A Position Sensitive Proportional Counter for Neutron Radiography

A prototype position-sensitive proportional counter was investigated for use in neutron imaging applications. The counter uses a converter foil of 6LiF and a collimation grid to achieve improved resolution compared to counters filled with a neutron-sensitive counting gas. The proportional counter consists of a grid of 36 1-mil pyrolytic carbon coated quartz fibers on 2.5 mm centers. Because of the high resistance of the collector wires and the distributed capacitance to ground, the rise time of a pulse will increase with distance along the collector. This rise-time information is used to determine the position of an event to accuracies less than 1 mm along the wire. Standard nuclear instrumentation techniques are utilized with readout on a multichannel pulse height analyzer or on a storage scope. Tests performed with alpha particles showed that a geometry using collector wires off center by 0.5 mm produced the best resolution. The best resolution achieved was 0.59 mm FWHM, but the off-center geometry was not used for the neutron imaging experiments because of difficulties in achieving stable operation. Neutron images produced by this counter had an unsharpness of 5.5 mm (3.3 mm FWHM) and a contrast of 55%; this was wijh a 6LiF converter foil (100¿). Images with 10B and Gd converter foils showed unsatisfactory contrasts.