Response of Moxon-Rae type gamma detectors for neutron capture cross section measurements

A detector devised by Moxon and Rae for the absolute measurement of (n,γ) cross sections is briefly described. This detector is supposed to have an efficiency per MeV of γ-ray energy independent of the energy of the γ-rays. Such a detector consists of an electron converter placed before a thin plastic scintillator which detects the electron emitted by interaction of the γ-ray in the converter. The performance of this type of detector depends on the thickness and composition of the converter. Detailed Monte-Carlo calculations of the response for γ-ray energies from 0.2 to 12 MeV has been carried out for elements ranging from C to Bi and for a mixture of elements as well as for a mixture of an element plus compound, to find out the suitable material and thickness of the converter. Among the elements studied for the converter, Ni, Mo and Sn have a uniform response over the photon energy range 1-12 MeV. Out of these elements Mo has a low neutron capture cross section in the energy range 1-1000 keV and is thus to be preferred. A mixture of C + Bi2O3 in the weight ratio 11.6 : 88.4 gives a uniform response over the photon energy range 1-12 MeV. (K.B.)