ASSESSMENT OF QUANTITIES FOR RADIATION PROTECTION IN THE ICRU STANDARD TISSUE SPHERE FOR INTERMEDIATE ENERGY NEUTRONS (DOSIMETRY, ICRU, MONTE CARLO)

The effect of the fluctuating cross section structure in the energy range of 0.4 - 10.0 MeV on the responses of neutrons in the ICRU standard tissue sphere is analyzed. The point-energy, coupled neutron-secondary particles Monte Carlo method was used in the direct estimation of the absorbed dose and the dose equivalent. A new approach was adopted where energy band-averaged responses were calculated instead of the previous method which generated them as point values. By analyzing the depth-dose distributions with an improved searching method, assessments were made for the important quantities in radiation protection; the index quantities, the newly defined operational quantities, and other relevant quantities defined in the standard receptor. The energy dependencies of the resulting response functions were consistent with the cross section structure and were dominated by the oxygen content in tissue due to both the oxygen resonance cross section and the characteristic behavior of the recoil proton quality factor. The relative differences between the present index quantities and the previous values vary from -17.0 to +3.4 percent with respect to the present results for the deep dose equivalent, and from -19.2 to +8.6 percent for the shallow dose equivalent. For the surface dose equivalent values on the principal axis, these relative differences deviate from +4.0 to +33.8 percent. Due to the completeness of the response function obtained in this study, the problem arising from interpolation of the point-valued response functions is inherently removed. For the energy range beyond the one studied here, a method of extending the band-averaging concept is suggested. The dosimetric data generated in this study is recommended for adoption and is expected to contribute to the undergoing efforts for the standardization of the neutron protection dosimetry.