Understanding the Radiation Soaking Effect in Neutron Survey Instruments

The drift in response of neutron survey meters that employ moderated 3He and BF3 proportional counters has been reported and investigated by instrument calibration laboratories for over 20 y. When within a constant radiation rate field of high intensity, dose rate readings of an instrument may steadily increase with a rate proportional to the field intensity before reaching a relatively stable plateau. This phenomenon, commonly referred to as the radiation soaking effect, has been attributed to the accumulation of charge on insulating surfaces inside the detector. This paper provides results of a study specifically conducted to better understand the effect and, hence, to possibly identify mitigation techniques. The magnitude of the effect in different instruments tested under the same conditions has been assessed, and a reason for the detector response variance has been clearly identified. In addition to measures used in calibration practice to compensate for the radiation soaking effect, a potential method to return an instrument sensitivity to its baseline level is also discussed.