Neutron and photon spectrometry with liquid scintillation detectors in mixed fields

Abstract

Liquid scintillation detectors of type NE213 or BC501A are well suited and routinely used for spectrometry in mixed n-γ-fields. Neutron- and photon-induced pulse height spectra may be simultaneously recorded making use of the n/γ-discrimination capability based on pulse shape analysis. The light output functions for the detected secondary charged particles, i.e. electrons, positrons, protons and other charged reaction products, and the pulse height resolution function must carefully be determined. This can be done experimentally, in part via an iterative procedure by comparison with calculations. The response functions can then be reliably calculated by Monte Carlo simulations. Photon response functions calculated with the PHRESP code, which was developed on the basis of the EGS4+PRESTA program package, are in very good agreement with calibrations up to 17 MeV, both in shape and absolute scale. Similarly, neutron response functions calculated with the NRESP7 code well describe the pulse height spectra for monoenergetic neutrons up to 20 MeV, although with some limitations for neutron energies beyond 10 MeV. In the case of measurements in mixed fields, the photon pulse height spectrum has to be corrected for neutron-induced photons generated in the detector assembly. The corresponding response functions may be determined numerically (with the MCNP and the PHRESP codes) or experimentally. Hence, the response matrices necessary for the deconvolution of measured pulse height spectra by the various unfolding procedures included in the HEPRO package are finally calculated with at least six functions per FWHM of the corresponding pulse height resolution. With appropriate counting statistics of the measured pulse height spectra, excellent energy resolution is achieved, e.g. about 20% of the pulse height resolution at the corresponding Compton or recoil proton edge. However, these specifications require that the entire detector system, i.e. scintillator, photomultiplier and associated electronics, exhibits a gain stable over time and independent of count rate and temperature.