Integral cross section of isomeric state formation in (neutron,nucleon) reactions using an Am–Be source

Abstract

The present work targeted the study of the isomeric state formation in (n,n′) and (n,p) reactions using the spectral emission of an 241Am–Be neutron source which favors the (neutron, nucleon) reactions over the (n, γ) and (n,2n) ones. The integral cross section of the isomeric nuclear states in the neutron-induced nucleon-emission reactions of fast neutrons on 115In, 199Hg, 58Ni, and 195Pt was measured experimentally. The monitor reaction of neutrons was the 115In(n,n′)115mIn using the evaluated integral cross section of 269−15+13 mb, which was confirmed to have a constant isomeric ratio in the energy range between ∼1.6 MeV and ∼10 MeV. Integral cross sections for the 199Hg(n,n′)199mHg, 58Ni(n,p)58mCo, 58Ni(n,p)58m+gCo and 195Pt(n,n′)195mPt reactions were found to be 449 ± 13 mb, 112 ± 34 mb, 343 ± 72 mb and 740 ± 55 mb, respectively. Each experimental value was based on independent measurements of samples irradiated with a constant neutron flux from an 241Am–Be source (having standard emission probability). The experimental results are compared with the integrated cross sections from a new evaluation for neutron induced reactions weighted to standard neutron emission probability distribution. Integral values estimated from theoretical calculations and evaluated libraries were used for the examination of our results. Results confirmed that formation of isomeric states is sensitive to the neutron energy distribution. The behavior of the isomeric ratio of neutron reaction leading to isomeric state formation was found to be useful in order to distinguish between reactions suitable for use as monitor ones (yielding a constant isomeric ratio throughout the emission energy range) and those used to probe the disturbance in the neutron spectral distribution.