Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) (2001) / Chapter Skim
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3. Thermal-Neutron and Fast-Neutron Measurements
3. Thermal-Neutron and Fast-Neutron Measurements
Pages 31-59
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From page 31... ...
Even thermal-neutron measurements are valuable in testing the transport in air because the thermal-neutron fluence at any site results primarily from downscattering of higher-energy neutrons. Thermal and epithermal neutrons have ranges of only a few meters in air and thus are produced locally.
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From page 32... ...
Near the epicenter at both Hiroshima and Nagasaki, 60Co and i52Eu activation data tended to be about 50% lower than calculated from the DS86 neutron fluence. RERF has surveyed the literature and communicated with investigators directly and has created a database of all known activation measurements (see Appendix A)
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From page 33... ...
The committee and its consultants have examined the various measurement data in great detail to determine whether part or all of the disagreement could be due to measurement errors (including failure to account properly for background contribution) and to determine better the exact extent of the potential discrepancy in calculated DS86 neutron fluence as a function of distance.
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From page 34... ...
A number of measurement problems might have resulted in reported activation data that were biased high at low activities. On the basis of this evaluation, we provide our current best estimate of the degree of the calculationmeasurement discrepancy relative to distance at Hiroshima and Nagasaki on the basis of best fits of activation measurements to calculated activation as a function of slant range, considering measurement and calculation uncertainties and appropriate background corrections.
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From page 35... ...
Letters requesting additional information regarding measurement protocols, uncertainty analyses, and background subtractions were sent to select investigators to clarify various issues. Committee consultants also visited the laboratories of several of the investigators and interviewed the principal investigators.
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From page 36... ...
Assuming that most of the neutrons originate from a point isotropic source at the epicenter, one would expect the thermal fluence, and thus thermal activation, to decrease approximately exponentially with distance, provided that the spectral distribution of neutrons in the epithermal and thermal region remained about the same. The DS86-calculated fluences show this near-exponential decrease, although the relaxation length increases slightly as the distance increases and the neutron spectrum becomes harder (Roesch 1987~.
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From page 37... ...
As shown in Table 3-2a, all the Shizuma data at the larger slant ranges are less than our estimated MDC. However, all the data at great distances might be biased high (and the uncertainty underestimated)
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From page 38... ...
38 63 m = V^ an V)
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From page 39... ...
39 ~ o o ~ ~ o ~ ~ ~ oo o ~ ~ ~ ~ .
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From page 40... ...
40 as c)
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From page 42... ...
| ,. ~ 'I.` :'~~~ _ _ T W;-' ' 400 600 800 1000 1200 1400 1600 1800 2000 Slant Range, m FIGURE 3-1 Hiroshima 60Co measurements (scaled)
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From page 43... ...
Only a few preliminary 63Ni measurements are available, but they suggest (see Figure 3-4) that the discrepancy for higher-energy neutrons is smaller than that for thermal neutrons; this perhaps reflects systematic uncertainty in the calculation of thermal activation at the more distant sites.
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From page 44... ...
Table 3-2a also lists estimates of the MDCs of 60Co and i52Eu obtainable by particular investigator methods for comparison with the reported values. Initial measurements of Hiroshima copper samples show promise that it will be possible to obtain good measurements of bomb-neutron fluence as a function of
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From page 45... ...
Figure 3-5 combines all the data for Hiroshima at slant ranges greater than 1000 m in a plot of M/C vs. slant range with approximate measurement errors and a + 30% assumed calculation uncertainty (1 SD)
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From page 46... ...
T 1 0.1 :1 ~ t: I i In. 400 500 600 700 800 900 Slant Range, m FIGURE 3-7 Nagasaki 152Eu.
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From page 47... ...
ca _ ~ 2 ~ 9= ~ ~ =" V)
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From page 48... ...
48 Cq an V)
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From page 49... ...
Because the neutron fluence at Nagasaki was much lower than that at Hiroshima, activation at 1000-1200 m at Nagasaki corresponds to that at about 1050-1350 m at Hiroshima. If the lack of a discrepancy up to 1300 m at Nagasaki is confirmed by additional data, it would strongly suggest that the discrepancy at Hiroshima at low activities cannot be due only to measurement error and background subtraction errors in that these errors would also have been expected to occur at comparable activities at Nagasaki.
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From page 50... ...
Cross Contamination of Samples Because the half-lives of i52Eu and 60Co are relatively short and it has been over 50 years since the events, the activation measurements reported in recent years, which include most of the data at larger slant ranges, are very low. The amounts of natural europium and cobalt in the samples are small, so enough must be extracted from the sample (that is the sample must be enriched sufficiently)
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From page 51... ...
In the case of environmental measurements to ascertain the neutron yield of the Hiroshima and Nagasaki bombs, sampling variance is minimized by considering only samples whose exact locations at the time of the bombs are well established. Sample shielding must also be known.
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From page 52... ...
Background Activity An important potential source of error in the reported activity at the time of bombing for sites at very large slant ranges is the correction for environmental background activity. All investigators corrected for background counting errors from their detector system due to radiation, but some of the corrections can be highly uncertain, particularly at low activities.
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From page 53... ...
The i52Eu cosmic-ray activity based on the reagent measurements and calculations appears to be too small to account for any significant error in the measurements, particularly because the decay correction due to the elapsed time between the bombing and sample measurement is only up to about a factor of 10 compared with about a factor of 1000 for 60Co. Owing to the short half-lives of 60Co and i52Eu, all the cosmic-ray activation took place in situ after the bombing and saturation (production rate = decay rate)
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From page 54... ...
Such samples as roof tiles were blown off buildings and later recovered, so their location at the time of the bombing is certain only to within several tens of meters. On the basis of a calculated DS86 free-field relaxation length of about 126 m from thermal activation at slant ranges of 500-1000 m, an error of 10 m in the slant range would result in an error of about 8% in the calculated activation.
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From page 55... ...
If the concentration of nickel in the sample is relatively high, the 63Ni created by thermal activation can also be high. Before the 63Ni data can be considered reliable, there must be a complete uncertainty analysis that accounts for potential errors due to the chemical separation of nickel from copper, AMS measurement, thermal-neutron activation of nickel, and uncertainty in total nickel content of the sample.
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From page 56... ...
The thermal activation will come primarily from a slowing of incident epithermal neutrons in the sampled For the case of 60Co, about 25% of the activation is due to neutrons above thermal energy from resonances in the cross section (see Table 3-1~. Thus, the uncertainty in the calculated activation is sensitive to the actual energy distribution of the fluence in the first few centimeters of the sample and to the change in this distribution from that calculated for the free field because of slowing in the sample itself.
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From page 57... ...
Furthermore, previous comparisons of M/C ratios as a function of slant range, using a nonweighted regression that failed to reduce the influence of the higher uncertainty in more-distant measurement data and to include the possible calculation uncertainty, might have unduly emphasized the distant data and led to an overestimate of the discrepancy in the DS86 neutron fluence at large slant ranges. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS A number of measurement issues might explain in part why the thermalactivation measurements reported for large slant ranges are too high.
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From page 58... ...
The nearly exponential decrease in both calculated and measured scaled activation (see Figures 3-1 through 3-4) at Hiroshima suggests that the remaining discrepancy at large distances can be explained by a slightly harder source spectrum that would allow more higher-energy neutrons to penetrate to greater distances and thus produce a larger local thermal and epithermal fluence at that distance.
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From page 59... ...
· For the committee to provide its best assessment of the most critical data sets, those at sites greater than 1000 m, it is essential that the investigators that have reported activation measurements be encouraged to provide the necessary information and agree to cooperate in sharing samples and participating in intercomparisons. · Substantial environmental data on isotopes produced by neutron activation in the Hiroshima and Nagasaki bombs exist and are documented in the RERF database (Appendix B
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