Prior to the selection of a disposal method for unexploded ordnance (UXO), a determination must be made of the ordnance type and what filler material it contains. The materials can range from standard military explosives to chemical agents to inert simulants. Currently, trained UXO experts perform this determination using external markings and visual examination. Many times the UXO has weathered or corroded and the markings and external visual cues are deteriorated or absent. If a positive determination cannot be made that the UXO is free of explosives or chemicals, all questionable UXO are required to be treated as explosive- or chemical-filled, so the cost of clearance and disposal operations is greatly increased. If a less conservative approach is used, accidents can occur that lead to injury or loss of life. There is the need for a means of quickly and accurately determining the fill of UXO to permit the rapid disposition of inert or empty rounds and proper handling of explosive- or chemical-filled UXO.

The objective of this project was to evaluate a neutron-based, non-intrusive inspection technique called Associated Particle Imaging (API) for use as a man-portable sensor that discriminates the fill of unexploded ordnance (UXO) found on the ground surface or partially buried. The goal is to have a sensor weighing less than 80 pounds (35 kilograms) that can be carried or wheeled into place and provide identification of the munitions fill within a few minutes. Objectives of this project were to conduct experiments with a lab-based system to determine the performance capability of API, including the probability of detection and false alarm.

Technical Approach

Monoenergetic 14 MeV neutrons are emitted from a small electronic generator that employs the deuteron-tritium nuclear reaction. An alpha particle is emitted directly opposite to the neutron. In the API technique, the alpha particle is detected using a position-sensitive detector so the direction of the outgoing neutron is known. An outgoing neutron that interacts in the target volume can produce gamma rays, the energy of which is measured by nearby detectors. By using the time of flight between detection of the alpha particle and the gamma ray, the location of the neutron reaction is known. The gamma ray spectra are analyzed for each voxel to determine the amount of carbon, oxygen, and nitrogen in the target volume and, therefore, the material present. Because of the “electronic collimation” of the neutrons, the background from surrounding material is greatly reduced, leading to improved performance compared to other non-API-based neutron sensors. Furthermore, a separate background measurement is not required, reducing overall inspection time.


The researchers initially conducted trade studies of various gamma ray and alpha particle detectors and high-speed electronics for use with a lab system. The API generator would be leased from its manufacturer and integrated into a lab system for the testing. SAIC later teamed with Applied Signal Technology, Inc., which provided a modified version of their API-based neu-VISION™ system used for explosives detection in vehicles. A number of experiments were conducted on a set of simulated UXO targets with inert and simulated explosives fills. Because of the lower output of the API neutron generator, only a subset of the desired measurements was made. However, the performance was good, with a probability of detection of 83% and a probability of false alarm of 3%. The performance and inspection time are expected to greatly improve with a stable, high-output API generator. Based on these results, a concept design and plan forward were developed.


The portable API system could be used during remediation efforts to quickly and safely identify UXO fills and separate explosives from inerts. The cost savings is significant compared to efforts where each of the munitions is assumed to be an explosive, leading to expensive excavation efforts. Based upon the results of the experiments, a concept design of a man-portable system was produced.