The European Physical Journal A最新文献

We study the fine structure constant dependence of the rates of some selected radiative capture reactions within the framework of so-called Halo Effective Field Theory in order to assess the adequacy of some assumptions made on the Coulomb penetrability. We find that this dependence deviates from that implied by a parameterization of the cross sections of this effect via a simple penetration factor. Some features of this fine-structure dependence are discussed, in particular its potential impact on the abundances of the light elements in primordial nucleosynthesis.

Fission dynamics of (^{188})Pt, a neutron deficient nucleus in newly discovered mass asymmetric sub-Pb region, have been explored via fission fragments mass-angle and mass-total kinetic energy distributions, at energies around and above the barrier. The observed correlations of fragment mass and emission angle indicate the presence of fission events originating from a non-equilibrated source. Mass-total kinetic energy spectra are relatively broader and the dependence of measured mean total-kinetic-energy on fragment mass is also broader than the expected parabolic dependence for liquid drop fission behaviour, at all studied energies. The measured mean total kinetic energy values are higher than the prediction of Viola systematics. The widths of measured total kinetic energy distributions are also inconsistent with the observed systematic behaviour of compound nucleus (CN) in this mass region. These observation of mass-angle and mass-total kinetic energy distributions revealed the clear signatures of the presence of slow quasifission in the fission of (^{188})Pt compound nucleus. These findings indicate the role of entrance channel parameters such as mass asymmetry and charge product (hbox {Z}_{P}) (hbox {Z}_{T}) in Fusion-Fission and quasi-fission dynamics. Dynamical model calculations do not predict the presence of quasi-fission for the present reaction system.

The origin of the modification of the quark structure of nucleons in the nuclear medium can be tested with tagged recoil nucleon measurements from deep inelastic scattering off electrons on deuterium. The LAD experiment at the Thomas Jefferson National Laboratory (JLab) will measure the modification of the neutron structure function for high-momentum, highly-virtual neutrons by measuring the spectator recoil protons in coincidence with the scattered electron. An update on the experimental setup and projected results is presented. The experiment will collect data in Fall 2024.

A few years ago, we theoretically studied the production of a stellar neutron spectrum at kT = 30 keV using a shaped proton beam impinging on a thick lithium target. Here, we first measure the proton distribution to better control the produced neutron spectrum. Then, we measure the forward-emitted angle-integrated neutron spectrum of the ⁷Li(p,n)⁷Be reaction via time-of-flight neutron spectrometry with such proton distribution. The result resembles a stellar neutron spectrum at kT = 30 keV. This method avoids in activation experiments the need for spectrum correction. In the case of spherical samples, no knowledge of the cross-section of the isotope being measured by activation would be necessary. Therefore, the present method is of interest for isotopes with unknown or poorly known cross-sections, such as branching points in astrophysics. The key point of our method is the experimental control of the proton distribution that impinges on the lithium target.

A comprehensive analysis of high spin band structures for odd mass (^{117-127})I nuclei is performed using Triaxial Projected Shell Model (TPSM) approach. Using suitable values for the relevant parameters, the estimated energy spectrum of odd mass (^{117-127})I agrees well with the experimental results The potential energy surfaces reveal that the isotopes are heading from (gamma )-softness towards rigidity. The current analysis further revealed that the typical band crossing along the yrast as well as the yrare line is caused by the three-quasipaticle band crossing the one-quasiparticle band. Further, transitional probabilities [B(E2) and B(M1)] have been computed and found to be consistent with the available experimental data. Chirality in (^{123}) I has also been discussed.

Large inconsistencies still exist in nuclear data libraries regarding the kinetic parameters of delayed neutron (DN) precursors. As an example, there is a 17% gap between ENDF-B/VIII.0 and JEFF-3.3 on the average lifetime T_1/2 of DN precursors from thermal fission of ²³⁵U. This parameter is of major importance for reactivity predictions of nuclear reactors in nominal or accidental configurations. In this context, CEA is actively participating to the ALDEN project (Average number and Lifetime of DElayed Neutrons) which aims at providing the nuclear data community with new data sets of DN from thermal and fast neutron induced fission of various actinides. A dedicated experimental setup was designed and optimized for that purpose and is presented in this paper. It consists of a “long counter” detector containing 16 proportional counters filled with ³He, embedded in a high density polyethylene matrix. The detector surrounds a fissile target prepared in the form of a miniature fission chamber, containing a few hundreds of micro-grams of fissile material. This set-up is connected to fast and efficient neutron shutters that can produce step-irradiations of variable durations. The equations driving the DN counting following step-irradiations of the fissile target are established and discussed in the perspective of DN yield or group parameter measurement. A comprehensive analysis of the different steps of data reduction is detailed: dead time characterization, Region of Interest (ROI) determination, absolute and relative efficiency calibration, fission rate estimation, irradiation time and background determination, DN decay curve production and physical parameter fitting. Following a prototype experiment performed in 2018 at the PF1b cold neutron beam line of Institute Laue Langevin (ILL, Grenoble, France), we discuss here the analysis of two campaigns occurring in 2019 and 2021 in which significant improvements were achieved in terms of background minimization, counting statistics and fission rate determination. The achievements of this work are the measurement of the delayed neutron emission per fission for the thermal neutron induced fission of ²³⁵U, estimated at (1.625 ± 0.010) % and the group parameters leading to an estimated lifetime of ({T}_{1/2}) = (8.87 ± 0.10) s. Those results are consistent with the values recommended by the IAEA/CRP work and they come with reduced uncertainties compared with previously published results.

A spectral solution method is proposed to solve a previously developed non-equilibrium statistical model describing partial thermalization of produced charged hadrons in relativistic heavy-ion collisions, thus improving the accuracy of the numerical solution. The particle’s phase-space trajectories are treated as a drift-diffusion stochastic process, leading to a Fokker–Planck equation (FPE) for the single-particle probability distribution function. The drift and diffusion coefficients are derived from the expected asymptotic states via appropriate fluctuation–dissipation relations, and the resulting FPE is then solved numerically using a spectral eigenfunction decomposition. The calculated time-dependent particle distributions are compared to Pb–Pb data from the ATLAS and ALICE collaborations at the Large Hadron Collider.

The isomeric ratios of (^{198})Au, (^{197})Hg and (^{195})Hg produced by (alpha )-particle induced reactions on natural platinum were investigated experimentally up to 29 MeV by using the standard stacked foil activation technique and (gamma )-ray spectrometry. The isomeric ratios of (^{197})Hg and (^{195})Hg determined by the conventional activation cross section formula showed strong cooling time dependence. The time dependence was resolved by adjusting the isomeric transition branching ratios for the two isotopes within a simultaneous decay curve analysis framework. Our analysis suggests 94.5±0.7% and 48.9±1.8% as the isomeric transition branching ratios of (^{197m})Hg (24 h) and (^{195m})Hg (42 h), respectively. The isomeric ratios and independent production cross sections of (^{198})Au, (^{197})Hg, (^{195})Hg and some other Hg, Au and Pt isotopes were also measured down to 6 MeV with these corrected isomeric transition branching ratios, and compared with predictions of statistical and pre-equilibrium models by TALYS-2.0 to discuss spin cutoff parameter dependence. We found the measured isomeric ratios are better predicted if we reduce the spin cutoff parameter to half or less from that estimated with the rigid body moment of inertia.

The distorted wave Born approximation (DWBA) with a nonlocal deuteron-nucleus d-A potential (Canadian Journal of Physics, 100(6):309–318, 2022) is used to calculate (d, p) transfer cross sections. We considered three target nuclei: light (^{16})O, intermediate (^{40})Ca and heavy (^{208})Pb. For each target nucleus we extracted spectroscopic factors and determined asymptotic normalization coefficients for various values of the single particle radius. The spectroscopic factors are reduced in agreement with previous works in the literature. Compared to the results of DWBA with a local model, the nonlocal model resulted in more peripheral transfer reactions for (^{40})Ca and (^{208})Pb target nuclei, but no significant effect is observed for the light (^{16})O target nucleus. The predicted transfer cross sections are in very good agreement with the experimental data particularly for (^{40})Ca and in the vicinity of the first peak. For the other two target nuclei the predictions of the DWBA with a local model are in slightly better agreement with experiment than the predictions of the nonlocal model at large angles beyond the first peak. For the (^{16})O(d, p)(^{17})O reaction, the fast decrease in the transfer cross section at small forward angles is better predicted by the nonlocal model than the local one. This suggests that the nonlocal model accounts, at least partially, for the channel coupling nonlocality resulting from deuteron break up in the entrance channel.

The present review studies total reaction cross sections from elastic scattering angular distributions of light stable and exotic projectiles ((2 le Z_P le 10), (4 le A_P le 24)) on ²⁰⁸Pb at energies around the Coulomb barrier. From the overall about 200 data it is found that the widely accepted simple grouping of the derived reduced cross sections into three categories—tightly bound, weakly bound, and exotic halo projectiles—is too simplistic and does not fully reflect the range of the experimental data. Furthermore, the energy dependence of the reduced cross sections shows unexpected properties which were hitherto only sparsely considered.