Mitigation of Outgas Effects in the Neutron-Capture /sup 6/Li Pulse-Mode Ionization Chamber Operation

Abstract

When a novel neutron-capture lithium pulse-mode ionization chamber (LiPMIC) was designed to replace He proportional counters as a more economical solution for some homeland security and counter-terrorism applications for large-scale, long-term remote sensors, this brought on a fundamental disadvantage of possibly exposing the ionization chamber to outgas effects from plastic detector components. The design of LiPMIC intends to reduce the outgas effects to a specified level for continuous operation by selecting the fill-gas mixtures where the electron dynamics do not vary within the range of outgas concentration. For example, Garfield simulation of electron transport properties inside LiPMIC indicates the convergence of electron drift velocity for a different concentration of methane between 0.06 and 0.09 V/cm/Torr. The sufficiently high drift velocity in this low electric field strength is characteristic of the Ar-CH/sub 4/ mixture, since mixtures of argon and other hydrocarbon gas indicate a higher target range of electric field strength for stable drift velocity. The electron drift velocity between 4% and 15% of methane concentration remains within 20% in the said field strength range. The Garfield simulation data of LiPMIC of analytic geometry have shown that when this criterion is met, the outgas effect from HDPE and other outgassing detector parts inside the detector can be mitigated for continuous operation.