An advanced coded imaging without side lobes

Passive detection of special nuclear material (SNM) at long range or under heavy shielding can only be directly achieved by observing the penetrating neutral particles that it emits: gamma rays and neutrons in the MeV energy range. The ultimate SNM standoff detector system would have sensitivity to both gamma and neutron radiation, a large area and high efficiency to capture as many signal particles as possible, and good discrimination against background particles via directional and energy information. We are exploring the use of time-modulated collimators that may lead to practical gamma-neutron imaging detector systems that are highly efficient with the potential to exhibit simultaneously high angular and energy resolution. We will present results from a large standoff SNM detection demonstration using a prototype high sensitivity time encoded modulation imager.

Time encoded fast neutron/gamma imager for large standoff SNM detection

Because of their penetrating power, energetic neutrons and gamma rays (∼1 MeV) offer the best possibility of detecting highly shielded or distant special nuclear material (SNM). Of these, fast neutrons offer the greatest advantage due to their very low and well understood natural background. We are investigating a new approach to fast-neutron imaging — a coded aperture neutron imaging system (CANIS). Coded aperture neutron imaging should offer a highly efficient solution for improved detection speed, range, and sensitivity. We have demonstrated fast neutron and gamma ray imaging with several different configurations of coded masks patterns and detectors including an “active” mask that is composed of neutron detectors. Here we describe our prototype detector and present some initial results from laboratory tests and demonstrations.

Results from the coded aperture neutron imaging system

: This dissertation explores the application of a mechanically collimated imaging device known as a rotating modulation collimator (RMC) to the orphan source search problem. Orphan sources are lost radioactive sources (accidentally or through material diversion) that can pose a threat to both human health and national security when they fall into the wrong hands. The RMC is a simple imaging system that can see through complex background scenes that can often mask these lost sources. Key developments for this research include the development of a new system model for predicting the system response when the source is located at any distance from the detector, the design, construction, and characterization of a prototype RMC, new methods for locating sources in three dimensions with a RMC, a new operating mode for imaging extended sources, an initial study on adaptive imaging with the RMC, and a method for predicting the time required to detect a hidden source of a known activity to a given level of confidence. Results from simulations and measurements are used to asses the performance of the RMC for the orphan source search problem.

A rotating modulation imager for the orphan source search problem

A new coded-aperture configuration for gamma-ray imaging is described. It measures a single Fourier component of the object distribution at a time and does not require a position-sensitive detector. If, however, a position-sensitive detector is used, three-dimensional information about the source can be obtained.

Gamma-ray imaging in Fourier space.

The spatial distribution of a radioactive fluid can be measured indirectly by observing the emerging gamma rays. A method is proposed and analyzed for gamma-ray imaging by stochastic time modulation and cross-correlation. Theoretical comparison is made to collimation and coded aperture techniques in gamma-ray image formation. Computed results are presented that illustrate the mean response and statistical error characteristics of this technique. Monte Carlo simulations are performed as a further verification. Because it relies upon a point-by-point reconstruction, rather than upon the integral properties of any particular aperture, the time modulation approach is seen to provide a theoretical basis for obtaining a smooth three-dimensional point response.

Randall S. May

Papers

Gamma-ray imaging with stochastic apertures.