Statistical study of the prompt-fission γ -ray spectrum for U238(n, f ) in the fast-neutron region

Abstract

Prompt-fission γ -ray spectra (PFGS) have been measured for the 238 U(n, f ) reaction using fast neutrons produced by the LICORNE directional neutron source. Fission events were detected with an ionization chamber containing actinide samples placed in the neutron beam, and the coincident prompt-fission γ rays were measured using a number of LaBr 3 scintillation detectors and a cluster of nine phoswich detectors from the PARIS array. Prompt-fission γ rays (PFGs) were discriminated from prompt-fission neutrons using the time-of-flight technique over distances of around 35 cm. PFG emission spectra were measured at two incident neutron energies of 1.9 and 4.8 MeV for 238 U(n, f )andalsofor 252 Cf (sf ) as a reference. Spectral characteristics of PFG emission, such as mean γ multiplicity and average total γ -ray energy per fission, as well as the average γ -ray energy, were extracted. The sensitivity of these results to the width of the time window and the type of spectral unfolding procedure used to correct for the detector responses was studied. Iteration methods were found to be more stable in low-statistics data sets. The measured values at E n = 1.9 MeV were found to be the mean γ multiplicity M γ = 6.54 ± 0.19, total released energy per fission E γ,tot = 5.25 ± 0.20 MeV, and the average γ -ray energy ǫ γ = 0.80 ± 0.04 MeV. Under similar conditions, the values at E n = 4.8 MeV were measured to be M γ = 7.31 ± 0.46, E γ,tot = 6.18 ± 0.65 MeV, and ǫ γ = 0.84 ± 0.11 MeV.

Prompt $$\gamma $$-ray characteristics from $$^{235}$$U(n, f) at $${\overline{E}}_{n}$$ = 1.7 MeV

“…This was discussed in Ref. [4], depending on the chosen unfolding technique and the number of recorded fission events. In Eq.…”

“…in Ref. [4]. The results for the average multiplicity M γ , the average total γ -ray energy E γ,tot , and the average energy per γ ray γ are given in the first column of Table 1, together with corresponding values in column three from another recent experiment on the same fissioning system, but induced by thermal neutrons [9].…”

“…One of the prerequisites mentioned above was the recent measurement of prompt γ rays from the reaction 238 U(n, f) at E n = 1.9 MeV [4]. Taking into account that the average promptfission γ -ray multiplicities for this reaction agree well within 1% for incident neutrons of 1.7 and 1.9 MeV kinetic energy, which may be inferred from the calculations in Ref.…”

“…In the very first experiment ever performed at the directional fast-neutron source LICORNE of IPN Orsay [1] some years ago, prompt-fission γ -ray spectra (PFGS) were measured with 14 hexagonal BaF 2 and three coaxial LaBr 3 :Ce scintillation detectors, which were calibrated in energy with different sources over an energy range between 121 keV and 4. 4 MeV [2]. The γ rays were produced in fission reactions induced by neutrons of average energy E n = 1.7 MeV hitting 235 U and 238 U samples of 10 mg approximate mass each, placed back-to-back at the central cathode position inside a cylindrical twin Frisch-grid ionization chamber.…”

The Los Alamos fission yield evaluation pipeline

“…Since this time, remarkable advancement has been made in theoretical modeling of fission [4][5][6][7] and measurements campaigns have produced modern data over a range of excitation energies, see e.g. [8][9][10]. Our novel evaluation effort seeks to combine these recent advances, focusing on the description of FPY, to independent (IFY) and cumulative (CFY) fission yields that take into account prompt and delayed emission consistent with decay data [11,12].…”