Experimental assessment of cosmic ray ionizing component at ground level by means of HPGe and NaI(Tl) spectrometry

摘要

Experimental assessment of cosmic ray ionizing component at ground level has been carried out by use of high purity germanium (HPGe) and NaI(Tl) scintillation spectrometry systems, and empirical equations connecting dose rate obtained by an ionization chamber to corresponding count rate measured by the spectrometers in the energy region of 3 to 100 MeV were derived. A spherical ionization chamber of 14 litre in volume was used simultaneously with a 3''(phi) spherical NaI(Tl) and GAMMA-X HPGe detectors in order to make distinction between the dose rate of gamma ray below 3 MeV and that of cosmic ray ionizing component detected in higher energy region. Various cosmic ray dose rates and corresponding count rates were made available by applying shielding effect of a multi-storied building, including measurements at outdoor flat and a mountain top. As a result Y = 30.25 X(sub G) + 1.42 and Y = 19.83 X(sub N) + 1.32 were derived, where Y is dose rate due to cosmic ray ionization in nGy (center dot)h(sup -1), and X(sub G) and X(sub N) are corresponding count rates in cps measured by HPGe and NaI(Tl) spectrometers, respectively, in the region of 3 to 100 MeV. And it appeared that 93 to 95 % of total spectrometirc counts were summed up in the energy region below 40 MeV of the cosmic ray spectra measured by both detector systems. Dual-valued slope of overall attenuation of cosmic ray ionization intensity was also found with a reflection point at penetrating depth equivalent of about 1600 kg (center dot)m(sup -2) on the basis of power law model. (author). (ERA citation 17:035016)