

In nuclear thermodynamics, the determination of the excitation energy of hot nuclei is a fundamental experimental problem. Instrumental physicists have been trying to solve this problem for several years by building the most exhaustive 4Pi detector arrays and perfecting their calorimetry techniques. In a recent paper, a proposal for a new calorimetry, called "3D calorimetry", was made. It tries to optimize the separation between the particles and fragments emitted by the Quasi-Projectile and the other possible contributions. This can be achieved by determining the experimental probability for a given nucleus of a nuclear reaction to be emitted by the Quasi-Projectile. It has been developed for the INDRA data. In the present work, we wanted to dissect and validate this new method of characterization of a hot Quasi-Projectile. So we tried to understand and control it completely to determine these limits. Using the HIPSE event generator and a software simulating the functioning of INDRA, we were able to achieve this goal and provide a quantitative estimation of the quality of the QP characterization.

Systematic study ofPo192,202,206,210compound nuclei using neutron multiplicity as a probe

In the present work we have measured pre- and post-scission neutron multiplicities $({M}_{\mathrm{pre}}$ and ${M}_{\mathrm{post}})$ from two compound nuclei, namely $^{192,202}\mathrm{Po}$ populated by $^{48}\mathrm{Ti}+^{144,154}\mathrm{Sm}$ systems at 72 MeV of excitation energy using the National Array of Neutron Detectors (NAND) facility at IUAC, New Delhi. Statistical model analysis has been performed for $^{48}\mathrm{Ti}+^{144,154}\mathrm{Sm}$ along with already existing data for $^{12}\mathrm{C}+^{194}\mathrm{Pt}$ and $^{18}\mathrm{O}+^{192}\mathrm{Os}$ covering compound nuclei of Po $(^{192,202,206,210}\mathrm{Po})$ with neutron number ${N}_{C}=108$, 118, 122, and 126 respectively. Variation of experimental ${M}_{\mathrm{pre}}$ with $N/Z$ of the compound nucleus does not show any special feature at neutron shell closure $({N}_{C}=126)$ at compound nucleus excitation energy around 72 MeV considered here. In particular, it is found that dissipation alone is unable to reproduce the experimental ${M}_{\mathrm{pre}}$ for $^{192}\mathrm{Po}$, and the role of entrance channel dynamics should be considered in future works.

Mapping the demise of collective motion in nuclei at high excitation energy

High energy gamma-rays from the 116Sn + 24Mg reaction at 23A MeV were measured using the MEDEA detector at LNS – INFN Catania. Combining this new data with previous measurements yields a detailed view of the quenching of the Giant Dipole Resonance as a function of excitation energy in nuclei of mass A in the range 120÷132. The transition towards the disappearance of the dipole strength, which occurs around 230 MeV excitation energy, appears to be remarkably sharp. Current phenomenological models give qualitative explanations for the quenching but cannot reproduce its detailed features.

Statistical analysis of excitation energies in actinide and rare-earth nuclei

Statistical analysis of distributions of the collective states in actinide and rare-earth nuclei is performed in terms of the nearest-neighbor spacing distribution (NNSD). Several approximations, such as the linear approach to the level repulsion density and that suggested by Brody to the NNSDs were applied for the analysis. We found an intermediate character of the experimental spectra between the order and the chaos for a number of rare-earth and actinide nuclei. The spectra are closer to the Wigner distribution for energies limited by 3 MeV, and to the Poisson distribution for data including higher excitation energies and higher spins. The latter result is in agreement with the theoretical calculations. These features are confirmed by the cumulative distributions, where the Wigner contribution dominates at smaller spacings while the Poisson one is more important at larger spacings, and our linear approach improves the comparison with experimental data at all desired spacings.

Shell-model method for Gamow-Teller transitions in heavy deformed odd-mass nuclei

A shell-model method for calculating Gamow-Teller (GT) transition rates in heavy deformed odd-mass nuclei is presented. The method is developed within the framework of the projected shell model. To implement the computation requirement when many multi-quasiparticle configurations are included in the basis, a numerical advancement based on the Pfaffian formula is introduced. With this new many-body technique, it becomes feasible to perform state-by-state calculations for the GT nuclear matrix elements of $\ensuremath{\beta}$-decay and electron-capture processes, including those at high excitation energies in heavy nuclei which are usually deformed. The first results, ${\ensuremath{\beta}}^{\ensuremath{-}}$ decays of the well-deformed $A=153$ neutron-rich nuclei, are shown as the example. The known $log(ft)$ data corresponding to the $B({\mathrm{GT}}^{\ensuremath{-}}$) decay rates of the ground state of $^{153}\mathrm{Nd}$ to the low-lying states of $^{153}\mathrm{Pm}$ are well described. It is further shown that the $B$(GT) distributions can have a strong dependence on the detailed microscopic structure of relevant states of both the parent and daughter nuclei.

Experimental signature of collective enhancement in nuclear level density

We present a probable experimental signature of collective enhancement in the nuclear level density (NLD) by measuring the neutron and the giant dipole resonance (GDR) $\gamma$ rays emitted from the rare earth $^{169}$Tm compound nucleus populated at 26.1 MeV excitation energy. An enhanced yield is observed in both neutron and $\gamma$ ray spectra corresponding to the same excitation energy in the daughter nuclei. The enhancement could only be reproduced by including a collective enhancement factor in the Fermi gas model of NLD to explain the neutron and GDR spectra simultaneously. The experimental results show that the relative enhancement factor is of the order of 10 and the fadeout occurs at $\sim$ 14 MeV excitation energy, much before the commonly accepted transition from deformed to spherical shape. We also explain how the collective enhancement contribution changes the inverse level density parameter ($k$) from 8 to 9.5 MeV observed recently in several deformed nuclei.

Investigation of the heavy nuclei fission with anomalously high values of the fission fragments total kinetic energy

Binary fission of 232Th and 238U induced by fast neutrons were under intent investigation in the IPPE during recent years. These measurements were performed with a twin ionization chamber with Frisch grids. Signals from the detector were digitized for further processing with a specially developed software. It results in information of kinetic energies, masses, directions and Bragg curves of registered fission fragments. Total statistics of a few million fission events were collected during each experiment. It was discovered that for several combinations of fission fragment masses their total kinetic energy was very close to total free energy of the fissioning system. The probability of such fission events for the fast neutron induced fission was found to be much higher than for spontaneous fission of 252Cf and thermal neutron induced fission of 235U. For experiments with 238U target the energy of incident neutrons were 5 MeV and 6.5 MeV. Close analysis of dependence of fission fragment distribution on compound nucleus excitation energy gave us some explanation of the phenomenon. It could be a process in highly excited compound nucleus which leads the fissioning system from the scission point into the fusion valley with high probability.

Synthesis of the Z=122 superheavy nucleus via Fe58 - and Ni64 -induced reactions using the dynamical cluster-decay model

Within the framework of the dynamical cluster-decay model (DCM), we have studied the nuclear system with $Z=122$ and mass number $A$ = 306 formed via two ``hot'' fusion reactions $^{58}\mathrm{Fe}+\phantom{\rule{0.16em}{0ex}}^{248}\mathrm{Cm}$ and $^{64}\mathrm{Ni}+\phantom{\rule{0.16em}{0ex}}^{242}\mathrm{Pu}$. The up-to-date measured data are available only for the first reaction, and for fusion-fission cross section ${\ensuremath{\sigma}}_{\mathrm{ff}}$ and quasifission cross section ${\ensuremath{\sigma}}_{\mathrm{qf}}$, only at one compound nucleus (CN) excitation energy ${E}^{*}=33\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$. In this study, we have included the deformation effects up to quadrupole deformations ${\ensuremath{\beta}}_{2i}$ and with ``optimum'' orientations ${\ensuremath{\theta}}_{i}^{\mathrm{opt}.}$ for coplanar ($\mathrm{\ensuremath{\Phi}}={0}^{0}$) configurations. The only parameter of the model is the neck-length parameter $\mathrm{\ensuremath{\Delta}}R$ whose value, for the nuclear proximity potential used here, remains within its range of validity ($\ensuremath{\sim}2\phantom{\rule{0.28em}{0ex}}\mathrm{fm}$). Using the best fitted $\mathrm{\ensuremath{\Delta}}R$'s to the observed data for ${\ensuremath{\sigma}}_{\mathrm{ff}}$, calculated for mass region $A/2\ifmmode\pm\else\textpm\fi{}20$, and ${\ensuremath{\sigma}}_{\mathrm{qf}}$ for the incoming channel of Fe-induced reaction at ${E}^{*}=33\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$, we have extended the DCM calculations to the other Ni-induced reaction, and to ${E}^{*}$'s in the energy range 25--68 MeV. The interesting result is that the predicted evaporation residue cross section ${\ensuremath{\sigma}}_{\mathrm{ER}}$ for 1--4 neutrons is largest for 4n decay at ${E}^{*}=45\phantom{\rule{0.16em}{0ex}}\mathrm{MeV}$, having the value ${\ensuremath{\sigma}}_{\mathrm{ER}}\ensuremath{\equiv}{\ensuremath{\sigma}}_{4n}\ensuremath{\sim}{10}^{\ensuremath{-}5}$ pb for both reactions, and that the $\mathrm{\ensuremath{\Delta}}R$'s for the three processes (ER, ff, and qf) are different, i.e., they belong to different time scales where ff occurs first, then qf and the ER at the end. Other results of interest are the predictions of the magic $N=82\phantom{\rule{4pt}{0ex}}^{136}\mathrm{Xe}$ fragment in the ff region of mass $A/2\ifmmode\pm\else\textpm\fi{}20$, and the doubly magic $^{208}\mathrm{Pb}$ in the qf region, in near close agreement with observed data (the observed fission fragment is of mass 132, instead of the predicted mass 136). The role of the weakly bound neutron-rich intermediate mass fragments and of the nucleus in the neighborhood of deformed magic $Z$ = 108 are also indicated in the DCM calculations, which need experimental verification. For the predicted ${\ensuremath{\sigma}}_{\mathrm{ER}}$, the largest value of CN fusion probability ${P}_{\mathrm{CN}}=0.2$, and its survival probability against fission ${P}_{\mathrm{surv}}\ensuremath{\rightarrow}0$. Further experiments are called for.

Structure evolution and phase transition in odd-mass nuclei

The evolution of level structures due to the unique parity orbitals ${g}_{9/2}$, ${h}_{11/2}$, and ${i}_{13/2}$ in odd-mass nuclei from Zn to Am is studied within a unified framework, by correlations between ratios of excitation energies in both odd-mass nuclei and their even-even core nuclei. These plots reveal regularities that can be understood in terms of the particle-plus-rotor model, as evolutions along its three limiting coupling schemes: weak coupling, decoupling, and strong coupling, and transitions between them. Peculiar transitions between the decoupling and strong coupling schemes are found in both ${i}_{13/2}$ structures of neutron-odd nuclei and ${h}_{11/2}$ structures of proton-odd nuclei, at neutron numbers around 90 and 70, respectively. These are correlated with the critical shape phase transitions from vibrator to rotor from the even-even nuclei in the same regions and are characterized as critical phase transitions too. This behavior is corroborated with a nonmonotonic behavior of the differential variation of the two-neutron separation energies in the same nuclear regions.

Experimental Challenge to the vp-Process in Type II Supernovae

The vp-process is a new nucleosynthetic scenario, proposed 2006, which supposed to take place at the very early epoch of type II supernova, involving nuclear reactions of proton-rich nuclei not only with protons and alphas, but also with neutrons due to the neutrino processes. The vp-process is one of the key processes for investigating the mechanism of type II supernovae, and the process could be possibly responsible for the anomalously abundant p-nuclei around mass 90-100. Specifically, the nuclear physics problems in the vp-process were discussed in this talk including our recent experimental results with low-energy RI beams and a simulation study. Alpha cluster resonances have been identified experimentally which play a crucial role for the stellar (α,p) and (α,γ) reactions just above the alpha threshold. Neutron induced reactions in the proton-rich nuclear regions in the vp-process are also suggested to play an important role, which will discard the waiting points, and accelerate the flow to heavier nuclei. This process involves nuclear structures of very high level density at high excitation energies in neutron deficient nuclei, and both of the projectile and the target are unstable, which is a quite difficult experimental challenge in nuclear astrophysics in the coming years. Some experimental challenges are discussed.

Softness of Sn isotopes in relativistic semiclassical approximation

Within the framework of relativistic Thomas–Fermi (RTF) and relativistic extended Thomas–Fermi (RETF) approximations, we calculate the giant monopole resonance (GMR) excitation energies for Sn and related nuclei. A large number of nonlinear relativistic force parameters are used in these calculations. We find a parameter set is capable to reproduce the experimental monopole energy of Sn isotopes, when its nuclear matter compressibility lies within 210–230 MeV, however, it fails to reproduce the GMR energy of other nuclei. That means, simultaneously a parameter set cannot reproduce the GMR values of Sn and other nuclei.