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

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.