Dependence of dose rate measurement in the environment on crystal configuration of scintillation detectors

Ambient dose equivalent （ H ＊ （ 10 ）） rates in the environment in Fukushima prefecture were measured by NaI （ Tl ） and CsI （ Tl ） scintillation detectors whose crystal shapes are different. The indicated values of H ＊ （ 10 ） measured by a CsI （ Tl ） scintillation detector with a rectangular parallelepiped crystal （ 13 × 13 × 20 mm 3 ） were approximately 40 ％ higher than those of a CsI （ Tl ） scintillation detector with a rectangular parallelepiped crystal （ 38 × 38 × 25 mm 3 ） and 20 ％ higher than those of a NaI （ Tl ） scintillation detector with a cylindrical crystal （ 25.4 mm in diameter and height ） in the case where G （ E ） functions were used in parallel irradiation geometry. It was found that cylindrical crystals are more appropriate than rectangular parallelepiped crystals with respect to directional dependence in environmental dose rate monitoring. However, using a spectrum-dose conversion operator determined in a rotational irradiation geometry, the values of H ＊ （ 10 ） agreed within 10 ％ among all the detectors. These results indicate that the directional dependence of scintillation detectors should be carefully considered for H ＊ （ 10 ） dose rate monitoring in the environment.