Abstract
A new prompt fission neutron spectra (PFNS) measurement in the 238U(n,f) reaction was performed at LANSCE/WNR facility. Evaluated data show discrepancies on the low (below 1 MeV) and high (above 5 MeV) energy parts in the PFNS for different major and minor actinides. The goal is to improve these measurements in a wide range of incident energy. The energy of the incoming neutron, inducing the fission, and the prompt neutron energies, are measured by time-of-flight method. A dedicated fission chamber was developed, in order to improve alpha-fission discrimination, timing resolution, actinide mass, and to reduce the amount of neutron scattering. To detect prompt neutrons, the 54 Chi-Nu scintillator cells array were surrounding the fission chamber. High statistics were recorded during this experiment, allowing a precise study of PFNS behavior as a function of incident neutron energy, from 1 MeV to 200 MeV. This experiment also showed that all the new tools developed to improve PFNS measurements are performing. Therefore, measurements of PFNS with others actinides such as 239Pu are planned.
Highlights
The energy spectrum of prompt neutron emitted in neutron-induced fission (PFNS) is a quantity of high interest, for instance, for reactor physics, global security and fundamental understanding of the fission process
Recent publications [1, 2] point out the needs on new experimental data to produce new prompt fission neutron spectra (PFNS) evaluations with uncertainties for actinide nuclei in accordance with the Coordinated Research Project (CRP) “Evaluation of Prompt Fission Neutron Spectra of Actinides” established by the IAEA Nuclear Data Section in 2009 [3]
But containing only one deposit of 252Cf was used for neutron detection efficiency measurement purpose, assuming that the PFNS of 252Cf(sf) is well known and recommended as a evaluated standard [12]
Summary
The energy spectrum of prompt neutron emitted in neutron-induced fission (PFNS) is a quantity of high interest, for instance, for reactor physics, global security and fundamental understanding of the fission process. The experimental data are rather scarce, for the high energy neutron induced fission reactions. New dedicated tools (fission chamber, electronics, data acquisition system) were developed and validated through new PFNS measurements. The PFNS are obtained with a time-of-flight measurement using the signals of fission chamber and scintillator. By using a pulsed beam, it is possible to measure fission by fission, the time-of-flight of the incoming neutron and its energy. This technique is called the double time-of-flight method, as illustrated on Fig. 1.
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