BERTHOLD TECHNOLOGIES LB 444, Operating Manual

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04/03
347
LB 444
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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
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 (ter-
minals 3 and 4, Figure 21).
Select menu item “Pt 100 Adjust”
Push
<sk2> and <clear> at the same time. The message “ Pt 100 adjustment run-
ning...” 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!
10.4.3 Plateau Check
Measure Plateau:
A detector plateau can be recorded automatically in steps of 60 V, e.g. to check the pro-
per function of the NaI crystal multiplier combination.
Working principle of scintillation counters:
The detector (scintillation counter) comprises:
a) sodium iodide crystal doped with Thallium
b) photomultiplier
c) 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 elec-
tronic 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. In the case of detectors with NaI-crystals, this error can be de-
tected only by means of a plateau check. The evaluation unit LB 444 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 den-
sity in the measuring path will remain absolutely constant during the time of the
check (approx. 10 min.). Otherwise, the scintillation counter and the shielding container
must be dismantled and the measurement has to be set up in the laboratory. The aver-
age density value normally occurring in the measuring path has to be simulated with
steel plates. Also, the distance between source and detector should be the same as under
actual operating conditions.