Excitation Energy Of Compound Nucleus Research Articles

Probing collective enhancement in nuclear level density with evaporation α particle spectra

Collective enhancement in nuclear level density (CELD) and its fadeout has been studied experimentally using neutron and high energy $\ensuremath{\gamma}$-ray spectra earlier. Attempts to probe CELD with $\ensuremath{\alpha}$-particle spectra were made for compound nucleus (CN) $^{178}\mathrm{Hf}$ in the excitation energy $54--124$ MeV, but no signature of CELD was found. The possible reason behind this nonobservance has been discussed in a few subsequent reports. In our previous study, evidence of CELD was found using neutron spectra and enhancement faded away near 25 MeV of excitation energy. This implies that the effect of CELD on $\ensuremath{\alpha}$ particles, if any, would be found at excitation lower than 25 MeV. With the aim to observe CELD and its fadeout with $\ensuremath{\alpha}$-particle spectra, two reactions $^{12}\mathrm{C}+^{116}\mathrm{Sn},^{159}\mathrm{Tb}$, forming CN in different mass region ($A\ensuremath{\approx}128$ and $A\ensuremath{\approx}171$) were studied. Evaporation $\ensuremath{\alpha}$-particle spectra were measured for these reactions in singles as well as in coincidence with neutrons. Experimental data were compared with statistical model calculations and inverse level density parameter ($k$) were obtained from $\ensuremath{\alpha}$-particle spectra. As a function of CN excitation energy, the $k$ value showed peak like structure for the reaction $^{12}\mathrm{C}+^{159}\mathrm{Tb}$ which indicates fadeout of CELD. No such evidence was found for the reaction $^{12}\mathrm{C}+^{116}\mathrm{Sn}$. Collective enhancement factor for daughter nuclei populated in $^{12}\mathrm{C}+^{159}\mathrm{Tb}$ reaction was extracted. Critical energy of the fadeout was found to be similar to that of mass $A\ensuremath{\approx}188$ region.

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.

Examining the entrance channel effects on the synthesis of the double deformed nucleus Hs270 : A theoretical study using the dynamical cluster-decay model including Skyrme forces

The fusion evaporation residue cross sections ${\ensuremath{\sigma}}_{xn}$ for the decay of the compound nucleus (CN) $^{274}\mathrm{Hs}^{*}$ via $3n$- to $5n$-decay channels, synthesized in $^{248}\mathrm{Cm}+^{26}\mathrm{Mg}, ^{238}\mathrm{U}+^{36}\mathrm{S}$, and $^{226}\mathrm{Ra}+^{48}\mathrm{Ca}$ reactions, are studied including deformations ${\ensuremath{\beta}}_{2i}$ for compact hot orientations ${\ensuremath{\theta}}_{i}$ at various excitation energies of ${E}^{*}=40$ to 51 MeV, which support symmetric fission, in agreement with experiments. For the nuclear interaction potentials, we use the Skyrme energy density functional based on the semiclassical extended Thomas-Fermi approach, which means an extension of our earlier study of excitation functions of $^{248}\mathrm{Cm}+^{26}\mathrm{Mg}, ^{238}\mathrm{U}+^{36}\mathrm{S}$, and $^{226}\mathrm{Ra}+^{48}\mathrm{Ca}$ reactions, based on the dynamical cluster-decay model (DCM). In the present work the above reactions have been investigated by using a hot compact configuration. The Skyrme forces used here are the conventional SIII and the newer GSkI and KDE0(v1) given for both normal and isospin-rich nuclei, with densities added in the frozen density approximation. Interestingly, with the inclusion of both conventional and new Skyrme forces, the DCM gives an excellent fit within one parameter fitting of $\mathrm{\ensuremath{\Delta}}R$ to the measured data on the fusion evaporation residue for all three incoming channels at the energy range ${E}^{*}=40$ to 51 MeV, independent of the entrance channel and the Skyrme force used. The possible fusion-fission and quasifission mass-regions of fragments on the DCM are also predicted. The DCM with Skyrme forces is further used to look for all the possible target-projectile combinations forming the ``cold'' CN $^{274}\mathrm{Hs}^{*}$ at the CN excitation energy of ${E}^{*}$ for ``hot compact'' configurations. The role of magic shells is shown in enhancing the evaporation residue cross sections, not only for the entrance channel $^{226}\mathrm{Ra}+^{48}\mathrm{Ca}$ but also for the residue $^{270}\mathrm{Hs}$, compared to its neighboring isotopes ${}^{269\ensuremath{-}271}\mathrm{Hs}$. The fusion evaporation residue cross sections for the proposed new reactions in synthesizing the CN $^{274}\mathrm{Hs}^{*}$ are also estimated for future experiments.

Analysis of the decay of the rare p nucleus Te*120 within a collective clusterization approach

The dynamics of one out of thirty-five known rare $p$ nuclei, i.e., the $^{120}\mathrm{Te}^{*}$ compound nucleus (CN), are worked out within the dynamical cluster-decay model (DCM) over incident $\ensuremath{\alpha}$-beam energies ${E}_{\ensuremath{\alpha}}\ensuremath{\approx}10.3$ to 30 MeV, constituting energies below, around, and above the Coulomb barrier. Experimental data are available for the decay of the $^{120}\mathrm{Te}^{*}$ CN formed in the $^{4}\mathrm{He}+^{116}\mathrm{Sn}$ reaction, to both ground $(g)$ and metastable $(m)$ states where the predominant decay mode is neutron emission $xn$ $(x=1,2)$, essentially a ``cold-fusion'' reaction. Both kinds of decays are independently addressed here, optimizing the neck-length parameter $\mathrm{\ensuremath{\Delta}}R$, the model's only parameter, to best fit the data for observed $xn$, $x=1$ or 2, channel cross sections while simultaneously predicting cross sections for other experimentally unobserved channels. The collective clusterization technique of the DCM involves a two step mechanism for analyzing a CN decay: first the preformation of the fragments within the parent CN and then the penetration of the preformed clusters through their respective interaction potential barriers, with probabilities ${P}_{0}$ and $P$, respectively. The quadrupole deformations of nuclei $({\ensuremath{\beta}}_{2i})$ with their respective ``optimum'' orientations ${\ensuremath{\theta}}_{i}^{\mathrm{opt}.}$ and coplanarity $(\mathrm{\ensuremath{\Phi}}={0}^{\ensuremath{\circ}})$, are included for the use of ``cold-elongated'' configurations. Interestingly, in complete agreement with experimental observations, the results of the theoretical model calculations require incorporation of significant quasifissionlike (qf-like) noncompound nucleus (nCN) effects above ${E}_{\ensuremath{\alpha}}\ensuremath{\approx}20$ MeV, corresponding to decay of $^{120}\mathrm{Te}^{*}$ to the ground state of $^{118}\mathrm{Te}$ via $2n$. Decay of $^{120}\mathrm{Te}^{*}$ to the metastable state $^{119m}\mathrm{Te}$ via $1n$ is a pure CN decay over the entire energy range studied here. The smooth variation of $\mathrm{\ensuremath{\Delta}}R$ values with CN excitation energy ${E}_{CN}^{*}$ is also well within the nuclear proximity limit of $\ensuremath{\approx}2.5\phantom{\rule{4pt}{0ex}}\text{fm}$. The trends of statistical CN formation probability and survival probability, ${P}_{CN}$ and ${P}_{\mathrm{surv}}$, respectively, with ${E}_{CN}^{*}$ match the known systematic of other compound nuclei of mass range $100l{A}_{CN}⪅200$ and ${Z}_{1}{Z}_{2}l460$ studied so far within the model, which gives further credence to the calculations.

Analysis of 113In(α,n)[formula omitted]Sb reaction at astrophysically interesting center-of-mass energies and synthesis of 117Sb⁎ within the Dynamical Cluster-decay Model

The Quantum Mechanical Fragmentation Theory (QMFT)-based Dynamical Cluster-decay Model (DCM) analysis of compound nucleus (CN) 117Sb⁎, formed in 4He+113In reaction is done at some astrophysically important energies, which is found to decay via 1n-emission to ground state (g.s.) and metastable state (m.s.) of 116Sb. It is for the first time DCM has been applied to analyze decays to metastable states in a “cold fusion” reaction. The DCM cross sections σ1n are calculated at various astrophysically relevant center-of-mass energies (Ec.m.=9.66–13.64 MeV) corresponding to available experimental data for α-induced reaction with the odd-Z, proton-rich (p-) nuclide 113In, measured with the activation method where, in addition to the radioactive alpha-capture (α,γ), some other reaction channels such as (α,n) and (α,p) of the same isotope can be measured at the same time. The only parameter of DCM is the neck-length parameter ΔR, that is optimized to best fit the experimental data for g.s. to g.s. and g.s. to m.s. decay of 117Sb⁎ CN and predict the cross sections for evaporation-residue (ER), intermediate mass fragments (IMFs) and fusion-fission (ff) fragments. For a best fit to the experimentally observed σ1n (ground and metastable states), we find that, ΔR-values vary smoothly with CN excitation energy ECN⁎ and thus is useful for making predictions at energies where experimental data is unavailable. Possible synthesis of this medically important radionuclide 117Sb⁎ is also studied via all the possible “cold” target-projectile (t–p) combinations, identifying 6Li+111Cd as the best possibility.

Skyrme forces and isotopic dependence of evaporation residue cross-section in the decay of 252,254−256No∗ formed in 204,206,207,208Pb+48Ca reactions

The fusion evaporation residue (ER) cross-sections [Formula: see text] for the decay of compound nucleus (CN) [Formula: see text]No[Formula: see text] via 1[Formula: see text]–4[Formula: see text] decay channels, synthesized in [Formula: see text]Pb[Formula: see text][Formula: see text][Formula: see text][Formula: see text]Ca reaction, are studied, including deformations [Formula: see text] for cold-optimum orientations [Formula: see text] at various [Formula: see text]Ca excitation energies of [Formula: see text] to 45[Formula: see text]MeV. For the nuclear interaction potentials, we use the Skyrme energy density functional (SEDF)-based on semi-classical extended Thomas Fermi (ETF) approach under frozen density approximation on our earlier study of fusion ER cross-section for the decay of [Formula: see text]No[Formula: see text], via 1[Formula: see text]–4[Formula: see text] decay channels synthesized in [Formula: see text]Pb[Formula: see text][Formula: see text][Formula: see text][Formula: see text]Ca reaction based on the Dynamical cluster-decay model (DCM) using the pocket formula for nuclear proximity potential in which the above reaction was investigated by using hot-optimum orientations. In this work the above reaction has been investigated in two parts, in the first part, [Formula: see text][Formula: see text]MeV is used for cold elongated configurations and in the second part, [Formula: see text][Formula: see text]MeV is used for hot compact configurations. The Skyrme forces used here are the old SIII, and new GSkI and KDE0(v1) given for both normal and isospin-rich nuclei, with densities added in frozen density approximation. Interestingly, independent of the Skyrme force used, the DCM gives an excellent fit within one-parameter fitting of [Formula: see text] to the measured data on fusion ER for cold fusion. Of all the three Pb-isotopes and three [Formula: see text] considered, at each [Formula: see text], the [Formula: see text] is largest for compound system with mass numbers 256 and 254, and smallest for 252, which means that the neutrons emission occur earliest for 256, then 255 followed by 254 and finally for 252, in complete agreement with experimental data. The possible fusion–fission (ff) and quasi-fission (qf) mass-regions of fragments on DCM are also predicted. The DCM with Skyrme forces is further used to look for all the possible target-projectile ([Formula: see text]-[Formula: see text]) combinations forming the “cold” CN [Formula: see text]No[Formula: see text] at the CN excitation energy of [Formula: see text] for “optimum cold” configurations. The fusion ER cross-sections, for the proposed new reactions in synthesizing the CN [Formula: see text]No[Formula: see text], are also estimated for the future experiments.

Systematic study of Po192,202,206,210 compound 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.

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.

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.

Evolution of uranium fission-fragment charge yields with neutron number

We use the Brownian shape-motion model, with its recent extensions, which allow modeling of odd-even staggering, to calculate the evolution of fission-fragment charge distributions with neutron number for the compound-system sequence 234U, 236U, 238U, and 240U. We compare to experimental data where available, for neutron- and electromagnetic-induced fission over a compound-nucleus excitation energy range from about 6 to 20 MeV. A notable result of the study is that the evolution of the location of the peak charge yield from $Z=54$ in 234U towards $Z=52$ in heavier isotopes, seen in the experimental data, is present also in the calculated yields. We further show that to describe yields at higher compound-nucleus excitation energies, then, already at 20 MeV, it is necessary to take multi-chance fission into account.

Effect of the energy spectrum and angular momentum of pre-scission neutrons on the prediction of fission fragment angular anisotropy by the models

Effects of the various neutron emission energy spectra, as well as the influence of the angular momentum of pre-scission neutrons on theoretical predictions of fission fragment angular anisotropies for several heavy-ion induced fission systems are considered. Although theoretical calculations of angular anisotropy are very sensitive to neutron emission correction, the effects of the different values of kinetic energy of emitted neutrons derived from the various neutron emission energy spectra before reaching to the saddle point on the prediction of fission fragment angular distribution by the model are not significant and can be neglected, since these effects on angular anisotropies of fission fragments for a wide range of fissility parameters and excitation energies of compound nuclei are not more than 10%. Furthermore, the theoretical prediction of fission fragment angular anisotropy is not sensitive to the angular momentum of emitted neutrons.

Excitation energy dependence of fragment-mass distributions from fission of 180,190Hg formed in fusion reactions of 36Ar + 144,154Sm

Mass distributions of fission fragments from the compound nuclei 180Hg and 190Hg formed in fusion reactions 36Ar + 144Sm and 36Ar + 154Sm, respectively, were measured at initial excitation energies of E⁎(Hg180)=33–66 MeV and E⁎(Hg190)=48–71 MeV. In the fission of 180Hg, the mass spectra were well reproduced by assuming only an asymmetric-mass division, with most probable light and heavy fragment masses A¯L/A¯H=79/101. The mass asymmetry for 180Hg agrees well with that obtained in the low-energy β+/EC-delayed fission of 180Tl, from our earlier ISOLDE(CERN) experiment. Fission of 190Hg is found to proceed in a similar way, delivering the mass asymmetry of A¯L/A¯H=83/107, throughout the measured excitation energy range. The persistence as a function of excitation energy of the mass-asymmetric fission for both proton-rich Hg isotopes gives strong evidence for the survival of microscopic effects up to effective excitation energies of compound nuclei as high as 40 MeV. This behavior is different from fission of actinide nuclei and heavier mercury isotope 198Hg.

Experimental study of the quasifission, fusion-fission, and de-excitation of Cf compound nuclei

Background: The fusion-evaporation reaction at energies around the Coulomb barrier is presently the only way to produce the heaviest elements. However, formation of evaporation residues is strongly hindered due to the competing fusion-fission and quasifission processes. Presently, a full understanding of these processes and their relationships has not been reached.Purpose: This work aims to use new fission measurements and existing evaporation residue and fission excitation function data for reactions forming Cf isotopes to investigate the dependence of the quasifission probability and characteristics on the identities of the two colliding nuclei in heavy element formation reactions.Method: Using the Australian National University's 14UD electrostatic accelerator and CUBE detector array, fission fragments from the ${}^{12}\mathrm{C}+{}^{235}\mathrm{U}$, ${}^{34}\mathrm{S}+{}^{208}\mathrm{Pb}$, ${}^{36}\mathrm{S}+{}^{206}\mathrm{Pb}$, ${}^{36}\mathrm{S}+{}^{208}\mathrm{Pb}$, and ${}^{44}\mathrm{Ca}+{}^{198}\mathrm{Pt}$ reactions were measured. Mass and angle distributions of fission fragments were extracted and compared to investigate the presence and characteristics of quasifission.Results: Mass-angle-correlated fission fragments were observed for the ${}^{44}\mathrm{Ca}+{}^{198}\mathrm{Pt}$ reaction; no correlation was observed in the other reactions measured. Flat-topped fission-fragment mass distributions were observed for ${}^{12}\mathrm{C}+{}^{235}\mathrm{U}$ at compound-nucleus excitation energies from 28 to 52 MeV. Less pronounced flat-topped distributions were observed, with very similar shapes, for all three sulfur-induced reactions at excitation energies lower than 45 MeV.Conclusions: A high probability of long-time-scale quasifission seems necessary to explain both the fission and evaporation residue data for the ${}^{34}\mathrm{S}+{}^{208}\mathrm{Pb}$ and ${}^{36}\mathrm{S}+{}^{206}\mathrm{Pb}$ reactions. Flat-topped mass distributions observed for ${}^{12}\mathrm{C}$- and ${}^{34,36}\mathrm{S}$-induced reactions are suggested to originate both from late-chance fusion-fission at low excitation energies and the persistence of shell effects at the higher energies associated with quasifission.

Determination of the compound nucleus survival probabilityPsurvfor various “hot” fusion reactions based on the dynamical cluster-decay model

After a successful attempt to define and determine recently the compound nucleus (CN) fusion/ formation probability ${P}_{\text{CN}}$ within the dynamical cluster-decay model (DCM), we introduce and estimate here for the first time the survival probability ${P}_{\text{surv}}$ of CN against fission, again within the DCM. Calculated as the dynamical fragmentation process, ${P}_{\text{surv}}$ is defined as the ratio of the evaporation residue (ER) cross section ${\ensuremath{\sigma}}_{\text{ER}}$ and the sum of ${\ensuremath{\sigma}}_{\text{ER}}$ and fusion-fission (ff) cross section ${\ensuremath{\sigma}}_{\text{ff}}$, the CN formation cross section ${\ensuremath{\sigma}}_{\text{CN}}$, where each contributing fragmentation cross section is determined in terms of its formation and barrier penetration probabilities ${P}_{0}$ and $P$. In DCM, the deformations up to hexadecapole and ``compact'' orientations for both in-plane (coplanar) and out-of-plane (noncoplanar) configurations are allowed. Some 16 ``hot'' fusion reactions, forming a CN of mass number ${A}_{\text{CN}}\ensuremath{\sim}100$ to superheavy nuclei, are analyzed for various different nuclear interaction potentials, and the variation of ${P}_{\text{surv}}$ on CN excitation energy ${E}^{*}$, fissility parameter $\ensuremath{\chi}$, CN mass ${A}_{\text{CN}}$, and Coulomb parameter ${Z}_{1}{Z}_{2}$ is investigated. Interesting results are that three groups, namely, weakly fissioning, radioactive, and strongly fissioning superheavy nuclei, are identified with ${P}_{\text{surv}}$, respectively, $\ensuremath{\sim}1,\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}{10}^{\ensuremath{-}6}$, and $\ensuremath{\sim}{10}^{\ensuremath{-}10}$. For the weakly fissioning group ($100l{A}_{\text{CN}}\ensuremath{\lesssim}200$), independent of the interaction potential, different isotopes and for coplanar or noncoplanar collisions, ${P}_{\text{surv}}$ decreases from one to zero as ${E}^{*}$ increases, whereas, independent of entrance channel effects, the same is surprisingly the reverse for the radioactive group (${A}_{\text{CN}}\ensuremath{\sim}200--250$), i.e., ${P}_{\text{surv}}$ increases with the increase of ${E}^{*}$. This is shown to be so due to the different relative magnitudes of ${\ensuremath{\sigma}}_{\text{ER}}$ and ${\ensuremath{\sigma}}_{\text{ff}}$ and their variations with ${E}^{*}$ in the two cases. For the superheavy nuclei also ${P}_{\text{surv}}$ is a decreasing function of ${E}^{*}$. Furthermore, of particular interest are the cases of ${}^{105}{\mathrm{Ag}}^{*}$, isotopes of ${\mathrm{Pt}}^{*}$, and ${}^{213,215,217}{\mathrm{Fr}}^{*}$ nuclei --- for ${}^{105}{\mathrm{Ag}}^{*}$, whereas the ${P}_{\text{CN}}$ belongs to the strongly fissioning superheavy group, ${P}_{\text{surv}}$ belongs to weakly fissioning nuclei; for ${\mathrm{Pt}}^{*}$ isotopes, the inverse of all the compound systems studied, both ${P}_{\text{CN}}$ and ${P}_{\text{surv}}$ decrease with the increase of ${E}^{*}$; for ${}^{213,215,217}{\mathrm{Fr}}^{*}$ nuclei, though fissility $\ensuremath{\chi}$ is nearly the same, ${P}_{\text{surv}}$ for ${}^{213,217}{\mathrm{Fr}}^{*}$ is of the same order as for weakly fissioning nuclei, but that for ${}^{215}{\mathrm{Fr}}^{*}$ is of the order of radioactive nuclei. Apparently, further calculations are called for.

Compound nucleus formation probabilityPCNdefined within the dynamical cluster-decay model

With in the dynamical cluster-decay model (DCM), the compound nucleus fusion/ formation probability PCN is defined for the first time, and its variation with CN excitation energy E* and fissility parameter χ is studied. In DCM, the (total) fusion cross section σfusion is sum of the compound nucleus (CN) and noncompound nucleus (nCN) decay processes, each calculated as the dynamical fragmentation process. The CN cross section σCN is constituted of the evaporation residues (ER) and fusion-fission (ff), including the intermediate mass fragments (IMFs), each calculated for all contributing decay fragments (A1, A2) in terms of their formation and barrier penetration probabilities P0 and P. The nCN cross section σnCN is determined as the quasi-fission (qf) process where P0=1 and P is calculated for the entrance channel nuclei. The calculations are presented for six different target-projectile combinations of CN mass A~100 to superheavy, at various different center-of-mass energies with effects of deformations and orientations of nuclei included in it. Interesting results are that the PCN=1 for complete fusion, but PCN <1 or ≪1 due to the nCN conribution, depending strongly on both E* and χ.

Compound nucleus formation probabilityPCNdetermined within the dynamical cluster-decay model for various “hot” fusion reactions

The compound nucleus (CN) fusion/formation probability ${P}_{CN}$ is defined and its detailed variations with the CN excitation energy ${E}^{*}$, center-of-mass energy ${E}_{\mathrm{c}.\mathrm{m}.}$, fissility parameter $\ensuremath{\chi}$, CN mass number ${A}_{CN}$, and Coulomb interaction parameter ${Z}_{1}{Z}_{2}$ are studied for the first time within the dynamical cluster-decay model (DCM). The model is a nonstatistical description of the decay of a CN to all possible processes. The (total) fusion cross section ${\ensuremath{\sigma}}_{\mathrm{fusion}}$ is the sum of the CN and noncompound nucleus (nCN) decay cross sections, each calculated as the dynamical fragmentation process. The CN cross section ${\ensuremath{\sigma}}_{CN}$ is constituted of evaporation residues and fusion-fission, including intermediate-mass fragments, each calculated for all contributing decay fragments (${A}_{1}$, ${A}_{2}$) in terms of their formation and barrier penetration probabilities ${P}_{0}$ and $P$. The nCN cross section ${\ensuremath{\sigma}}_{\mathit{nCN}}$ is determined as the quasi-fission (qf) process, where ${P}_{0}=1$ and P is calculated for the entrance-channel nuclei. The DCM, with effects of deformations and orientations of nuclei included in it, is used to study the ${P}_{CN}$ for about a dozen ``hot'' fusion reactions forming a CN of mass number $A\ensuremath{\sim}100$ to superheavy nuclei and for various different nuclear interaction potentials. Interesting results are that ${P}_{CN}=1$ for complete fusion, but ${P}_{CN}&lt;1$ or ${P}_{CN}\ensuremath{\ll}1$ due to the nCN contribution, depending strongly on different parameters of the entrance-channel reaction but found to be independent of the nuclear interaction potentials used.

Isomeric yield ratios of 87m,gY from different nuclear reactions

The independent isomeric yield ratios of 87m,gY produced from the 93Nb(γ, α2n) and natZr(γ, pxn) reactions with the end-point bremsstrahlung energy of 45–70 MeV have been determined by an off-line γ-ray spectrometric technique using 100 MeV electron linac at the Pohang accelerator laboratory, Korea. The isomeric yield ratios of 87m,gY were also determined from the natZr(p, αxn) and the 89Y(p,p2n) reactions with EP = 15–45 MeV as well as those from the 89Y(α, α2n) reaction with Eα = 32–43 MeV using the MC-50 cyclotron at the Korea Institute of Radiological and Medical Science, Korea. The isomeric yield ratios of 87m,gY from the present work in the 93Nb(γ, α2n), natZr(γ, pxn), natZr(p, αxn), 89Y(p,p2n), and 89Y(α, α2n) reactions were compared with those of the literature data in the 85Rb(α, 2n), the 86,87,88Sr(d, xn), 89Y(n,3n), and the 89Y(γ, 2n) reactions to examine the role of target, projectiles, and ejectiles through compound nucleus excitation energy and input angular momentum. The isomeric yield ratios of 87m,gY in the above eleven reactions were also calculated using the computer code TALYS 1.4 and compared with the experimental data. The different behaviors between photon- and neutron-induced reactions and charged-particle-induced reactions are discussed from the viewpoint of compound and non-compound (pre-equilibrium) process.

Synthesis of the doubly magic deformed nucleus108270Hs162in the decay of274Hs*formed via hot fusion reactions: Entrance-channel effects and role of magicity of48Ca and270Hs

Quantum mechanical fragmentation theory (QMFT) is used to look for all possible target-projectile (t,p) combinations forming the ``cold'' compound nucleus (CN) ${}^{274}$Hs${}^{*}$ at the CN excitation energy ${E}^{*}$ of ``hot, compact'' configuration. The predicted reactions, referring to potential energy minima, include all the three reactions ${}^{248}$Cm + ${}^{26}$Mg, ${}^{238}$U + ${}^{36}$S, and ${}^{226}$Ra + ${}^{48}$Ca already used in experiments, and a few more. The optimum ``cold'' and ``compact'' (t,p) combination, corresponding to lowest interaction barrier and smallest interaction radius, is one with largest mass asymmetry, but because of the doubly magic ${}^{48}$Ca nucleus, the evaporation residue cross sections for the ${}^{226}$Ra + ${}^{48}$Ca reaction are shown to be further enhanced. For the decay of CN ${}^{274}$Hs${}^{*}$, synthesizing ${}^{269--271}$Hs via 3$n$--5$n$ emission, we use the dynamical cluster-decay model (DCM) with effects of quadrupole deformations and ``hot'' compact orientations included in it, which support symmetric fission, in agreement with experiments. The fusion evaporation residue cross sections ${\ensuremath{\sigma}}_{xn}$, for $x=3$, 4, and 5 neutrons emission from the above-mentioned three entrance channels, are calculated within one parameter fitting, namely, the neck length. For best fitted neck-length parameter, the roles of entrance channel and that of magic shells are analyzed. In spite of different entrance channels resulting in different evaporation residue cross sections, the neck-length parameter at a given ${E}^{*}$ is shown to be independent of the entrance channel. The role of magic shells is shown in enhancing evaporation residue cross sections, not only for the entrance channel ${}^{226}$Ra + ${}^{48}$Ca, but also for the residue ${}^{270}$Hs, compared to its neighboring isotopes ${}^{269,271}$Hs. The fusion evaporation residue cross sections for the proposed new reactions, in synthesizing CN ${}^{274}$Hs${}^{*}$, are also estimated for future new experiments.

Decay Mechanism of 290,292114* Superheavy Nuclei Formed in 48Ca-Induced Reactions

We calculate the neutron-evaporation residue cross sections σ3n, σ4n, and σ5nin the hot-fusion reactions 48Ca+242,244Pu →290,292114 ∗ over a wide range of compound-nucleus excitation energies (\(E_{\text{CN}}^{*}\) = 34–53 MeV). We work with the dynamical cluster-decay model (DCM), with a single parameter, the neck-length parameter ΔR. To calculate neutron-evaporation cross sections, we choose the superheavy proton magic Z = 126 and neutron magic N = 184. Among the 3n, 4n, and 5n production cross sections for 290, 292114∗, only the 3n decay cross sections of 292114∗ correspond to spherical fragmentation. The 4n and 5n cross sections of 292114∗ and 3n, 4n, and 5n cross sections of 290114∗ could only be fitted after the inclusion of quadrupole deformations β2i within the optimum orientation approach. Changes in the angular momentum and N/Z ratio do not significantly influence the fragmentation paths of 290, 292114∗ superheavy nuclei. Larger barrier modification is required for the lower angular momentum states and lighter neutron clusters. The contribution of the fusion–fission component is also computed for the compound nucleus 292114∗ in the energy range \(E_{\text{CN}}^{*}\) = 27–47 MeV.