Includes two active disc sources, one blank

This set is used for determining the resolving time of GM Counters. It consists of three half discs, two of which contain 5 micro curies of Tl-204 plus a third half disc with no activity. The count-rate of each half disc plus the blank (to maintain constant geometry) is measured and then both active half discs are combined for a measurement with high count-rate. The counting loss may now be calculated by adding the rates from each half disc and comparing the result to the count-rate with both halves combined.