5.6 Nuclide Calibration Selection
In order to carry out a radiological assessment of the data obtained during contamination monitoring, it is
necessary to know the efficiency response of the monitor for the radionuclide under investigation
and to make the appropriate calculations.
The Tracerco T401 has been specially developed to provide a direct reading of the level of
contamination in Bq/cm
2
for a number of radionuclides which the operator can pre-select.
The efficiency calibration response functions of the monitor for various radionuclides are pre-
programmed into the instrument software. These functions are also checked and reset during
the annual inspection and calibration process.
In order to carry out a ‘statistically’ effective measurement of surface activity it is necessary to
accumulate counts over a reasonable timeframe and to make appropriate corrections for
‘background’ levels of radiation. The following procedures should be followed when carrying out a
radiological measurement.
5.7 Carry Out a “Background” Count
After switch on press the yellow “
MODE
” button to select the background function (allow 5 seconds for the
display function to normalise). The “
BGnd
” message will appear on the screen. Confirm this choice
by pressing the “
SELECT / RESET
” button.
The monitor will commence a background radiation count which will continue for a period which,
based on environmental levels, is sufficient to provide a statistically acceptable count. A typical
count duration is 60 seconds. The unit has been pre-programmed to accumulate and terminate this
count. During the count period, the display will show the real time integration of counts. After the
count has been automatically terminated, the display will show the mean value of the ‘Background’
count in counts per second. This value will be automatically stored in memory.
Press the “
SELECT / RESET
” button to accept this count and move to Bq/cm
2
monitoring. Alternatively
press the ‘
MODE
’ button to repeat a background count.
The background count will remain in memory after the monitor has been turned off. This value
will only be updated when the above procedure for carrying out a background count is
repeated.
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