Unexploded ordnance (UXO)-contaminated sites often include ordnance filled with inert substances that were used in dummy rounds. During UXO surveys, it is difficult to determine whether ordnance is filled with explosives or inert material (e.g., concrete, Plaster of Paris, or wax) or whether it is empty. Without verification of the filler material, handling procedures often necessitate that the object be destroyed in place, which has potential impacts to the environment, personnel, communities and survey costs. The Department of Defense (DoD) needs a reliable, timely, non-intrusive and cost-effective way to identify filler material before a removal action. A new technology that serves this purpose would minimize environmental impacts, personnel safety risks and removal costs; and, thus, would be especially beneficial to remediation activities.

The objective of this project was to demonstrate that a portable neutron spectrometer utilizing scintillating fiber detectors is capable of distinguishing between inert and explosive filler material in UXO.

The spectrometer utilizes a novel neutron detector which improves on the portability and speed of current designs. Neutron spectrometry can indicate the relative hydrogen, carbon, nitrogen and oxygen ratios in dense containers remotely and non-destructively. The technology is similar to devices used commercially to determine soil moisture, characterize oil wells and measure asphalt quality. A neutron source interacts with the target object, some neutrons are backscattered into the detector, and their energies inferred. The differences in the incident and backscattered neutron energies are indicative of the relative hydrogen, carbon, nitrogen and oxygen contents of the target. These data can then be used to differentiate between inert filler materials and explosives. It is not necessary to fully determine the neutron energy spectra, or deconvolve the instrument response, in order to determine if given targets match certain criteria related to chemical ratios. A statistical analysis of the raw data is adequate to sort targets into inert and explosive categories.

In determining the content of UXO, PNNL's neutron spectrometer analyzes the energies of neutrons that are backscattered to the detector.

The researchers confirmed that alteration of neutron momentum spectra includes information about the chemical content of the intervening materials. A high-efficiency moderating detector reduces data collection times to a few minutes, or less, and enables utilization of smaller neutron sources than previous efforts. Three inert fillers and an empty container were separated by analysis of the detector response without regard to container effects. Separating inert fillers from each other is more challenging than separating ordnance from inert fillers. Technical difficulties prevented the researchers from collecting measurements to determine the minimum mass of ordnance filler and measurements in other geometries.

The filler identification system developed in this project is broadly applicable to many installations and sites that are currently contaminated by UXO. The researchers recognize the need to successfully develop, customize, deploy, and commercialize the neutron spectrometer technology to meet U.S. industry and government needs. Toward this end, the researchers have licensed the core scintillating fiber technology to industry and have invested in the patent protection of hydrogen content detectors based on this technology for use in nuclear safeguards.