Current mode wire chambers for cold neutron detection at the SNS FNPB

A $^3$He chamber is a multi-wire proportional counter for detecting neutrons. A $^3$He nucleus that captures a neutron will break up by the reaction $n+^3He\rightarrow p+T+765\ keV$ which is detected by gas ionization inside the chamber caused, by the reaction products. The 765 keV is released as kinetic energy of the proton and triton, allowing a consistent signal from each capture. The chamber gas is a mixture of gases with a fraction of a $^3He$, the amount of which is used to adjust the neutron thickness; the fraction of beam that is captured in the monitor. I will report on the design, construction, and testing of a new set of beam monitors for the Spallation Neutron Source Fundamental Neutron Physics Beam line (FNPB), which use this technology. The $^3$He chambers will be used to monitor the neutron flux at various positions along the neutron beam, as it passes through cold neutron experiments planned at the SNS. In addition, I will report on the design of a $^3$He wire chamber that will be used in the n$^3$He experiment at the SNS. This chamber uses the same neutron detection process as described above, but will be black to neutrons (high $^3He$ content) with a small amount of ionization gas, to allow the protons to range out over as long a distance as possible. This chamber will be used to measure the parity violating longitudinal asymmetry in the number of protons emitted in the capture reaction.