Leak-Rate Determination Using Krypton 85

An account is given of the krypton 85 test for determining leak rates of tubes evacuated to a pressure of ~10-8 torr. The minimum detectable air leak rate is of the order of 10-12 atm cc s-1, the limit being set by the background radiation and by the inaccuracy of dispensing a known-activity calibration standard. A description is given of the soak procedure and of the operation and calibration of two alternative counting equipments. An essential requirement for unambiguous counting is the facility for efficient removal of unwanted contamination that adheres to the exterior surface of the tube as a result of the soak. Determination of the optimum cleaning process is, of course, a matter for experiment, and must be proved for each particular application.