A position-sensitive ionization chamber for thermal neutrons

A position-sensitive neutron detector based on ³He is being developed for a national crystal backscattering spectrometer (CBS) at a pulsed neutron source. It comprises a single gas volume, of constant depth, in the form of an annulus that is also part of a truncated cone. Charge from each event is collected on one or two anode strips, out of a total 336 around the annulus, yielding an angular resolution of about 1°. Low-noise signal processing, in the form of a monolithic preamplifier/shaping amplifier connected to each anode strip, provides electronic noise that is small enough to permit detector operation in an ionization mode. In addition, the segmented anodes ensure near complete induced charge collection. This new detector therefore has significant advantages over previous detectors used in CBS instruments, which primarily consisted of arrays of individual ³ He filled cylindrical counters with dead regions and nonuniform detection efficiency. The optimization of gas mixture/pressure, drift field, and shaping time is significant in achieving successful performance in the new detector