Mark A. Smith-Nelson

Evaluation of key detector parameters for isotope identification

The algorithm used for isotope identification onboard a radioisotope identifier (RIID) plays a key role in obtaining a correct identification. The majority of RIIDs deployed by the United States Department of Homeland Security are based on NaI spectrometers. Their performance in isotope identification has been well-documented. It has been demonstrated that the secondary analysis of spectra by a trained spectroscopist is frequently necessary to resolve problems in the RIIDs identification. It is also clear that trained spectroscopists are capable of identifying complicated, multiple-line sources with even the poorest resolution detectors such as Nal. This paper seeks to understand the factors in detector performance such as energy resolution that play an important role in isotope identification.

Uncertainties of the Yn Parameters of the Hage-Cifarelli Formalism

One method for determining the physical parameters of a multiplying system is summarized by Cifarelli [1]. In this methodology the single, double and triple rates are determined from what is commonly referred to as Feynman histograms. This paper will examine two methods for estimating the uncertainty in the parameters used in inferring these rates. These methods will be compared with simulated data in order to determine which one best approximates the sample uncertainty.

Uncertainties in the Standard Moments, the Derivation of the Y1 to Y8 parameters, and the Derivation of w1 to w8 for Feynman Histograms

Momentum is a neutron multiplicity analysis software package that calculates a variety of parameters associated with Feynman histograms. While most of these parameters are documented in Cifarelli and Smith-Nelson, there are some parameters which are not. Most prominent of these are the uncertainties in the standard moments, and this paper will explicitly document these parameters. This paper will also document the higher-order Yn parameters and their associated ωn functions because they may be useful for future applications. The generation of what is referred to as Feynman histograms is not explained here.

Termination effect on pulse shape discrimination

NE213 liquid scintillation detectors have been used in mixed radiation fields with great success due to their pulse shape discrimination (PSD) ability. Numerous PSD techniques using various analog equipments had been proposed and developed by various individuals in past years. A common method for PSD is to evaluate the fall-time of the voltage across a resistor terminating the output of the photomultiplier tube (PMT) attached to the NE213 cell. The product of the terminating resistor and the terminating capacitance is known as the time- constant. Additionally, the combination of the terminating resistance and capacitance create a high-pass filter whose characteristics depend upon the value of the time constant. The greater the time-constant the less attenuation of the longer frequencies in a given signal occur. This paper will present a quantitative comparison of the fall time PSD technique using various terminating resistors. Specifically, 50 ohm, 500 ohm and a 1 kohm termination schemes are tested. Furthermore, due to nonlinearities in the system, a linear PSD spectrum may not be possible to obtain. In such cases, a traditional figure of merit (FOM) may not be usable to quantify the PSD capability of the system. A modified version of FOM is explored and used to describe the PSD capability of the current system.

Temperature dependency analysis of light output from an NE-213 liquid sintillator

NE-213 liquid scintillation detectors are currently used in radiation fields consisting of both gamma rays and fast neutrons and are an excellent tool for differentiating between each type of radiation via their respective interactions. In this experiment, an analysis was performed on an NE-213 liquid scintillation detector to investigate the effects of temperature changes on the light output. The two effects measured were the amount of the individual light decay components have to the total pulse height and the total gain of the system as a function of temperature.