Long term performance of a gas filled ΔE-counter and the role of electronegative elements and moisture in counter deterioration

The long term performances of metal body (Al, brass, SS) proportional counters (non-flow type) operating at sub-atmospheric filling pressure (130 Torr) for CO2 and P-10 gases have been investigated. The causes of deterioration of counter performance with time have been attributed to the dynamic effects like percolation, dissociation, chemical reaction and outgassing from the counter walls which essentially increase the fraction of electronegative impurities like oxygen, CO, H2O vapour, etc. in the counter volume. Among them the role of moisture was found to be most damaging particularly in the case of CO2-filled counters. The possible measures to arrest deterioration have been discussed. Inspite of a very slow deterioration (peak channel shift) in counter performance over several days, it has been concluded that it is still possible to use filled ΔE-gas counters in reaction experiments provided a reasonably good energy resolution is retained.