Neutron and Gamma Ray Imagers
Our Legacy Imagers
Many of developments in radiation imaging date back over 30 years to our involvement with Nuclear Medicine in the development of instrumentation for gamma-ray imaging. Since the medical instrumentation industry represents one of the largest commercial outlets for radiation instrumentation, we expect that our present involvement in this area will continue to grow in the foreseeable future. The camera shown in the figure above is our first radiation imager applied for environmental imaging, and the success initiated generations of more sophisticated and complex radiation imagers. The radioactive materials were produced in our nuclear reactor, and the images shown required 1 graduate-student-honeymoon, acquired remotely. That was quite a feat in 1993.
After exploring simple mechanical collimators to form images, we led the development of Compton scattering techniques that avoid the efficiency loss due to the collimator in conventional gamma-ray cameras. While there appears to be fundamental limits in the use of this technology at the low gamma ray energies currently predominate in nuclear medicine studies, we recognized there would be decided advantages in their application to higher energy gamma rays in either a medical or industrial context. Currently, there is a great deal of interest in techniques for higher energy gamma ray imaging for use in the decontamination of nuclear facilities or in fast neutron imaging for the detection of clandestine nuclear materials. For these reasons, imaging studies have become an even more active research field.
Over the past three decades, we have developed families of mechanically-collimated (e.g., parallel-hole collimator, coded-aperture) and electronically-collimated (i.e., Compton) radiation imagers. We then moved into hybrid cameras that utilized both types of imaging in a single instrument (e.g., SORDS) More recently, we have been engaged in developing portable dual-particle imagers (to detector both gamma rays and fast neutrons for nonproliferation applications) using time-encoding. These devices have the advantage of simplicity and cost-effectiveness for field applications.
Time-encoded imaging (TEI) was invented at Michigan for nuclear medicine imaging. The technique has the advantage of encoding source radiation through a time-varying mask pattern on a single detector rather than encoding the source radiation through a fixed mask in space on an array of detectors. The advantage of the TEI technique is simplicity (it requires only a single radiation detector) and cost, while the disadvantage is typically a longer acquisition time. When multiple imaging systems are desired, the TEI advantages may prove dominant.
Time-Encoded Gamma Ray and Fast Neutron Imaging with an Asymmetric Mask (MATADOR)
The MATADOR imaging system is a 1D, dual-particle, adaptive cylindrical TEI system. MATADOR utilizes a dual-layer mask and two non-position sensitive detectors to image both gamma rays and fast neutrons. To modulate gamma rays, the inner layer of the mask is made of 0.635 cm of tungsten, and to modulate fast neutrons, the outer layer is made of 6 cm of high-density polyethylene. Both mask layers are arranged in a uniformly redundant array pattern with 35 elements, thus the angular width of an element is ∼10.3◦. The outer radius of the mask is 25.7 cm and MATADOR can collect a full revolution of data in 90 s.