Level Densities in the actinide region and indirect n,y cross section measurements using the surrogate method

¹ Institut de Physique Nucléaire d'Orsay, Bât 100, 15 rue G. Clémenceau, 91406 Orsay, France
² DSM/IRFU, CEA Saclay, Bât 141 , F-91191 Gif-sur-yvette Cedex, France
³ University of Oslo, Department of Physics, P.O. Box 1048, Blindern 0316 Oslo, Norway
⁴ Lawrence Livermoore National Laboratory, 7000 East Avenue, Livermore, CA 94550-9234, USA
⁵ Themba LABS, P.O. Box 722, 7129 Somerset West, South Africa

^a e-mail: wilson@ipno.in2p3.fr

Abstract

Results from a program of measurements of level densities and gamma ray strength functions in the actinide region are presented. Experiments at the Oslo cyclotron involving the Cactus/Siri detectors and ²³²Th(d,x) and ²³²Th(³He,x) reactions were carried out to help answer the question of which level density model is the most appropriate for actinide nuclei, since it will have an impact on cross section calculations important for reactor physics simulations. A new technique for extracting level densities and gamma ray strength functions from particle-gamma coincidence data is proposed and results from the development of this technique are presented.

In addition, simultaneous measurements of compound nuclear gamma decay probabilities have been performed for the key thorium cycle nuclei ²³³Th, ²³¹Th and ²³²Pa up to around 1MeV above the neutron binding energy and have enabled extraction of indirect neutron induced capture cross sections for the ²³²Th, ²³¹Pa and ²³⁰Th nuclei using the surrogate reaction method. Since the neutron capture cross section for ²³²Th is already well known from direct measurements a comparison provides a stringent test of the applicability of the surrogate technique in the actinide region.