SPURIOUS PULSES IN METHANE FILLED PROPORTIONAL COUNTERS

The time distribution of spurious pulses in methane filled proportional counters under various conditions is presented. In all cases the spectra were found to be continuous up to the positive ion transit time at which point a weak ion-cathode effect was observed. The average mobility of the positive ions in methane was found to be 2·4 ± 0·1 cm/sec per volt/cm. Possible mechanisms for the production of spurious pulses prior to this transit time are discussed; an ion-molecule reaction is believed to be responsible for spurious pulses in the middle time range while those produced within the first few tens of microseconds may result either from electrons becoming trapped in the dense positive ion cloud or from a two stage process involving hydrogen radicals. An upper limit for the probability of a spurious pulse occurring when a couter was operated at the centre of the plateau (for Fe-55 X-rays) was estimated as 2 × 10-5 per genuine pulse. © 1969.