A spectrometer for 14 MeV neutron capture spectroscopy

Abstract

A telescopic scintillation pair spectrometer for the measurements of energy spectra of gamma rays from the radiative capture of 14 MeV neutrons is described. The main parts of the spectrometer are: a lead foil as a converter of gamma rays to electron-positron pairs, a 12.5 cm dia. × 12.5 cm long plastic scintillator in which the energy of pairs is absorbed and an arrangement of two thin plastic scintillators. Two thin scintillators, together with the appropriate electronics, form a coincidence-anticoincidence shield which appreciably reduces the background due to charged particles created in the main scintillator or coming into the spectrometer from outside. An average ratio of the net effect to the background of about 1:1 was achieved for medium weight nuclei like Fe. The meson background in the whole energy range of the spectrometer is reduced to about 0.5 cpm. The energy resolution of the spectrometer is 12.5%, 9.5% and 8.0% for gamma rays of 12.1 MeV, 16.5 MeV and 20.4 MeV, respectively. An average value of the spectrometer efficiency in this energy region is 0.5%. The spectrum of gamma rays from the capture of 14 MeV neutrons in 56Fe is shown as an example of the application of the spectrometer.