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Estimate of Radiation Exposure Dose |
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 Physical
estimates of dose
The dosimetry system provides individual dose estimates based on
information regarding each survivor's location and shielding situation
at the time of the bombings. The current dosimetry system, referred
to as DS02, was introduced in 2002 and is based on theory of modern
nuclear physics validated by the measurements of existing exposed
materials, such as brick and tiles.
For survivors who were within about 2 km of the hypocenter,
DS86 information is based on detailed shielding histories obtained
through interviews conducted during the late 1950s and early 1960s.
Estimates for other survivors are based on less detailed information
provided in response to questionnaires. Dose estimates are presently
available for 92% of the approximately 100,000 survivors in the
entire cohort and about 80% of those who were exposed within 2 km
of the hypocenters. Those for whom estimates are not available had
shielding situations that were difficult to assess.
There is some imprecision in dose estimates due to many factors.
Survivor location is not always known precisely, and it would be
impossible to account in detail for all aspects of shielding. In
addition, yields of the bombs and some technical issues regarding
the characteristics of the radiation can only be estimated. Generally
speaking, it is believed that typical random errors in dose estimates
may be on the order of ±35%. Special statistical methods have been
developed to reduce the systematic effect on risk estimation that
would arise from such random errors. |
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If inside an average Japanese
house, radiation dose is reduced to nearly one-half. |
| The relationship
between distance from the hypocenter and
radiation dose in air (without shielding) according to
DS02 (Dosimetry System 2002)
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Most radiation exposure was through gamma rays,
but there was a small neutron component. In Nagasaki, this neutron
component is believed to be virtually negligible, but for Hiroshima,
it is somewhat larger, and there is some indication that the neutron
exposures in Hiroshima may need revision. Neutrons are believed
to have a greater biological effect per unit dose than gamma rays,
and many analyses use a weighted total dose, such as the sum of
the gamma dose in gray (Gy) and 10 times the neutron dose, intended
to approximate an equivalent pure gamma ray dose, ie, units in weighted
dose (Gy).
DS02 provides estimates of gamma ray and neutron doses to 15 organs.
These organ doses account for shielding of the organs by the body
and consider the survivors' orientation and position at the time
of the bombings as well as the external shielding referred to above.
Analyses for specific cancer sites, such as stomach cancer, are
based on these organ doses. |
Biological
estimates of dose
Some effects of A-bomb radiation are “recorded” in blood cells and
tooth enamel and can be measured even now. Such measurements allow
scientists to better estimate the doses of radiation received by
individual survivors. (See table below.) |
Biological dosimetry
for measuring radiation dose |
Method |
Necessary
materials |
Characteristics |
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Chromosome
aberration of lymphocytes
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Blood
(2 cc) |
Possible any time after
exposure |
| T-cell
Receptor (TCR) mutation of lymphocytes |
Blood (1 cc) |
Possible only within several
years of exposure |
| Electron
spin resonance (ESR) of tooth enamel |
Extracted tooth |
Possible any time after
exposure |
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Measurement using
blood cells
This method focuses on the damage that radiation causes to
the genetic material (DNA) of cells. In principle, measurement
can be made of either damage to specific genes (mutated cells)
or chromosome aberrations
(observed at the time of cell division). However, these abnormalities
are not caused by radiation alone. They also arise naturally
at a certain rate when cell division takes place. Consequently,
the frequency of these abnormal cells usually increases with
age. Because the increase in frequency of these abnormal cells
varies from person to person, considerable differences are
often seen among aged individuals when compared to younger
persons. In fact, this is a major obstacle to studying the
effects of low-dose radiation. Furthermore, cells are capable
of repairing DNA damage. For this reason, the extent of damage
is not related to total dose alone, but depends on the time
course over which it is received. The effects of instant exposure
to radiation (acute exposure), as in the case of the A-bomb,
are greater than the effects of the same total amount of radiation
received gradually over a long period of time (chronic exposure),
as in the case of people living in the area contaminated by
the Chernobyl accident. Nevertheless, measurements of such
damage can be helpful in assessing radiation exposure, especially
in conjunction with other indicators. |
Measurement using
tooth enamel
Recently, it became known that measuring radicals
recorded in tooth enamel is very effective for assessing radiation
dose. In this case, enamel is separated from teeth that have
been extracted for medical reasons and the presence of radicals
is quantified by a method called electron spin resonance (ESR).
Because the ESR signal intensity is linearly correlated with
the amount of radiation received, we can estimate the dose
from the signal intensity. This method can be used equally
well for either acute exposure or chronic exposure to measure
total dose. There is great hope that this method will contribute
to assessing doses of radiation received by people who were
exposed in the Chernobyl accident. |
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