System for 14Co2 radioassay in the presence of a high background radiation

The detection system consist of a 30 cm3 spherical proportional counter to measure the beta rays from 14C labelled gas. This counter is built inside a plastic scintillator. Pulses from the photo-multiplier tube which views this scintillator and from the wire anode of the proportional counter are amplified and fed to an anti-coincidence circuit which passes the proportional counter pulses only when no pulse occurs in the scintillator. The purpose of the scintillator and anti-coincidence circuitry is to reduce sensitivity to background radiation. Rejection factors of as high as 80 have been achieved for gamma radiation. Under optimum operating conditions, the detection system achieves a figure of merit of 6.2 in the presence of a SNAP-19 unit and 27 in the laboratory environment. This allows lower limits of detectability of 7 pCi and 1.4 pCi of 14CO2, respectively, at a confidence level of 0.999999. The system has been designed for possible use on a Mars mission.