Neutron-induced capture cross sections of short-lived actinides with the surrogate reaction method

M Aïche,N Pillet,E Bauge,F Gunsing,O Sérot,G Boutoux,L Gaudefroy,G Barreau,O Roig,S Czajkowski,J Taieb,L Matthieu,B Jurado,V Méot,D Dassie,T Faul,B Haas

doi:10.1051/epjconf/20100206002

Abstract

Determination of neutron-capture cross sections of short-lived nuclei is opening the way to understand and clarify the properties of many nuclei of interest for nuclear structure physics, nuclear astrophysics and particularly for transmutation of nuclear wastes. The surrogate approach is well-recognized as a potentially very useful method to extract neutron cross sections for low-energy compound-nuclear reactions and to overcome the difficulties related to the target radioactivity. In this work we will assess where we stand on these neutron-capture cross section measurements and how we can achieve the short-lived Minor Actinides nuclei involved in the nuclear fuel cycle. The CENBG collaboration applied the surrogate method to determine the neutron-capture cross section of 233Pa (T1/2 = 27 d). The 233Pa (n,γ) cross section is then deduced from the measured gamma decay probability of 234Pa compound nucleus formed via the surrogate 232Th(3He,p) reaction channel. The obtained cross section data, covering the neutron energy range 0.1 to 1 MeV, have been compared with the predictions of the Hauser-Feshbach statistical model. The importance of establishing benchmarks is stressed for the minor actinides region. However, the lack of desired targets led us to propose recently the 174Yb (3He,pγ) reaction as a surrogate reaction for the (n,γ) predetermined benchmark cross section of 175Lu. An overview of the experimental setup combining gamma ray detectors such as Ge and C6D6 in coincidence with light charged particles ΔE-E Telescopes will be presented and preliminary results will be discussed.

Highlights

IntroductionAccording to the surrogate reaction method, this measurement of the fission or capture probability permits to determine the neutron-induced cross section for the nucleus A-1 according to the equation: σdAe−c1ay(En) = σCAN (En).PdAe,ecxapy(E∗)
Neutron-induced data of short-lived nuclei are of great interest in applied science and fundamental physics
In this work we will assess where we stand on these neutron-capture cross section measurements and how we can achieve the short-lived Minor Actinides nuclei involved in the nuclear fuel cycle

Summary

Introduction

According to the surrogate reaction method, this measurement of the fission or capture probability permits to determine the neutron-induced cross section for the nucleus A-1 according to the equation: σdAe−c1ay(En) = σCAN (En).PdAe,ecxapy(E∗). It may be noted here that J,π Pnfeourtmron(E∗, J, π) = 1 and the cross section for the desired reaction takes the simple product form of the equation (2) This hypothesis is known as the WeisskopfEwing approximation and is justified for excitation energies where the decay of the compound-nucleus is dominated by statistical level densities. The use of the Weisskopf-Ewing approximation in the analysis of Surrogate experiments is typically justified a posteriori by comparing the extracted cross sections to the existing ”direct” measurements In this contribution, we focus on capture cross section measurements which, compared to fission, may be more sensitive to the difference of spin-parity distributions populated in neutron-induced and surrogate reactions. Note that the WeisskopfEwing approximation should not be valid when the angular momentum of the CN is not much larger than the spincutoff parameter of the level density distribution, which, for the actinide region, is about 7

Capture cross sections for 233 Pa

Capture cross sections for 175 Lu

Conclusion