Neutron Evaporation Research Articles

Study on the reaction channels in the 6Li+89Y system with multi-angular proton and deutron-γ coincidence analysis

The 6Li+89Y experiment was conducted at the Legnaro National Laboratory in Italy to explore the influence of breakup and transfer reactions on the fusion process induced by the weakly bound projectiles. Due to the competition between neutron and proton evaporation, complete and incomplete fusion might produce identical residues, leading to the difficulties in identification of different reaction process. In this work, the High-Purity-Germanium (HPGe) detector array (GALILEO) was employed to measure γ rays, and the silicon detector array (EUCLIDES) was utilized to capture light charged particles. Exclusive measurements of prompt γ rays from residuals with various light charged particles at an energy near the Coulomb barrier are reported. In the p−γ coincident measurements, observed 91Nb, 92Nb, and 93Nb is considered from neutron evaporation channel in complete fusion reaction, and 90Y is generated through 1n stripping reaction. A two-step, breakup followed by fusion, in case of the capture of α is inferred to be the dominant mechanism to yield the 92Nb and 91Nb in the deutron coincident exclusive measurement.

Software compensation for highly granular calorimeters using machine learning

A neural network for software compensation was developed for the highly granular CALICE Analogue Hadronic Calorimeter (AHCAL). The neural network uses spatial and temporal event information from the AHCAL and energy information, which is expected to improve sensitivity to shower development and the neutron fraction of the hadron shower. The neural network method produced a depth-dependent energy weighting and a time-dependent threshold for enhancing energy deposits consistent with the timescale of evaporation neutrons. Additionally, it was observed to learn an energy-weighting indicative of longitudinal leakage correction. In addition, the method produced a linear detector response and outperformed a published control method regarding resolution for every particle energy studied.

Pre-neutron fragment mass yields for 235U(n, f ) and 239Pu(n, f ) reactions at incident energies from thermal up to 20 MeV* *Supported by the National Natural Science Foundation of China (12065003), the Natural Science Foundation of Guangxi (2019GXNSFDA185011), the Key Laboratory of Neutron Physics China Academy of Engineering Physics (2018BA03), the Scientific Research and Technology Development Project of Guilin

Pre-neutron fragment mass yields in the vicinity of the thermal neutron energy are highly important for applications because of the larger fission cross sections of the U(n, f) and Pu(n, f) reactions. In this paper, pre-neutron fragment mass yields at incident energies from thermal up to 20 MeV are systematically studied using an empirical fission potential (EFP) model, the potential parameters of which are obtained from the measured data. The energy dependences of the peaks and valleys of the pre-neutron fragment mass yields are described by exponential and linear functions for the U(n, f) and Pu(n, f) reactions, respectively. The energy dependences of the evaporation neutrons, which play a crucial role in the reasonable description of pre-neutron fragment mass yields, are also obtained from the fission cross sections. The pre-neutron fragment mass yields in this study are not only consistent with the results of previous studies in regions of several MeVs but also agree well with existing measured data at incident energies from thermal up to 20 MeV. The results show that the feasibility of this EFP model is verified in this extended energy region.

Angular and energy dispersion in neutron-rich nuclei by neutron evaporation: A Monte Carlo simulation


 In this paper we present the results of a Monte Carlo simulation of the effects of neutronevaporation on the angular and energy dispersions in neutron-rich nuclei as a function of the massnumber A, the kinetic energy E and the number of emitted neutrons. We based our simulation on theassumption of an isotropic random distribution for the neutron emission angles in space and on aMaxwell-Boltzmann distribution for the kinetic energy of the evaporated neutrons. Our resultsconfirm large angular and energy dispersions for light, low-energy nuclei and for high neutronchannels.

Predictions for the synthesis of the Z=120 superheavy element

The possible stable and radioactive beam-induced hot fusion reactions for the production of new superheavy element (SHE) $Z=120$ are investigated within the dinuclear system model. Our investigation shows that the reaction $^{45}\mathrm{Sc}+^{252}\mathrm{Es}$ has a relatively large evaporation residue cross-section (ERCS) for the synthesis of SHE $Z=120$ due to its slightly larger mass asymmetry. In addition, we analyzed the effects of fission barrier and neutron separation energy of superheavy nuclei on ERCS. To synthesize the double-magic $Z=120$ and $N=184$ nucleus predicted by the relativistic mean-field model, some radioactive beam-induced fusion reactions, which can be performed in available experimental equipment, for producing superheavy nucleus $^{304}120$ in neutron evaporation channels are investigated systematically. We find that the hot fusion reaction $^{50}\mathrm{Ca}+^{257}\mathrm{Fm}$ in the $3n$ evaporation channel is optimal for synthesizing the superheavy nucleus $^{304}120$. We hope these predictions will shed new light timely for the recent experiments on the synthesis of $Z=120$ superheavy element and the search for superheavy stability islands.

Fragmentation analysis of Z=112 –116 nuclei using a Skyrme energy density formalism within the collective clusterization approach

The Skyrme energy density formalism is applied to address the capture cross sections of $_{112}^{286}\mathrm{Cn}^{*}$, $_{114}^{292}\mathrm{Fl}^{*}$, and $_{116}^{296}\mathrm{Lv}^{*}$ superheavy nuclei formed in $^{48}\mathrm{Ca}\phantom{\rule{0.16em}{0ex}}+\phantom{\rule{0.16em}{0ex}}^{238}\mathrm{U},^{244}\mathrm{Pu},^{248}\mathrm{Cm}$ reactions. The dynamics of $^{48}\mathrm{Ca}$-induced reactions is investigated by including GSkI and SSk forces for the hot-optimum configurations of decay fragments. The neutron evaporation $(3n \mathrm{and} 4n)$, compound nucleus fission (CN fission) and quasifission (QF) cross sections are calculated with the application of both forces, but the GSkI force seems to be appropriate only for the neutron evaporation channels. However, by including SSk force various decay processes can be handled by including the only parameter of the model, called the neck length parameter $(\mathrm{\ensuremath{\Delta}}R)$. The fragment mass distribution of $Z=112$--116 nuclei shows considerable modifications in the fission, symmetric quasifission $({\mathrm{QF}}_{\mathrm{sym}.})$ and asymmetric quasifission $({\mathrm{QF}}_{\mathrm{asym}.})$ regions with the inclusion of the SSk force as compared to analysis based on the density independent potentials. In addition to this, the role of ${\ensuremath{\beta}}_{2}$ deformations and magic shell effects (around the Pb isotope) is scrutinized through the fragmentation behavior and preformation probability of $_{112}^{286}\mathrm{Cn}^{*}$, $_{114}^{292}\mathrm{Fl}^{*}$, and $_{116}^{296}\mathrm{Lv}^{*}$ nuclei. Finally, the distribution of the average total kinetic energy of fragments is calculated with the density dependent SSk force and compared with experimental data and earlier work based on the proximity potential.

Inference on fission timescale from neutron multiplicity measurement in 18O + 184W

The pre-scission and post-scission neutron multiplicities are measured for the 18O + 184W reaction in the excitation energy range of 67.23–76.37 MeV. Langevin dynamical calculations are performed to infer the energy dependence of fission decay time in compliance with the measured neutron multiplicities. Different models for nuclear dissipation are employed for this purpose. Fission process is usually expected to be faster at a higher beam energy. However, we found an enhancement in the average fission time as the incident beam energy increases. It happens because a higher excitation energy helps more neutrons to evaporate that eventually stabilizes the system against fission. The competition between fission and neutron evaporation delicately depends on the available excitation energy and it is explained here with the help of the partial fission yields contributed by the different isotopes of the primary compound nucleus.

Theoretical description of fission yields: Toward a fast and efficient global model

Background: A quantitative microscopic understanding of the fission-fragment yield distributions represents a major challenge for nuclear theory as it involves the intricate competition between large-amplitude nuclear collective motion and single-particle nucleonic motion. Purpose: A recently proposed approach to global modeling of fission fragment distributions is extended to account for odd-even staggering in charge yields and for neutron evaporation. Method: Fission trajectories are obtained within the density functional theory framework, allowing for a microscopic determination of the most probable fission prefragment configurations. Mass and charge yields distributions are constructed by means of a statistical approach rooted in a microcanonical ensemble. Result: We show that the proposed hybrid model can reproduce experimental mass and charge fragment yields, including the odd-even staggering, for a wide range of fissioning nuclei. Experimental isotopic yields can be described within a simple neutron evaporation scheme. We also explore fission fragment distributions of exotic neutron-rich and superheavy systems, and compare our predictions with other state-of-the art global calculations. Conclusion: Our study suggests that the microscopic rearrangement of nucleons into fission fragments occurs well before the scission, and that the subsequent dynamics is mainly driven by the thermal excitations and bulk features of the nuclear binding. The proposed simple hybrid approach is well suited for large-scale calculations involving hundreds of fissioning nuclei.

Laser-nucleus interactions in the sudden regime

The interaction between medium-weight nuclei and a strong zeptosecond laser pulse of MeV photons is investigated theoretically. Multiple absorption of photons competes with nuclear equilibration. We investigate the sudden regime. Here the rate of photon absorption is so strong that there is no time for the nucleus to fully equilibrate after each photon absorption process. We follow the temporal evolution of the system in terms of a set of rate equations. These account for dipole absorption and induced dipole emission, equilibration (modeled in terms of particle-hole states coupled by the residual nuclear interaction), and neutron decay (populating a chain of proton-rich nuclei). Our results are compared with earlier work addressing the adiabatic regime where equilibration is instantaneous. We predict the degree of excitation and the range of nuclei reached by neutron evaporation. These findings are relevant for planning future experiments.

Nuclear level density and thermal properties of $$^{115}\hbox {Sn}$$ from neutron evaporation

The nuclear level density of $^{115}$Sn has been measured in an excitation energy range of $\sim $2 - 9 MeV using the experimental neutron evaporation spectra from the $^{115}$In($p,n$)$^{115}$Sn reaction. The experimental level densities were compared with the microscopic Hartree-Fock BCS (HFBCS), Hartree-Fock-Bogoliubov plus combinatorial (HFB+C), and an exact pairing plus independent particle model (EP+IPM) calculations. It is observed that the EP+IPM provides the most accurate description of the experimental data. The thermal properties (entropy and temperature) of $^{115}$Sn have been investigated from the measured level densities. The experimental temperature profile as well as the calculated heat capacity show distinct signatures of a transition from the strongly-paired nucleonic phase to the weakly paired one in this nucleus.

Determination of the Zn60 level density from neutron evaporation spectra

Nuclear reactions of interest for astrophysics and applications often rely on statistical model calculations for nuclear reaction rates, particularly for nuclei far from $\beta$-stability. However, statistical model parameters are often poorly constrained, where experimental constraints are particularly sparse for exotic nuclides. For example, our understanding of the breakout from the NiCu cycle in the astrophysical rp-process is currently limited by uncertainties in the statistical properties of the proton-rich nucleus $^{60}$Zn. We have determined the nuclear level density of $^{60}$Zn using neutron evaporation spectra from $^{58}$Ni($^3$He, n) measured at the Edwards Accelerator Laboratory. We compare our results to a number of theoretical predictions, including phenomenological, microscopic, and shell model based approaches. Notably, we find the $^{60}$Zn level density is somewhat lower than expected for excitation energies populated in the $^{59}$Cu(p,$\gamma$)$^{60}$Zn reaction under rp-process conditions. This includes a level density plateau from roughly 5-6 MeV excitation energy, which is counter to the usual expectation of exponential growth and all theoretical predictions that we explore. A determination of the spin-distribution at the relevant excitation energies in $^{60}$Zn is needed to confirm that the Hauser-Feshbach formalism is appropriate for the $^{59}$Cu(p,$\gamma$)$^{60}$Zn reaction rate at X-ray burst temperatures.

Preliminary analysis of neutron time-delay histograms from Changvan latitude surveys

The name “Changvan” is given to a transportable neutron monitor, modernized by researchers in Thailand and housed inside a standard-size shipping container. It contains three neutron-sensitive proportional counters, which contain enriched 10BF3 gas, to detect cosmic ray showers. Primary cosmic rays are high-energy particles arriving from outer space. By colliding with atoms in the atmosphere, they produce a shower of sub-atomic particles, called secondary particles. Some particles can reach the Earth’s surface, including neutrons. The Changvan is designed to measure atmospheric neutrons during a latitude survey. The side counter tubes are ringed with lead producer to produce evaporation neutrons when secondary particles collide with it. The center counter lacks the lead producer but can receive neutrons from nearby counters. The reflector and moderator made from polyethylene thermalize the evaporation neutrons and protect against disturbance by external low-energy neutrons. In this work, we pursue methods to determine spectral changes in Galactic cosmic rays using data from the Changvan, avoiding the systematic inconsistencies of cross-station comparisons. We examine neutron time delay histograms from the three counter tubes, as recorded by specially designed readout electronics, where the time delay refers to the time interval between consecutive neutron detections in one counter. We perform a preliminary analysis of such histograms to measure the leader fraction (L) of neutrons that do not follow a previous neutron from the same primary cosmic ray. The electronic dead time will be considered in the L calculation.

Energy sharing based on microscopic level densities

The transformation of a moderately excited heavy nucleus into two excited fission fragments is modeled as a strongly damped evolution of the nuclear shape. The resulting Brownian motion in the multi-dimensional deformation space is guided by the shape-dependent level density which has been calculated microscopically for each of nearly ten million shapes (given in the three-quadratic-surfaces parametrization) by using a previously developed combinatorial method that employs the same single-particle levels as those used for the calculation of the pairing and shell contributions to the five-dimensional macroscopic-microscopic potential-energy surface. The stochastic shape evolution is followed until a small critical neck radius is reached, at which point the mass, charge, and shape of the two proto-fragments are extracted. The available excitation energy is divided statistically on the basis of the microscopic level densities associated with the two distorted fragments. Specific fragment structure features may cause the distribution of the energy disvision to deviate significantly from expectations based on a Fermi-gas level density. After their formation at scission, the initially distorted fragments are being accelerated by their mutual Coulomb repulsion as their shapes relax to their equilibrium forms. The associated distortion energy is converted to additional excitation energy in the fully accelerated fragments. These subsequently undergo sequential neutron evaporation which is calculated using again the appropriate microscopic level densities. The resulting dependence of the mean neutron multiplicity on the fragment mass, as well as the dependence of on the initial excitation energy of the fissioning compound nucleus, exhibit features that are similar to the experimentally observed behavior, suggesting that the microscopic energy sharing mechanism plays an important role in low-energy fission.

Monte-Carlo simulation of the response of bare neutron counters at the South Pole to vertical secondary particles from cosmic rays

Neutron monitors (NM64) are standard ground-based detectors that measure the flux of primary cosmic rays at GeV energies in space by counting secondary particles (mostly neutrons) from atmosphere cascades. The atmospheric neutrons are detected by induced nuclear fission in a gas proportional counter. In the standard design, there is a lead ring to generate evaporation neutrons that are moderated by polyethylene before being detected in the 10BF3 gas counter. By omitting the lead, so called “bare counters” respond to lower energy particles on average and can be used in conjunction with NM64 to estimate the energy spectrum of the primary cosmic rays. The specific objective of this research is to refine the understanding of the lead-free neutron monitor now installed at the South Pole using Monte-Carlo FLUKA simulations. This design uses paraffin and wood to moderate high-energy neutrons and detects them with 3He gas-filled proportional counters. Latitude surveys have shown that they have different detection efficiency from either the NM64 or polyethylene moderated bare counters, but they have never been adequately modelled. Understanding the differences quantitatively is the goal of this work. We will also report the detection efficiency of the paraffin bare for other atmospheric particles.

Investigation of the cold valley paths for the synthesis of isotopes of Ubh in optimum orientations

Cold valley paths have been investigated for the synthesis of isotopes of superheavy element Unbihexium using quantum mechanical fragmentation theory, which suggests the fusion of fission fragments belonging to the cold valleys in the potential energy surfaces with at least one spherical closed shell nucleus as the reaction partner for the nuclear synthesis. For the suitability of the cold valley fission fragments for the synthesis of isotopes of Ubh, first the fragmentation potentials, preformation probabilities and fission barriers of the fission fragments of 314Ubh and 326Ubh compound systems in their ground states have been compared by keeping the fragments either in hot or cold optimum orientations. The comparison suggests 138Ba + 176Yb and 132Sn + 194Os reactions in hot optimum orientations for the synthesis of isotopes of Ubh due to their highest values of preformation probabilities, lowest values of the fragmentation potentials and relatively good values of the fission barriers. Also, the reactions 68Ni + 246Cf and 70Ni + 256Cf have been considered for the comparison with 138Ba + 176Yb and 132Sn + 194Os reactions due to their relatively smaller values of the Coulomb potential, higher mass-asymmetry and fission barrier, and shell closures next to 48Ca which has been used in most of the experiments for the synthesis of superheavy elements. Finally, the excitation functions for the neutron evaporation from the compound systems formed in these reactions, the probabilities of the formation and survival of the compound nuclei have been compared to suggest the suitable cold valley paths energies for the synthesis of the isotopes of Ubh.

On the isotopic composition of fission fragments

We use a newly developed method, based on the original Brownian shape motion (BSM) model, to calculate fission-fragment isotopic yields Y(Z,N). Recent measurements of post-neutron isotopic distributions with better than 1 u resolution make it possible, for the first time, to test in detail this model. We here report on the comparison between the measured and the calculated fission-fragment average neutron number <N> as a function of proton number, as well as isotopic distributions for selected proton numbers. The calculated primary yields are corrected for post-scission neutron evaporation by the fragments. The highly variable dependence of <N> with Z seen in the measurements is also present in the calculations.

Safety Evaluation of a Long-Lived Tumour-Specific Gadolinium (Gd)-Based Imaging Agent in Proton Therapy

We assessed the dosimetric safety of a potential diapeutic Gadolinium-based contrast agent for diagnostic MR imaging and MR-guided radiotherapy by calculating depth-dependent reactions rates in a spread-out Bragg peak (SOBP) in water. Energy-dependent cross-sections for inelastic proton reactions on Gadolinium were folded with proton energy spectra at depth. Particle transport, dose, and phase space scoring was performed using Geant4-based TOPAS Monte Carlo software. The isotopic Terbium yield at depth was calculated to be between 103 to 105 atoms/mmolnatGd per Gray SOBP dose in a 2.4 cm3 volume. At currently achievable Gadolinium concentrations of 0.2 μmol/cm3 this yields approximately 26 atoms/(cm3 GySOBP), corresponding to less than 1 Bq activity. Additional dose from evaporation neutrons and subsequent capture gammas is on the order of 1 μSv/GySOBP. Thus, use of this long-lived contrast agent is dosimetrically safe at current concentrations and induced radioactivity is negligible.

Anomalous neutron yields confirmed for Ba-Mo and newly observed for Ce-Zr from spontaneous fission of Cf252

We reinvestigated the neutron multiplicity yields of Ba-Mo, Ce-Zr, Te-Pd, and Nd-Sr from the spontaneous fission of $^{252}$Cf; by (i) using both $\gamma$-$\gamma$-$\gamma$-$\gamma$ and $\gamma$-$\gamma$-$\gamma$ coincidence data, (ii) using up to date level scheme structures, and (iii) cross-checking analogous energy transitions in multiple isotopes, we have achieved higher precision than previous analyses. Particular attention was given to the Ba-Mo pairs where our results clearly confirm that the Ba-Mo yield data have a second hot fission mode where 8, 9, 10, and now 11 neutron evaporation channels are observed. These are the first observations of the 11 neutron channel. These 8-11 neutron channels are observed for the first time in the Ce-Zr pairs, but are not observed in other fission pairs. The measured intensities of the second mode in Ba-Mo and Ce-Zr pairs are $\sim$1.5(4)$\%$ and $\sim$1.0(3)$\%$, respectively. These high neutron number evaporation modes can be an indication of hyperdeformation and/or octupole deformation in $^{143-145}$Ba and in $^{146,148}$Ce at scission to give rise to such high neutron multiplicities.

Excitation energy partition in fission

The transformation of an atomic nucleus into two excited fission fragments is modeled as a strongly damped evolution of the nuclear shape. As in previous studies, it is assumed that the division of mass and charge is frozen in at a critical neck radius of c0=2.5fm. In order to also determine the energetics, we follow the system further until scission occurs at a smaller neck radius, at which point the shapes of the proto-fragments are extracted. The statistical energy available at scission is then divided on the basis of the respective microscopic level densities. This approach takes account of important (and energy-dependent) finite-size effects. After the fragments have been fully accelerated and their shapes have relaxed to their equilibrium forms, they undergo sequential neutron evaporation. The dependence of the resulting mean neutron multiplicity on the fragment mass, ν¯(A), including the dependence on the initial excitation energy of the fissioning compound nucleus, agrees reasonably well with observations, as demonstrated here for 235U(n,f), and the sawtooth appearance of ν¯(A) can be understood from shell-structure effects in the level densities.

A Numerical Solution of Low-Energy Neutron Boltzmann Equation

A multigroup method using a straight ahead approximation is created to calculate low energy neutron fluence due to the elastic scattering of evaporation neutrons produced in interactions of high energy particles with target nuclei. This multigroup method is added to NASA Langley Research Center's HZETRN particle transport code. This new code is used to calculate the energy spectra of the neutron fluence in several different materials. The multigroup method is found to be an efficient way of calculating low energy neutron fluence in multiple atom materials as well as single atom materials. Comparisons to results produced by Monte Carlo methods show that the straight ahead multigroup method is accurate for larger depths but less accurate for small depths due to leakage at the boundary. For this reason, an improved multigroup method is created which propagates neutrons in two directions, forward and backward approximately accounting for the isotropic distribution of the evaporation source. This new multigroup method compares well with the Monte Carlo method at all depths. For this reason, the multigroup method is considered an accurate method which is highly computationally efficient for calculating low energy neutron fluence.