Berthold LB 442, Operating Manual

Page: 103 / 158

Section 11 Service Instructions
101
11.5.3 Checking the Crystal-Multiplier Assembly
Faults in the crystal-multiplier assembly are indicated by the pla-
teau becoming too small or too steep. They can often be detected
through visual inspection.
To do this, take the crystal-multiplier assembly apart. Proceed as
follows:
o
Switch off the scintillation counter before opening the
instrument.
o
Do not perform the check in bright daylight, as this may
damage the photocathode of the photomultiplier.
o
Open the scintillation counter by removing the cover of
the connection box first and then the screws of the
socket. The entire electronics (1) with the
crystal/multiplier combination can then be detached
from the housing.
o
Remove the photomultiplier combination from the
socket (2) and unscrew the ring nut (3) on the front
panel. The multiplier (4) including crystal (5) can now
be detached from the Mu-metal shielding (6).
o
There is a thin layer of silicone oil between crystal and
multiplier to ensure optical coupling; silicone oil is
rather viscous, particularly at low temperatures.
Carefully detach the crystal from the multiplier window
by gently sliding the crystal sideways. Do not wipe off
the silicone oil!
o Check:
The crystal must be perfectly clear inside and not show
any dull areas. The normal coloring is slightly greenish.
A yellowish to brownish coloring is a sign of thermal
overload and indicates that the crystal must be re-
placed. The surrounding white reflecting layer must not
be damaged. You can only check the photomultiplier for
glass breakage or other mechanical damages. Other
faults cannot be identified by visual checks alone.
However, if the crystal does not show any faults, a bad
plateau indicates that the photomultiplier is faulty.
Figure 33: Scintillation counter
1 = Electronics part with (a) CPU,
(b) HV, (c) voltage divider
2 = Base
3 = Ring nut
4 = Photomultiplier
5 = Crystal
6 = Mu-metal shielding
(a)
(b)
(c)