Fluke Biomedical 956A-201-M2 - Manual (Page 77)

4-5
8. Remove the detector from the field calibrator.
9. The values recorded in steps 5, 6, and 7 should be between the values calculated in step 1.
If the values recorded are not within the calculated range, then
the detector is out of calibration. Return the detector to Cardinal
Health, Radiation Management Services for evaluation and
determination of proper course of action. Do not attempt to
change the conversion constant to "fit" the readings.
4.5 DETECTOR CALIBRATION
Plateau Verification
Plateau verification does not need to be performed if the detector
is supplied with a calibration data sheet, identifying the factory
high voltage setting. It may, however, be performed as a
preventative maintenance test for monitoring changes in the
length of the detector plateau over time. It should be performed if
the detector calibration data sheet is not available. If the detector
is a replacement part, the high voltage should be set to the value
shown on the calibration data sheet supplied with the detector.
To determine the optimum high voltage setting for the G-M detector, perform the following:
1. Connect an electrostatic voltmeter to the high voltage section of the readout. Adjust the high voltage
to 500 Vdc.
2. Connect a counter to the input pulse circuitry at the readout. Adjust the counter for a 1-minute time
base.
3. Place a source near the detector and position it so that a reasonable count rate will be displayed on
the readout.
4. Using the counter, take a 1-minute count. Record the high voltage setting and the gross count rate.
Do not exceed 650 Vdc under any condition. Damage to the G-
M tube will occur.
5. Repeat step 4, increasing the voltage in 20 Vdc increments until the maximum operating voltage is
reached.
6. Plot the resulting data, gross count rate vs. high voltage. Locate the plateau where the slope is
minimized (between the minimum voltage of 500 Vdc and the maximum voltage 650 Vdc). The
operating point should be the mid point of this plateau. Figure 4-1 is an example of the detector
plateau.
7. If the plateau is not within the 500 Vdc to 650 Vdc range, return the unit to Cardinal Health, Radiation
Management Services for G-M tube replacement and factory NIST source range calibration.
NOTE
NOTE
CAUTION