Neutron Induced Fission Of Actinides Research Articles

New developments of a fission chamber for very high radioactivity samples

A new fission chamber was developed and built for prompt-fission-neutron spectra (PFNS) measurements in neutron-induced fission of actinides, and is described in Taieb et al. (2016). It allowed us to minimize incident- and outgoing-neutron scattering, and optimize both time resolution and alpha to fission fragment discrimination. The fission chamber was validated and used for 238U and 252Cf PFNS measurements. In order to measure PFNS of samples with a natural alpha activity of the order of 10 MBq, further improvements to the detector were done and are presented in this work. Tests of the improved fission chamber lead to a fission fragment detection efficiency of 91% for a 14.41 MBq 240Pu deposit, and 99% for a 247 kBq 242Pu deposit. The fission chamber was successfully used for 239Pu PFNS measurements at the Weapons Neutron Research facility of the Los Alamos Neutron Science Center (WNR@LANSCE).

Neutron-neutron correlations in the photofission of U238

Background: In the fission of actinides, the nearly back-to-back motion of the fission fragments has a strong effect on the kinematics of fission neutrons. This leads to a favoring of opening angles near ${0}^{\ensuremath{\circ}}$ and ${180}^{\ensuremath{\circ}}$ in the neutron-neutron $(n\text{\ensuremath{-}}n)$ opening angle distribution of correlated neutron pairs from the same fission event.Purpose: To measure the $n\text{\ensuremath{-}}n$ opening angle and energy correlations in the photofission of $^{238}\mathrm{U}$. As of this writing, measurements of correlated $n\text{\ensuremath{-}}n$ opening angle distributions have been reported only for the spontaneous and neutron-induced fission of actinides. This work is the first to report such a measurement using photofission and will provide useful experimental input for photofission models used in codes such as Monte Carlo N-Particle (MCNP) and the Fission Reaction Event Yield Algorithm (FREYA).Methods: Fission is induced using bremsstrahlung photons produced via a low-duty-factor, pulsed, linear electron accelerator. The bremsstrahlung photon beam impinges on a $^{238}\mathrm{U}$ target that is surrounded by a large neutron scintillation detection system capable of measuring particle position and time of flight, from which the $n\text{\ensuremath{-}}n$ opening angle and energy are measured. Neutron-neutron angular correlations are determined by taking the ratio between a correlated $n\text{\ensuremath{-}}n$ distribution and an uncorrelated $n\text{\ensuremath{-}}n$ distribution formed by the pairing of neutrons produced during different beam pulses. This analysis technique greatly diminishes effects due to detector efficiencies, acceptance, and experimental drifts.Results: The angular correlation of neutrons from the photofission of $^{238}\mathrm{U}$ shows a high dependence on neutron energy as well as a dependence on the angle of the emitted neutrons with respect to the incoming photon beam. Angular correlations were also measured using neutrons from the spontaneous fission of $^{252}\mathrm{Cf}$, showing good agreement with past measurements.Conclusions: The measured angular correlations reflect the underlying back-to-back nature of the fission fragments. An anomalous decline in $n\text{\ensuremath{-}}n$ yield was observed for opening angles near ${180}^{\ensuremath{\circ}}$ for $^{238}\mathrm{U}$.

Measurement of the angular distribution of fission fragments using a PPAC assembly at CERN n_TOF

A fission reaction chamber based on Parallel Plate Avalanche Counters (PPACs) was built for measuring angular distributions of fragments emitted in neutron-induced fission of actinides at the neutron beam available at the Neutron Time-Of-Flight (n_TOF) facility at CERN. The detectors and the samples were tilted 45° with respect to the neutron beam direction to cover all the possible values of the emission angle of the fission fragments. The main features of this setup are discussed and results on the fission fragment angular distribution are provided for the 232Th(n,f) reaction around the fission threshold. The results are compared with the available data in the literature, demonstrating the good capabilities of this setup.

Angular distribution in the neutron-induced fission of actinides

Fission 2013 – Fifth International Workshop on Nuclear Fission and Fission Product Spectroscopy

Measurement of Fragment Mass Yields in Neutron-Induced Fission of 232Th and 238U at 33, 45, and 60 MeV

Incineration of spent nuclear fuel in Accelerator Driven Systems requires nuclear data on neutron-induced fission of actinides at energies above 20 MeV - the upper limit of conventional evaluated nuclear data files. Over the past years, a significant effort has been devoted to measurements of neutron-induced fission cross-sections at intermediate energies but there is a lack of experimental data on fission yields. Here we describe recent measurements of pre-neutron emission fragment mass distributions from intermediate energy neutron-induced fission of (232)Th and (238)U. The measurements have been carried out at the neutron beam of the Louvain-la-Neuve cyclotron facility CYCLONE. Quasi-monoenergetic neutrons with peak energies at 32.8, 45.3 and 59.9 MeV were produced by protons impinging on a thin lithium target. A multi-section Frisch-gridded ionization chamber was used as a fission fragment detector. The measurement results are compared with available experimental data as well as with predictions of the multi-modal random neck-rupture model implemented into the TALYS code.

Development of the track registration technique in solid state nuclear track detectors from solution media and its applications

The development of the track registration technique in solid state nuclear track detectors from solution media, its special features, advantages and applications in nuclear science and technology are reviewed. Experimental procedures for sample preparation, track recording, etching and evaluation are described. Special precautions necessary for proper utilization of the technique and various aspects of track registration from solution media are discussed. A brief account of its applications in the analysis of nuclear materials in diverse matrices, monitoring of neutron fluence and measurement of absolute fission yields in the spontaneous and neutron-induced fission of actinides is given. Suggestions for possible further work in this area are indicated.