J.D. Valentine

Light yield nonproportionality of CsI(Tl), CsI(Na), and YAP

CsI(Tl), CsI(Na), and YAP light yield nonproportionality has been characterized using the Compton Coincidence Technique. Measured electron responses were used to calculate photon responses of the scintillators studied. These calculated photon responses were then compared with measured photon responses. In addition results from electron response measurements and photon response calculations were compared with previously reported results. While the CsI(Na) electron response was observed to have the largest deviation from proportionality (about 40%), YAP was observed to have a nearly proportional response. The CsI(Tl) calculated photon response was observed to agree to within 1% with measured photon response in this study for all measured photon energies. While the CsI(Na) calculated photon response agreed to within 1% of measured data for photon energies above 60 keV, deviations up to 4% were observed below 60 keV.

The light yield nonproportionality component of scintillator energy resolution

The scintillator energy resolution component which is due to light yield nonproportionality has been characterized for NaI(Tl) and LSO. Results are based on a discrete convolution of measured electron response data and the electron energy distribution resulting from full-energy absorption events. The behavior of this energy resolution component as a function of energy is observed to be strongly dependent on the shape of the electron response. Furthermore, in some energy regions, the light yield nonproportionality component is observed to be larger than the resolution predicted by assuming Poisson photoelectron statistics. Characterization of this energy resolution component will facilitate deconvolution of other components from the total energy resolution.

Scintillator light yield nonproportionality: calculating photon response using measured electron response

To study the effects of scintillator light yield nonproportionality, a technique has been developed to calculate photon response. A discrete convolution of measured electron response and the electron energy distribution for a particular scintillator yields the photon response. By establishing the ability to accurately calculate photon response, the experimental implications of scintillator light yield nonproportionality and geometry effects can be studied without the requirement for experimental measurements. This technique also provides a more detailed characterization of photon response than experimental techniques that rely on the use of multiple gamma-ray and X-ray sources. To demonstrate this technique, NaI(Tl), CaF/sub 2/(Eu), and LSO(Ce) photon responses have been calculated. By comparing calculated results to both measured photon responses and previously published photon responses for these scintillators, this technique has been validated.

Design of a Compton spectrometer experiment for studying scintillator non-linearity and intrinsic energy resolution

Abstract A Compton spectrometer experiment has been designed and modeled for the purpose of studying the light yield non-linearities and intrinsic energy resolution of scintillation materials that are used to detect gamma rays. This coincidence method is used to create a nearly monoenergetic internal electron source within the scintillator by recording pulses from the primary detector only when a simultaneous pulse is generated by the coincidence detector. Such an electron source is necessary to accurately quantify the electron response of a scintillator, and has been previously identified as a requirement for quantifying scintillator non-linearity and intrinsic energy resolution. The ability to quantify these scintillator characteristics using this technique and the characterization of the Compton spectrometer geometry, including collimation of primary and scattered gamma rays, using Monte Carlo simulation are discussed.

Calculating lens dose and surface dose rates from 90Sr ophthalmic applicators using Monte Carlo modeling

Using a 90Sr applicator for brachytherapy for the reduction of recurrence rates after pterygium excisions has been an effective therapeutic procedure. Accurate knowledge of the dose being applied to the affected area on the sclera has been lacking, and for decades inaccurate estimates for lens dose have thus been made. Small errors in the assumptions which are required to make these estimates lead to dose rates changing exponentially because of the attenuation of beta particles. Monte Carlo simulations have been used to evaluate the assumptions that are now being used for the calculation of the surface dose rate and the corresponding determination of lens dose. For an ideal 90Sr applicator, results from this study indicate dose rates to the most radiosensitive areas of the lens ranging from 8.8 to 15.5 cGy/s. This range is based on different eye dimensions that ultimately corresponds to a range in distance between the applicator surface and the germinative epithelium of the lens of 2-3 mm. Furthermore, the conventional 200 cGy threshold for whole lens cataractogenesis is questioned for predicting complications from scleral brachytherapy. The dose to the germinative epithelium should be used for studying radiocataractogenesis.

An energy-subtraction Compton scatter camera design for in vivo medical imaging of radiopharmaceuticals

A Compton scatter camera (CSC) design is proposed for imaging radioisotopes used as biotracers. A clinical version may increase sensitivity by a factor of over 100, while maintaining or improving spatial resolution, as compared with existing Anger cameras that use lead collimators. This novel approach is based on using energy subtraction (/spl Delta/E=E/sub 0/-E/sub SC/, where E/sub 0/, /spl Delta/E, and E/sub SC/ are the energy at the emitted gamma ray, the energy deposited by the initial Compton scatter, and the energy of the Compton scattered photon) to determine the amount of energy deposited in the primary system. The energy subtraction approach allows the requirement of high energy resolution to be placed on a secondary detector system instead of the primary detector system. Requiring primary system high energy resolution has significantly limited previous CSC designs for medical imaging applications. Furthermore, this approach is dependent on optimizing the camera design for data acquisition of gamma rays that undergo only one Compton scatter in a low-Z primary detector system followed by total absorption of the Compton scattered photon in a high-Z secondary detector system. The proposed approach allows for a more compact primary detector system, a more simplified pulse processing interface, and a much less complicated detector cooling scheme as compared with previous CSC designs. Analytical calculations and Monte Carlo simulation results for some specific detector materials and geometries are presented.

Energy-subtraction Compton scatter camera design considerations: a Monte Carlo study of timing and energy resolution effects

An energy-subtraction Compton scatter camera (ESCSC) was previously proposed for medical imaging applications. This ESCSC consists of a primary detector system (silicon) and a secondary detector system (cadmium-zinc-telluride) for preferred detection of Compton scatter and photoelectric absorption interactions, respectively. To further evaluate the usefulness of this ESCSC for medical imaging, the following characteristics have been simulated: the random emission of gamma-rays in time; detector timing, energy and spatial resolution; list mode data acquisition; and post-acquisition coincidence analysis. The resulting optimization of detector characteristics, data acquisition and analysis techniques, and administered activity is presented and discussed. One significant result of these simulations is that a localized activity of about 1.0 mCi allows for recovery of the majority of preferred events while eliminating the majority of interfering events when 10 and 50 ns FWHM timing resolutions for silicon and cadmium-zinc-telluride, respectively, are assumed. Consequently, the Proposed ESCSC should be capable of acquiring data for administered activities similar to those used with current mechanically-collimated imaging cameras.

Pilot study to determine levels of contamination in indoor dust resulting from contamination of soils

In order to develop more realistic risk assessments, an experimental program was conducted to characterize indoor, residential environments and the relationship between the indoor environment and contaminants that originated from the outdoor environment. Parameters measured included concentration of uranium in soils, mass loading of dust on indoor surfaces, and concentrations of uranium in indoor dust. Samples of indoor dust were collected using a personal air sampler modified to act as a low flow rate vacuum cleaner. The concentrations of uranium in indoor dust were measured using kinetic phosphorescence analysis, while the concentrations of uranium in outdoor soil were measured by analyzing the thorium‐234 activity using gamma‐ray spectrometry. This pilot study derived an estimate of 20 to 30% of indoor contamination resulting from sources in soil.

An evaluation of radioxenon detection techniques for use with a fluid-based concentration system

A portable monitoring system to measure the quantity of radioxenon (/sup 131m/Xe, /sup 133/Xe, /sup 133m/Xe, and /sup 135/Xe) in the atmosphere is being developed which incorporates a fluid-based concentration system with a detection system. To this end a number of radioxenon detection techniques have been evaluated to determine the best method of analyzing the output of the concentration system, which may contain significant amounts of radon in addition to concentrated xenon. Three detector configurations have been tested to measure the characteristic electron/photon coincidence radiation: gas proportional detector/NaI(Tl), plastic scintillator/NaI(Tl), and liquid scintillator/NaI(Tl). In addition to standard coincidence measurements, some additional gating criteria were also used: pulse height discrimination, pulse shape discrimination, and delayed coincidence. While the lowest relative minimum detectable activity was achieved using the liquid scintillator with delayed coincidence gating, the best performance for fieldable detection systems depends on the ratio of xenon to radon in the output of the concentration system. A high ratio favors the use of a gas proportional/NaI(Tl) detector using coincidence gating with pulse height discrimination. The use of a plastic scintillator/NaI(Tl) detector using coincidence gating with pulse shape discrimination is preferred when the ratio is low.

The modified three point Gaussian method for determining Gaussian peak parameters

Abstract A modified three point Gaussian (MTPG) method for analyzing histogrammed peaks which are assumed to have an underlying Gaussian distribution has been developed. The original three point Gaussian method introduces systematic biases in estimating area A and full-width at half maximum (FWHM) of Gaussian peaks in histogrammed spectra, in addition to resulting in relatively large uncertainties in these estimates. This bias is caused by choosing the midpoint of each bin as the x-coordinate when calculating the three parameters of a Gaussian distribution (A, centroid μ, and FWHM). The MTPG method applied an iterative procedure to more accurately determine the x-coordinate for each bin used in three point Gaussian method. In the FWHM range of about 0.5–2 bins, the MTPG method converged after several iterations (typically 5 to 6), in the process eliminating the bias introduced by the original method. Additionally, the uncertainties of A and FWHM estimates in this same FWHM range were minimized and approached the statistical limits. A detailed description of the MTPG method and results over a range of peak areas and FWHMs are presented.