Confirming the rule of thumb that has been mentioned in this thread, our experience at TMI-2 matches the ratio exactly (at least in the RadCon HP context!!), although we used it as 100 mrad/hr measured on a smear/swipe or air filter with an RO-2 was indicative of 1 microcurie of Cs-137/Sr-90 activity. Paul Ruhter ruh@inel.gov