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, provides electronic noise that is small enough to permit detector operation in ionization mode, and without the need for an electron-transmitting Frisch grid. This new detector therefore has significant advantages over previous detectors used in CBS instruments, which primarily consisted of arrays of individual ³He filled cylindrical counters with dead regions and nonuniform detection efficiency. The correct choice of gas mixture/pressure, drift field and shaping time is significant in achieving a successful performance in the new detector