SLIDE 31
Absorbed dose estimates. External thyroid absorbed dose estimates were based
on integrated photon exposure and based on an adjustment for living pattern in
a variable exposure rate environment.
External beta radiation penetrating to the depth of the thyroid was not
thought by us to be important. The thickness of tissue overlying the thyroid
ranges from 0.4 to 2.0 cm (0.2 to 0.8 in), average 0.82cm (0.3 in), and does
not correlate with age or body weight very well (ICRP74).
The minimum
beta
energy for penetration of 0.82 cm (0.3 in) of tissue was estimated to be 1.8
MeV. At Rongelap Island about 70% of the population had skin lesions on some
part of the neck appearing about 21 days post exposure (Cr56). This would
imply a skin surface dose of tens of gray (several thousand rad).
Only a
small per cent of the beta flux was above 1.8 MeV in kinetic energy. Of this
higher energy flux, only a small fraction would penetrate 0.82 cm (0.3 in) of
tissue and deposit energy in the thyroid. Thus thyroid dose from this pathway
was considered insignificant.
For internal emitters,
thyroid absorbed dose per unit intake was compiled
from values generated by Johnson (Jo82). An exponential interpolation of nonadult values was performed. Thyroid absorbed dose commitment was calculated
because the nuclides of interest had half-lives much shorter than 50 years.
Absorbed dose was generated based on the assumption of a quality factor of
one.
The absorbed dose per unit activity intake values for adults were those
given by Johnson directly (Jo82).
The values for the tellurium isotopes were
generated from reference man data contained in Limits for Intakes of
Radionuclides by Workers (ICRP79).
Tellurium isotope values for the ages less
than adult were generated by ratio of the Johnson values for the appropriate
iodine daughters.
The thyroid absorbed dose for any age per unit intake of
any tellurium isotope was assumed directly proportional to the product of the
adult value and the ratio of the itodine value.
ABSORBED DOSE
ESTIMATES
33