Neutron Capture Probabilities for Thick Samples at Resonance Energies

Capture and scattering probabilities for neutrons impinging on thick samples were measured by the Japan Atomic Energy Research Institute Linac time-of-flight spectrometer and were compared with those by Monte Carlo calculation. Sweeping the incident neutron energy, the capture probability shows peaks at resonance energies in the case of a thin sample, whereas it shows dips for a thick sample, i.e., saturation occurs just at resonance energies. This saturation phenomenon is analyzed by Monte Carlo calculation for a distribution of path lengths of incident neutrons in the sample until capture in the sample. The saturation values of capture probability at resonance energies P_co are defined, and their dependence on the resonance parameters Γ_n/Γ is examined. The relations between P_co and Γ_n/Γ, with parameters including recoil energy, are obtained by Monte Carlo calculation. The relations are verified by measurement of P_co for many resonances of various Γ_n/Γ values. With the relation, Γ_n/Γ can be determined from Pco, which is not sensitive to sample thickness.