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Reset Detector:
To reset the detector you have to push the buttons <sk1> and <clear> at the same
time. This will interrupt the plateau recording and reset the detector electronics. A reset
also has to be performed when the microprocessor of the detector has been replaced.
Pt 100 Adjust (N/A)
The Pt 100 input is adjusted by the manufacturer. New adjustment is necessary only
when detector boards or the microprocessor of the detector have been replaced.
Apply resistance with 100 Ω (with lowest tolerance) at Pt 100 input of the detector
(terminals 3 and 4,).
Select menu item “Pt 100 Adjust”
Push <sk2> and <clear> at the same time. The message “Pt 100 adjustment
running...” is displayed. The adjustment is finished after approx. 5 s.
Then connect resistance thermometer to detector, terminals 3 and 4.
Pt 100 adjustment should be performed by trained personnel only!
10.2.1 Plateau Check
Measure Plateau
A detector plateau can be recorded automatically in steps of 60 V, e.g. to check the
proper function of the NaI crystal multiplier combination.
Working principle of scintillation counters
The detector (scintillation counter) is comprised of:
a) an organic scintillator (PVT)
or
b) a sodium iodide crystal doped with Thallium
c) photomultiplier
d) electronics unit
As the gamma radiation hits the detector, it triggers minute flashes of light in the crystal
which are invisible to the naked eye. These flashes of light release electrons from the
light-sensitive cathode (photo cathode) of the photomultiplier. This electron current is
amplified via a dynode system resulting in an electrical impulse at the photomultiplier
output for each light flash.
The high voltage required for operation of the photomultiplier is generated in the
electronic unit.
The scintillation counter is equipped with a drift compensation which compensates for the
effects of aging and temperature-related gain variations, thus ensuring high long-term
stability.
Malfunctions of the scintillation counter are not always indicated by a missing pulse rate.
It is also possible that the specific Gamma sensitivity appears to have changed or
obvious instabilities are apparent. This error can be detected only by means of a plateau
check. The evaluation unit LB 444 K-40 includes a function for automatic plateau
recording.
Please keep in mind:
The scintillation counter may remain at the measuring point if it is certain that the
concentration in the measuring path will remain absolutely constant during the time
of the check (approx. 10 min.). Otherwise, the check has to be carried out in the
workshop. An appropriate amount of potassium salt (10 kg) has to be provided directly in
front of the detector.
Select Plateau / Measure Plateau in the Service menu.
Upon completion of the plateau measurement, you may invoke the function Request
Plateau to display the value pairs HV-steps/count rate.
The detector works properly if a clear plateau is visible; the position of the plateau is
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