Appendix C
Cosmic-ray Neutron Contribution to Sample Activation

Cosmic rays that reach the Earth’s atmosphere produce a variety of secondary particles from interactions with nitrogen, oxygen, and argon atoms. (NCRP 1987; UNSCEAR1994; UNSCEAR 2000). The fluence of neutrons produced extends to very high energies (more than several gigaelectonvolts (Goldhagen and others 2000). 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). The fraction of the total fluence that is below about 1 keV at ground level, and can thus slow down sufficiently in the sample to contribute to the thermal or resonance activation, is about 15–30%. The fraction can vary widely depending on local scattering and spallation effects.

The cosmic-ray neutron fluence is known to vary with geomagnetic latitude and with altitude because of the effect of the earth’s magnetic field on the incident cosmic-ray particles. The total fluence is much lower (by about a factor of 2) near the equator than at the poles (UNSCEAR 1994). Estimates of the variations in total fluence with geomagnetic latitude are uncertain because of the scarcity of measurement data. UNSCEAR (1994) estimated that the total fluence in Tokyo was about half that at 45° N on the basis of a single measurement of dose-equivalent reported for Tokyo. However, Lal (1991) developed a polynomial fit to available low-energy neutron fluence measurements at various sites around the world that indicate that the fluence at sea level at 25° N (the geomagnetic latitude of Tokyo) should be about 75% of the value at 45° N.

The cosmic rays incident on the earth’s atmosphere also vary with time because of variations in solar activity with an 11-year cycle. The time variation at sea



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Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) Appendix C Cosmic-ray Neutron Contribution to Sample Activation Cosmic rays that reach the Earth’s atmosphere produce a variety of secondary particles from interactions with nitrogen, oxygen, and argon atoms. (NCRP 1987; UNSCEAR1994; UNSCEAR 2000). The fluence of neutrons produced extends to very high energies (more than several gigaelectonvolts (Goldhagen and others 2000). 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). The fraction of the total fluence that is below about 1 keV at ground level, and can thus slow down sufficiently in the sample to contribute to the thermal or resonance activation, is about 15–30%. The fraction can vary widely depending on local scattering and spallation effects. The cosmic-ray neutron fluence is known to vary with geomagnetic latitude and with altitude because of the effect of the earth’s magnetic field on the incident cosmic-ray particles. The total fluence is much lower (by about a factor of 2) near the equator than at the poles (UNSCEAR 1994). Estimates of the variations in total fluence with geomagnetic latitude are uncertain because of the scarcity of measurement data. UNSCEAR (1994) estimated that the total fluence in Tokyo was about half that at 45° N on the basis of a single measurement of dose-equivalent reported for Tokyo. However, Lal (1991) developed a polynomial fit to available low-energy neutron fluence measurements at various sites around the world that indicate that the fluence at sea level at 25° N (the geomagnetic latitude of Tokyo) should be about 75% of the value at 45° N. The cosmic rays incident on the earth’s atmosphere also vary with time because of variations in solar activity with an 11-year cycle. The time variation at sea

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Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) level is fairly small—about 10% peak-to-peak (NCRP 1987) —and because the exposures of interest take place over an entire cycle or more, the temporal variations can generally be neglected. Activations of environmental samples at any site will depend not only on the neutron fluence in air near the sample, but also on the amount of local scattering, buildup, and attenuation that will affect the fraction of the fluence that can cause activation in the sample (that is neutrons slowing down) and by any shielding of the sample by overlying material. Measurements in rock of 36Cl activation (Linus and others 1999) indicate that the thermal activation first increases with depth to a depth of about 30–50 g/cm2 and then decreases exponentially with a relaxation length of about 230 g/cm2. 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 cosmogenic radionuclides, including 10Be, 26Al, and even some 36Cl (from reactions with argon (NCRP 1987)). One can roughly estimate the amount of thermal activation that will occur in an unshielded sample from the estimated incident fluence, assuming that the sample has been exposed continuously to the same cosmic ray fluence for at least 3 or 4 half-lives of the activation product under consideration. If so, an equilibrium condition (saturation) will be achieved in which the rate of production of the decaying product will become equal to the production rate. Thus, the activity (A) at equilibrium will be given by A=φσN where φ is the effective thermal-neutron fluence, σ is the effective activation cross section, and N is the number of target atoms. N=(Nav./W)f, where f is the isotopic fraction of the target (natural abundance), W is the atomic weight of the target atom, and Nav. is Avogadro’s number. The appropriate cross section depends on the spectral distribution of thermal neutrons. Averaging over a 300°K Maxwellian distribution with a 1/E tail gives a weighted cross section about one-third lower than the published “thermal” cross section at 300°K (Kaul and others 1994) (see Table 3–1 of Chapter 3). Several investigators attempted to calculate the cosmic-ray production of 60Co and 152Eu due to thermal neutron activation of 59Co and 151Eu. However, for the fluence term in the above equation, they used the older UNSCEAR (1994) estimate for high geomagnetic latitudes of 0.008 n/cm2-s instead of the more recent estimate of 0.012 n/cm2-s (Goldhagen and others 2000; UNSCEAR 2000). This value was an estimate of total fluence, however, and only incident thermal and epithermal neutrons (which slow to thermal in the sample) will activate 151Eu and 59Co (some 152Eu and a substantial fraction of the 60Co is formed by incident neutrons above thermal because

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Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) of resonance in the region below 1 KeV (see Table 3–1 of Chapter 3). They also used the thermal-activation cross section at 300°K that might overestimate the activation (Kaul and others 1994). Because of the variations with geomagnetic latitude, the total neutron fluence at sea level in Japan is probably around 0.008 n/cm2s–1 as was used, however, only a fraction of that will be converted to thermal energies in the sample.1 Thus, one would expect that these calculated activation estimates could be too high by a factor of 2–3. 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) but have not been exposed to bomb neutrons. Because of its long half-life, the copper samples analyzed for 63Ni would not probably have been exposed in situ for sufficient time for equilibrium to have been achieved. An estimate of 35Cl thermal activation by cosmic rays is even more dependent on background measurements because of the long half-life of 36Cl (300,000 y). Because of that long half-life no natural chlorine samples have been exposed in situ for even a small fraction of their cosmic-ray exposure and most sources of chlorine present in concrete or granite originated from sources that have probably been heavily shielded from cosmic rays for all but the last 50,000–100,000 y. Thus, the fraction of saturation reached would generally be only a few percent of the equilibrium level and can be expected to vary widely, depending on the exposure history of the chlorine-containing materials that are present in the sample. The contributions to environmental 36Cl from spallation reactions occurring continuously in the atmosphere and alternative production reactions (such as 39K(n,α)), as well as the fact that the Earth’s magnetic field has not remained constant over several hundred thousand years, make a true estimate of the possible level in any real sample suspect. Values of 36Cl in sands and rocks in the Northern Hemisphere have been reported to range from less than 100×10–15 to 600×10–15 36Cl/Cl (Straume and others 1992). The estimated background levels of 36Cl in the bomb samples analyzed by Straume and others (1992) —around 100–130×10−15 —are roughly the same as the reported values in rocks and sands in the Northern Hemisphere, accounting for variations with geomagnetic latitude.2 Except for 36Cl, environmental background measurements of the cosmic-ray contribution to the reported activation measurements have not been performed. 1   However, if the sample is surrounded by relatively high Z material, additional evaporation neutrons will be generated, as discussed by Linus and others (1999), and thus the thermal and epithermal fluence incident on the sample might be even higher. 2   For example, the surface activity of 36Cl in rock measured at an altitude of 2 km in the Sierra Nevada by Linus and others (1999) after using the model of Lal (1991) to correct to sea level and 25° geomagnetic latitude was about 300×10–15 36Cl/Cl.

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Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) Shizuma attempted to measure 60Co in a sample of steel obtained far from the epicenter (army storehouse), but the amount of 59Co in the sample was too low to allow a reasonable measurement of cosmic-ray activity (see Appendix B-2). Measurements of the cosmic ray production of 63Ni in copper at a church in Germany and in samples from other sites are in progress (Ruehm and others 2000a). Shizuma (1999) and Komura (2000) have reported measurements of cosmic-ray activation in laboratory reagents. For 60Co, Shizuma reported a value of 0.57± 0.06×10–6 Bq/mg of CoO, which corresponds to about 0.72×10–6 Bq/mg of cobalt; Komura reported a value of 3.5×10–6 Bq/mg of cobalt. For the europium reagent, Shizuma reported a value of 4.2±0.08×10–6 Bq/mg Eu203, corresponding to about 5×10−6 Bq/mg of natural europium; Komura reported a value of 2.3× 10−5. In both cases, the values reported by Shizuma are about one-fifth of those reported by Komura. The large discrepancy might indicate that the Komura samples were exposed to a higher neutron fluence due to the amount and composition of shielding material surrounding the storage location. Shizuma suggested that the relatively low values he measured for 152Eu might indicate that the reagent had not been exposed long enough to reach equilibrium. Using the above equation, and assuming that saturation had been reached, that the average thermal cross sections were 30 and 4400 barns for 59Co and 151Eu (see Table 3–1 of Chapter 3), and about 25% and 10% of the 60Co and 152Eu activation were from epithermal (resonance integral) neutrons, and that the natural 151Eu isotopic fraction was 0.51, one can estimate the 60Co and 151Eu produced based on a cosmic-ray neutron fluence of 0.008 to be 3×10–6 Bq/mg for 60Co and 7×10–5 Bq/mg for 152Eu. The measurements by Shizuma thus correspond to an effective thermal plus epithermal fluence of about 2×10−3 from the 60Co measurement and about 0.5×10–6 from the 152Eu measurement, or about one-fourth and one-fifteenth of the total incident fluence. The estimated effective thermal fluence from the Komura 152Eu data is about one-fourth of the expected total fluence. Thus, the Shizuma estimate of cosmic-ray 60Co activation and the Komura estimate of 152Eu activation are in reasonable agreement with what one might expect, whereas the Shizuma 152Eu 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, absorbtion of thermal neutrons (such as boron in laboratory glassware), and attenuation—effects that are unknown for these reagent samples—we have adopted the Komura reagent values for 152Eu (2.3×10–5) and for 60Co (3.5×10–6), each with an estimated uncertainty of ±25% (1 SD). Note that the above equation suggests that the ratio of 152Eu to 60Co should be about 20–25 for cosmic rays, whereas both Shizuma’s and Komura’s measurements indicated a ratio of only about 8. A possible explanation is that the 152Eu sample was not in equilibrium in either sample and the actual 152Eu activation is therefore higher by a factor of 2–3 than the Komura measurement of about 6×10–5. It is interesting that Ichikawa (2000) reported results of measuring thermal neutrons using a 3He detector at various locations in Hiroshima and Nagasaki. The

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Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) values ranged from 123 to 200 counts/h in his detector, corresponding to an average thermal fluence near the ground of about 1.5×10–3 n/cm2-s, about 20% of the estimated total fluence at Hiroshima given above. That is in reasonable agreement with the fraction of total fluence in the thermal range measured by Goldhagen and others (2000). 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–1 estimated, but it does indicate that the total cosmic-ray neutron fluence in Hiroshima is within the expected range.