a method and apparatus for verifying operation and achieving gain stabilization in a helium - 3 proportional counter radiation detector utilized in nuclear logging operations . the detector includes a beta - minus particle source selected to provide a predetermined number of radiation counts for a specified time period to verify detector operation . the beta - minus source is further utilized to achieve detector gain stabilization by defining two energy windows on either side of the characteristic beta - minus energy peak . the difference in counts for the two energy windows is used to define an error signal which is applied to negative feedback loop in the detector electronics . the beta - minus source is preferably selected such that the type of source and activity is exempt from nuclear regulatory commission licensing and safety requirements .