The sealed radioactive 137Cs check source of 3.7 MBq (100 µCi) activity is designed for routine checking of isotope calibrators to detect instability. The gamma emitting 137Cs nuclide of 662 keV with 30.17 years half-life has a long live span, so that only little correction for decay is required. The source matches the form of the samples to be assayed. It consists of an outer plastic (PVC) holder, shaped to resemble a 10 ml multidose vial, type P6 (diameter 20 mm, height 57 mm). This contains the 137Cs source, which is incorporated in a ceramic pellet and welded in a stainless steel capsule.