A proportional chamber to measure the screening potential in 3H β decay

A gas proportional chamber has been assembled to make precise observations of the low-energy region of the 3H β spectrum. Backgrounds have been measured and understood, and are maintained at ∼2% of the data rate. Wall effects have been simulated for this chamber and are predicted at <0.5% of events. Systematic effects which may introduce errors into the observed spectrum have been carefully measured at a level below ∼10−4 per ADC channel. This apparatus will be used to make precise measurements of the effects of screening of the nuclear charge by orbital electrons on the β spectrum of 3H, and to attempt to observe the effect of a hypothesized admixture of heavy neutrinos in the 3H spectrum.