BERTHOLD TECHNOLOGIES Uni-Probe LB 490, Руководство пользователя

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4 Repair, Maintenance and Service Volume 2
4.14
Check Detector
Scintillation counters do not include any wearing parts and their
service life is not limited, provided they are used under normal
operating conditions. Malfunctions in the scintillation detector can
only be caused by excessive mechanical or thermal stress. See also
Volume 2, sections 2.5 and 7.
Scintillation counters are used as detectors, since only these detec-
tor systems provide the required high sensitivity to Gamma radia-
tion and their service life is independent of the intensity of the radi-
ation field.
The number of light flashes per time unit is a measure of the inten-
sity of the radiation field. The individual flashes of light are very
short, so that a high resolution is obtained; this detector can be
employed for high count rates.
The flashes of light are converted into electrical signals in a photo-
multiplier , which is optically coupled to the detector.
Figure 4-10 Scintillation detector
In rod detectors , the scintillator consists of a special transparent
plastic material, which generates flashes of light when absorbing
Gamma radiation. Depending on the required measurement range,
plastic detectors up to a length of 2 meters are available. Multi-
detector arrangements are chosen for larger measurement ranges.
Point detectors use a 2" NaI crystal as scintillator.
4.14.1 Checking the NaI Point Detector
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 detectors with NaI crystals, these errors can be
detected only by means of a plateau check. The Uni-Probe includes
a function for automatic plateau recording. The check can be per-
formed using the source at the measuring site or better a test
source. Plot the measurement results in a curve (Figure 4-11). The
detector works perfect when you get a clearly visible plateau; the
Radiation Scintillator Amplifier Dynodes
Light flashes
Photo cathode