Thermo Electron EPD MK2 - Page 94

EPD/HB/40521/ 000
ELECTRONIC PERSONAL DOSEMETER HANDBOOK
MAINTENANCE
ISSUE 5
Page 80
Thermo
A change in EPD calibration is usually indicative of a fault in the EPD and, due to
the complexity of achieving calibration over the full energy range, it is
recommended that any EPDs that fail calibration checks are returned to Thermo
for repair, recalibration or replacement.
During calibration the calibration constants, and the threshold levels for each of
the four channels are set. The threshold levels determine the minimum pulse size
that will be considered a "count". The calibration constants ensure that the
channels are combined in the correct fashion to give correct dose readings.
Calibration Constants
. Calibration constants convert the counts accumulated on
the four detector channels into dose equivalents. These constants are:
HGSens10
SGSens10
HGSens07
SGSens07
FBSens07
BCSens07
Where EPDs are provided with calibration in the unlocked state (see above), it is
these six factors which may be adjusted within the defined percentages from the
manufacturers original calibration. These ‘gain’ adjustments are made using
software facilities available via Thermo Electron but should only be considered in
consultation with Thermo.
Detector Thresholds
. Detector thresholds discriminate between radiation pulses
received by the EPDs detectors. These thresholds are set as course and fine
values measured as:
Hard Gamma (HG)
Soft Gamma (SG)
Full Beta (FB)
Beta Compensating (BC)
From software version 11 the detector thresholds were changed to improve the
precision and accessible range. No facilities are provided to adjust the detector
thresholds and these can only be set by the manufacturer.
EPD calibration may be checked using a Thermo irradiator, or by exposure to a
known radiation field, in a manner approved by the appropriate regulating
authority or local rules. It is recommended that calibration is checked annually.