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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
h i t
ti
d It
b
t h t
l t
t
t
c arge n egra on mo e. may e necessary o ave wo e ec rome ers o
cover the full range of brachytherapy sources to be calibrated.