Construction of Response Matrices for Various Cylindrical and Spherical NaI(Tl) Scintillation Detectors for Gamma Rays and the Test Results

Abstract

Gamma ray spectra analyses using NaI(Tl) scintillation and Ge semiconductor detectors are widely performed in the fields of gamma radiation monitoring, dose rate evaluation as well as radiation physics research in the environment. These detectors are usually used for spectroscopic measurements in the environment due to their high gamma detection efficiencies, high full energy absorption peak efficiencies, and excellent energy resolution especially in Ge semiconductor detectors. In Japan a series of guide manuals for environmental radiation and radioactivity measurement methods are prepared by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), and manual No. 20 entitled "The gamma ray spectra measurement methods in air" provides representative response matrices for NaI(Tl) scintillation detectors in tables. However, except for a few examples, the energy intervals are relatively broad and the upper energy is limited up to 3 MeV for the measurements of natural gamma radiations. Furthermore the shapes and sizes of the most commonly available NaI(Tl) detectors are limited to a few types, such as 3'' 3'' cylindrical and 3'' spherical types, so the response functions of the other shapes and sizes of NaI(Tl) scintillation detectors are not available, even if we would like to choose the other types depending on the fluence rate level and the energy range. During the past twenty years, new dose concepts and quantities , such as air kerma (Gy) as well as equivalent dose (Sv), have been proposed by ICRU and ICRP, and we have experienced drastic changes from measurable physical quantities to non-measurable conceptual quantities in the field of radiation dose evaluation. Corresponding to these changes, several radiation dose evaluation methods and techniques have been developed to adapt to these changes. Among them we consider that a spectrometric technique that evaluates the radiation dose through spectrum analysis from the measured pulse height spectra is the most versatile and flexible tool. The relation of radiation dose to the fluence in the environment changes with the gamma ray energy and the incident direction. The International Commission on Radiation Units and Measurements (ICRU) 2 4) and the International Committee on Radiation Protection (ICRP) 5, 6) have provided through theoretical analyses according to radiation physics various flu-ence-dose conversion factors under the typical irradiation conditions over the energy range 0.01 10.0 MeV, and the relationship among various radiation dose units. As mentioned above, we experienced historical changes of dose concepts, and according to these changes, we have continuously revised the dose evaluation procedures to new dose Jpn.