Martin R. Sweet

Development of high efficiency, multi-element CdZnTe detectors for portable measurement applications

We describe the development of detector arrays and electronics for large-volume, hand-held CdZnTe detectors with the same counting efficiency as portable NaI(Tl) detectors presently used for nuclear material measurement applications. The pulse-height resolution of the multi-element detectors is at least three times better than NaI(Tl) over a wide energy range (from 100 keV to several MeV), enabling more accurate measurements of gamma-rays emitted by special nuclear material. Arrays of up to eight coplanar grid detectors can be combined to make detectors ranging in size from 4 to 14 cm3. Because the number of spectroscopy channels is small, low-power, hand-held detectors can be manufactured with conventional printed circuit board technology, thus keeping the cost of multi-element detectors to a minimum. The design and performance of an 8-element detector is presented.

Neutron detection and applications using a BC454/BGO array

Neutron detection and multiplicity counting has been investigated using a boron-loaded plastic scintillator (BC454)/bismuth germanate (BGO) phoswich detector array. Boron-loaded plastic combines neutron moderation (H) and detection ({sup 10}B) at the molecular level, thereby physically coupling increasing detection efficiency and decreasing die-away time with detector volume. Separation of the phoswich response into its plastic scintillator and bismuth germanate components was accomplished on an event-by-event basis using custom integrator and timing circuits, enabling a prompt coincidence requirement between the BC454 and BGO to be used to identify neutron captures. In addition, a custom time-tag module was used to provide a time for each detector event. Time-correlation analysis was subsequently performed on the filtered event stream to obtain shift-register-type singles and doubles count rates.

Radiation exposure effects on the performance of an electrically trainable artificial neural network (ETANN)

The effects of radiation exposure on an analog neural network devices are examined. The neural network implements a fully parallel architecture integrating 10240 analog nonvolatile synapses fabricated in a CMOS process. Graceful degradation of forward propagation performance was observed in units that were exposed to absorbed doses of up to 26 krads (Si) of 10-MeV electrons. The units were exposed without bias, except for that due to the floating gates. Single-chip solutions to two pattern recognition problems representing two levels of difficulty are employed for testing. Over the weeks following exposure, postirradiation effects due to a latent charge trapping mechanism in the oxides of the nonvolatile floating gate structures are observed. It is shown that units with apparently permanent damage can be retrained to 100% recognition performance. >

A validation payload for space and atmospheric nuclear event detection

We describe an experimental flight validation payload for detecting atmospheric and space nuclear events with a planned launch date in 2004. The five detector subsystems in the payload employ 27 sensors including Si, CdZnTe, gas proportional counter tubes, photomultiplier tubes, channel electron multipliers and photodiodes. Detection of events is based on simultaneous measurements of gamma rays, neutrons, and charged particles with wide dynamic ranges of deposited energy and count rates. The sensors and electronics are housed in one package with approximate mass and power consumption of 27 kg and 50 watts, respectively. The instrument uses sophisticated on-board digital signal processing and multi-layer triggering algorithms to detect and assess the validity of small signals in a large background radiation environment. This paper presents system configuration and preliminary test data from the first of the two units in development.

Gamma and neutron detector performance in a MOX fuel fabrication plant environment

Effectively implementing safeguards in fuel reprocessing and fuel fabrication plants requires the extensive use of state-of-the-art instrumentation for accurate material characterization. The ability to rapidly and continuously determine the material balance in a large facility is essential to control and reduce the risk of diversion. Multiple detection technologies are implemented that provide the necessary Continuity of Knowledge (CoK) and inventory control required for effective safeguards. In a Pu/U-oxide (MOX) fuel processing facility, much of the detection technology is based on moderated 3He neutron detectors. Recent concerns with respect to the possible lack of sufficient quantities of 3He has resulted in the interest in developing alternative detection technologies for current and future needs and applications. To address this situation, we have begun a series of experiments at a MOX fuel fabrication facility to determine the performance and suitability of liquid scintillator neutron detectors. MOX fuel in the form of reactor fuel assembly fuel rods and large volume MOX powder containers typically found in such production facilities were measured. In addition, high-purity Germanium (HPGe) and cadmium zinc telluride (CZT) detectors will be included to determine their respective performance characteristics in a large and complex fuel production environment. The results of these experiments will be used for consideration in the development and deployment of new safeguards systems in large, complex facilities such as the Japan Mixed-Oxide Fuel Fabrication Facility (J-MOX) or the Rokkasho Reprocessing Plant (RRP). This paper will present and discuss the results of the first series of experiments, conducted at PPFF (Plutonium Fuel Development Center Plutonium Fuel Facility) of JAEA, as well as describe the next steps in this process.

Performance of multielement CdZnTe detectors

Single-element CdZnTe detectors are limited in size, and therefore efficiency, by the poor hole transport, even with a coplanar grid. We are investigating the possibility of a 27-element array using 15 mm X 15 mm X 15 mm elements for gamma-ray energies to 10 MeV for NASA planetary missions. We present experimental results for combinations of various size coplanar grid detectors using NIM electronics and energies to 6.1 MeV. Summation of the signals after linear gating and requiring coincidence produces only a small increase in the energy resolution. Our results indicate that good efficiency and spectrum not complicated by a large Compton continuum can be achieved by simply summing the spectra from 15 X 15 X 15 mm3 detectors for gamma-ray energies below about 2 MeV. Above 2 MeV, 2-fold coincidence might be required, depending on the spectrum, to suppress the Compton continuum and escape peaks. We use a Monte Carlo calculation to predict the performance of the 27-elements array for a lunar highlands spectrum. Such ambient-temperature, high-efficiency, good- resolution arrays will facilitate new NASA mission to determine elemental composition of planetary bodies and terrestrial applications requiring high-efficiency, good- resolution portable instruments.

Reagentless optical biosensor

Critical to our ability to respond effectively to a biothreat attack is the development of sensitive and specific sensor systems that can easily be used for rapid screening of potential victims for infection due to biothreat agents and detection of pathogens in the environment. To help address these needs, we have developed a Reagentless Optical Biosensor (ROB) based on protein specific assays and waveguide-based evanescent fluorescence excitation. Modeled on host pathogen interactions, the sensors membrane based assay provides rapid, sensitive detection without the addition of reagents. We report here the development of two waveguide based detection systems: a laboratory sensor test-bed system and a handheld, battery operated, prototype. Evanescent fluorescence excitation using planar optical waveguides provides spatial filtering of background auto-fluorescence found in many natural samples, thereby permitting direct analysis of complex environmental and medical samples. The waveguide based assay is fully self-contained in a small, exchangeable cartridge that is optically coupled to the sensor detection system making ROB simple to use and offering the possibility of inexpensive, disposable sensor elements. Using assays for cholera toxin we compare results using flourimetry of vesicle solutions against results for our waveguide based test-bed and prototype sensor systems.

A semi-custom dual channel peak hold circuit for spaceborne instrumentation

A monolithic dual-channel peak hold circuit is developed using a semi-custom application specific integrated circuit (ASIC). The circuit is designed specifically for spaceborne instrumentation that requires low-power operation and low-mass packaging. Each independent circuit holds positive pulses and consists of a differential transconductance amplifier followed by a one-way current amplifier. Input gate and output hold functions are enabled by standard transistor-transistor-logic (TTL) or CMOS logic levels. To accommodate a range of applications, quiescent power is adjustable for performance-power tradeoff or can be disabled for single-channel use. Fabricated with dielectrically isolated vertical geometry NPN and PNP transitors, the circuit is inherently radiation-hard and immune to transient upset.<<ETX>>