The project was aimed at developing and testing a neutron-based detector system for detecting and measuring chloride in in-service concrete. The nondestructive test method is based on prompt gamma neutron activation (PGNA). Neutrons from a portable source are used to irradiate the concrete structure. The neutrons are captured by atoms in the material, and in this process gamma rays are emitted with characteristic energies. The gamma rays travel out of the concrete and are then counted by detectors. The size of each gamma ray peak in the spectrum is proportional to the concentration of the element in the concrete. The original work plan involved three stages: (1) Numerical modeling and simulations to optimize the design of the system; (2) Instrument assembly and calibration in the laboratory on test specimens with known chloride concentrations; and (3) Field testing on actual concrete bridges. Work on the design calculations (Stage 1) was satisfactorily completed. This consisted of specifying the dimensions of the planar gamma ray detector, selecting the type of neutron generator, and modeling the moderator using the Monte Carlo N-Particle (MCNP) software. However, a major obstacle to the completion of Stages 2 and 3 was the lack of a thermal neutron source in the timeframe of this project. The nuclear reactors at the University of Maryland and at the National Institute of Standards and Technology (NIST) were not operable. Consequently, most of the work proposed in Stage 2 of the original work plan, which involved calibration of the system in the laboratory, could not be accomplished. Still, some experimental investigations were carried out using radio-isotope gamma-ray sources that confirmed the principle of electronic collimation and verified the improved directionality of the system. In addition, experiments using the cold neutron PGNA station at NIST provided data that can be used to estimate the performance of a portable field PGNA system. A number of state departments of transportation have expressed interest in using the PGNA system when it becomes available. Current plans are to continue the research on laboratory testing of the system using the portable neutron generator at NIST.
