Abstract:
Soreq NRC and PTB are developing dual-probe (gamma-ray and fast neutron) high-spatial-resolution radiography for detection of explosives and special nuclear materials (SNM).
The explosives are detected by Fast Neutron Resonance Radiography (FNRR), that exploits the specific energy-dependent cross-section characteristics of several low-Z elements in the neutron energy-range En=1-10 MeV. The neutrons are produced in 1-2 ns bursts using a pulsed beam of 12 MeV deuterons impinging on a thick Be target. Neutron spectroscopy is performed by means of a novel, integral-mode variant of the time of flight (TOF) technique. This method holds promise for detecting a broad range of conventional and improvised explosives, by determining the identity and density distribution of light elements such as C, N and O within an inspected object. It is expected that the high spatial resolution and the capability of operating at very high counting rates particular to this method will ultimately permit a significant improvement in baggage screening capabilities at airports, in terms of enhanced throughput and operator-independent detection of sheet explosives.
In addition to neutrons, the d-Be reaction also yields prompt gamma-rays in the 0.5-6 MeV range. These gamma-rays are readily distinguished from neutrons by TOF. By taking the attenuation ratio of neutrons to gamma-rays through the inspected object it is possible to distinguish materials according to their atomic number. For example, the ratio for tungsten is higher by a factor of 4.2 and 2.3 than that of water and iron, respectively. The method produces high resolution images and also has the potential to detect small quantities of SNM.
In this paper we shall describe the time-resolved neutron-gamma ray imaging detector and present studies of various detector parameters, such as spatial and energy resolution, imaging and material discrimination capabilities.