Table 6.1. Effective Depth of Tissue for Various Organs
Depth of Tissue mg/cm2 .
Lens of eye
Recently, DOE has made significant changes in the methodology used for
radiation protection. Previously, DOE used the concept of dose equivalent. For
whole body irradiations, dose equivalent was the product of absorbed dose
multiplied by the quality factor, which was evaluated by Monte Carlo
calculations in a cylindrical phantom of 30-cm diameter and 60-cm height. For
monoenergetic neutrons or photons normally incident on the phantom model, the
dose equivalent was the highest value calculated anywhere in the phantom. Dose
equivalent was non-additive because the maximum values occur at different
depths in the phantom for different energies. A detailed explanation of the
calculations can be found in an article by Auxier et al. (1968).
In 1977, the ICRP introduced a major revision in recommended radiation
protection with the introduction of ICRP Publication 26 (ICRP, 1977). The new
methodology establishes a "risk-based" system of dose limitation. The ICRP
introduced the terms stochastic and nonstochastic for radiation effects and set
limits for both types of effect. Stochastic effects are defined as those for which
the probability of the effect occurring (as opposed to the degree or severity of
effect) is a function of radiation dose. Nonstochastic effects were defined as
those for which the severity of the effect is a function of the dose; a threshold
may exist. Limits were established such that the risk of stochastic effects
occurring was equivalent to about the same risks faced by workers in "safe"
industries who were not occupationally exposed to radiation in the workplace.
Limits were also established for nonstochastic effects that prevented these effects
from occurring even if the exposure occurred at the annual limit over the lifetime
of the worker.
The ICRP specified in Publication 26 that radiation exposure be limited by the
effective dose equivalent, HE, which can be expressed by the relation:
H E = ∑ w TD TQ T