Response of IRSN following ACRO questioning of the use of RPL

24 March, 2015
Response of IRSN following ACRO questioning of the use of
RPL-dosimeters in the contaminated territories of the
Fukushima region
In the chapter on the "return of the people", the internet file "Fukushima insurmountable
challenges" published by ACRO evokes the problem of significant discrepancies between on
one hand, the estimated doses through the use of passive individual dosimeters calibrated
for radiological monitoring of workers, and on the other hand, the doses measured by
ambient dosimeters placed in the vicinity of people exposed to radiation in the
contaminated territories. The value provided by the ambient dosimeter being higher by
about 40% compared to the individual dosimeter, the document implicitly considers that
the first value is correct, and that therefore the use of passive individual dosimeters is to
the detriment of the protection of persons. However, in this case, and despite all
appearances, the reverse is true: the individual dosimeters give a correct estimate of
the effective dose representative of the actual exposure of people, and the ambient
dosimeters overestimate substantially the effective dose.
The explanation is as follows: the observed difference comes from the definition of
operational quantities (Hp(10) for the individual dose and H*(10) for the ambient dose)
which are used to estimate the effective dose (E) which is the reference quantity (not
directly measurable) used to define the radiation risk to the whole body. These operational
quantities are more or less representative of the effective dose depending on the exposure
configurations (radiation from one or all directions…) and its energy; they are defined by
the ICRU (International Commission on Radiation Units and Measurements) in the report 57
through conversion factors. In most situations, these operational quantities provide an
envelope estimation of the effective dose. The dosimeters are then calibrated to provide
results which are representative of the effective dose under the usual conditions of
exposure to assess.
In a contaminated area, a person is exposed to radiation coming from all directions. But at
energies of gamma rays that are present in the contaminated territories (mainly 137Cs
emitting photons of 0.662 MeV), the measurement made by an ambient dosimeter
significantly overestimates the effective dose as the ratio between the quantities H*(10)
and E is equal to 1.5 (see Figure 64 on page 72 of the report ICRU 57). However, for this
energy, the measurement provided by a personal dosimeter in general, especially type
RPL, calibrated in Hp(10) is a very good estimate of the effective dose as the ratio
between E and Hp(10) is very close to 1.
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Consequently, the personal dosimeter RPL provides a measure (in Hp(10)), which is a very
good estimate of the dose received by an individual on contaminated areas. This measure
is lower than the ambient dose (expressed in H*(10)) from 30 to 40%, not because it
underestimates the dose received by an individual but because the ambient dose
overestimates the effective dose by about 50% at energies and exposure conditions of a
contaminated area.
Extract from ICRU Report No. 57 on conversion factors used in radiation protection in case
of external exposure to ionizing radiation: Figure 64 shows the conversion factors for the
various operational quantities and the effective dose for configuration "ROT" that is
representative of an exposure in case of contamination of soil where the radiation comes
from all directions. In blue, the dose with a RPL-dosimeter, in orange the dose with an
ambient dosimeter.
Extract from ICRU Report N°57
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