Effects of cross-section structure on the dosimetric response functions for 0.4 to 10.0 MeV neutrons in the ICRU tissue sphere.

The effect of the fluctuating cross-section structure in the energy range of 0.4 to 10.0 MeV on the dosimetric response functions of neutrons in the ICRU standard tissue sphere is analyzed. A Monte Carlo method with point-energy cross-section values, including coupled transport for neutrons and secondary charged particles, was used in the direct estimation of the absorbed dose and the dose equivalent. An approach was adopted in which source-energy band-average responses were calculated instead of the more usual approach involving monoenergetic source neutrons. Data were obtained for the newly defined term, ambient dose equivalent, at various depths, as well as the older index quantities. Such data generated were compared with information from other research workers. In general, good agreement was found, with due consideration to the differences engendered by the use of the source-energy band-average approach. Agreement was poorest for very shallow depths, corresponding to outer skin thickness, this being a most difficult depth to calculate accurately. The dosimetric data generated in this study should contribute to the ongoing efforts for the standardization of neutron protection dosimetry.

Considerations of coherent scattering and electron binding in incoherent scattering, in computation of dose deposition in tissue from low-energy photon beams.

Two different sets of Monte Carlo computations were carried out for the study of dose penetration of monoenergetic, low-energy (10 to 100 keV) photon beams incident on slabs of tissue. One program took into account coherent scattering and considered electron binding when finding the angle of scattering during incoherent scattering; the other simpler program, customarily used at higher energies, largely ignored these effects. For calculations at the source photon energy of 100 keV, it was found that there was negligible difference in dose distribution in the slab between the more and less complex type of calculations. The same thing was found to be true for the 30 and 10-keV source photon energies only for shallow penetration distances; and at deeper penetrations the simple approach tended to overestimate the dose appreciably. It is concluded that for penetration of low-energy photon beams into tissue, accurate calculational results cannot be assured with the neglect of coherent scattering effects and electron binding considerations in determining the scattering angles except for shallow depths of penetration.

Calculation of low-energy neutron dose indices and depth doses in the ICRU tissue sphere.

Detailed neutron dose distributions were calculated for the ICRU tissue sphere for broad, parallel beams of incident neutrons with 10 different energies ranging from thermal to 0.3 MeV. The calculation was carried out by the Monte Carlo method on the CYBER 175 computer of the University of Illinois. From these calculated data, a set of fluence-to-dose-index conversion factors, needed to establish the allowable limits for exposure to external neutron beams, was obtained. Normalized depth doses along the principal axis at depths of 0.007, 0.3 and 1.0 cm are also provided. The fluence-to-dose-index conversion factors are compared to previous conversion factors based on cylindrical phantoms, and any differences are explained.