Design considerations of a coded source neutron imaging system at the PULSTAR reactor

A Neutron Imaging Facility is operational at beam tube #5 of the 1-MW PULSTAR Reactor at North Carolina State University. The neutrons are collimated through a 3.66 cm × 4.45 cm rectangular aperture, the effective diameter (D) of which is 4.57 cm. If the image plane is set to be at the end of the beam line, the distance from the aperture to the image plane (L) can be maximized to be 6.5 m. At this location the L/D ratio is approximately 140 and the neutron flux is 1.8 × 10⁶ n/cm²·sec. To explore high resolution imaging at the current facility, a coded source technique is under development. In this case, the neutron source is encoded using a mosaic of pinholes. The collection of pinholes represents an aperture (or mask) that generates an encoded image of the object at the detector. Consequently, the use of a large number of pinholes ensures sufficient signal-to-noise performance. In addition, the L/D ratio of the system is increased, which greatly improves the geometric degradation of the image. Coded masks with pinholes of 50 μm diameter are being evaluated using Monte Carlo neutron transport simulations that take into account facility characteristics. Using the existing system dimensions, it is estimated that for a coded mask placed in an open area of 4 cm × 4 cm, a field of view that is 2.7 cm × 2.7 cm in size can be established. Furthermore, assuming the use of a detector with an inherent spatial resolution of 100 μm, the simulations demonstrate that an overall spatial resolution of approximately 50 μm can be achieved. Experimental implementation of the coded imaging arrangement is currently underway.