1.4 Measurement Results as Raw Data and Calculated Units
The LB 134 is able to output raw data as counts per second or,
depending on the application, different measured physical
quantitities calculated using a calibration factor. Typical units for
calculation are for
Contamination: Bq/cm², pCi/cm²
Dose rate: µSv/h, mrem/h, µGy/h, R/h
For most probes, the calculation is very simple:
Measured value = Calibration factor * (gross rate – background)
Of course, the gross rate is corrected with respect to dead time
or linearity (as in a ZnS scintillator). The displayed accuracy of
the measurement due to counting statistics also takes into ac-
count the background and the background measuring time. The
uncertainty in the calibration factor is not included. In the coun-
ter/timer mode, the statistical accuracy G is:
(%)
In the ratemeter mode:
(%)
Where:
Rb gross count rate cps
Ro background count rate cps
Tp sample measurement time s
To background measurement time s
tau time constant s
With preset time constant (also see below under Ratemeter),
tau is the entered time constant, with preset accuracy, tau is de-
termined every second from the current count rate and the en-
tered accuracy. For a high accuracy, one should always meas-
ure the background as long as possible, e.g. one hour. With in-
creasing sample measurement time in the counter/timer mode,
the accuracy is the better, the lower the accuracy value. In the
ratemeter mode with preset time constant, the accuracy, with a
given tau, also depends on the count rate. To get a higher ac-
curacy with the same count rate, we need to enter a larger time
constant. With preset accuracy, you just have to enter a smaller
number.
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