James H. McQuaid

Delay Line Readouts for High Purity Germanium Medical Imaging Cameras

High purity germanium offers excellent potential for use in nuclear medical imaging cameras. A position and energy readout technique using two inexpensive delay lines has been developed for these cameras. Results obtained with a 1-cm2 , 4 mm deep, 5×5 strip high purity germanium detector are 2.1 mm full width spatial resolution, a measured single strip resolution of 0.65 mm full width half maximum (FWHM), a 25 element uncollimated energy resolution of 2.95 keV FWHM, and 2.65 keV FWHM for a single central element at 140 keV.

Imaging with a Small Ultra Pure Germanium Gamma-Camera

Semiconductor detector gamma-cameras promise marked improvement in spatial resolution, compared to NaI based system. In addition, solid state systems offer the potential of simultaneous imaging of multiple isotopes, or polychromatic nuclides. Beciuse semiconductor systems offer improved resolution, and expand the spectrum of radionuclides applicable to diagnostic imaging, their impact on nuclear medicine will be significant.

A High-Rate Spectroscopy System for Use with Ge(Li) Coaxial Detectors

A high-rate gamma-ray spectroscopy system with a 40 cc Ge(Li) coaxial detector is described which utilizes a direct-coupled recycling preamplifier, a unipolar Gaussian filter amplifier with only one differentiating network, and a pileup rejector and baseline inspection circuit, to yield excellent resolution. A family of resolution curves is shown as a function of the input count rate (from 50K cps to 175K cps) and of the actual data acceptance rate. For an input rate of 100K cps and an acceptance rate of 5.5K cps the FWHM and FWl/10M for the 1.33-MeV peak of 60Co was 2.2 keV and 4.24 keV, respectively. The system lends itself to high counting rates by eliminating the need for pole-zero compensation and baseline restoration.

Recycling Integrator for Measuring Nanoampere Currents

A current‐to‐frequency converter for gamma MeV measurements is described. In this application a photo‐multiplier tube is used as the current source. Currents as low as 10−9 A have been measured with a long term stability of 0.09 %/8h period. Extremely low leakage is achieved by the use of a field effect transistor at the input. The current to be measured is integrated and digitized. The output pulses are then fed to a frequency counter for digital readout.

High rate spectroscopy for on-line nuclear coal analyzer (nucoalyzer TM conac)

A high count rate, time-variant Ge(Li) spectrometer has been developed for on-line coal analysis. The analyzer is being fabricated for use in a power generating station. Prompt neutron activation of coal samples is the basis of analysis, with 252Cf as the source for irradiation. The spectroscopy system allows counting rates up to 150 k counts per second without significant loss in energy resolution or peak shape. The high data throughput allows the coal analyzer to be used for on-line process control. The coal analyzer will be discussed, with emphasis on the high-rate signal processing system. Results of analysis of coal samples will be presented.

A High-Rate Direct-Coupled Preamplifier for High Energy Ge Detector Systems

The fast counting of high energy gamma events with excellent resolution is made possible by the design of a recycling preamplifier and a direct-coupled spectrometer system. Measurements were performed with a 40 cc Ge(Li) coaxial detector. Data were observed with and without pile-up rejection for count rates up to 150,000 counts/s. Resolution without pile-up rejection for 60Co was 2.50 keV FWHM at 75,000 counts/s (1.5-?s shaping). The exceptional performance of this direct-coupled system is mainly attributed to the preamplifier which requires no pole-zero compensation. After single differentiation the pulses return precisely to the baseline.

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. >

On-Line Nuclear Analysis of Coal and its Uses

On-Line Nuclear Analysis of Coal based on prompt neutron activation represents a technological breakthrough for real-time process control necessary for optimum efficiency in the use of coal. These analyzers are presently being installed to solve a variety of current problems in coal usage. This paper describes the general features of these instruments and gives a detailed discussion of a high counting rate spectroscopy system used with an analyzer based on a germanium detector. A brief discussion of various applications of these analyzers include control of coal blending, control of coal burning efficiency, and quality control in coal beneficiation and synfuel processes.

Manufacturing Process for Ceramic Encapsulated Junction Field Effect Transistors (JFETs) and Statistical Evaluation of JFET Header Dielectric Materials

Electronic noise is the ultimate resolution limiting factor in non-dispersive low energy X-ray spectrometers. Lossy dielectric materials in the input of the spectrometers charge-sensitive preamplifier contribute significant amounts of noise. In particular, the glass insulating header of the input JFET produces a major portion of this dielectric noise. A previous small experimental lot of JFETs encapsulated with low loss ceramic headers exhibited significantly lower electronic noise than JFETs with lossy glass headers containing the same type JFET chip. A larger lot of JFETs (~350) encapsulated with 99% BeO ceramic headers has been prepared under standard transistor production conditions. Processing details are reported as well as statistical analyses of noise and other electrical measurements obtained on a random sample from this production lot. Similar data are reported for commercial JFETs containing the same type chip (TI-2N4416) encapsulated in an epoxy body and encapsulated with glass headers. In addition, measurements have been obtained for JFET chips mounted on high purity Al2O3 (99.5%) thin flat substrates. These substrates, normally used for integrated circuit production, exhibit very low loss at high frequencies. Previous results are confirmed in that JFET chips encapsulated with ceramic headers (under production conditions) produce significantly lower noise than epoxy or glass encapsulated JFETs. For a 2-?sec main amplifier Gaussian shaping time, the high purity A12O3; flat substrates produce less noise than the BeO headers. For 5-?sec shaping, no significant difference is observed between the two ceramic materials.

Preliminary Uranium Enrichment Analysis Results Using Cadmium Zinc Telluride Detectors

Lawrence Livermore National Laboratory (LLNL) and EG&G ORTEC have jointly developed a portable ambient-temperature detection system that can be used in a number of application scenarios. The detection system uses a planar cadmium zinc telluride (CZT) detector with custom-designed detector support electronics developed at LLNL and is based on the recently released MicroNOMAD multichannel analyzer (MCA) produced by ORTEC. Spectral analysis is performed using software developed at LLNL that was originally designed for use with high-purity germanium (HPGe) detector systems. In one application, the CZT detection system determines uranium enrichments ranging from less than 3% to over 75% to within accuracies of 20%. The analysis was performed using sample sizes of 200 g or larger and acquisition times of 30 min. The authors have demonstrated the capabilities of this system by analyzing the spectra gathered by the CZT detection system from uranium sources of several enrichments. These experiments demonstrate that current CZT detectors can, in some cases, approach performance criteria that were previously the exclusive domain of larger HPGe detector systems.