Analysis of response characteristics of NaI(Tl) scintillation detectors to cosmic ray

Observation of cosmic ray were made using several kinds of NaI(Tl) scintillation detector on board an airplane (DC-8) upto altitude 37000 ft, including level flights at 16000, 23000, 31000 and 37000 ft within latitude 34000'N - 35030'N. Two sets of instrument were used for the cosmic ray observation; one consists of DBM exposure rate meter equipped with 3'' dia.x 3'' cylindrical NaI(Tl) scintillation detector for evaluation of exposure rate equivalent due to cosmic ray, the other consists of multi-channel pulse height analyzer. In latter set, five kinds of NaI(Tl) scintillation detector were exchanged one after another, and deposited energy spectra due to cosmic ray were measured. Our principal objectives of this flight measurement are to obtain useful information on the response characteristics of NaI(Tl) scintillation detectors to cosmic ray in high altitide and to estimate the ground surface value of cosmic ray contribution. In this experiment, vertical profile of cosmic ray intensity, variation of energy loss spectrum with altitude, and response characteristics of these detectors to cosmic ray in terms of their weight were clarified. The ground surface values of cosmic ray contribution into energy region below 3 MeV for each detector were estimated as follows; 0.23(μR/h eq.) for 3'' dia. spherical, 0.22(μR/h eq.) for 3'' dia.x 3'' cylindrical, 0.23(μR/h eq.) for 2'' dia. spherical, 0.26(μR/h eq.) for 2'' dia.x 2'' cylindrical, and 0.29(μR/h eq.) for 1'' dia.x 1'' cylindrical NaI(Tl) scintillation detector. These data are expected to be very useful for evaluation of cosmic ray contribution in measurement of environmental gamma ray using NaI(Tl) scintillation detectors. (author)