Executive Summary

The system now used for estimating radiation doses to individual survivors of the atomic bombs is called DS86 (for Dosimetry System 1986). It was introduced in 1986 as the first comprehensive system to provide dose estimates for the nearly 100,000 survivors being studied by the Radiation Effects Research Foundation (RERF). It replaced an earlier system T65D (Tentative 1965 Dosimetry). The main component of the radiation dose to organs of exposed people (over 98% of the absorbed dose) is gamma radiation, and DS86 gamma-ray calculations have been verified by direct experimental measurements using thermoluminescence. DS86 estimates the dose to each of several organs of a given survivor, allowing for his or her shielding by house, terrain, and his or her own tissues. Major components of the radiation field include prompt and delayed neutrons as well as early and late gamma rays (Roesch 1987).

Uncertainty in DS86 estimates of organ dose was provisionally discussed by Roesch (1987) and more comprehensively by Kaul and Egbert in 1989, but a complete evaluation of uncertainty in all aspects of DS86 is still needed. One important uncertainty in DS86 concerned the neutron component. Measurements of cobalt-60 (60Co) and europium-152 (152Eu) activated by thermal neutrons suggested that there were more fast neutrons at great distances (>1500 m) than DS86 had indicated. The discrepancy became greater in Hiroshima when chlorine-36 (36Cl) activation measurements were added and apparently became smaller or nonexistent in Nagasaki when more detailed calculations were made. The DS86-calculated neutron component of the dose in Hiroshima is only 1–2% of the total organ absorbed dose, and that in Nagasaki only one-third of this, so the impact of a discrepancy in the neutron component is not necessarily large, depending on the choice of relative biological effectiveness (RBE) for the neutrons.



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Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) Executive Summary The system now used for estimating radiation doses to individual survivors of the atomic bombs is called DS86 (for Dosimetry System 1986). It was introduced in 1986 as the first comprehensive system to provide dose estimates for the nearly 100,000 survivors being studied by the Radiation Effects Research Foundation (RERF). It replaced an earlier system T65D (Tentative 1965 Dosimetry). The main component of the radiation dose to organs of exposed people (over 98% of the absorbed dose) is gamma radiation, and DS86 gamma-ray calculations have been verified by direct experimental measurements using thermoluminescence. DS86 estimates the dose to each of several organs of a given survivor, allowing for his or her shielding by house, terrain, and his or her own tissues. Major components of the radiation field include prompt and delayed neutrons as well as early and late gamma rays (Roesch 1987). Uncertainty in DS86 estimates of organ dose was provisionally discussed by Roesch (1987) and more comprehensively by Kaul and Egbert in 1989, but a complete evaluation of uncertainty in all aspects of DS86 is still needed. One important uncertainty in DS86 concerned the neutron component. Measurements of cobalt-60 (60Co) and europium-152 (152Eu) activated by thermal neutrons suggested that there were more fast neutrons at great distances (>1500 m) than DS86 had indicated. The discrepancy became greater in Hiroshima when chlorine-36 (36Cl) activation measurements were added and apparently became smaller or nonexistent in Nagasaki when more detailed calculations were made. The DS86-calculated neutron component of the dose in Hiroshima is only 1–2% of the total organ absorbed dose, and that in Nagasaki only one-third of this, so the impact of a discrepancy in the neutron component is not necessarily large, depending on the choice of relative biological effectiveness (RBE) for the neutrons.

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Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) The National Research Council’s Committee on Dosimetry for the Radiation Effects Research Foundation was formed at the request of the Department of Energy (DOE) soon after DS86 was introduced. The committee was charged with monitoring the status of DS86 and assessing its relevance in light of any new evidence. It has continued to be both the repository and the forum for discussion of revisions of DS86. In a 1996 letter report to DOE, the committee recommended a program to help solve the neutron-discrepancy problem through direct measurement of activation of nickel-63 (63Ni) due to release of fast neutrons from both bombs. That program and a program to evaluate thermal-neutron measurements (also stimulated by the committee) are still in progress. Cooperation between dosimetry working groups from Japan and the United States and committees from both countries has been fostered by joint meetings like those in 1996 in Irvine and in March 2000 in Hiroshima, the latter supported largely by Japan’s Ministry of Health and Welfare. Both countries, on the advice of these committees, have set up formal working groups to pursue issues related to DS86 and to deliver an updated dosimetry for RERF. This report describes the status of DS86—specifically, the discrepancies that should be investigated, some of the approaches recommended to remedy them, and some preliminary results. GAMMA-RAY MEASUREMENTS When DS86 was adopted, the main component of the dose, gamma rays, had been directly measured by thermoluminescence in quartz that was inside bricks and tiles in structures up to about 2 km from the hypocenter in both cities. These and other measurements since 1986 are described in Chapter 2, which discusses and considers the magnitude of various sources of uncertainty in the measurements, including fading, calibration, energy response, and background. The measurements are slightly higher than calculations in some regions and lower in others. The uncertainty in the gamma-ray fluences measured and calculated is around ±20% (well within the range of uncertainty to be expected in thermoluminescence measurements), but arbitrarily reducing the measurements by 20% does not improve agreement with DS86 calculations. NEUTRON MEASUREMENTS As noted above, discrepancies have been reported in fast-neutron calculations of fluence and dose compared with measured thermal-neutron activation of 60Co, 152Eu, and 36Cl; the ratio of measured to calculated fluence can reach 10:1 at 1500m in Hiroshima and higher at greater distances. Discrepancies in Nagasaki seem small or nonexistent. The latter finding suggests a problem with the Hiroshima source term. Attempts to model a new source that fits the measurement data have not resulted in a plausible source. Thermal neutrons result from fast neutrons but

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Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) do not contribute significantly to the dose as fast neutrons do. Consequently, direct measurements of fast neutrons at various distances from the epicenter would constitute a superior test of the nature and magnitude of the problem. A method involving fast-neutron activation of copper to nickel—63Cu(n,p)63Ni—has been proposed and is now in use. The 63Ni is being measured in Japan on the basis of its radioactivity and in the United States and Germany with accelerator mass spectrometry of copper samples from known locations in Hiroshima. Preliminary results are available but have not yet been fully evaluated. Samples from Nagasaki will, hopefully, also be available for measurement. On the recommendation of this committee in its 1996 letter report, a joint US-Japan team of experienced measurement personnel (T.Maruyama, W.Lowder, and, later, H.Cullings and H.Beck) was set up to reexamine all aspects of all gamma and neutron measurements with regard to how uncertainties are represented and what is included in them. Particularly important are how background is measured and subtracted and how data are selected at low activity levels. In some cases, background is greater than the sample signal. REEVALUATION OF DS86 Since 1986, many revisions in the parameters of DS86 have been proposed (but not incorporated) that would improve the calculations. These have included changes in transport cross sections and transport codes and refinement of the calculations (increases in numbers of gamma-ray and neutron energy groups utilized). The changes, when tested, greatly improved the agreement with thermal-neutron measurements in Nagasaki, essentially removing the discrepancies, but the major discrepancy in Hiroshima beyond 1200 m remained. Many scenarios for the Hiroshima bomb were explored to try to understand the problem. It was found that no addition of fast (more penetrating) neutrons, either from leakage through a cracked bomb casing, or from an alternative plausible source term, could account for the increase in thermal-neutron activation and still agree with the well-known fast-neutron activation measurements of sulfur-32 (32S) made in situ soon after the bomb explosion. BIOLOGICAL DOSIMETRY Biological dosimetry is not generally expected to achieve the detailed precision of good physical measurements, but biological assays can provide a sound perspective on whether the physical dosimetry has led to reasonable results. They can be used to test the dosimetry system and provide evidence to confirm DS86 or to suggest the presence of a problem. In the RERF research program in Hiroshima and Nagasaki, biological methods of evaluation of doses have been especially important. For example, cancer-risk estimates based on epidemiological data have been about twice as great for the

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Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) Hiroshima survivors as for the Nagasaki survivors. Many uncertainties are associated with these estimates for both cities, and there are many possible reasons for the differences between them, including the much more complex terrain-shielding problems in Nagasaki; but one possible reason for the differences is the dosimetry itself. The differences might be attributable, at least in part, to the greater number of neutrons in Hiroshima than in Nagasaki. DS86 predicts that the neutron organ doses are 3 times greater in Hiroshima than in Nagasaki. As noted previously, DS86 might substantially underestimate the neutron fluence in Hiroshima, to judge from preliminary measurements. Two techniques have been used for direct biological measurement. Measurements of stable chromosomal aberrations can be compared with DS86 dose calculations but these aberration measurements show considerable scatter, indicating only a rough correspondence with DS86 estimates. Measuring electron-spin resonance in tooth samples has been possible in about 60 survivors and yields estimates in good agreement with estimates based on chromosomal aberrations for the same individuals. Within broad uncertainty limits, however, both methods yield results consistent with DS86 estimates for the same people, except for the Nagasaki factory workers. UNCERTAINTY In estimating doses, it is essential to consider sources of and contributions to uncertainty. DS86 included a preliminary assessment of these uncertainties. The assessment was based on fractional standard deviations estimated for various key parameters and on nonparametric methods derived mainly from the judgment of the DS86 authors, with little analytical support. The National Research Council’s initial review of DS86 recommended a rigorous uncertainty analysis that would use improved uncertainty-input values for each aspect of the dosimetry system (NRC 1987). A report produced in response to that recommendation, indicated fractional standard deviations of around 25–40% (Kaul and Egbert 1989). The present report indicates (in Chapters 2 and 3) that uncertainties might be larger when all relevant factors are taken into account, item by item, for both gamma rays and neutrons. Uncertainty analysis has become more feasible because of the availability of new information on possible sources of uncertainty and the availability of faster computers, which permit benchmark and sensitivity studies. IMPLICATIONS FOR RISK ASSESSMENT One potentially important aspect of the possibly greater neutron fluence in Hiroshima is whether such neutrons have an important effect on estimates of risk posed by gamma rays, which are the main component of the dose and therefore the primary cause of late effects, such as cancer. It is particularly important because DS86 considers neutrons to be a small component that might be ignored with regard to their tumorigenic effect. But neutrons might still be significant in DS86,

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Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) in view of their potentially large relative biological effectiveness (RBE). In the illustration given in Chapter 7, the DS86-estimated neutron fluence with reasonable RBEs of 20–50 at low doses might cause 23–43% of the effect in Hiroshima, leaving 57–77% for gamma rays. In Nagasaki, the corresponding proportion of neutron effect is 9–20%. The effects would therefore be expected to be 1.2–1.4 times greater in Hiroshima because of the extra neutrons in Hiroshima alone, and indeed that is roughly the case. However, so many other complex factors are involved in differences between the two cities, including terrain and shielding, that it is highly questionable whether that difference can be attributed solely to the neutrons. If the neutron component is actually larger in Hiroshima than in DS86—for example, 3 times larger at 1500 m—it would imply an even greater proportion of the effect for neutrons and less for gamma rays. In any case, the gamma-ray risk might be lower than previously estimated because of the contribution of the neutrons at their present level in DS86 in Hiroshima (see illustration in Chapter 7). The gamma-ray risk (i.e. lifetime attributable cancer mortality risk) might drop from the present 5–6% Sv−1 to 4–5% Sv−1 —still well within the current range of uncertainty of these derived values. If more neutrons were present at Hiroshima, the effect could be to reduce the gamma-ray risk estimates somewhat further. CONCLUSIONS AND RECOMMENDATIONS Although DS86 is a good system for specifying dose to the survivors and for assessing risk, it needs to be updated and revised. Uncertainties have not been fully evaluated and might amount to more than the 25–40% in fractional standard deviations of parameters (Kaul and Egbert 1989). While the calculated gamma-ray fluences agree well with values measured with thermoluminescence and constitute the main component of the dose to the survivors, more work needs to be done to establish the magnitude of the neutron component and to assess the extent to which the neutron component (small in DS86) affects (lowers) the estimates of gamma-ray risk. The committee offers the following recommendations regarding the revision of DS86 that is clearly needed and that hopefully will be completed in 2002: The present program of 63Ni measurements should be pursued to completion. All thermal-neutron activation measurements, particularly those with 36Cl and 152Eu, should be reevaluated with regard to uncertainties and systematic errors, especially background (see Chapter 3). Critical efforts to understand the full releases from the Hiroshima bomb by Monte Carlo methods should be continued. Adjoint methods of calculation (i.e., going back from the field situation to the source term) should be pursued to see whether they help solve the neutron problem.

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Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) Local shielding and local-terrain problems should be resolved. The various parameters of the Hiroshima explosion available for adjustment, including the height of burst and yield, should be reconsidered in the light of all current evidence in order to make the revised system as complete as possible. A complete evaluation of uncertainty in all stages of the revised dosimetry system should be undertaken and become an integral part of the new system. The impact of the neutron contribution on gamma-ray risk estimates in the new system should be determined.