Constancy
Daily constancy check is performed by measuring a long-lived radioactiv-
ity (e.g.,^137 Cs) in the dose calibrator and observing the variation not to
exceed ±10% relative to the previous day reading. If the variation exceeds
±10%, the unit must be repaired or replaced.
Accuracy
Accuracy of the dose calibrator is determined by measuring the activity of
at least two long-lived radionuclides (e.g.,^137 Cs and^57 Co) certified by the
National Institute of Standards and Technology (NIST) in the dose cali-
brator and comparing the measured activity with the activity reported by
the NIST. The measured value should not differ from the standard value by
more than ±10%. If it exceeds ±10%, the unit must be repaired or replaced.
Linearity
Decay Method
The linearity test indicates the dose calibrator's ability to measure the activ-
ity accurately over a range of values. It is performed by measuring a
radioactive source (e.g.,99mTc), containing the highest activity normally
used in the clinical setting, in the dose calibrator at different time intervals
until the source decays down to less than 30mCi (1.1 MBq). The measured
activities are plotted against time on a semilog paper and the “best fit” line
is drawn. If the deviation of any point from the line exceeds ±10%, the dose
calibrator needs to be replaced, or a correction factor must be applied to
the data in the nonlinear region.
Shielding Method
The advantage of this method is that it is less time-consuming and is easy
to perform. The method utilizes a commercial kit, called Calicheck, that
contains seven concentric tubes or “sleeves.” All sleeves except the inner-
most one are lead-lined with increasing thickness simulating the various
times of decay. When an activity source is measured by using first the inner
sleeve, followed sequentially by increasingly thick sleeves, the data repre-
sent the activities at different decay times. Calibration factors are calculated
by dividing the innermost tube reading by each outer tube reading. For sub-
sequent linearity tests, identical measurements are made using the sleeves,
and each measurement is multiplied by the corresponding calibration
factors. Each corrected sleeve reading should give an identical value, and
the average of all values is calculated. If an individual reading exceeds the
average value by ±10%, then the calibrator needs replacement, or a cor-
rection factor needs to be applied.
It should be noted that before the shielding method can be instituted, the
linearity test must be first performed by the decay method for a new dose
calibrator.
76 7. Gas-Filled Detectors