Statistical Model Calculations Research Articles

New signature of non-equilibrium fission from pre-scission α-particle emission

The pre-scission α-particle multiplicity (αpre), measured in 16O (96 MeV) + 232Th reaction, is observed to be significantly lower than the global trend reported earlier. Statistical model calculations also show a much shorter fission-time to reproduce the experimental αpre. By assimilating the previous measurements, it is observed that the αpre makes a changeover from high to a very low value while crossing the Businaro Gallone point in mass asymmetry in the entrance channels in heavy-ion induced fission of 232Th. Similar discontinuous behavior with respect to the entrance channel mass asymmetry was observed earlier in fission fragment angular anisotropy data, and it was attributed to “pre-equilibrium fission”. The present results on αpre provide compelling evidence for an admixture of non-equilibrium fission which depends on the entrance channel dynamics.

Studies of the 2α and 3α channels of the 12C+12C reaction in the range of E c.m.=8.9 MeV to 21 MeV using the active target Time Projection Chamber* *Supported in part by the Strategic Priority Research Program of Chinese Academy of Sciences (XDB34020200), the National Key Research and Development program (MOST 2016YFA0400501) from the Ministry of Science and Technology of

The 12C+12C fusion reaction was studied in the range of E c.m.=8.9 to 21 MeV using the active-target Time Projection Chamber. With full information on all tracks of the reaction products, cross sections of the 12C(12C,8Be)16Og.s. channel and the 12C(12C,3α)12C channel could be measured down to the level of a few milibarns. The 12C(12C,8Be)16Og.s. reaction channel was determined to be 10 mb at E c.m.=11.1 MeV, supporting the direct α transfer reaction mechanism. The 12C(12C,3α)12C reaction channel was studied for the first time using an exclusive measurement. Our result does not confirm the anomaly behavior reported in the previous inclusive measurement by Kolata et al. [Phys. Rev. C 21, 579 (1980)]. Our comparisons with statistical model calculations suggest that the 3α channel is dominated by the fusion evaporation process at E c.m. > 19 MeV. The additional contribution of the 3α channel increases the fusion reaction cross section by 10% at energies above 20 MeV. We also find that an additional reaction mechanism is needed to explain the measured cross section at E c.m. < 15 MeV at which point the statistical model prediction vanishes.

Statistical model calculation of the compound nuclear fission timescale in fusion-fission reactions

In the decay of an excited heavy nucleus, both the fission or evaporation residue (ER) cross sections and the fission time are determined by the competition between the various evaporation channels and fission. Statistical model (SM) calculations for several heavy-ion-induced fusion-fission reactions are presented which reproduce the fission or ER excitation functions. The corresponding calculated fission times are found to depend upon the compound nuclei and their excitation energies. The multichance fission probabilities and the corresponding fission times are also obtained, and they are shown to depend sensitively on the shell correction energies and the neutron separation energies of the compound nuclei involved. The experimental neutron multiplicities are reproduced by adjusting the saddle-to-scission transition time interval. An additional delay time for compound nuclear formation is found necessary to fit the experimental neutron multiplicities for a system with higher entrance channel mass symmetry.

Effects of entrance channels on breakup fusion induced by F19 projectiles

The study of breakup fusion of $^{19}\mathrm{F}$ with $^{154}\mathrm{Sm}$ target was studied through offline $\ensuremath{\gamma}$ ray spectrometry. Partial cross sections of evaporation residues produced in this reaction were measured in center-of-mass energies ranging $\ensuremath{\approx}3--30$ MeV above the fusion barrier. The excitation functions of the evaporation residues populated through $xn/pxn$ channels were found to be satisfactorily reproduced by statistical model calculations, whereas for the $\ensuremath{\alpha}$ emitting channels the cross sections show an enhancement over the theoretical predictions. The critical angular momentum deduced from the measured cross sections was found to be in good agreement with statistical model calculations. The degree of fusion incompleteness in the $^{19}\mathrm{F}+^{154}\mathrm{Sm}$ reaction is estimated by comparing the fusion excitation functions with coupled channels calculations and the extracted fusion function with the universal fusion function. The large cross sections observed for incomplete fusion products support the interpretation that this suppression of fusion is caused by $^{19}\mathrm{F}$ breaking up into charged fragments before reaching the fusion barrier. The incomplete fusion probability was also found to increase with the reduced mass and charge of the entrance channel, indicating the influence of entrance channel mass asymmetry and Coulomb repulsion on incomplete fusion. The present analysis shows the presence of strong clustering in the $^{19}\mathrm{F}$ projectile as $\ensuremath{\alpha}$ and $^{15}\mathrm{N}$.

Constraining the astrophysical p process: Cross section measurement of the Kr84(p,γ)Rb85 reaction in inverse kinematics

One of the biggest questions in nuclear astrophysics is understanding where the elements come from and how they are made. This work focuses on the p process, a nucleosynthesis process that consists of a series of photodisintegration reactions responsible for producing stable isotopes on the proton-rich side of stability. These nuclei, known as the p nuclei, cannot be made through the well-known neutron-capture processes. Currently p-process models rely heavily on theory to provide the relevant reaction rates to predict the final p-nuclei abundances and more experimental data is needed. The present work reports on an experiment performed with the SuN detector at the National Superconducting Cyclotron Laboratory, NSCL, at Michigan State University using the ReA facility to measure the $^{84}$Kr(p,$\gamma$)$^{85}$Rb reaction cross section in inverse kinematics. The reverse $^{85}$Rb($\gamma$,p)$^{84}$Kr reaction is a branching point in the p-process reaction network that was highlighted as an important reaction in sensitivity studies in the production of the $^{78}$Kr p nucleus. A new hydrogen gas target was designed and fabricated and a new analysis technique for background subtraction and efficiency calculations of the detector were developed. The experimental cross section is compared to standard statistical model calculations using the NON-SMOKER and TALYS codes.

Measurement of the Ne18(α,p)Na21 reaction with the ANASEN active-target detector system at Ec.m.=2.5–4 MeV

The $^{18}\mathrm{Ne}(\ensuremath{\alpha},p)^{21}\mathrm{Na}$ reaction plays a significant role in Type-I X-ray bursts. It is a major path in the breakout from the hot-CNO cycles to the synthesis of heavier elements in the $\ensuremath{\alpha}p$- and $rp$-processes. An experiment to determine the cross section of this reaction was performed with the ANASEN active-target detector system, determining the cross section at energies between 2.5 and 4 MeV in the center-of-mass frame. The measured cross sections for reactions populating the ground state in $^{21}\mathrm{Na}$ are consistent with results obtained from the time-inverse reaction, but significantly lower than the previously published experimental data of direct measurements. The total cross sections are also compared with those derived from indirect methods and statistical-model calculations. This experiment establishes a new experimental data set on the excitation function of the $^{18}\mathrm{Ne}(\ensuremath{\alpha},p)^{21}\mathrm{Na}$ reaction, revealing the significance of the excited states' contributions to the total reaction cross section and allowing us to separate the contribution of the $(\ensuremath{\alpha},2p)$ reaction. The impact of the measured cross section on thermal reaction rates is discussed.

Production of high-energyγrays in proton- andα-induced reactions

The high-energy $\ensuremath{\gamma}$ rays have been measured in proton and $\ensuremath{\alpha}$-induced reactions on medium mass target nuclei $^{115}\mathrm{In}$ and $^{112}\mathrm{Sn}$, respectively. Theoretical analyses of the spectra have been performed within the Hauser-Feshbach statistical model and the Akkermans-Gruppelaar exciton model formalisms. It is observed that the proton-induced reaction has significant contribution from the direct-semidirect capture in the region ${E}_{\ensuremath{\gamma}} \ensuremath{\approx}10--20$ MeV. The $^{4}\mathrm{He}$-ion-induced reaction could be described reasonably well by the results of the statistical model calculations. A reduced level density parameter, as compared to that used for the proton-induced reaction, was required to explain the high-energy $\ensuremath{\gamma}$-ray spectrum in the $^{4}\mathrm{He}$-ion-induced reaction.

Evaporation residue cross section measurements for the Si30+Yb176 reaction

Evaporation residue (ER) cross sections are measured for the reaction $^{30}\mathrm{Si}+^{176}\mathrm{Yb}$, which forms the compound nucleus $^{206}\mathrm{Po}^{*}$, over the excitation energy range from 47.68 to 113.73 MeV. Dependence of noncompound nuclear reaction on entrance channel parameters such as charge product (${Z}_{1}{Z}_{2}$), mass asymmetry ($\ensuremath{\alpha}$), deformation (${\ensuremath{\beta}}_{2}$) of the target, and isospin asymmetry ($\mathrm{\ensuremath{\Delta}}\frac{N}{Z}$) is explored. To analyze the experimental data the coupled-channels and the statistical model calculations are used. The measured ER cross sections are compared with the system,$^{12}\mathrm{C}+^{194}\mathrm{Pt}$ forming the same compound nucleus and also with $^{28}\mathrm{Si}+^{176}\mathrm{Yb}$, forming $^{204}\mathrm{Po}^{*}$ in the neighborhood. Observed suppression in the evaporation residue cross sections at higher energies may be attributed to the presence of the noncompound nuclear process.

The New Physics in LILITA_N21: An Improved Description of the Reaction 190 MeV 40Ar + 27Al

In this paper, light charged particle emission in the evaporation residue channel for the 190 MeV 40Ar + 27Al reaction leading to 67Ga composite nuclei at Ex = 91 MeV and angular momentum up to 46 ℏ has been re-analyzed. The main goal was to study the decay of 67Ga on the basis of an extended set of observables in order to provide a description of the evaporative decay cascades using the multistep Monte Carlo approach. The proton and α-particle energy spectra along with their angular distributions and ratios of differential multiplicities have been considered. The measured observables were compared with statistical model calculations. Having used a single-step Monte Carlo approach and standard parameters decades ago, the model does not provide a good description of the full dataset. Only a subset of the data was reproduced by assuming emitting nuclei with very large deformed shapes in a previous work published in the late 1980s. In the reported analysis, better agreement has been observed. Using the new transmission coefficients from the Optical Model, the parameters of which have recently been derived, the multi-step approach and the introduction of a nuclear shape description based on the nuclear stratosphere allowed us to realize a significant improvement.

The Preliminary Statistical Analysis of LAMOST DR8 Low Resolution AFGK Stars

Abstract We download the Large sky Area Multi-Object fiber Spectroscopic Telescope Data Release 8 (LAMOST DR8) low resolution catalog for 6,478,063 AFGK type stars and plot the figures of effective temperature, gravitational acceleration and metal abundance. Some small and medium mass stars are evolved from pre-main sequence or main sequence stage to planetary nebula stage or white dwarf stage by the stellar evolution code MESA. We analyze the observed statistical data and model calculation results, and then obtain some basic conclusions preliminarily. Most red giant and asymptotic giant branch stars with log g less than 0.85 have poor metal abundance. Most hot A type main-sequence stars are metal-rich stars with log g from 3.5 to 4.5. The conclusions are reasonable within a certain error range. The theory of a gap area in the Hertzsprung–Russell diagram for stellar evolutions of medium mass stars is reflected in the statistical plots. The central core hydrogen burning stage and the central core helium burning stage correspond to the peak structures in the statistical plots for gravitational acceleration. The metal abundances among A, F, G and K type stars have a wide distribution. We cannot simply replace the metal abundances of these stars with the metal abundance of the Sun when doing fine research work.

Cross-section measurement of the Cd114(p,γ)In115m reaction for nuclear reactor and astrophysical applications

The present study, carried out for the first time, contributes to the multidisciplinary work in reactor design and nuclear astrophysics. Cadmium has been chosen for the present study for its significant importance in the development of nuclear reactors and for the astrophysical $p$ process. A stacked foil activation technique followed by offline $\ensuremath{\gamma}$-ray spectroscopy were used to carry out the present work. Cross sections were measured for the $^{114}\mathrm{Cd}{(p,\ensuremath{\gamma})}^{115m}$ reaction due to the scarcity of data observed in the literature. The $S$ factor was calculated from the cross-section data. The $^{114}\mathrm{Cd}(p,n)^{114m}\mathrm{In}$ reaction was also investigated using the same spectra. Statistical model calculations were performed using the Hauser-Feshbach (HF) code of the latest version of talys and were compared with the measured data and the previously measured experimental data available in the literature. A good agreement between theoretical and experimental data was found. The present data follow the trend of theoretical predictions and are in good agreement with the EXFOR database.

Measurements of neutron and photon induced cross sections for the production of medical isotopes of strontium

The cross sections of the 88Sr(n,2n)87Srm, 88Sr(n,p)88Rb, 88Sr(n,α)85Kr, 86Sr(n,2n)85Srm, 86Sr(n,2n)85Srg and 84Sr(n,2n)83Sr nuclear reactions at 14.77 ± 0.17 MeV neutron energy and the average cross sections of the 88Sr(γ,n)87Srm and 86Sr(γ,n)85Srm nuclear reactions at bremsstrahlung of 15 MeV endpoint energy were measured with the activation method and offline gamma-ray spectroscopy. The measured cross sections were reported with a detailed covariance analysis for uncertainty and correlation coefficients. The experimental results were compared with the previously reported experimental data from the EXFOR database and the evaluated data from the TENDL-2019 library. Statistical model calculations were performed with the latest TALYS-1.95 code with optimised input parameters to better reproduce reported experimental data. The measured cross sections were found to be in good agreement with the literature and theoretical calculations. These results highlight the alternate production routes for the medical radioisotopes of 87Srm and 85Srg.

Systematic study of the (n, 2n) reaction cross section for 121Sb and 123Sb isotopes

The cross sections of the 121Sb 120Sbm and 123Sb 122Sb reactions were measured at 12.50, 15.79 and 18.87 MeV neutron energies relative to the standard 27Al 24Na monitor reaction using neutron activation and offline γ-ray spectrometry. Irradiation of the samples was performed at the BARC-TIFR Pelletron Linac Facility, Mumbai, India. The quasi-monoenergetic neutrons were generated via the 7Li reaction. Statistical model calculations were performed by nuclear reaction codes TALYS (ver. 1.9) and EMPIRE (ver. 3.2.2) using various input parameters and nuclear level density models. The cross sections of the ground and the isomeric state as well as the isomeric cross section ratio were studied theoretically from reaction threshold to 26 MeV energies. The effect of pre-equilibrium emission is also discussed in detail using different theoretical models. The present measured cross sections were discussed and compared with the reported experimental data and evaluation data of the JEFF-3.3, ENDF/B-VIII.0, JENDL/AD-2017 and TENDL-2019 libraries. A detailed analysis of the uncertainties in the measured cross section data was performed using the covariance analysis method. Furthermore, a systematic study of the reaction cross section for 121Sb and 123Sb isotopes was also performed within 14–15 MeV neutron energies using various systematic formulae. This work helps to overcome discrepancies in Sb data and illustrate a better understanding of pre-equilibrium emission in the reaction channel.

Удаан нейтроноор цэнэгт бөөм үүсэх урвалын астрофизикийн хэрэглээ

Results of statistical model calculations for average (n,a) cross sections arc compared with experimental values at E=30 keV. Using the results of theoretical calculations for the (n,a) and (n,p) cross sections were proposed new measurement for nuclear astrophysics study on the IREN neutron source projected at the Frank Laboratory of Neutron Physics, JINR, Dubna.

Isomeric cross sections of the (n, α) reactions on the 90Zr, 93Nb and 92Mo isotopes measured for 13.73 MeV–14.77 MeV and estimated for 10 MeV–20 MeV neutron energies

Isomeric cross sections for the 90Zr(n, α)87Srm, 93Nb(n, α)90Ym and 92Mo(n, α)89Zrm reactions were measured at five neutron energies over the range 13.73 MeV–14.77 MeV using the activation technique in combination with high resolution γ-ray spectrometry. In the present work, the cross sections are measured for the 90Zr(n, α)87Srm and 93Nb(n, α)90Ym reactions are referenced to the 27Al(n, α)24Na standard reaction cross section whereas those measured for 92Mo(n, α)89Zrm reaction are referenced to the 56Fe(n, p)56Mn standard reaction cross section. The cross sections for these reactions were also theoretically estimated using the EMPIRE-3.2 and TALYS 1.8 codes over the neutrons energy range of 10 MeV–20 MeV and matched with the experimental cross sections by making a proper choice of the model parameters. A minimum eight different sets of these statistical model calculations were performed by using the consistent sets of model parameters along with the pre-equilibrium mechanism in addition to the direct-reaction and the statistical Hauser–Feshbach (HF) compound nucleus ones. The measured cross sections for these three reactions increase with the increase in neutron energy from 13.73 MeV to 14.77 MeV. As the proton number increased by one when we go from zirconium to niobium or from niobium to molybdenum, the probability of alpha particle emission also increases at each corresponding neutron energy. The present results indicate that the measured cross section at each neutron energy for the 92Mo(n, α)89Zrm reaction is found to be the highest as compared to the other two reactions whereas, for the 90Zr(n, α)87Srm reaction, the measured cross section is found to be the lowest as compared to the other two reactions studied. The results obtained from the present measurement are found to be in good agreement with the calculated reaction cross section based on theoretical models and also with the work reported by earlier authors.

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.

Measurements of evaporation residue cross-sections for 48Ti + 140,142Ce reactions

Evaporation residue (ER) cross-sections have been measured for the 48Ti + 140,142Ce reactions to understand the influence of neutron shell closure on compound nucleus (CN) formation. The measured ER cross-sections for 48Ti + 140Ce (NT = 82) system are found to be very close to those of 48Ti + 142Ce (NT = 84) near the Coulomb barrier. The results show that the effect of shell closure in the target nucleus on fusion cross-sections is negligible for the present systems which can be attributed to the small values of the shell correction energies in the entrance channel. Statistical model calculations are performed to interpret the experimental results. Further, comparison of the present results with those from a reaction with higher mass asymmetry in the entrance channel shows no evidence of quasifission in the present systems.

Exploring the uncertainties of (α, xn) reactions for the weak r-process

“Light” heavy elements (Z = 38 − 47) can be synthesized in the neutrino–driven ejecta of core–collapse supernovae via the weak r–process. This nucleosynthesis scenario exhibits uncertainties from the absence of experimental data from (α, n) reactions on neutron–rich nuclei, and are mostly based on statistical model calculations. We present preliminary results from a recent sensitivity study, using the Atomki–V2 α-nucleus potential to identify the most important (α, n) reactions that can affect the production of “light” heavy elements between strontium and silver under different astrophysical conditions. We also discuss the planning of studies to experimentally determine (α, xn) reaction rates using the MUSIC detector at Argonne National Laboratory and the SECAR recoil separator at FRIB.

Impact of nucleon transfer channels on complete fusion in the Li6,7+Ni58 reactions near the Coulomb barrier

Reactions involving weakly bound nuclei, both stable and radioactive, especially at energies near the Coulomb barrier remain at the forefront as topics to be understood, both experimentally and theoretically. Of special interest is the interplay between complete fusion, breakup, and direct reactions for these cases and the complete understanding of their influence on fusion, which arises because of the bottleneck in experimentally identifying complete fusion events. In this work we address this issue by measuring individual residue cross sections in the interaction of $^{6,7}\mathrm{Li}$ on a thin $^{58}\mathrm{Ni}$ target at energies around the barrier. Comparison with statistical model calculations and coupled reaction calculations allowed us to delineate the reaction products into those arising from complete fusion and those from direct transfer. At energies below the barrier where cross section enhancement is found to occur, the contribution of transfer channels to the total cross section is found to be large and had to be accounted for before assessing the enhancement above the one-dimensional barrier penetration predictions.

Measurements of {Xi left( 1530right) ^{0}} and {overline{Xi }left( 1530right) ^{0}} production in proton\u2013proton interactions at sqrt{s_{NN}}\xa0=\xa017.3\xa0text{ GeV } in the NA61/SHINE\xa0experiment

Double-differential yields of {Xi left( 1530right) ^{0}} and {overline{Xi }left( 1530right) ^{0}} resonances produced in p+p interactions were measured at a laboratory beam momentum of 158 text{ GeV }!/!c. This measurement is the first of its kind in p+p interactions below LHC energies. It was performed at the CERN SPS by the NA61/SHINE collaboration. Double-differential distributions in rapidity and transverse momentum were obtained from a sample of 26times 10^6 inelastic events. The spectra are extrapolated to full phase space resulting in mean multiplicity of {Xi left( 1530right) ^{0}} (6.73 pm 0.25pm 0.67)times 10^{-4} and {overline{Xi }left( 1530right) ^{0}} (2.71 pm 0.18pm 0.18)times 10^{-4}. The rapidity and transverse momentum spectra and mean multiplicities were compared to predictions of string-hadronic and statistical model calculations.