The NaI (Tl) scintillation detector which was used for environmental radiation monitoring had to be concealed with a protection case so as to keep given functions under severe atmospheric conditions. In order to establish a correction method for gamma-ray energy spectra modified by the protection case, T-S (transmission-scattering) matrix whose elements were the transmission or scattering probabilities of incident photons in the protection case was constructed with the aid of the irradiation experiments using some mono-energetic gamma-ray sources and the calculation following the KLEINNISHINA formula.
The correction method based on the T-S matrix was adopted for determination of the true energy spectra of gamma-rays when the NaI (Tl) scintillation detector, which was covered with the aluminum case of 4mm in thickness, was employed for the radiation monitoring in the natural environment, and it was found that the T-S matrix method was successful to correct the gamma-ray energy spectra modified by the aluminum protection case.
