The authors aim to establish a method for predicting radiation environment and facilitating architectural measures to reduce the radiation space dose rate in a building prior to the construction of a new building in the area affected by the nuclear power plant accident. In order to clarify the formation mechanism of the dose rate distribution in the building, in situ measurement of gamma ray incidence characteristics on the outer wall is necessary. Several methods for directional radiation measurement have been proposed in the past; however, there are no measuring instruments perfectly suited for this purpose. This paper reports a directional gamma ray measurement instrument developed by the authors.
In this study, the authors developed a directional radiation air dose rate measuring instrument using a lead shielding disk against gamma rays from radionuclide 137Cs which is dominant in the affected area. Prior to the development of the instrument, two kinds of directional radiation measuring methods intended for field survey were examined. One method is shielding the radiation by a cylindrical collimator excluding a specific direction of a scintillator (Fig. 2), and the other method is obtaining the difference by subtracting the measurement result with lead disk shielding a specific direction, from the measurement result without shielding (Fig. 3, 4). Matters taken into account in the selection of a method include the configuration of the instrument consisting of a 3 × 3φ inch NaI (Tl) scintillator and a lead shield, the instrument’s viewing angle, sensitivity cutoff performance and weight. In this research, the emphasis is placed on the ease of handling in the field measurement, thus the latter method was chosen which is easy to realize good cutoff performance with a lightweight shield although measuring errors possibly occur due to the radiation from outside of the viewing angle.
Since the viewing angle of the directional radiation is determined by the shielding disk diameter D and the distance L between the disk and the scintillator, the combinations of D and L were investigated using the radiation analysis code PHITS by JAEA.
The authors defined the equivalent viewing angle φeq and the ratio of sensitivity concentration RC to evaluate the combination of D and L. When the integrated value of the directional sensitivity of a certain combination is the same as the ideal viewing angle φ, the viewing angle of the combination is defined as the equivalent viewing angle φeq. The coefficient RC is the ratio of integrated sensitivity within the angle φeq to the total sensitivity, and the sharpness of sensitivity cutoff near the viewing angle was evaluated. Using these coefficients, the characteristics based on some combinations of D and L were compared and studied. We designed D and L to make viewing angle +/- 45 °. Finally, the lead shielding disk of 150 mm diameter and 50 mm thickness was selected and placed with the scintillator 35 mm apart, realizing the equivalent viewing angle φeq to be +/- 45 °. In order to verify the validity of the designed measuring instrument, experiments were carried out using a 137Cs point source and the actual measuring instrument, confirming good agreement between the PHITS analysis and the experimental results.
