Wayne D. Ruhter

Directional-correlation results for 134Ba and 110Cd using a megachannel gamma- gamma coincidence system☆

Abstract The development of high-resolution Ge(Li) detectors has created a need for gamma-ray coincidence analysis systems able to process and store the large quantities of data generated. We describe a megachannel pulse-height analysis system that can sort and store these data on line. This is accomplished using a 32K-word PDP-8/E minicomputer and two moving-head disks that provide a storage capacity of 2 18 events at any of 2 20 data locations. An X -or Y -pulse-height spectrum, in coincidence with one to four arithmetically combined pulse-height peak and background windows, can be assembled for CRT display and spectral analysis within 2–20 s. The rapid recovery of data allows convenient set-up and monitoring of experiments and anlyysis of the accumulated data. To test the performance of the system, we made directional-correlation measurements on gamma-ray cascades in 134 Ba and 110 Cd. Analyses of these data gave E2/M1 mixing ratios that for most transitions agreed well with published results. Using this system with its improved energy resolution and data processing capabilities, we obtained new E2/M1 mixing ratio results of 13.3 ± 2.1 for the 563 keV transition in 134 Ba and −1.24 ± 0.20 for the 1505 keV transition in 110 Cd.

Application of cadmium-zinc-telluride detectors in 235U enrichment measurements☆

Abstract High-resolution, gamma- and X-ray spectrometry are used routinely in nuclear safeguards verification measurements of plutonium and uranium in the field. These measurements are mostly performed with high-purity germanium (HPGe) detectors, that require cooling to liquid-nitrogen temperatures, thus limiting their utility in field and unattended safeguards measurement applications. NaI scintillation detectors do not require cooling, but their moderate energy resolution (10% at 122 keV) is insufficient in most cases for reliable verification measurements. Semiconductor detectors that operate at room temperature, such as cadmium-zinc-telluride (CdZnTe) detectors, with energy resolution performance reaching 2.0% at 122 keV may complement HPGe detectors for certain safeguards verification applications. We used a 5 × 5 × 5 mm CdZnTe detector to measure 235 U enrichments ranging from 3% to 75%. We use a spectrum analysis technique that fits 235 U, 238 U, and U K X-ray response profiles to data in the 89- to 100-keV region of a gamma-ray spectrum. From the relative magnitudes of the 235 U and 238 U profiles we determine the 235 U enrichment with an accuracy of about 10% with CdZnTe detectors.

CADMIUM ZINC TELLURIDE SPECTRAL MODELING

Cadmium Zinc Telluride (CZT) detectors are the highest resolution room-temperature gamma ray detectors available for isotopic analysis. As with germanium detectors, accurate isotopic analysis using the spectra requires peak deconvolution. The CZT peak shapes are asymmetric, with a long low energy tail. The asymmetry is a result of the physics of the electron/hole transport in the semiconductor. An accurate model of the physics of the electron/hole transport through an electric field will allow the parameterization of the peak shapes as a function of energy. In turn this leads to the ability to perform accurate spectral deconvolution and therefore accurate isotopic analysis. The model and the peak-shape parameterization as a function of energy will be presented.

Uranium and plutonium solution assays by transmission-corrected X-ray fluorescence spectrometry

An x-ray fluorescence analysis technique was developed for uranium and plutonium solutions, which compensates for variations in the absorption of the exciting gamma rays and fluorescent x-rays. 57 Co is used to excite efficiently the K lines of the elements, and a mixed 57 Co plus 153 Gd transmission source is used to correct for variations in absorption. The absorption correction is a unique feature of the technique which permits accurate calibration with a single solution standard and the measurement of a wide range of concentrations (up to 300 g l -1 ). Without this correction procedure, up to six solution standards are required to correct for non-linearity over this concentration range. In addition, other elements present in the specimens and not present in the standards would otherwise reduce the accuracy or even invalidate the analyses. Specially designed equipment incorporates a planar intrinsic germanium detector, excitation and transmission radioisotopes and specimen holder. The apparatus can be inserted into a rubber glove of a glove-box, keeping the apparatus outside and the solutions inside the glove-box, thereby protecting the user and the equipment from possible contamination. This technique will be tested at the Bochvar Institute of Inorganic Materials in Moscow for possible use in the Russian complex of nuclear facilities. This is part of a cooperative program between laboratories in the USA and Russia to strengthen systems of nuclear materials protection, control and accountability (MPC&A) in both countries. A part of this program is to measure accurately and track inventories of materials, hence the need for good non-destructive analytical techniques.

Development of a portable ambient temperature radiometric assaying instrument

There is a strong need for portable radiometric instrumentation that can accurately confirm the presence of nuclear materials and allow isotopic analysis of radionuclides in the field. To fulfill this need we are developing a hand-held, non-cryogenic, low-power gamma- and X-ray measurement and analysis instrument that can both search and then accurately verify the presence of nuclear materials. We will report on the use of cadmium zinc telluride detectors, signal processing electronics, and the new field-portable instrument based on the MicroNOMAD Multichannel Analyzer from EG&G ORTEC. We will also describe the isotopic analysis that allows uranium enrichment measurements to be made accurately in the field. >

Three dimensional imaging of a molten-salt-extracted plutonium button using both active and passive gamma-ray computed tomography

Abstract We are developing a new technique using both active (A) and passive (P) computed tomography (CT) to overcome problems in the quantitative analysis of plutonium contents in the molten salt extract (MSE) plutonium buttons. Our first three dimensional image of the MSE button using ACT scan has already shown heterogeneous behavior throughout the button as opposed to the model of a homogeneous button as previously thought. An image from a PCT scan, although it does not have sufficient spatial resolution and data, also seems to support the heterogeneous picture.

Monte Carlo simulation of plutonium gamma-ray standards

Abstract We are developing a new method which uses Monte Carlo simulation of photon transport to synthesize radioactive gamma-ray standards. Our first study involves simulations of energy spectra observed of seven plutonium gamma-ray standards from a high-purity germanium (HPGe) detector. We have found good agreement (in both isotopic contents and spectral behavior) between simulated and experimentally measured gamma-ray spectra of these seven standards. We also addressed a problem in the simulation which involves a modification to the Klein-Nishina correction for photon scattering with electrons in a crystal lattice. We have developed a method to correct this problem.

Nondestructive Assay Instrumentation for a Savannah River Plant Upgrade Project

We have designed and are developing three different computer-based spectrometer systems. Two will measure the concentration of Pu solutions by gamma-ray and by stimulated x-ray fluorescence emissions. The third will measure relative isotopic abundances from the gamma-ray emissions of solid samples in closed containers. All systems are coupled to remote terminals and bar code readers, and also to mini-computer based MCAs, which in-turn are linked to another computer to provide a state-of-the-art NDA measurement capability. Installation at SRP is planned in late 1985.

Pulse Height Analyzers

The sections in this article are 1 Pulse Processing with the PHA 2 Principles of Operation 3 Acknowledgement

A Portable Microcomputer Unit for the Analysis of Plutonium Gamma-Ray Spectra

A portable microcomputer has been developed for the IAEA to perform in-field analysis of plutonium gamma-ray spectra. The unit includes a 16-bit LSI-11/2 microprocessor, 32K words of memory, a 20-character display for user prompting, and a 20-character thermal printer for hardcopy output. Only the positions of the 148-keV Pu-241 and 208-keV U-237 peaks are required for spectral analysis. The unit was tested against gamma-ray spectra taken of NBS plutonium standards and IAEA spectra. Results obtained are presented.