Scattering of fast neutrons on238U,average energy and angular distributions of fission neutrons

Angular distributions of neutrons elastically and inelastically scattered from²³⁸U have been measured with a time-of-flight spectrometer at seven incident neutron energies between 1.5 and 5.5 MeV. Inelastic angular distributions for groups of unresolved levels are given for incoming neutron energies of 1.5, 1.9 and 2.3 MeV. The corresponding neutron cross-sections were obtained relative to then-p scattering cross-sections. The average energies and angular distributions of the fission neutrons were extracted from the measured fission neutron spectra at 1.5,1.9 and 2.3 MeV. Cross-section calculations based on a spherical optical model have shown to be inadequate to describe the neutron-nucleus interaction in case of strong nuclear deformation. The experimental reality may be better approached, instead, if the calculations are made using a potential which takes into account the deformation of the target nucleus. Some of the present measurements are interpreted in this theoretical perspective.     

Neutron emission from the reaction 232Th(n, xn′f)

The energy distributions of neutrons accompanying the fission of ²³²Th are measured by the time-of-flight technique at the bombarding-neutron energies of E _n=14.6 and 17.7 MeV. The data obtained in this way are compared with the results of previous investigations. An excess of soft neutrons that was observed in the experimental spectra of neutrons from ²³⁸U fission at E _n=13.2, 14.7, 16.0, and 17.7 MeV in relation to the results of the calculations based on the model of two sources is also present in the spectra for ²³²Th. The discrepancy between the results of the calculations and experimental data disappears as soon as one assumes the presence of a third source that is related to neutron emission from nonaccelerated fragments.     

Mean energies of prompt fission neutrons in the neutron-induced fission of <Superscript>235</Superscript>U at primary energies in the range <Emphasis Type="Italic">E</Emphasis><Subscript>n</Subscript> < 20 MeV

The spectra of neutrons accompanying the induced fission of ²³⁵U target nuclei are described theoretically. It is confirmed that a third neutron source must be introduced in order to reproduce the shape of experimental distributions at high energies of primary neutrons (previously, a third source was used in describing the spectra of neutrons emitted in ²³²Th and ²³⁸U fission). On the basis of experimental results and their analysis, the mean energy of fission neutrons is estimated as a function of the bombarding-neutron energy up to E _n = 20 MeV.     

Photoproduction of high-energy neutrons in thick targets by electrons in the energy range 150 to 270 MeV

Photoneutron spectra with energies greater than 12 MeV produced by electrons incident on a thick lead target have been measured for primary electron energies between 150 and 266 MeV and at a fixed angle of 90 ° to the beam axis. Measurements of the neutron yield have furthermore been performed at a primary energy of 234 MeV as a function of target depth for the same lead target and as a function of the mass number for C, Al, Cu, Cd and Pb targets. The results were obtained with three independent neutron detectors: two proton recoil counters and one time-of-flight set-up. The high-energy regions of the spectra are compared with the predictions of the phenomenological quasi-deuteron model and a satisfactory agreement in shape and scale of the spectra is found. Additional high-energy neutrons from pion reabsorption processes were observed at electron energies of 234 and 266 MeV.     

Angular-distribution neutron-emission spectra of niobium following bombardment by fast neutrons

Neutron-emission spectra at ten angles between 20 and 160 degrees and incident neutron energies of 5.9, 7.1 and 8.4 MeV were measured relative to the neutron field emitted after spontaneous fission of ²⁵²Cf. The angular distribution of inelastically scattered neutrons appears to be essentially isotropic where it can be separated from the dominant elastic scattering peak. Above an excitation energy of 4 MeV the spectra are well described by a Maxwellian temperature distribution.     

Investigation of fast neutron interaction with235U

The angular distribution of neutrons emitted by elastic, inelastic and fission processes on²³⁵U were measured at the incident neutron energies of 1.5, 1.9, 2.3, 4.0, 4.5, 5.0 and 5.5 MeV using nanosecond time-of-flight technique. The differential elastic scattering cross sections and their angular distributions at all the seven energies are presented. The total elastic scattering cross sections, angle and energy integrated cross sections for the inelastically scattered neutrons in energy bands of 200 keV, fission cross sections and the angular distributions of fission neutrons were extracted at 1.5, 1.9 and 2.3 MeV incident neutron energies. The energy distributions of the prompt fission neutrons and of the inelastically scattered neutrons are given at the incoming neutron energies of 1.5, 1.9 and 2.3 MeV; and the average fission neutron energies and the inelastic neutron evaporation temperatures were also evaluated at these energies.     

Angular correlation of neutrons from thermal-neutron fission of 235U

The neutron-neutron angular correlation for neutrons with energies ? 1 MeV emitted in thermal-neutron-induced fission of ²³⁵U has been measured. The correlation could be adequately described by a simple model, assuming isotropic neutron emission in the fully accelerated fragment frames together with a 20% isotropic scission component and using binomial distributions to simulate the neutron multiplicity (instead of the usual gaussian form) for each fission fragment. The results give no indication of enhanced polar emission.     

Interactions of neutrons with239Pu in the energy range between 1.5 and 5.5 MeV

Neutron scattering angular distributions of²³⁹Pu were measured at seven primary neutron energies between 1.5 and 5.5 MeV. Absolute differential scattering cross-section results are plotted and tabulated. Cross section calculations using a central optical model were made and the results were compared with the experimental values. Moreover, nuclear temperatures of the fission and the inelastic neutron evaporation processes were extracted from the measured spectra.     

Aspects of α-emission from the bombardment of 58Ni with 14.7 MeV neutrons

An enriched ⁵⁸Ni target has been bombarded with 14.7 MeV neutrons and the energies and angular distributions of the outgoing α-particles have been measured. A statistical theory analysis has been made which yields a value of 1.022 ± 0.15 MeV for the nuclear temperature and a value of 9.7 ± 0.45 MeV^?1 for the level density parameter of ⁵⁵Fe. The total cross section has also been measured. The results have been compared with previous measurements.     

Dynamics of nuclear interaction in the reaction <Superscript>56</Superscript>Fe(<Emphasis Type="Italic">α</Emphasis>, <Emphasis Type="Italic">xn</Emphasis>)

The spectra and angular distributions of neutrons from the reaction ⁵⁶Fe(α, xn) were measured at the alpha-particle energies of 12.3, 16.3, 18.3, 26.8, and 45.2 MeV. The measurements were performed with time-of-flight fast-neutron spectrometers at pulsed accelerators of charged particles. The measured data were analyzed within models of equilibrium, preequilibrium, and direct reaction mechanisms. The calculations were performed by using an exact formalism of Hauser-Feshbach statistical theory. The required nuclear-level densities in nickel isotopes excited in this reaction were determined on the basis of new experimental data on low-lying levels, neutron resonances, and spectra of evaporated neutrons. The contributions of equilibrium, preequilibrium, and direct neutron emission were studied over a broad range of alpha-particle energies.     

Cumulative fission yield measurements with 14.7 MeV neutrons on 238U

The fission yield data in the 14 MeV energy neutron induced fission of ²³⁸U play an important role in decay heat calculations and generation-IV reactor designs. In order to accurately measure fission product yields (FPYs) of ²³⁸U induced by 14 MeV neutrons, the cumulative yields of fission products ranging from ⁹²Sr to ¹⁴⁷Nd in the ²³⁸U(n, f) reaction with a 14.7 MeV neutron were determined using an off-line γ-ray spectrometric technique. The 14.7 MeV quasi-monoenergetic neutron beam was provided by the K-400 D-T neutron generator at China Academy of Engineering Physics (CAEP). Fission products were measured by a low background high purity germanium gamma spectrometer. The neutron flux was obtained from the ⁹³Nb (n, 2n)^92mNb reaction, and the mean neutron energy was calculated using the cross-section ratios for the ⁹⁰Zr(n, 2n)⁸⁹Zr and ⁹³Nb(n, 2n)^92mNb reactions. With a series of corrections, high precision cumulative yields of 20 fission products were obtained. Our FPYs for the ²³⁸U(n, f) reaction at 14.7 MeV were compared with the existing experimental nuclear reaction data and evaluated nuclear data, respectively. The results will be helpful in the design of a generation-IV reactor and the construction of evaluated fission yield databases.     

Energy deposition distributions caused by fast neutrons in converter type silicon detectors

Polyethylene converter silicon detectors have been developed as part of a dosimetry system for the application in neutron fields. Signals have been investigated which are mainly caused by reactions occurring when neutrons hit the PE converter or the silicon diode directly. In this work, neutron interactions in the components of a PE converter silicon sensor have been calculated to determine energy deposition distributions for neutron energies from 1.2 v MeV to 14.8 v MeV. Experiments in quasi-monoenergetic neutron fields have been performed. For the simulation of the neutron interactions with the detector layers, the GSF neutron interaction code NISD has been applied. The transport of ions has been calculated separately by means of the program TRIM. It has been shown that by the use of these concepts and models, a rather good agreement of measured and computed pulse height distributions can be obtained and, consequently, the response of converter type detectors to neutron radiation can be determined.     

Energy and angular momentum transfers in equilibrium and pre-equilibrium 158Gd(α, xn) reactions

Neutron time-of-flight spectra and γ-ray multiplicities were measured in coincidence with discrete γ-rays for specific exit channels in the ¹⁵⁸Gd(α, xn) reactions at E_α = 70 MeV. The neutron energy spectra and angular distributions were analysed in terms of equilibrium and non-equilibrium decay processes. Significant emission of non-equilibrated neutrons was found, amounting to about 40 %, 25 % and 15 % of the total neutron emission in the 4n, 5n and 6n exit channels, respectively. The corresponding average kinetic energies in this precompound phase were around 12, 8 and 4.5 MeV. The angular momentum carried away by the neutrons was found to be rather constant for all exit channels and on the average about 3? units. It is found that the total energy and angular momentum removed by the neutrons and γ-rays agrees within the experimental errors with the calculated values for the compound nucleus.     

Response of a Si-diode-based device to fast neutrons

Semiconductor devices based on a Si-detector are frequently used for charged particle's detection; one application being in the investigation of cosmic radiation fields. From the spectra of energy deposition events in such devices, the total energy deposited by the radiation in silicon can be derived. This contribution presents the results of studies concerning the response of this type of detector to fast neutrons. First, the spectrum of energy deposition was established in fast neutron radiation fields with average energies from 0.5 to 50 MeV. It was found that these spectra vary significantly with the neutron energy. The comparison with the spectra registered in photon beams permitted an estimation of the part of energy deposited that could be attributed to neutrons. It was found that this part increases rapidly with neutron energy. The possibilities to use this type of detector for neutron detection and dosimetry for radiation protection are analysed and discussed.     

Spectra of solar neutrons with energies of ~10–1000 MeV in the PAMELA experiment in the flare events of 2006–2015

The first results from measuring the spectra of solar neutrons with energies of ~10–1000 MeV in the solar flares of 2006–2015 observed by the PAMELA international space experiment are presented. The PAMELA neutron detector with ³He counters and a moderator with an area of 0.18 m² allows us to estimate the flux of solar neutrons during solar flares. Solar neutrons with energies of ~10–1000 MeV likely occurred in 21 out of the 24 analyzed flares of 2006–2015.     

Diabatic emission of neutrons: A probe for the energy-dissipation mechanism in nucleus-nucleus collisions

The precompound emission of neutrons in central nucleus-nucleus collisions is investigated within the framework of dissipative diabatic dynamics. For ⁹²Mo + ⁹²Mo at bombarding energies between 7.5 and 20 MeV/u the differential neutron multiplicities $\text{d}\text{M}{}_{\mathrm{<mtext>n</mtext>}}\text{d}\text{E}{}_{\mathrm{<mtext>n</mtext>}}$ are estimated from the decay of highly excited diabatic single-particle states. The energy spectra have an almost exponential high-energy tail with effective temperatures up to 10 MeV for 20 MeV/u bombarding energy.     

Polarization of recoil neutrons from single-pion photoproduction off protons in the resonance region

The polarization of the recoil neutrons from the reaction γp→π⁺n was measured at a c.m. angle of 105° for incident photon energies between 675 and 1125 MeV. A scattered π⁺ meson and a recoil neutron were detected in coincidence with a magnetic spectrometer and a neutron polarimeter system using liquid hydrogen. Results are compared with recent phenomenological analyses and the other existing data measured by the double polarization measurement technique.     

Charged particle production on 12C by intermediate energy neutrons

Energy spectra of protons, deuterons and tritons produced on ¹²C by 545 MeV neutrons have been measured at 7 angles between 73° and 165°. The slopes of the invariant cross sections and absolute yields are compared to corresponding proton induced reactions at similar energies and angles.