S.E. Garner

Wavelet analysis of sodium iodide spectra

Wavelet analysis is a mathematical technique that was presented in the mid-1980s to solve a variety of problems in signal analysis where the signal is aperiodic, noisy, transient, etc. More recently, wavelets have been applied to other problems such as feature detection and localization, making it a very promising tool for the analysis of gamma-ray spectra. Recent results have also shown that this technique has the potential to benefit over other approaches due to the fact that the signal can simultaneously be analyzed over multiple scales by using wavelet analysis, thus eliminating potential false isotope identifications from artifacts such as the Compton edge and backscatter peaks. This implies that this peak localization algorithm is no longer a function of detector resolution, which changes with energy. We will present our results evaluating the technique of wavelet analysis for low-resolution (NaI) gamma-ray spectra. Emphasis will be placed on wavelet selection and the incorporation of a simple algorithm to the problem of isotope identification

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.

Wavelet analysis of gamma-ray spectra

Since September 11, 2001, there has been increasing interest in providing first responders with radiation detectors for use in the search for and isotope identification of potentially-smuggled special nuclear material (SNM) or radiological dispersal devices (RDDs). These devices are typically comprised of low-resolution detectors such as NaI, thus limiting their identification abilities. We present a new technique of wavelet analysis of low-resolution spectra for the use in isotope identification. Wavelet analysis has the benefit of excellent feature localization while, unlike with Fourier analysis, maintaining the signal frequency and time characteristics. We will demonstrate this technique with a series of gamma-ray spectra obtained from typical hand-held isotope identifiers, illustrate figures-of-merit to be applied to these results, and discuss future algorithm optimization.

Long-range alpha detector (LRAD) sensitivity to beta contamination and soil moisture

Long-range alpha detector (LRAD) systems are designed to monitor alpha contamination by measuring the ionization in air formed by the alphas. Recent tests have been performed to determine the sensitivity of LRAD systems to beta contamination and soil moisture levels. These results and the general technology are discussed in this paper. >

Long-range alpha detector sample monitoring

Abstract Long-range alpha detector (LRAD) systems are designed to monitor alpha sources and contamination by measuring the number of ions created in air by ionizing radiation. Traditional alpha detectors are designed to detect alpha particles directly and must be passed slowly within about 3 cm of an alpha source to operate effectively. LRAD detectors collect the ions created from alpha interactions with air. Therefore, they are better able to monitor equipment and complex surfaces and can be operated at a much greater distance from an alpha source than traditional alpha detectors. Furthermore, because LRAD detectors remain stationary during monitoring, they are less subject to operator error than traditional alpha detectors. This paper will discuss the basic operation as well as recent advances that have been made to LRAD Sample Monitors.

Radiation detection evaluation: RadAssessor characterizes integrated findings

Radiation handheld instruments perform an integral role in first responder programs to detect illicit radioactive materials and reduce terrorist attacks. A comprehensive understanding of each commercial instruments performance facilitates rapid and accurate data interpretation and threat assessment. RadAssessor was developed as a radiation detector database to characterize and evaluate commercial handheld instrument findings. Under laboratory conditions, previously reported instrument performance data have been expanded both in terms of the number of instruments tested and sources measured as well as including gamma isotope identification and neutron response findings. Instrument performance data and findings captured within RadAssessor include: radiation detectors, radioactive sources, shielding, experiments, detailed measurements, spectra, and measurement performance categorizations. Measurement performance categorizations are an approach to categorizing gamma identification results based on defining the most abundant isotopes (MAIs) identified during a measurement. There are two category systems that provide both a high-level understanding of instrument performance and a more detailed understanding with additional granularity. RadAssessor features a web-based interface for selecting instruments, isotopes, and shielding combinations. Users are provided with dynamically generated screens based on the most recent data loaded in the RadAssessor database. Graphical and textual results are generated and presented instantaneously for enhanced decision aides

238PuO2 Fuel and Dosimetry

238Pu is an ideal material for use as a heat source with its half-life of 87.7 years and copious particle emissions. 238Pu radioisotope thermoelectric generators (RTGs) have found use for pacemakers, Apollo Space missions, Mars rovers, and Voyager spacecraft. In evaluating the dose to personnel and components near a 238Pu-based RTG, a number of additional nuclides and their daughter products must be considered to get an accurate estimate for γ-dose, and the amount of 17O and 18O for the neutron-dose must be considered. This paper looks at the contributing nuclides and their daughter products that add the most to the dose rates.

Simulation of Photon energy Spectra Using MISC, SOURCES, MCNP and GADRAS

The detector response functions included in the Gamma Detector Response and Analysis Software (GADRAS) are a valuable resource for simulating radioactive source emission spectra. Application of these response functions to the results of three-dimensional transport calculations is a useful modeling capability. Using a 26.2 kg shell of depleted uranium (DU) as a simple test problem, this work illustrates a method for manipulating current tally results from MCNP into the GAM file format necessary for a practical link to GADRAS detector response functions. MISC (MCNP Intrinsic Source Constructor) and SOURCES 4C were used to develop photon and neutron source terms for subsequent MCNP transport, and the resultant spectrum is shown to be in good agreement with that from GADRAS. A 1 kg DU sphere was also modeled with the method described here and showed similarly encouraging results.

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.