Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) (2001) / Chapter Skim
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6. Uncertainty in DS86
6. Uncertainty in DS86
Pages 79-86
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From page 79... ...
The temporary uncertainty assessment yielded estimated fractional standard deviations (FSDs) for various key parameters and preliminary crude estimates of the correlation among system components.
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From page 80... ...
Evidence from biodosimetry data and the Nagasaki factory-worker effects history indicates further that the preliminary DS86 estimates of uncertainty in individual gamma doses are lower than they should be, even though the overall agreement between DS86 calculations and TED measurements for gamma rays is very good. Uncertainty in DS86 can be divided into three types: systematic uncertainty that would affect estimates of doses to all people or groups at about the same distance in the same manner, random errors resulting from the method, and random errors resulting from the input data.
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From page 81... ...
, would tend to reduce the uncertainty associated with the delayed-neutron contribution to kerma, according to the observed improvement in the agreement between activation calculations and measurements at Nagasaki and for NTS tests of devices similar to the Nagasaki device. The delayed neutrons were a relatively small contributor to the neutron kerma at both Hiroshima and Nagasaki (<5%)
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From page 82... ...
at 1500 m; the prompt and secondary gamma CVs were estimated to be about 3% and 6%, respectively. The uncertainty in nitrogen and oxygen cross sections and improvements in the transport code energy bin structure have been extensively investigated since 1986 and appear to have a substantial impact on the calculated kerma in air.
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From page 83... ...
RANDOM ERRORS RESULTING FROM INPUT DATA Random error due to uncertainty in input data arises in connection with survivor location, shielding, and survivor orientation. Survivor location and shielding description were estimated to have the greatest contribution to total random uncertainty, primarily because of uncertainty in survivor recall.
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From page 84... ...
However, the possible discrepancy in the gamma fluence might be directly or at least partially related to the apparent discrepancy in the neutron fluence. The preliminary uncertainty assessment in DS86 recognized the apparent discrepancy between the Hiroshima neutron calculations and measurements, and was one of the reasons for the decision to defer a final uncertainty assessment until its cause was resolved.
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From page 85... ...
Finally, a comprehensive uncertainty analysis should include a rigorous estimate of the uncertainty in the calculations of neutron activation and TED dose used to confirm the transport models; additional uncertainties that are not included in the uncertainty model for kerma come into play in these calculations. These include uncertainties in activation cross sections, attenuation in samples, sample orientation, backscattering effects, and the shape of the calculated neutron spectrum at low energy; the latter uncertainty is due to the limited energy bin structure of the transport models.
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From page 86... ...
The apparent discrepancy between measured and calculated activation and TED measurements might not be completely resolved by a revised dosimetry system that incorporates improved source and transport calculations. Nevertheless, a thorough uncertainty assessment can provide credible estimates of the confidence limits on the major component of the dose, the gamma rays, and for the lesser neutron component for representative exposed subjects.
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