Design and construction of a prototype rotation modulation collimator for near-field high-energy spectroscopic gamma imaging

Neutron Stimulated Emission Computed Tomography (NSECT) is being developed for in vivo measurement of the concentration and location of biologically relevant elements. NSECT is a spectroscopic imaging technique whereby the body is illuminated via high-energy neutrons that excite elemental nuclei that then relax through characteristic gamma radiation. This imaging technique requires high-resolution spectroscopy, thereby eliminating the use conventional scintillation gamma cameras. Consequently, high-purity germanium (HPGe) semi-conductor detectors are utilized, providing no spatial information. To obtain 2D elemental concentration images, we are adapting high-energy solar spectroscopy technology. A rotating modulation collimator (RMC) consisting of two parallel-slat collimators is placed in front of the detector to modulate the incoming signal in a manner predicted by its geometry. Reconstruction of 2D images is feasible by counting the number of incident gammas at each rotation angle. The challenge is to identify a RMC geometry that allows this method to work in the near-field environment, which has far fewer assumptions and simplifications than the infinite focus of solar imaging. Herein we describe construction of a prototype RMC and experiments conducted with a radioactive ²²Na point source. These experiments verified that the RMC modulates the signal in manner consistent with its geometric and physical properties.