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Chapter 5: Calibration at the SSDL and Hospital Level

5.5. Guidance on constancy limits for well type chambers

Chamber output stability should be checked at least 4 times per year. If the

calibration factor from

137

Cs re-calibrations, and periodic constancy checks,

remain the same within 1% for high-energy photon sources, or within 1.5% for

low-energy photon sources, it can reasonably be assumed that the calibration

factor for other sources has not changed. Recalibration is recommended if it

is observed that response changes by more than the limits given above.

5.6. Electrometer to be used

IEC 60731 describes desired characteristics of electrometers. They also shall

be capable of measuring up to 0.2 μA for HDR sources and have a signal

resolution of 0.1%. For LDR sources, signal resolution should be < 10 fA or

less; this may be achieved by charge resolution of 0.2 pC when used in

charge integration mode. It may be necessary to have two electrometers to

cover the full range of brachytherapy sources to be calibrated.