A method is disclosed for operating a well logging tool having a controllable radiation source operatively coupled to a power source. The method includes monitoring at least one condition in a wellbore, and operatively disconnecting the power source from the radiation source when the at least one condition changes. Also disclosed is a downhole measurement assembly, which includes a radiation source, a power source, and at least one interlock operatively connected between the power source and the radiation source, where the at least one interlock is adapted to selectively arm and disarm the radiation source.