Proton Recoil Neutron Spectrometry With Organic Scintillators

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.

Organic scintillation detectors are useful for neutron spectroscopy because pulses induced by gamma rays and those induced by neutrons can be distinguished by electronic means. Computer analysis can be used to obtain the energy spectrum of the neutrons from the measured recoil-proton pulse-height distribution. The report describes a practical neutron spectrometer with improved electronic circuitry for pulseshape discrimination. A detailed study of the interaction of neutrons with the scintillator is presented in relation to pulse-height analysis. A computer program that uses a straightforward method of pulse-height analysis is given in FORTRAN with suggestions for adaptation for use on a small computer. Spectrometer applications are presented for measuring neutron spectra of monoenergetic neutrons, radioactive neutron sources, scattered neutrons and neutrons from a photonuclear reaction and also for detection of fission neutrons. (Author).

The goal of this thesis was to construct and test a neutron detector to measure the energy spectrum of 1 to 14-MeV neutrons in the presence of gammas. A spectrometer based on the process of pulse shape discrimination (PSD) was constructed, in which the scintillator NE-213 was used. The primary neutron/ gamma sources used were 78-mCi and 4.7-Ci 239PuBe sources, while 4.7-micro-Ci and 97.6-micro-Ci 22Na gamma sources were used for energy calibration and additional testing of the detector. Proton recoil spectra and Compton electron spectra were unfolded with the neutron and gamma unfolding code FORIST to generate the incident neutron and gamma spectra, respectively. FORIST, which was written for a CDC computer, was modified to run on a VAX 6420. The experimental spectra were compared to those in the literature. The locations of the peaks in the 239PuBe spectrum agreed with the literature to within 8.3%, the 239PuBe gamma spectrum agreed to within 0.7%, while the 22Na gamma spectrum agreed exactly. Uncertainties in the detection system and unfolding procedure are on the order of 5-10%. This thesis is intended to be a summary of the relevant literature and a user's guide to the PSD spectrometer ... Neutron unfolding code, Gamma spectrometry, Neutron spectrometry, Pulse shape discrimination.