Precision measurement of ionization current from a long half-life radioactive source

We present precision measurements of the ionization current produced by a sample of carbon-14. Using specialized ammeters with a gain uncertainty of less than a part per million (ppm), the decay of the ∼400 pA ionization current, ∼1 fA/week, is resolved by the measurement system for experimental runs of 50 days. When the apparatus is temperature controlled to ±0.05 °C, fluctuations in instrumental offsets are suppressed, and the random uncertainty is limited by Poisson noise, reaching ∼2.5 ppm of the ionization current for 24 h of data integration. We discuss the possibility of a measurement of the carbon-14 half-life by directly observing the decay in the ionization current.