P. L. Phelps

A High-Efficiency Multidetector System for Tumor Scanning

A high-efficiency detector system developed especially for medical imaging has three speciall cut Ge(Li) coaxial detectors (total volume 249 cm3). At 122 keV, the peak efficiency is 93% of that of a 7.6 × 7.6 cm NaI (T1) detector. Degradation of the paralleled energy resolution is avoided and resolution is improved by 35% over that of conventional output-summing techniques by gating the detector outputs. In effect this multiplexes them to a single line output.

A Detector System for In-Situ Spectrometric Analysis of 241Am and Pu in Soil

This system for quantitative analysis of Pu in soil via 241Am has four 2.5-mm high-purity Ge detectors of 33 cm2 total detecting surface area. These detectors are paralleled by gating circuitry to avoid the degradation of energy resolution associated with electronic output summing. In field tests the system was approximately three times as sensitive as a 70-cm3 Ge(Li) detector and approximately an order of magnitude more sensitive than the FIDLER system.

A Portable Ge(Li) Detector for Field Measurement of Radionuclides in the Environment

A description of a portable Ge(Li) field spectrometer and its application to in situ measurements is given. The mathematical techniques of calibrating the spectrometer so that field spectrometer counting rates may be converted into units of soil activity are developed. The quantification of gamma-ray emitting radionuclides in the field has proven to be more sensitive than laboratory analysis using Ge(Li) anticoincidence shielded spectrometers. This was demonstrated by obtaining approximately the equivalent statistical accuracies for quantifying several radionuclides from a 30 min field count as compared to a 1437 min laboratory analysis. A comparison between the results obtained from field analysis with those obtained by returning the samples to the laboratory are given for natural radionuclides and 137Cs. An example of qualitative spectral data obtained from field measurements of gaseous emissions from a BWR reactor is also given.

A Review of Measurement Techniques for Stack Monitoring of Long-Lived Alpha Emitters

As a result of the promulgation of new guidelines by the Environmental Protection Agency (40 CFR 190) for releases of long-lived, alpha-emitting substances, the stack-monitoring requirements for measuring long-lived alpha particles may change in terms of both monitored isotopes and the detection levels. This paper briefly reviews stack-monitoring requirements for long-lived alpha-emitting particles. It also examines the currently deployed alpha-particulate, stack-monitoring systems and discusses prototype systems that may be applicable to stack monitoring.

Sensitivity Problems in Biological and Environmental Counting

Analysis of biological and environmental samples differs from other physics applications in the need for very high sensitivity. A number of factors, including efficiency, geometry, resolution, and an interrelation of these, all affect sensitivity. An analysis, based on both experimental data and computations, indicates that in the range of equal detector volumes < 40 cm3, planar detectors are to be preferred over coaxial detectors due to their much better ability to quantitate data at energies below 500 keV without loss of performance at higher energies. For equal detector volumes in excess of 40 cm3, coaxial detectors may be preferred for high energy use due to their better resolution, but performance at lower energies will still favor the planar detectors.

Experience in the Use of an Anticoincidence Shielded Ge(Li) Ganma-Ray Spectrometer for Low Level Environmental Radionuclide Analysis

An evaluation of a low level, high resolution anticoincidence shielded Ge(Li) spectrometer, constructed over five years ago, has been made. The evaluation was based upon results obtained in assaying a variety of biological and environmental samples, each contained in a 200 cm3 aluminum can. The spectrometer used an 18 cm3 planar type Ge(Li) detector constructed in our Laboratory. An example of the very high sensitivity possible with the spectrometer was in the assaying for 137Cs in a 296 gram soil sample which was determined to have a concentration of 0.027 pCi of 137Cs per gram of soil. Description of the spectrometer, methods used in data analysis and results of counting low level samples are given.

Ge(Li) Low Level In-Situ Gamma-Ray Spectrometer Applications

Currently we are employing a Ge(Li) spectrometer for in-situ measurements of radionuclides contained in soil. This is being done at nuclear reactor sites and in complex radionuclide fields at the Nevada Test Site. The methodology and precision of the in-situ spectrometric technique has previously been established for analysis of radionuclides in soil. Application of the technique to gaseous and liquid effluents containing radionuclides has shown a great deal of promise.

Gamma-Ray Spectrometers for the Assay of Complex Mixtures of Low Concentration of Radionuclides in Environmental and Biological Materials

Solid state detectors, especially Ge(Li) drifted diodes, have given the biologist a new and powerful tool for determining the identity of radioactive species in biological samples. Often these samples contain very low quantities of radioactivity in a complex mixture of radionuclides. High resolution, maximum counting efficiency, suppression of background and Compton radiation and enhancement of photopeaks are required. A series of gamma-ray spectrometers has been developed to assay radionuclides in biological materials. These spectrometers have incorporated features that facilitate achieving these requirements. Samples up to 8.5 cm in diameter and 2.5 cm thick are now routinely analyzed. A range of gamma and x-ray energies from a few kilovolts to 2.5 Mev can be analyzed. Since the probability becomes rather high above 1 Mev that the favored gamma interaction within the Ge(Li) detector will be a Compton event with the resultant photoelectron interfering with the spectrum below the initial gamma-ray energy, it is necessary to suppress this type of interaction. This has been achieved by surrounding the Ge(Li) detectors with a scintillator for detecting scattered Compton photons. A method is described to enhance both anticoincidence and coincidence events. Both NaI(Tl) and plastic scintillators are used as anticoincidence shields. The optimum anticoincidence shield may not be a simple annulus. Some arguments and preliminary results are given to show the advantages of totally surrounding the Ge(Li) with a scintillator for 4 ? Compton suppression. The Compton shields vary from 9 inches in diameter for NaI(Tl) to 30 inches for plastic.

A Transuranic Aerosol Measurement System: Preliminary Results

We have completed the design, fabrication, and assembly of a computer-based prototype system for the measurement of transuranic aerosols in the workplace and environment. This system (called WOTAMS for Workplace Transuranic Aerosol Measurement System) incorporates two detectors: (1) an in-line solid-state alpha detector that sends out an alarm the moment a transuranic release occurs, and (2) an in-vacuum detector that increases off-line-analysis sensitivity. The in-line sensitivity of the system is better than 5.0 MPC-h, and the in-vacuum sensitivity exceeds 0.5 MPC-h.

A Transuranic-Aerosol-Measurement System for the Workplace or Stack Monitoring

This compact transuranic-aerosol-measurement system, using alpha spectroscopy at vacuum, has a sensitivity of less than 0.5 MPC-hr for 239Pu. In addition, a detector is mounted in the inlet pipe that responds immediately if there is a release of radioactivity; then, after an appropriate time, the filter is moved to the vacuum chamber for more sensitive off-line analysis. The system is very efficient for particles as large as 10 ¿m in diameter. A micro-computer controls the filter transport, operates as a 256-channel PHA, performs calculations, checks calibration, and drives a matrix display and a central computer. This instrument, called WOTAMS, has been developed for use in facilities that process transuranics with the expectation it will further evolve into a commercial product to replace those less sensitive instruments now widely used.