We have developed an improved system to measure Cs-137 in wildlife at the Savannah River Site. This field-portable system consists of a shielded 5 cm by 10 cm by 40 cm NaI detector controlled by an Ametek Ortec Digibase. Measurement of an animal's radioactivity is made by placing the animal at a predefined location on the detector system for a one minute count-time. The counts, animal type, and animal weight are then used as inputs to an algorithm which calculates the amount of Cs-137 within the whole animal and within the edible meat portion of the animal. The results from these calculations are used to estimate the received dose from eating this animal and is included in the Savannah River Site's Hunter Dose Tracking System. This system has a detection limit of 0.60 pCi/g (22.20 Bq/kg) with a typical measurement uncertainty of less than 0.32 pCi/g (11.84 Bq/kg).

Author(s): Aucott, Timothy John | Advisor(s): Vetter, Kai | Abstract: Gamma-ray background radiation significantly reduces detection sensitivity when searching for radioactive sources in the field, such as in wide-area searches for homeland security applications. Mobile detector systems in particular must contend with a variable background that is not necessarily known or even measurable a priori. This work will present measurements of the spatial and temporal variability of the background, with the goal of merging gamma-ray detection, spectroscopy, and imaging with contextual information--a nuclear street view of the ubiquitous background radiation.The gamma-ray background originates from a variety of sources, both natural and anthropogenic. The dominant sources in the field are the primordial isotopes potassium-40, uranium-238, and thorium-232, as well as their decay daughters. In addition to the natural background, many artificially-created isotopes are used for industrial or medical purposes, and contamination from fission products can be found in many environments. Regardless of origin, these backgrounds will reduce detection sensitivity by adding both statistical as well as systematic uncertainty. In particular, large detector arrays will be limited by the systematic uncertainty in the background and will suffer from a high rate of false alarms.The goal of this work is to provide a comprehensive characterization of the gamma-ray background and its variability in order to improve detection sensitivity and evaluate the performance of mobile detectors in the field. Large quantities of data are measured in order to study their performance at very low false alarm rates. Two different approaches, spectroscopy and imaging, are compared in a controlled study in the presence of this measured background. Furthermore, there is additional information that can be gained by correlating the gamma-ray data with contextual data streams (such as cameras and global positioning systems) in order to reduce the variability in the background.This is accomplished by making many hours of background measurements with a truck-mounted system, which utilizes high-purity germanium detectors for spectroscopy and sodium iodide detectors for coded aperture imaging. This system also utilizes various peripheral sensors, such as panoramic cameras, laser ranging systems, global positioning systems, and a weather station to provide context for the gamma-ray data. About three hundred hours of data were taken in the San Francisco Bay Area, covering a wide variety of environments that might be encountered in operational scenarios. These measurements were used in a source injection study to evaluate the sensitivity of different algorithms (imaging and spectroscopy) and hardware (sodium iodide and high-purity germanium detectors).These measurements confirm that background distributions in large, mobile detector systems are dominated by systematic, not statistical variations, and both spectroscopy and imaging were found to substantially reduce this variability. Spectroscopy performed better than the coded aperture for the given scintillator array (one square meter of sodium iodide) for a variety of sources and geometries. By modeling the statistical and systematic uncertainties of the background, the data can be sampled to simulate the performance of a detector array of arbitrary size and resolution. With a larger array or lower resolution detectors, however imaging was better able to compensate for background variability.

Compton imaging has been demonstrated to provide excellent detection and localization capabilities in the search and characterization of radiation sources. However, the currently achievable sensitivity is limited by the Compton cone, which is backprojected. By measuring the initial trajectory of the Compton electron, the cone may be reduced to a cone segment with a corresponding increase in sensitivity.We have demonstrated the ability to measure electron trajectories (tracks) in thick (650 mu m), fully depleted silicon scientific CCDs, with a spatial resolution of 10 mu m in 2D. These measured tracks have been used to benchmark simulations of electron physics and detector response. We have developed an electron track algorithm to measure the initial electron direction in 3D from the CCD image, and utilized the modeled electron tracks to evaluate the angular resolution as a function of energy and initial direction for electrons up to similar to 500 key. For electrons above 150 keV and 30 degrees out-of-plane, we have achieved an in-plane angular uncertainty of sigma(alpha) less than or similar to 40 degrees, and an out-of-plane uncertainty of sigma(beta) less than or similar to 30 degrees in each hemisphere. (C) 2011 Elsevier B.V. All rights reserved.

The emerging threats created by a global expansion of nuclear technologies and terrorism demand improved nuclear materials detection systems to aid in nuclear security and nonproliferation. This project develops the idea of using machine vision combined with a large-area gamma-ray imager to improve sensitivity to threats and their rapid localization in a crowded environment (e.g., subway stations, airports, and bridges). We have achieved our first coded-mask images with a 1 m 2 array of 100 NaI(Tl) detectors. In addition, two video cameras have been used in stereo to create a three-dimensional map of points in front of the array, and image segmentation is being implemented to distinguish and track individual objects in the field of view. We are currently gearing up to simultaneously perform real-time gamma-ray imaging and object tracking so that we can eventually merge the two data streams and achieve the expected increase in sensitivity of this method.

The massive earthquake and tsunami off the coast of Japan on March 11, 2011 caused extensive damage at the Fukushima Daiichi nuclear power plant. During subsequent venting and explosions at the reactor site, there were releases of fission products such as 131I, 134Cs, 136Cs, 137Cs, and 132Te. Trace amounts of these isotopes were detectable in California around March 17. In the days after the disaster, the Berkeley Radiological Air and Water Monitoring (BRAWM) Project was started to measure the amounts of radioisotopes in the local environment around Berkeley. BRAWM has detected radioactive isotopes from Fukushima Daiichi in the air, rainwater, creek runoff, milk, soil, berries, and leafy vegetables. The team continues to monitor fallout levels in order to understand the nature of the radioactive releases from Fukushima as well as quantify the dilution or accumulation of the radioisotopes as they make their way through the environment and food chain.