A Simplified Technique for Measurement and Evaluation of Indoor Cosmic Ray Dose Rate Using a 3″ dia. Spherical NaI(T1) Scintillation Detector

Two kinds of major techniques are usually applied to measurement of the ionizing component of the cosmic ray dose rate. One is a “precise method” by which the cosmic ray dose rate is obtained from the difference between the reading of an ionization chamber and that of a gamma ray detector such as a NaI(T1) scintillation detector. The former detects both gamma ray and cosmic ray, and the latter only gamma rays. The other is the “simplified method” by which the cosmic ray dose rate is obtained from the count rate above 3MeV of the gamma ray detector based on the assumption that the cosmic ray dose rate is proportional to the count rate. It is reasonable to apply the simplified method in the open air near the ground since the cosmic ray spectrum does not change much. However, attenuation of low energy component by building materials must be considered when the simplified method is applied to indoor measurement because the cosmic ray spectrum may be changed.From a series of measurements by the precise method with a spherical air equivalent ionization chamber of 14 liters and a 3″ diameter spherical NaI(T1) scintillation detector under several environmental coditions, proportionality was found between the cosmic ray dose rate and the count rate above 3MeV. The proportional constant was evaluated to be 20±1.4nGy/h/cps. Also, no apparent difference in the proportional constants was found between indoor and outdoor. It is concluded that the simplified method can be applied to indoor measurement as well as outdoor measurement of the cosmic ray dose rate.