A Propane Bubble Chamber as a Low Level Counter for Tritium and CARBON-14

Abstract

A bubble chamber has been operated as a counter with the propane filling being highly superheated so that it is sensitive to the low energy (average 6 kev) electrons emitted in tritrium beta decay. Experimental techniques are described using quartz chambers of 10 ml volume operated at 55°C. Three plateaus are observed in a plot of the chamber counting rate against the superheat pressure (superheat pressure equals vapor pressure at 55°C minus expansion pressure). These were due to alpha particles (30-40 psi) recoil protons (50-90 psi) and electrons (140-220 psi). The chamber background counting rate was stable and reproducible for long periods of time. The counting rate was shown to be proportional to the activity in a tritiated propane filling; the detection efficiency was 65% on a plateau from 200 to 220 psi superheat. Cosmic rays could be eliminated from the background counting rate using anticoincidence techniques. The background rate for a 10 ml quartz chamber shielded by 2 of borated paraffin and 8 of iron was 4.5 cpm. We conclude that bubble chambers of somewhat larger size are suitable for the measurement of very low specific activities of such isotopes as carbon-14 and tritium with a sensitivity comparable to the best available gas counting systems.