One inch crystals are not always the same.

When you have to replace a bad crystal in a scintillator you have to put in one
that is the same size as the original one. The calibration of the meter is made for a
crystal of a given size. If a smaller one is used the sensitivity will be affected.
A bigger crystal than what was originally in the counter can give false readings. The
circuitry in a counter is matched to the size of the crystal. A larger crystal will
produce more pulses per second for the same amount of radiation. This can overload the
circuit and you can get readings that do not match the radiation.
Also if you replace the SI crystal with a Plastic crystal the counter will not have
the same responce.
When buying a replacement you have to specify the diameter and the length.
 Many counters use a one inch diameter crystal but the length can be from 1/2" to 1 1/2".
Some models use 2" X 1 1/2". 

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