Non-Intrusive Mercury Detection and Measurement

Contamination from mercury and other heavy metals can present a health risk to humans and wildlife. Sources of these contaminants can be difficult to locate, especially if lost beneath the surface of building materials and equipment where mercury was used. This work will develop a portable imaging spectrometer for non-intrusive detection and localization of mercury globules trapped beneath the surface of materials. A penetrating neutron radiation source will be used to stimulate nuclear and atomic transitions within the mercury, causing it to emit identifiable and penetrating photon signatures that are detected by a pixelated CdZnTe sensor. With enough collected data, the direction of the contaminant can also be resolved via Compton imaging. Phase I investigated the feasibility of using prompt gamma activation analysis for mercury detection in building materials using Monte Carlo simulation validated by measurement. A CdZnTe detector was chosen for the study because of its portability, good energy resolution, and imaging capabilities. A measurement of mercury within a 2-in steel pipe was recorded, and the data was used to calculate detection limits for various sizes of mercury globules. For the spectrometer used in this work, which contained 19 cm3 CdZnTe, a globule the size of a quarter dollar could be detected within about 30 minutes. This result is in rough agreement with Monte Carlo simulation. Imaging of the neutron-capture gamma from mercury was also experimentally demonstrated, but an improved background subtraction algorithm is needed for imaging outside the lab. The results from Phase I show that mercury detection is feasible for steel objects containing globules of mercury, and work continues for other building materials, equipment, and other pipe sizes.The goal of this Phase-II effort is to develop, build, and test a prototype imaging spectrometer for active interrogation of mercury and other materials. The unit will be hand portable and contain about six times the volume of CdZnTe compared to the detector used during Phase-I, which could improve detection limits by more than a factor of five. A 9-MeV-dynamic-range analog readout chip will be used in the system, allowing for measurement of the entire spectrum of neutron-activation gammas from mercury and other materials. Research and development of new algorithms for neutron activation analysis including isotope identification, multi-MeV gamma imaging, and three-dimensional imaging will also be developed. The final prototype will be tested using a neutron source and activated materials to verify performance. The project will deliver a handheld prototype system for detection and imaging of neutron-activated mercury and other materials.The high-efficiency and high-dynamic range imaging spectrometer proposed for development will also be sensitive to chemicals, explosives, and other contraband of interest for national security and military applications. Other applications of the instrument include range verification for proton beam therapy, detection of chlorine intrusion in roadways and bridges to improve US transportation infrastructure, and vehicle-mounted sensors for large-area mapping of gamma rays. The detection and imaging algorithms developed under this program will advance the state of the art in portable gamma spectrometers by improving spectroscopic and imaging performance of CdZnTe detectors for gammas >3 MeV.