RSDetection
User’s Manual
March 2016 S131-200-UM Rev A
Page 22
4.1.4 Measurement Characteristics
Table 5 provides measurement characteristics.
Table 5 Measurement Characteristics
Accuracy is often misunderstood and can be confused with the
statistical characteristics of the measurement. Accuracy is a
measurement of how well the detector reports a reading relative to
the actual radiation present, assuming the sampling time is long
enough such that statistical errors are not significant.
Another component of accuracy is drift, primarily caused by
temperature variations in the environment or long-term drifting over
time.
The calibration technique is very important since it determines the
underlying accuracy of the measurement. The accuracy of the
calibration is affected by three major components: the radiation
source accuracy, radiation field stability during calibration, and
measurement error.
The radiation source used is nominal 12 mCi of Cs137 that is certified
by NIST to an accuracy of 3%. This certification establishes the best
accuracy achievable.
Field variations during calibration are minimized by performing
measurements with and without a lead shield in the path of the
primary beam of radiation. By subtracting these two measurements,
the background and the scattered components of the source are
eliminated.
Last is the measurement error as a result of statistical and
experimental error. Two techniques are used to minimize these errors:
long averaging times and repeated calibrations at multiple distances.
Averaging times are selected to reduce the statistical errors to a
negligible amount in comparison to the 3% source accuracy.
Temperature drift is minimized in two ways in the design of the ion
chamber system.
The first is by incorporating components that have low temperature
coefficients. However, some characteristics of the electrometer are
still sensitive to temperature, and in order to meet the drift
specifications, the microprocessor firmware compensates for drift by
using a series of constants that represent the characteristics of the
electrometer.
These constants are determined by a test procedure performed on
each electrometer. These values are entered into the electrometer’s
non-volatile memory. Storing these values within the electrometer
means, in the situation where an electrometer requires replacement,
no other action is necessary.
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