Plateau Correction in High Precision Half-Life Measurements

The test of the unitarity of the Cabibbo-Kobayashi-Maskawa (CKM) matrix via β decays requires measurements accurate to better than 0.1%. Often reported half-lives reach accuracies well below this limit, bringing to light distortions at a level of ~0.02%. For measurements using proportional gas counters, even when operated in the "plateau" region, there is a small increase in the detection efficiency with the bias voltage. As the counting rate decreases during a decay measurement, so do the current and voltage across the resistor that polarizes the anode wire; thus, the actual potential of the wire increases during the decay measurement, making the detector less efficient at the beginning of the decay. This leads to a weak rate-dependent loss whose impact on the extracted half-life becomes comparable to the accuracy of the final result. A method to correct for these losses will be presented.