Gamma and X-Ray Effects in Multiplier Phototubes

The effects of gamma and X-ray exposure rate have been determined in 4 common multiplier phototube types over an intensity range of about 1 to 1010 r/hr. Radiation-induced anode current was essentially a linear function of exposure rate up to about 104 r/hr and reached values of 10-4 to 10-3 amperes at 105 r/hr steady-state. Above 105 r/hr, peak values of anode current as high as 50 ma were observed with pulsed X-rays of 0.2 , ¿sec duration. The output of all tubes was essentially space-charge-limited at peak exposure rates greater than 107 to 108 r/hr; however, anode current recovery time to the pre-irradiation levels in three tube types increased with exposure rate, and recovery times of 6-30 ¿sec were observed at 1010 r/hr. This delay could not be explained clearly from the results but may be related to tube insulator spacer charging due to large rapid increases of anode current. The principal radiation sensitivity of all tubes was due to radiation-induced luminescence of the face plates and glass envelopes, and tube response increased with anode luminous sensitivity. The dynode structure and its configuration did not contribute significantly to the radiation-induced response. Radiation sensitivity can be reduced significantly through the use of non-luminescent materials in tube construction.