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If the check spot contains only "Radium D+E" i.e. Pb-210 or radiolead, it has a half life of 22 years so the readings will be less over time. It is
predictable and can be accounted for. Such spots installed in 1962 would be about 2 half-live decayed by now or ~1/4 of the original activity.
If the check spot uses radium or any type of uranium, there is no appreciable decay in the reading over time.
Some Russian units use Sr-90 as do certain American ion-chamber units, as well as Kr-85 and other isotopes. Each of these
would have to have their reading recalculated according to their individual half-lives.
Depending on the probe and the CPM rate, an isotope that is 1/2 as strong as the original may not read exactly 1/2 the CPM, due to
dead-time issues in the probe.
Geo
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