11.5 Scintillation Detector
102
The multiplier window is coated with a vapor-deposited layer
acting as photo cathode. This layer gives the window a brownish
tint similar to smoked glass. If this layer is no longer present or if
it is stained, then the photo cathode has been destroyed (e.g. by
overheating, glass breakage, or incident light), and the multiplier
must be replaced. Faults caused by damage to the dynode systems
(e.g. by excessive vibration) cannot be identified by appearance. If
in doubt, replace the multiplier.
The glass pane at the mating faces of the photomultiplier must not
show any cracks.
Before re-assembly, apply a drop of clean silicon oil between crys-
tal and multiplier, and distribute it evenly by gentle rubbing to en-
sure a sound optical connection between the two components. Us-
ing the adhesive tape, replace the Mu-metal screen, making sure
that it is only under light tension.
Re-assembly:
Set the crystal onto the front face of the photomultiplier and twist
both components several times counter-clockwise. If you have to
replace a component, apply 1 or 2 drops of silicone oil on the mat-
ing faces of the new and old component. It may also suffice to
press the new component against the mating face of the old one
and to turn it several times. Reassemble the parts properly.
Multiplier
Magnetic shielding
Optical connection
Crystal
Figure 34: Crystal multiplier combination
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