Berthold LB 442, Operating Manual

Page: 101 / 158

Section 11 Service Instructions
99
11.5.2 Malfunctions
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 ap-
parent. In the case of detectors with NaI-crystals, this error can be
detected only by means of a plateau check. The evaluation unit LB
442 includes a function for automatic plateau recording (see Ser-
vice menu). The check can be performed using the source at the
measuring location or better a test source. Plot the measured re-
sults in a curve (Figure 32).
840 930 1020 1110 1200 1290 1380 1470
0
5.000
10.000
15.000
20.000
25.000
High voltage in V
cps
Figure 32: Plateau curve
The detector works perfect when you get a clearly visible plateau;
the position of the plateau is irrelevant.
While taking the plateau curve, change the high voltage for the
photomultiplier at constant radiation intensity and measure the re-
sulting count rate. The plateau measurement is started in the Ser-
vice menu , submenu Plateau .