Eric M. Becker

A Phoswich Detector With Compton Suppression Capability for Radioxenon Measurements

A phoswich detector with Compton suppression capability has been developed and tested for measuring xenon radioisotopes via a beta-gamma coincidence measurement technique. The phoswich detector has been designed with three scintillation layers. Beta-gamma coincidence events from radioxenon isotopes are identified when a coincidence energy absorption is detected in the first (BC-400) and second (CsI(Tl) crystal) scintillation layers. To identify and reject scattered photons from the CsI(Tl) crystal, the crystal is surrounded by a BGO scintillation layer. Our measurements show that the Compton suppression mechanism reduces the Compton continuum from 662 keV photons by 20-50% in the low-energy region of spectrum. Our beta-gamma coincidence measurements with 135Xe and 133Xe radioisotopes show energy resolutions (FWHM) of 13%, 46% and 24% for 250 keV, 30 keV and 80 keV gamma-ray peaks, respectively. In this paper, the detector design, assembly steps, digital pulse shape discrimination technique, and our recent measurements with radioactive lab sources and xenon radioisotopes are discussed.

A compton-suppressed phoswich detector for gamma spectroscopy

A phoswich detector with two scintillation layers has been designed and assembled at Oregon State University. This detector is able to identify and reject Compton events and ultimately reduce the Compton continuum in gamma energy spectra. In this detector, CsI(Tl) crystal is used to primarily detect photoelectric events. The CsI(Tl) crystal is partially surrounded by a BGO crystal layer to capture and identify Compton-scattered photons. Both crystals are optically coupled to a single photomultiplier tube. A real-time, FPGA-based digital pulse shape analysis was developed to discriminate and reject Compton-induced pulses from the CsI(Tl) crystal. All the digital pulse processing functions including pulse shape discrimination analysis, pile-up rejection and energy measurement were implemented in an on-board FPGA device. In this paper, the results of recent measurements using radioactive lab sources will be presented and discussed.

A CZT-based radioxenon detection system in support of the Comprehensive Nuclear-Test-Ban Treaty

In this work, a prototype radioxenon detection system was designed, developed and tested at Oregon State University to study the response of CdZnTe (CZT) detectors to xenon radioisotopes for monitoring nuclear explosions. The detector utilizes two coplanar CZT detectors and measures xenon radioisotopes through beta–gamma coincidence detection between the two detection elements. The CZT-based detection system offers excellent energy resolution and background count rate compared with scintillator-based beta–gamma coincidence detectors currently in operation at the IMS stations. In this paper, we briefly discuss the detector design and report our recent measurement results with 131mXe, 133mXe, and 133Xe produced in the TRIGA reactor at OSU.

Characterization of CLYC Detectors for a Next-Generation Unattended Sensor

We are developing a next-generation unattended sensor for detecting anomalous radiation sources. The system uses a scintillator material with dual sensitivity to gamma rays and neutrons, Cs2LiYCl6:Ce (CLYC), to reduce the size and complexity of the design. CLYC also offers a best-case energy resolution under 4% full width at half maximum at 662 keV, and allows for particle discrimination by pulse amplitude as well as pulse shape. The unattended sensor features sixteen one-inch CLYC detectors, each read out by a photomultiplier tube and custom readout electronics. A field-programmable gate array implements a suite of efficient processing algorithms for anomaly detection and isotope identification, and transmits alarm information to a base station via a wireless link. The system is designed to operate on battery power for several weeks. In this paper, we report the energy resolution, linearity, and temperature stability of the first CLYC detectors acquired for the project. Rather than characterizing the scintillator material under ideal conditions, we evaluate the detectors with the components selected for the unattended sensor, acknowledging the tradeoffs imposed by small size, limited power budget, and uncontrolled environmental conditions.

Small Prototype Gamma Spectrometer Using CsI(Tl) Scintillator Coupled to a Solid-State Photomultiplier

Small solid-state photomultipliers (SSPMs) are an alternative scintillator light-detection technology to traditional photomultiplier tubes that offer advantages such as lower bias voltages and insensitivity to magnetic fields. A digital spectrometer using a commercially available SSPM was constructed and characterized at Oregon State University as a prototype for small, highly-mobile, low-power, robust spectroscopy devices. The SSPM has over 19,000 microcells in a photo-sensitive area of 6.32 × 6.32 mm and was coupled to 6 × 6 × 10 mm reflectively-coated CsI(Tl) crystals. The rest of the spectrometer consists of a fast preamplifier and 200 MHz, 12-bit digital pulse processor based around a field-programmable gate array (FPGA). The efficiency, resolution, linearity, and peak-to-Compton ratio of the system were characterized.