The use of organic scintillators is an established method for the measurement of neutron spectra above several hundred keV. Fast neutrons are detected largely by proton recoils in the scintillator resulting from neutron elastic scattering with hydrogen. This leads to a smeared rectangular pulse-height distribution for monoenergetic neutrons. The recoil proton distribution ranges in energy from zero to the incident neutron energy. In addition, the pulse-height distribution is further complicated by structure due to energy deposition from alpha particle recoils from interactions with carbon as well as carbon recoils themselves. In order to reconstruct the incident neutron spectrum, the pulse-height spectrum has to be deconvoluted (unfolded) using the computed or measured response of the scintillator to monoenergetic neutrons. In addition gamma rays, which are always present when neutrons are present, lead to Compton electron recoils in the scintillator. Fortunately, for certain organic scintillators, the electron recoil events can be separated from the heavier particle recoil events in turn to distinguish gamma-ray induced events from neutron-induced events. This is accomplished by using the risetime of the pulse from the organic scintillator seen in the photomultiplier tube as a decay of light. In this work, an organic scintillator detection system was assembled which includes neutron-gamma separation capabilities to store the neutron-induced and gamma-induced recoil spectra separately. An unfolding code was implemented to deconvolute the spectra into neutron and gamma energy spectra. In order to verify the performance of the system, a measurement of two reference neutron fields will be performed with the system, unmoderated Cf-252 and heavy-water moderated Cf-252. After the detection system has been verified, measurements will be made with an AmBe neutron source.

In this report the neutron-gamma discrimination capabilities of the new type of solid organic scintillator, EJ-299-33, was investigated using several pulse-shape discrimination (PSD) techniques. Among others, the analog zero-crossing method andthe digital charge-comparison and integrated-rise-time method were tested. The parameters of the digital PSD methods were optimised individually and the figure-of-merit was measured for each method and compared in different energy windows. The photoelectron yield of the setup was measured using two different photomultiplier tubes (PMT), a 3 inch diameter ET 9821 and a 5 inch diameter ET 9390KB. The highest photoelectron yield was measured with the ET 9390KB, which was the PMT used for the neutron-gamma discrimination capability measurements. In this work, four decay constants were found for the scintillator decay times. These were found by fitting average neutron and gamma-ray waveforms with the convolution of severeal exponential functions, that describes the light emission intensity of the scintillator, with an approximation of the PMT response function. Thebest agreement was found for the assumption that the scintillator light emission intensity is governed by four decay constants. The intensity of the two slowest components contain information about the incident particle.

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