|
The Remote Sensing laboratory (RSL) of National Security Technologies Inc. has built an array of large (5.08 - cm
x 10.16 – cm x 40.6 - cm) thallium doped sodium iodide (NaI: Tl) scintillators to locate and screen gamma-ray
emitting radioisotopes that are of interests to radiological emergency responders [1]. These vehicle mounted
detectors provide the operators with rapid, simple, specific information for radiological threat assessment.
Applications include large area inspection, customs inspection, border protection, emergency response, and
monitoring of radiological facilities. These RSL mobile units are currently being upgraded to meet the Defense
Threat Reduction Agency mission requirements for a next-generation system capable of detecting and identifying
nuclear threat materials.
One of the challenging problems faced by these gamma-ray detectors is the unambiguous identification of medical
isotopes like 131I (364.49 keV [81.7%], 636.99 keV [7.17%]), 99Tcm (140.51 keV [89.1%]) and 67Ga (184.6 keV
[19.7%], 300.2 [16.0%], 393.5 [4.5%] that are used in radionuclide therapy and often have overlapping gamma-ray
energy regions of interest (ROI). The problem is made worse by short (about 5 seconds) acquisition time of the
spectral data necessary for dynamic mobile detectors. This article describes attempts to identify medical isotopes
from data collected from this mobile detection system in a short period of time (not exceeding 5 secs) and a large
standoff distance (typically ~ 10 meters)
The mobile units offer identification capabilities that are based on hardware auto stabilization of the amplifier gain.
The 1461 keV gamma-energy line from 40K is tracked. It uses gamma-ray energy windowing along with embedded
mobile Gamma Detector Response and Analysis Software (GADRAS) [2] simultaneously to deconvolve any
overlapping gamma-energy ROIs. These high sensitivity detectors are capable of resolving complex masking
scenarios and exceed all ANSI N42.34 (2006) requirements for the identification of bare, shielded and multiple
isotopes.
|
