Neutron Evaporation Research Articles

Neutron-emission mechanisms in the 115In(α, xn) reactions

Spectra and angular distributions of neutrons from the 115In(α, xn) reactions were measured at α-particle energies of 16, 18, 27, and 45 MeV. The measurements were performed with time-of-flight neutron spectrometers at pulsed accelerators of charged particles. The data obtained in this way were analyzed within the models of equilibrium, preequilibrium, and direct reaction mechanisms. The exact formalism of Hauser-Feshbach statistical theory was used in the calculations, level densities in residual nuclei excited in the reactions under consideration being found from the neutron evaporation spectra in the (p, n) reactions on tin isotopes. Contributions from equilibrium, preequilibrium, and direct neutron emission were studied in a wide range of α-particle energies.

Prompt fission neutron multiplicity and spectrum calculations for thermal and fast neutron induced reactions on 233,231Pa nuclei

The prompt fission neutron multiplicity and spectra of 233Pa(n, f) and 231Pa(n, f) were calculated for incident neutron energies up to 50 MeV and 30 MeV, respectively using refined neutron evaporation models. Those models have been already successfully applied to calculate the prompt neutron multiplicities and spectra for many actinides. The unique experimental data concerning the fission fragment mass distribution and total kinetic energy for the 234,233,232Pa fissioning nuclei were exploited in the frame of the Point-by-Point treatment. The total average prompt neutron multiplicity, the fission fragment pair multiplicity and the multiplicity dependence on fragment mass (sawtooth) for the reactions 233,232,231Pa(n th, f) and the average values of the model parameters for 234-232Pa fissioning nuclei were calculated. For the other Pa fissioning nuclei involved in the studied reactions relevant parameters were established following systematic trends. Due to a total lack of experimental prompt fission neutron multiplicity and spectrum data for Pa isotopes, the present results were compared with systematic trends following from neighboring actinides. The calculated prompt fission neutron multiplicity as well as the prompt γ-ray energy of the 233,231Pa(n, f) reactions are obtained in very good agreement with all systematic trends.

Francium sources and traps for fundamental interaction studies

Francium is one of the best candidates for atomic parity nonconservation (APNC) and for the search of permanent electric dipole moments (EDMs). APNC measurements test the weak force between electrons and nucleons at very low momentum transfers. They also represent a unique way to detect weak nucleon-nucleon interactions. EDMs are instead related to the time-reversal symmetry. Preliminary to these fundamental measurements are precision studies in atomic spectroscopy and the development of magneto-optical traps (MOT), which partially compensate for the lack of stable Fr isotopes. At LNL Legnaro, francium is produced by fusion of 100-MeV 18O with 197Au in a thick target, followed by evaporation of neutrons from the compound nucleus. Francium diffuses inside the hot target (1200 K) and is surface ionized for injection at 3 keV in an electrostatic beamline. Typically, we produce 1×106 (210Fr ions)/s for a primary flux of 1.5×1012 particles/s. We have studied Fr yields as a function of primary beam energy, intensity, and target temperature. Information on the efficiency of bulk diffusion, surface desorption and ionization is deduced. The beam then enters a Dryfilm-coated cell, where it is neutralized on a heated yttrium plate. The escape time of neutral Fr (diffusion + desorption) is approximately 20 s at 950 K, as measured with a dedicated setup. In the MOT, we use 6 orthogonal Ti:sapphire laser beams for the main pumping transition and 6 beams from a stabilized diode repumper. Fluorescence from trapped atoms is observed with a cooled CCD camera, in order to reach noise levels from stray light equivalent to approximately 50 atoms. Systematic tests are being done to improve the trapping efficiency. We plan to further develop Fr traps at LNL; in parallel, we will study APNC and EDM techniques and systematics with stable alkalis at Pisa, Siena, and Ferrara.

Pre-compound neutron evaporation in low energy heavy ion fusion reactions

Inclusive and exclusive neutron evaporation spectra have been studied from the fusion reactions at 80 MeV 12C on 46Ti and 131 MeV 31P on 27Al, populating the compound nucleus 58Ni at the excitation energy of 79.5 MeV but in different angular momentum ranges. The evaporation residues 53Fe, 55Fe and 56Co were identified by their characteristic γ-rays. The measured energy spectra of evaporated neutrons were compared with the predictions from statistical-model calculations. In case of the asymmetric system 12C + 46Ti, the neutron spectra are well reproduced, when compared with the results from statistical-model calculations with level density parameter a = A / 8 MeV −1 . On the contrary, the experimental spectra for the symmetric system 31P + 27Al are found to be harder than the theoretical predictions with a = A / 8 MeV −1 . In this case, a lower value of the level density parameter a = A / 10 MeV −1 seems to be required to reproduce the experimental results. The dependence of the energy spectra on the entrance channel is taken as an indication that, in case of the symmetric system, the neutrons are in part emitted from a temperature equilibrated di-nuclear complex at a higher temperature before the formation of the compound nucleus.

Bulk properties of iron isotopes

Nuclear level densities and radiative strength functions (RSFs) in 56Fe and 57Fe were measured using the 57Fe(3He, αγ) and 57Fe(3He, 3He′γ) reactions, respectively, at the Oslo Cyclotron Laboratory. A low-energy enhancement in the RSF below 4-MeV energy was observed. This finding cannot be explained by common theoretical models. In a second experiment, two-step cascade intensities with soft primary transitions from the 56Fe(n, 2γ) reaction were measured. The agreement between the two experiments confirms the low-energy enhancement in the RSFs. In a third experiment, the neutron evaporation spectrum from the 55Mn(d, n)56Fe reaction was measured at 7-MeV deuteron energy at the John Edwards Accelerator Laboratory at Ohio University. Comparison of the level density of 56Fe obtained from the first and third experiments gives an overall good agreement. Furthermore, observed enhancement for soft γ rays is strengthened by the last experiment.

Nuclear-charge polarization at scission in the 12 MeV proton-induced fission of 232Th

The most probable charges of secondary fragments, produced after neutron evaporation from primary fragments, have been evaluated using fractional cumulative and mass yields in the 12MeV proton-induced fission of 232Th . The nuclear-charge polarization of primary fragments at scission has been obtained by correcting the most probable charge of secondary fragments for neutron evaporation. The fragment mass dependence of the nuclear-charge polarization at scission shows good agreement with that for thermal neutron-induced fission of 235U , indicating that the nuclear-charge polarization is nearly insensitive to mass and excitation energy of the fissioning nucleus for asymmetric fission in the actinide region.

Excitation functions for complete fusion and transfer reactions in the interaction of 6He with 197Au nuclei

The excitation functions for the fusion reactions with subsequent neutron evaporation, 197Au(6He, xn)203 − xnTl (x = 2−7), and the neutron transfer reactions with production of the 196Au and 198Au isotopes have been measured. Unusually high cross section of the 198Au isotope production at energies below the Coulomb barrier has been observed. Possible mechanisms of production and decay of the transfer reaction products are considered.

Some peculiarities in the interaction of 6He with 197Au and 206Pb

Excitation functions were measured for fusion followed by the evaporation of neutrons in the reactions 206Pb(6He, 2n)210Po and 197Au(6He, xn)203−xnT1, where x = 2−7, as well as for the transfer reactions on a 197Au target with the formation of the 196Au, 198Au, and 199Au isotopes. The experiment was carried out at the Dubna Radioactive Ion Beams (DRIBs) complex of FLNR, JINR. The 6He beam intensity was about 5 × 106 pps, the maximum energy being (60.3 ± 0.4) MeV. A significant increase in the cross section was observed below the Coulomb barrier for the fusion reaction with the evaporation of two neutrons compared to statistical model calculations. Unusual behavior for the production of 198Au is observed, whereas the cross section for the formation of 199Au is very low. The analysis of the data in the framework of the statistical model for the decay of excited nuclei, which took into account the sequential fusion of 6He, has shown good agreement between the experimental and calculated values of the cross sections for the case of sub-Coulomb-barrier fusion in the 206Pb + 6He reaction.

Fissility of actinide nuclei by 60-130 MeV photons

Nuclear fissilities obtained from recent photofission reaction cross section measurements carried out at Saskatchewan Accelerator Laboratory (Saskatoon, Canada) in the energy range 60-130 MeV for 232Th, 233U, 235U, 238U, and 237Np nuclei have been analysed in a systematic way. To this aim, a semiempirical approach has been developed based on the quasi-deuteron nuclear photoabsorption model followed by the process of competition between neutron evaporation and fission for the excited nucleus. The study reproduces satisfactorily well the increasing trend of nuclear fissility with parameter Z2/A.

Level density ofFe56and low-energy enhancement of γ-strength function

The $^{55}$Mn$(d,n)^{56}$Fe differential cross section is measured at $E_d=7$ MeV\@. The $^{56}$Fe level density obtained from neutron evaporation spectra is compared to the level density extracted from the $^{57}$Fe$(^3$He,$\alpha\gamma)^{56}$Fe reaction by the Oslo-type technique. Good agreement is found between the level densities determined by the two methods. With the level density function obtained from the neutron evaporation spectra, the $^{56}$Fe $\gamma$-strength function is also determined from the first-generation $\gamma$ matrix of the Oslo experiment. The good agreement between the past and present results for the $\gamma$-strength function supports the validity of both methods and is consistent with the low-energy enhancement of the $\gamma$ strength below $\sim 4$ MeV first discovered by the Oslo method in iron and molybdenum isotopes.

Energy dependence of nuclear charge distribution in neutron induced fission ofZ-even nuclei

For the first time the distribution of nuclear charge of fission products with mass numbers 87, 88, 89, 91, 93, 94, 95, 137, 138, 139, and 140 and their complementary products have been studied for neutron induced fission of $^{235}\mathrm{U}$ and $^{239}\mathrm{Pu}$ in the energy range from thermal up to 1.2 MeV. The energy dependences of the cumulative yields of $^{87}\mathrm{Br}$, $^{88}\mathrm{Br}$, $^{89}\mathrm{Br}$, $^{91}\mathrm{Br}$, $^{93}\mathrm{Kr}$, $^{94}\mathrm{Rb}$, $^{95}\mathrm{Rb}$, $^{137}\mathrm{I}$, $^{138}\mathrm{I}$, $^{139}\mathrm{I}$, and $^{140}\mathrm{I}$ have been obtained by delayed neutron measurements. The most probable charge ${Z}_{P}(A)$in the appropriate isobaric \ensuremath{\beta}-decay chains was estimated. The results were analyzed in terms of the deviation $\ensuremath{\Delta}{Z}_{P}({A}^{'})$ of the most probable charge of isobaric \ensuremath{\beta}-decay chains from the unchanged charge distribution before prompt neutron emission (nuclear charge polarization) and they are compared with experimental data of other authors and with predictions from Nethaway's ${Z}_{P}$-formula and Wahl's ${Z}_{P}$-model. We show that the nuclear charge polarization of primary fission fragments $\ensuremath{\langle}\ensuremath{\Delta}{Z}_{P}({A}^{'})\ensuremath{\rangle}$ before prompt neutron evaporation decreases as the excitation energy of the compound nucleus increases. This decrease is more pronounced for fission of $^{235}\mathrm{U}$. The energy dependencies of $\ensuremath{\Delta}{Z}_{P}({A}^{'})$ and $\ensuremath{\Delta}{Z}_{P}({Z}_{P})$ obtained in the present work show an attenuation of the odd-even effects in the charge distribution as the excitation energy of the compound nucleus increases.

Nuclear-level densities around Z = 50 from neutron evaporation spectra in (p, n) reactions

Neutron excitation functions, spectra, and angular distributions in the (p, n) reactions on the isotopes 116Sn, 118Sn, 122Sn, and 124Sn were measured in the proton-energy range 7–11 MeV. The measurements were performed by the time-of-flight method with the aid of a fast-neutron spectrometer at the EGP-15 pulsed tandem accelerator of the Institute of Physics and Power Engineering (Obninsk). A high resolution (about 0.6 ns/m) and a high stability of the time-of-flight spectrometer made it possible to identify reliably low-lying levels along with the continuous section of the neutron spectra. The data obtained in this way were analyzed on the basis of the statistical equilibrium and preequilibrium models of nuclear reactions. The respective calculations were performed with the aid of the precise Hauser-Feshbach formalism of statistical theory. The nuclear-level densities in the isotopes 116Sb, 118Sb, 122Sb, and 124Sb were determined, along with their energy dependences and model parameters. In the excitation-energy range 0–2 MeV, the energy dependence of the nuclear-level densities exhibits a structure that is associated with the shell inhomogeneities of the spectrum of single-particle states near filled shells. The isotopic dependence of the nuclear-level density is discovered and explained. It is also shown that the data obtained here for the nuclear-level density differ markedly from the predictions of model systematics of nuclear-level densities.

Influence of angular momentum on fission fragment mass distribution: Interpretation within Langevin dynamics

Dependence of fission fragments mass distribution on the angular momentum within Langevin dynamics is studied. The calculations are performed in the framework of the rotating temperature-dependent finite-range liquid drop model. The calculations are done for the five nuclei, representing heavy fissioning nuclei, medium fissioning nuclei and light fissioning one with the angular momentum varied in the wide range from l = 0 to l = 70 ℏ . The dependence coefficients d σ M 2 / d l 2 for the investigated nuclei are extracted. The comparison of the extracted values with the experimental data reveals a good agreement for all the cases (the heavy, medium, and light fissioning nuclei). It is found out that the obtained dependence of σ M 2 on l can be explained with the help of temperature at scission as a function of l. The latter dependence is determined by dependence of the mean prescission neutron multiplicity on l. The analysis of this dependence is done as a competition between fission process and neutron evaporation. “Remembering of the former large fluctuations of mass asymmetry coordinate during descent from the saddle to scission” is considered. It is shown that the “remembering effect” takes place, but does not play a crucial role for the investigated dependence of σ M 2 on l.

A new view to the structure of 19C

The observation of longitudinal momentum distribution (P||) from two-neutron removal in 19C with a Be target at 64 A MeV is reported. Analysis in terms of Glauber model considering 19Cgs( Jπ = 1/2+) shows that neutron evaporation is necessary to explain the data.

Influence of multichance fission on fragment angular anisotropy in the 232Th( [formula omitted]) and 238U( [formula omitted]) reactions at intermediate energies

Fission fragment angular distributions have been measured for the 232Th( n , f ) and 238U( n , f ) reactions in the neutron energy range 20–100 MeV. The fragment angular anisotropy for 232Th was found to be systematically larger than that for 238U. The obtained results have been analyzed in the framework of the statistical saddle-point model combined with pre-equilibrium and Hauser–Feshbach calculations of partial fission cross sections. The calculations have revealed that fission following multiple neutron emission takes place in both reactions, resulting in a considerable contribution of high chances to the total fission fragment angular anisotropy. This gives grounds to expect that the observed difference is mainly due to nuclei fissioning at the end of the neutron evaporation chain.

A model for fitting peaks induced by fast neutrons in an HPGe detector

Inelastic neutron scattering in the HPGe detector produces wide, triangular-shaped peaks in the spectrum. We develop an accurate model for the peak shape and show that the inclusion of the model in the gamma spectrum analysis makes it possible to quantify fast neutron scattering in the Ge crystal and improves the estimation of the baseline. This in turn facilitates the detection of fission products present at trace levels in environmental samples. The model, together with simulations, is used to deduce some properties of the underlying neutron energy distribution. The neutron evaporation temperature of 1.1 MeV is obtained from the analysis of environmental monitoring gamma spectra.

Anomalous behavior of the level density parameter in neutron and charged particle evaporation

The compound nucleus $^{76}\mathrm{Kr}*$ was populated at the excitation energy of $75\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ and angular momentum of $39\ensuremath{\hbar}$ in fusion reactions with two complementary, mass-symmetric $(^{31}\mathrm{P}+^{45}\mathrm{Sc})$ and mass-asymmetric $(^{12}\mathrm{C}+^{64}\mathrm{Zn})$ entrance channels. The neutron evaporation spectra were measured and compared with the predictions of statistical model calculations using the transmission coefficients for the spherical nuclei in the inverse absorption channel, the rotating liquid drop model moment of inertia, and the normal systematic value of $A∕8$ for the level density parameter $a$. The results for the mass-asymmetric reaction are found to be consistent with the predictions of the statistical model calculations. However, for the mass-symmetric reaction $(^{31}\mathrm{P}+^{45}\mathrm{Sc})$, the experimental spectra are found to be harder than the theoretical neutron spectra and the statistical model calculations require a lower value of $A∕10$ for the parameter $a$ to reproduce the shape of the experimental spectra, indicating the neutron to be evaporated at higher temperature for the same excitation energy and angular momentum in symmetric system. According to the dynamical model, the formation time $(37\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}22}\phantom{\rule{0.3em}{0ex}}\mathrm{sec})$ of the compound nucleus for the symmetric $^{31}\mathrm{P}+^{45}\mathrm{Sc}$ system is significantly higher than that $(29\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}22}\phantom{\rule{0.3em}{0ex}}\mathrm{sec})$ for the asymmetric $^{12}\mathrm{C}+^{64}\mathrm{Zn}$ system. This may probably lead to the formation of a temperature-equilibrated dinuclear complex that may be responsible for neutron emission at higher temperature in the case of the symmetric system.

Feature Article: On the Discovery of Superheavy Elements

Remarkable progress was made in recent years in the field of Superheavy Element (SHE) research. Figure 1 gives an overview of isotopes presently (November 2004) known or under investigation at the upper end of the chart of nuclei. The isotopes are marked in different gray nuances indicating _ decay, β- decay, β+ decay or electron capture, and spontaneous fission. Also given are the measured half-lives. Dots and circles mark Compound Nuclei (CN) formed in complete fusion reactions. From the CN the evaporation residues which are detected in the experiments emerge by evaporation of neutrons.

Monte Carlo simulations of the Neutron Wall detector system

A Monte Carlo simulation of the Neutron Wall detector system has been performed using GEANT4. Mechanisms for the detection of neutrons in an aggregate of liquid scintillator detectors have been investigated in details, with the main aim to define optimum conditions for the detection and identification of multiple neutrons. Special emphasis was put on studying the scattering of neutrons between different detectors, which is the main source of the apparent increase of the number of detected neutrons, and hinders the selection of events with more than one emitted neutron. The results of the simulations were compared to experimental data obtained at the EUROBALL spectrometer using fusion–evaporation reactions. The quality of γ-ray spectra gated by 2 neutrons was improved 40 times, in terms of the relative intensity of γ-ray lines from nuclei produced with the emission of 2 and 1 neutrons. A similar increase of the relative γ-ray intensity was achieved in case of lines associated with the emission of 3 neutrons, with respect to lines from both 2 and 1 neutron evaporation channels. The improvements were only possible with a significant reduction of the intensity of the true 3 neutron emission γ-ray lines. The influence of small amounts of γ-rays mis-interpreted as neutrons was studied. It was found that it dramatically reduces the quality of the reduction of the effects due to scattered neutrons.

Prompt fission neutron multiplicity and spectrum model for 30–80 MeV neutrons incident on 238U

The improved Los Alamos model is developed for the first time in order to provide prompt fission neutron multiplicity, prompt fission neutron spectra and other quantities at high incident neutron energies where the fission of secondary compound nuclei formed by charged particle emission occurs. In this model (exemplified by the n+ 238U reaction up to 80 MeV incident energy) the fission of the secondary nuclei formed by proton emission, neutron evaporation from the nuclei formed by proton emission, deuteron emission, alpha emission and neutron evaporation from the nuclei formed by alpha emission is taken into account. Input model parameters and related excitation energy dependences are determined using available experimental information and systematics as well as total and partial neutron induced fission cross-sections and their ratios obtained separately from a recent evaluation performed up to medium energies. Our present model predictions are in good agreement with the measured prompt neutron spectra and multiplicities.