Daniel L. Stephens

Detection of moving radioactive sources using sensor networks

A variety of recent applications have led to a great interest in the development and application of sensor networks with the goal of providing more effective detection of moving radioactive sources. This paper endeavors to analyze and evaluate the costs and benefits associated with the use of a network of radiation detectors for applications involving the detection of a moving radioactive source. This analysis is restricted to the one-dimensional case, i.e., to the case where the moving source is constrained to move along a single path. It is found that the relative advantage resulting from sensor dispersal depends upon the goals, objectives, and constraints of the measurement scenario. The dispersal of sensors into a network may be advisable or required for operational reasons, but from a statistical perspective does not directly lead to improved performance in terms of detection efficiency and false detection rate.

Carrington flare of 1859 as a prototypical worst-case solar energetic particle event

Recent analyses of ice core samples indicate that the Carrington flare of 1859 was the largest event observed in the past 500 years. These ice core data yield estimates of the proton fluence for energies greater than 30 MeV, but provide no other spectrum information. Assuming that the proton energy distribution for such an event is similar to that measured for other recent, large events, total ionizing doses in deep space are estimated for these hypothetical worst-case spectra. These estimated doses, as large as 50 krad (Si), could be catastrophic for sensitive electronic devices unless substantial shielding is provided.

Neutron Detector Gamma Insensitivity Criteria

. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or Battelle Memorial Institute. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. PACIFIC NORTHWEST NATIONAL LABORATORY

Field tests of a NaI(Tl)-based vehicle portal monitor at border crossings

Radiation portal monitors are commonly used at international border crossings to detect illicit transport of radioactive material. Most monitors use plastic scintillators to detect gamma rays, but next-generation monitors may contain NaI(Tl). In order to directly compare the performance of the two types of detectors, a prototype NaI(Tl) monitor was tested at two international border crossings adjacent to a comparable plastic scintillator monitor. The NaI(Tl) monitor housed four large detectors, each 10.2 cm /spl times/ 10.2 cm /spl times/ 41 cm. The empirical data set from the two field tests contains approximately 3800 passages with known cargo loads for each vehicle. For a small subset of the vehicles, high purity germanium detector spectra were also collected. During the survey period several vehicles containing commercial products with naturally occurring radioactive material (NORM) passed through the monitor. Typical NORM cargo included pottery, large granite slabs, rock-based floor tiles, construction stone blocks, abrasive material, and fertilizer. Non-NORM sources included a large source of /sup 60/Co (200,000 GBq) and a shipment of uranium oxide, both items being legally transported. The information obtained during the tests provides a good empirical data set to compare the effectiveness of NaI(Tl) and plastic-scintillator portal monitors. The capability to be sensitive to illicit materials, but not alarm on NORM, is a key figure of merit for portal monitors.

Induced temporal signatures for point-source detection

Detection of radioactive point sources is inherently divided into two regimes encompassing stationary and moving detectors. The two cases differ in their treatment of background radiation and its influence on detection sensitivity. Stationary detectors are limited by the statistical fluctuation of the background, while moving detectors may be subjected to widely and irregularly varying background radiation as a result of geographical and environmental variation. This significant systematic variation, in conjunction with the statistical variation of the background, requires a very conservative threshold in order to yield the same false-positive rate as the stationary detection case. This manuscript discusses a novel detector geometry that induces a unique time-encoded signature (TES) when exposed to point sources. The identification of temporal signatures for point sources using TES has been demonstrated and compared with the canonical method. This work demonstrates that temporal signatures mitigate systematic background variation and thus increase point-source detection in a moving detector system.

Algorithm Implementation for a Prototype Time-Encoded Signature Detector

The authors constructed a prototype Time-Encoded Signature (TES) system, complete with automated detection algorithms that can be used to detect point-like gamma-ray sources in search applications where detectors observe large variability in background count rates beyond statistical (Poisson) noise. The person-carried system consists of two cesium iodide scintillators placed on opposite sides of a lead shield. This geometry mitigates systematic background variation and induces a unique signature upon encountering point-like sources. This manuscript focuses on the development of detection algorithms that identify point-source signatures while remaining computationally simple. The latter constraint derives from the instruments mobile (and thus low power) operation. The authors evaluated algorithms using both simulated and field data. The results of this analysis demonstrate the capability to detect sources at a wide range of source-detector distances using computationally simple algorithms.