Doctoral Dissertations

Chong Cao

Date of Award

12-1997

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Nuclear Engineering

Major Professor

Laurence F. Miller

Committee Members

P. G. Groer, J. T. Mihalczo, D. J. Downing

Abstract

New methods are developed that extend the state-of-art in neutron-photon pulse shape discrimination (PSD) in the neutron low energy region (< 500 keV). These include implementing a time-of-flight selection method for acquisition of "pure" neutron spectra for calibration at low energies, establishing PSD dynamic bias using neural networks, and modifying statistical models for use in evaluating PSD scintillation materials and methods. As a result, the lower energy limit for neutron-photon PSD with commercially available equipment, is reduced to 100 keV neutron energy in this work.

First, a time-of-flight selection method using a Cf-252 neutron source is developed to calibrate the cross talk between neutrons and photons in PSD at low energies. This method permits "pure" photon and "pure" neutron spectra to be obtained, which are used to construct calibration data with known ratios of photons to neutrons. These spectra are subsequently used to construct training and testing data at 15 - 150 keV equivalent electron energies for neural network classifiers. Next, perceptron, back- propagation neural networks and Bayes statistical classifiers are developed to implement dynamic bias methods over a wide energy range. All demonstrate superior neutron- photon PSD performance to the fixed bias used in conventional methods. The perceptron is simplest to use and can be easily incorporated into an IC chip. However, the Bayes classifier performs better when neutron source conditions are significantly changed. Our tests show that implementing dynamic bias classifiers enables PSD to be successfully performed at neutron energy down to 100 keV using a 1.5"BC501A-XP2020 PMT detector.

Finally two statistical models including the transit time spread and the electronic noise contributions are established to evaluate of scintillation materials and the PSD methods. The models are used to optimize parameters of the zero-crossing and the charge comparison methods. The calculated best CFD settings for NE213, trans-stilbene and Borexino scintillators are 0.8, 0.8 and 0.9 respectively. The best fast window settings for NE213, trans-stilbene and Borexino scintillators are 0-40, 0-22 and 0-10 ns, respectively. The effects of the transit time spread and electronic noise are evaluated. The intrinsic PSD low energy limitations for different scintillation materials are estimated. Under the optimal condition, the low energy PSD limitations of early NE213, trans-stilbene and Borexino scintillators are 100, 65 and 60 keV neutron energy, respectively. Results show that under the optimal condition the zero-crossing method is better for NE213 and the trans-stilbene crystal, and the charge comparison is better for the Borexino. Using these models to predict the PSD performance under different conditions, scintillation materials can be evaluated and parameter optimization can be completed with much less experimental effort. Further efforts to improve neutron-photon PSD at low energies should concentrate on the development of new scintillation materials.

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