The Model 844-36 and associated reference sources are used to field calibrate Fluke Biomedical’s pulse mode process monitor scintillation and GM detectors. The Model 844-36 consists of three (3) lead filled “doughnuts” clamped together with four (4) bolts, to provide a lead shielded cylinder, into which a scintillation detector or a GM detector (when installed in the appropriate adaptor) may be inserted. The assembled calibration fixture provides two (2) inches of lead shielding to minimize the detector response induced by ambient background radiation. At the bottom of the fixture, a removable slide drawer is provided to permit installation and withdrawal of reference calibration disc sources. The drawer is reversible, with one side machined for beta sources and the other side machined for gamma sources.
An optional tau drawer, machined to accept the two sources required to measure detector dead time, is also available. A stop is provided in the fixture to position the detector at a fixed, reproducible distance from the reference disc source. To perform a field re-calibration, the detector is inserted into the Model 844-36, the appropriate calibration disc source is inserted into the slide drawer, and the counts from the detector for a fixed time period are recorded. To achieve a 1% counting error, the counting time should be long enough to yield a minimum of 10,000 counts. A number if field calibration sources are available for field calibrating both beta and gamma scintillation detectors.
For GM detectors, the detector is inserted into a Lucite® adaptor designed for the detector, the reference source is inserted into a cavity in the adaptor, and the adaptor, with GM detector and reference source is inserted into the Model 844-36. In this case, the Model 844-36 functions as a shield, to reduce ambient background radiation.