Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) (2001) / Chapter Skim
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Appendix C: Cosmic-ray Neutron Contribution to Sample Activation
Appendix C: Cosmic-ray Neutron Contribution to Sample Activation
Pages 169-173
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From page 169... ...
The spectral distribution is relatively constant at atmospheric depths greater than a few hundred grams per square centimeter at geomagnetic latitude of 45 deg. N; the fluence rate is about 50% higher than previously reported measurements that probably did not account for the fluence of very high energy neutrons accurately (UNSCEAR 2000~.
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From page 170... ...
The initial increase to about 30% over that at the surface is due to spallation reactions of the high-energy portion of the neutron fluence with atoms in the rock that produce showers of lower energy neutrons, i.e., an increase in the total low-energy neutron fluence. These high-energy nuclear reactions also occur in the atmosphere due to interactions of incident particles with atmospheric nuclei, producing a variety of cosmogonic radionuclides, including i°Be, 26A1, and even some 36C1 (from reactions with argon (NCRP 1987~.
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From page 171... ...
If saturation had not been reached in the sample owing to its not being exposed to the estimated fluence for a long enough period, the true activity would be lower. Because of these local scattering and shielding effects, the uncertainty in any calculated value is very high; thus, a good estimate of cosmic-ray activation can be obtained only through measurement of environmental samples that have been exposed to cosmic radiation and are identical in almost all respects with the samples of interest (exposure time, shielding, materials, scattering, geomagnetic latitude, altitude, and so on)
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From page 172... ...
Thus, the Shizuma estimate of cosmic-ray 60Co activation and the Komura estimate of i52Eu activation are in reasonable agreement with what one might expect, whereas the Shizuma i52Eu estimate appears to be too low and the Komura 60Co estimate too high. However, because of the considerable uncertainty in the measurements and the expected variations in thermal fluence due to local scattering, spallation, absorption of thermal neutrons (such as boron in laboratory glassware)
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From page 173... ...
However, the exact thermal fraction of the total is known to be highly sensitive to the local scattering medium and would be expected to be higher near soil, particularly wet soil, than far from the ground in higher Z material. It thus does not keep the total fluence at Hiroshima from being somewhat higher or lower than the 0.008 n/cm2s-i estimated, but it does indicate that the total cosmic-ray neutron fluence in Hiroshima is within the expected range.
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