A segmented $^3$He ion chamber for n-spin rotation experiment

Searches for parity violation effects in few nucleon systems often require current-mode detectors. We describe a high efficiency $^3$He/Ar ion chamber designed to operate in current mode at a CW cold n source, It uses $^3$He to absorb the neutrons and Ar to limit the range of the ions from the n+$^3$He reaction. It is longitudinally partitioned to gain information on the n velocity spectrum and transversely segmented to monitor the spatial distribution of the n beam. We adjust the $^3$He and Ar density to obtain an approximately even absorption of the beam in the 4 longitudinal partitions. A similar detector is described by Penn et al$^{[1]}$ and was tested at the LANSCE for possible use in the NPD$\gamma$ experiment$^{[2]}$. We will present details on the design and performance as measured on the NG6 beam at NIST. [1] S.D. Penn et al, Nucl. Instr. and Meth. A 457 (2001) 332-337. [2] C. Blessinger, PhD thesis, Indiana Univ. 2000