Comparison of external dose coefficients used by the European Model for Inhabited Areas (ERMIN) and ICRP Publication 144

European Model for Inhabited Areas (ERMIN)及びICRP144で使用される外部被ばくの線量換算係数の比較

廣内 淳 ; Charnock, T.*

Hirouchi, Jun; Charnock, T.*

近年、ICRPは環境線源からの外部被ばくに対する線量換算係数をICRP Publ.144として発表した。European Model for Inhabited Areas(ERMIN)は、ICRP Publ.144の「地面上および地面内の平面状線源」の状況に非常に近い「オープンエリア」環境を含む、いくつかの理想的な居住環境下の線量換算係数を整備している。この研究では、ガンマ線による外部被ばくにおける線量換算係数、空気カーマから実効線量への換算係数、ベータ線による線量換算係数について、ERMINでの値とICRP Publ.144での値とを比較し、最新知見に基づいてERMINの幾つかの改良点を示した。まずICRP Publ.144に基づいて新しい「オープンエリア」環境を作成した。新しい線量換算係数を作成し、以前は深さに依存していないものから、土壌の深さを考慮した9つの係数に変更した。ERMINでは制動放射線が考慮されていなく、Cs-137、Pr-143、Pr-144、Ru-106、Sr-89、Sr-90、Y-90、Y-91の線量換算係数がERMINとICRP Publ.144で大きく異なっていた。ICRP Publ.144に基づく補正係数を用いて、制動放射線を考慮した線量換算係数を作成した。

The recent ICRP publication of dose coefficients for external exposures to environmental sources, ICRP Publ. 144, provided an opportunity to evaluate and improve the dose coefficients and dose conversion factors used by the European Model for Inhabited Areas (ERMIN). ERMIN contains several idealised built environments with the "open area" environment being very similar to the ICRP Publ. 144 "planar sources on and within the ground" situation. The study compares the values given in ERMIN with those derived from ICRP Publ. 144 for the external gamma dose coefficients, the dose conversion factors to convert from air kerma to effective dose and the beta dose coefficients. This study has enabled ERMIN to be improved in several ways. A new "open area" environment was developed based on ICRP Publ. 144. New dose conversion factors were developed moving from a single radionuclide specific one to nine that account for depth in the soil. The dose coefficients for Cs-137, Pr-143, Pr-144, Ru-106, Sr-89, Sr-90, Y-90, and Y-91 were significantly different between ERMIN and ICRP Publ. 144 because bremsstrahlung was not considered in ERMIN. 144, dose coefficients considering bremsstrahlung were developed using correction coefficients based on ICRP Publ. 144.