1
Introduction

In March 1986, at a meeting in Hiroshima, the senior dosimetry committees of the United States (Chair F.Seitz) and Japan (Chair E.Tajima) approved the adoption of a new dosimetry system, DS86 (Dosimetry System 1986), for use in estimating the doses received by people exposed to the Hiroshima and Nagasaki atomic bombs in August 1945. The dosimetry committees and those who worked with them (in the U.S. and Japanese working groups, chaired by R.Christy and E.Tajima, respectively) were at the time well aware of the critical importance of the dosimetry system, not only for the reconstruction of the doses to the survivors, but also for estimating the risk of late cancer effects in the survivors and possibly, as noted more recently (Shimizu and others 1999), the risk of some noncancer health effects. Those estimates depend critically on the dosimetry. The estimates of risk are the main basis of our knowledge of the late health effects to be expected in any future population exposed to substantial doses of ionizing radiation. They are therefore of great importance to the people of the entire world. The risk estimates also are the main basis of recommended radiation-protection dose limits for radiation workers and for the public after inadvertent exposure to ionizing radiation (ICRP 1991; NCRP 1993).

DS86

DS86 was the first fully comprehensive computerized dosimetry system to be recommended for use with the atomic-bomb survivors. It replaced the former T65D system with state-of-the-art knowledge of all known measures related to the Hiroshima and Nagasaki explosions. DS86 incorporates computations and models that describe the yield and radiation output of the bombs, the free-field radiation



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Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) 1 Introduction In March 1986, at a meeting in Hiroshima, the senior dosimetry committees of the United States (Chair F.Seitz) and Japan (Chair E.Tajima) approved the adoption of a new dosimetry system, DS86 (Dosimetry System 1986), for use in estimating the doses received by people exposed to the Hiroshima and Nagasaki atomic bombs in August 1945. The dosimetry committees and those who worked with them (in the U.S. and Japanese working groups, chaired by R.Christy and E.Tajima, respectively) were at the time well aware of the critical importance of the dosimetry system, not only for the reconstruction of the doses to the survivors, but also for estimating the risk of late cancer effects in the survivors and possibly, as noted more recently (Shimizu and others 1999), the risk of some noncancer health effects. Those estimates depend critically on the dosimetry. The estimates of risk are the main basis of our knowledge of the late health effects to be expected in any future population exposed to substantial doses of ionizing radiation. They are therefore of great importance to the people of the entire world. The risk estimates also are the main basis of recommended radiation-protection dose limits for radiation workers and for the public after inadvertent exposure to ionizing radiation (ICRP 1991; NCRP 1993). DS86 DS86 was the first fully comprehensive computerized dosimetry system to be recommended for use with the atomic-bomb survivors. It replaced the former T65D system with state-of-the-art knowledge of all known measures related to the Hiroshima and Nagasaki explosions. DS86 incorporates computations and models that describe the yield and radiation output of the bombs, the free-field radiation

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Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) environment, the shielding circumstances of the survivors, and the body shielding of the various organs. It is a modular system with separate databases for each of the free-field radiation components, for each of several distinct shielding environments, and for each of many different organs. The free-field components include prompt neutrons, early gamma rays (prompt-fission gamma rays and gamma rays from inelastic scattering and capture of prompt neutrons), late gamma rays (from fission products and from delayed neutrons), and delayed neutrons. A new or revised treatment of any of those components can readily be introduced by appropriately substituting a new database for an existing one. The shielding databases include models for all survivors with nine-parameter shielding and all survivors with globe-data shielding descriptions (Roesch 1987). Uncertainties were estimated for the shielding and organ environments by calculating fractional standard deviations among the many shielding and phantom environments that had been computed. They were combined with uncertainties in the free-field radiation fluences to provide a preliminary estimate of uncertainty in the computed doses. Uncertainty evaluation was incomplete at the time of the adoption of DS86 (Roesch 1987). Later, Kaul and Egbert (1989) presented the US dosimetry committee with a draft of a preliminary uncertainty analysis; this analysis was revised in 1992 but is still regarded as preliminary. GAMMA RAYS In the DS86 calculations of kerma and dose to exposed people, the codes and data used were superior to any used previously. Especially for doses to organs deep in the body, gamma rays dominate, amounting in Hiroshima to 98–99% of the total absorbed dose. In Nagasaki, where the neutron fluence at a specified total dose is only about one-third that in Hiroshima, the percentage contribution of neutrons is even lower. Not only were the calculations of gamma rays believed to be improved in DS86, but a most important consideration was the experimental confirmation of gamma-ray doses by measurements (with thermoluminescent dosimetry or TLD) of the gamma-ray signal in quartz, brick, and tile samples in both cities. Agreement between measurement and calculation is quite good over a wide range of distances from the hypocenter in both cities. That bears repeating for emphasis: the most important component of the dose, gamma rays, is experimentally well verified (see Chapter 2). NEUTRON MEASUREMENTS AND DISCREPANCY The contribution of fast neutrons to the dose in organs deep in the body is estimated in DS86 at around 1–2% of the total absorbed dose in Hiroshima in the dose range of about 0.5–2 Gy. Nevertheless, especially because of the potentially

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Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) high relative biological effectiveness (RBE) of neutrons compared with gamma rays, the neutron component, although small, might be a contributor to the effects of ionizing radiation from the atomic bombs. It was known by the dosimetry committees and the working groups (Roesch 1987) at the time (in 1986) that DS86 contained possible flaws. In particular, questions had arisen because some induced radionuclides, such as 60Co, activated by thermal neutrons in steel structures (Hashizume and others 1967; Loewe 1984; Roesch 1987), presented problems that at that time were unresolved. Notably, they seemed to indicate the possibility that at increasing distances from the hypocenter of both explosions the numbers of thermal neutrons exceeded those predicted by the DS86 calculations. Eventually because these measurements could not be easily or quickly verified or explained, it was decided to proceed with the application of DS86 to the survivors, since DS86 appeared superior to predecessor systems, and the corrections involved in the new system needed to be implemented without delay. The impact of the changes resulting from the application of DS86 on the risk estimates for cancer (NRC 1990; UNSCEAR 1988) and on the recommendations for radiation protection (ICRP 1991; NCRP 1993; NRPB 1993) is now well known. With other factors that were included in the reassessment, the changes contributed to the increase (by about a factor of 3) in the risk estimates for occupational exposures and exposures of the public. The apparent discrepancies between calculation and measurement of thermal-neutron activation first noted with 60Co were emphasized by measurements of 152Eu in Japan (Nakanishi and others 1983; Okajima and Miyajima 1983) and are discussed in the DS86 report (Roesch 1987). More recent measurements of 36Cl in concrete in Hiroshima (Straume and others 1992) appear to be consistent with those results; these 36Cl measurements culminated in a review of all the activation measurements known at the time (Straume and others 1992). The review further emphasized the higher measured values at large distances in Hiroshima (a factor of about 10 between measured and calculated values at a 1500 m slant range and still higher at longer distances). In Nagasaki, after revised calculations of neutron fluences and additional 36Cl measurements, the discrepancy appeared smaller and possibly nonexistent (Straume and others 1994). If the measurements were indeed correct for both cities, the problem might well rest in the physical characteristics of the source term of the Hiroshima bomb rather than in transport or other possible problems between source and detector. Possible new source terms for the Hiroshima bomb have been vigorously pursued and are still being considered, but no new and plausible source term that fits all the data has yet been proposed. Even though the fast-neutron component of the absorbed dose is small, with a high RBE (20–50 at the low neutron doses in question), the neutron equivalent dose in Hiroshima might not be negligible. Any increase in fast-neutron fluence implied by the thermal-activation measurements could therefore have an important effect on the total equivalent dose in Hiroshima and correspondingly on the

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Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) risk estimates for the Hiroshima portion (two-thirds) of the survivor population (see Chapter 7). Such uncertainties in the neutron contribution to the absorbed dose have also cast broad suspicion on DS86 for assessing the doses and the risks to the survivors and for deriving the gamma-ray risk estimates from the epidemiological studies at the Radiation Effects Research Foundation (RERF). With regard to the survivors themselves the concerns are largely unfounded because the effects of the actual radiation in Hiroshima and Nagasaki, including its neutron component, have been directly observed and are known without need to separate the effects of the gamma rays and the neutrons. Uncertainties in the neutron dose can however be important to the derivation of the risk due to the gamma rays. It has, however, been pointed out (NCRP 1997; Pierce and others 1996; Preston and others 1993) that the impact on risk estimates probably could not exceed 20%, because those estimates are derived mainly from comparatively high doses (such as 1 Gy and higher, which correspond to ranges at most 1200 m from the hypocenter) and because the discrepancies occur only in Hiroshima, apparently not in Nagasaki. Other workers (Kellerer and Nekolla 1997; Rossi and Zaider 1990; Rossi and Zaider 1996; Rossi and Zaider 1997) point out, however, that at low doses, the health effects, if any (depending on the dose-response relationship in that region), might be due to a significant extent to the neutrons in Hiroshima and not to gamma rays, for which the risk estimates are derived. Again, Nagasaki seems not to be subject to this uncertainty, but risk estimates for Nagasaki alone are less certain, because the sample is only about half the size of the Hiroshima sample and because of other factors, especially related to shielding and terrain, than risk estimates for the combined Hiroshima-Nagasaki sample. In 1996 (Pierce and others 1996), estimates of risk based on Nagasaki alone were substantially lower than those based on the combined sample. In any event, the idea has persisted in some quarters that DS86 is in question and needs to be reexamined and amended or replaced if necessary. At the very least, the “neutron problem” has contributed to the greater uncertainties now recognized in the risk estimates derived from this important source (NCRP 1997). A fuller understanding of the features of the Hiroshima explosion—which, unlike the Nagasaki explosion, has no counterpart for comparison among other (test) weapons—would obviously be highly desirable but might not in fact be possible. In the meantime, it would clearly be important to be able to measure fast neutrons at distances from the hypocenter in Hiroshima directly and to establish better the magnitude, if not the explanation, of the thermal-neutron discrepancy. It would also be highly desirable to measure fast neutrons at Nagasaki to confirm that a similar discrepancy does not exist there. During the time since DS86 was introduced, it became evident (Ruehm and others 2000b; Shibata and others, 1994) that a method to assay fast neutrons from the bombs directly by using 63Ni from an (n,p) reaction in 63Cu was feasible either with radioactivity measurement (T1/2=100y) or with mass spectrometry (Ruehm and others 2000b; Straume and others, 1996).

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Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) THE NATIONAL RESEARCH COUNCIL COMMITTEE ON DOSIMETRY FOR THE RERF The National Academies’ National Research Council had set up the US Senior Dosimetry Committee with Frederick Seitz as chair to oversee the work of the Department of Energy (DOE) working group (R.Christy, chair), which, with the Japanese working group, produced DS86. The National Academies essentially disbanded the Senior Dosimetry Committee in 1987 after the production of the report An Assessment of the New Dosimetry for A-bomb Survivors (NRC 1987). Later, it was recognized that there were likely to be continuing problems and issues in dosimetry that required a standing committee on dosimetry for RERF to advise the National Academies and others. A new Committee on Dosimetry for the RERF was set up in 1988, with Alvin Weinberg as chair and Warren Sinclair as vice-chair. The committee undertook such duties as approving a relaxation of the stringent requirements on dosimetric factors for DS86 dosimetry assignment, which enabled the survivors with assigned DS86 doses to be increased from about 76,000 to about 86,000 in a total sample of some 92,000 survivors. In 1992, with the problem of thermal-neutron activation studies coming to the fore on both sides of the Pacific, Sinclair and Weinberg exchanged roles, and Sinclair became chair. Addressing the neutron problem became the top priority of the committee although other features of DS86 had, in the meantime, been addressed by members of the former working group, such as shielding issues, revisions in yield, height of burst, and new measurements of neutron cross sections for which the committee was the focal point for discussion and review. The thermal-neutron activation studies continued to suggest larger numbers of neutrons in Hiroshima than could be accounted for by DS86. A meeting of the Committee on Dosimetry for the RERF held at Irvine, CA, on May 22–23, 1996, was attended (by invitation) by five members of the Japanese committee who were working on this problem. Later, a formal letter report from the NRC Committee was sent to DOE (Appendix D, NRC 1996). The letter recommended a number of actions to be taken to lead to renewed confidence in DS86 or to its revision. These included a review of all existing thermal-neutron measurements by a joint US-Japan team, initiation of 63Ni measurements of fast neutrons (based on radioactivity in Japan and mass spectrometry in the United States), revisions in other measures related to the Hiroshima bomb, and a thorough examination of uncertainties in all aspects of DS86. Many groups have come to recognize the importance of the dosimetry in Hiroshima and Nagasaki in the political arena because it underlies current risk estimation for radiation protection and because standards are based on it. That has led to more US government support for investigations to settle issues about DS86. The Committee on Dosimetry for the RERF was reconstituted in 1998 (Warren Sinclair, chair) with additional expertise and has since continued to concentrate on solving the neutron problem, acting as an advisory group first for the scattered

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Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) scientists working on the issue and, as of 2000, for the more cohesive US working group of scientists chaired by Robert Young. The efforts of the working group are expected to result in collaboration with a parallel Japan working group in a report detailing a revision or confirmation of DS86 and including a comprehensive uncertainty evaluation. The present NRC report describes in some detail the current situation with DS86 and recent measurements that give some preliminary indications of results for fast neutrons, and it sets the stage for the subsequent work of the joint US and Japan working groups. UNITED STATES-JAPAN INTERACTIONS The Committee on Dosimetry for the RERF has interacted closely over the years with corresponding colleagues in Japan, and interactions have been fostered by personnel at RERF itself, including Dale Preston, Shiochiro Fujita, and, recently, Harry Cullings. Those people have provided an invaluable service in obtaining samples for thermal-neutron and fast-neutron measurements in Japan, the United States, and Germany. Japanese colleagues have also attended some US meetings. As a result of the good offices of Shigenobu Nagataki, chairman of RERF, and assistance from the Ministry of Health and Welfare, Japan, for travel costs, a joint scientific meeting of Japanese and US workers and committees was held in Hiroshima on March 13–14, 2000. The meeting considered many aspects of DS86, including the recent work in Japan, the United States, and Germany on 63Ni assays in copper samples from Hiroshima (Ruehm and others 2000a). The results so far demonstrate the feasibility of applying these methods to measurements at large slant ranges in Hiroshima. Samples from Nagasaki are not yet available but are being eagerly sought, as are additional samples from Hiroshima. It is clear that appropriate background assessment in both fast-neutron and thermal-neutron measurements, with inevitably small signal-to-noise ratios, will be an important feature of the final measurements. Scientists in the United States and Japan had fruitful exchanges, and the meeting resulted in further dedication among the US and Japanese scientists to accelerate and complete investigations related to the dosimetry. In the 14 years since DS86 was approved, many other physical characteristics related to DS86 have been reassessed, and some improved values have been proposed. These include carrying out calculations with many more neutron and gamma-ray energy groups; reevaluating neutron cross sections in air, nitrogen, and iron over a broad range of energies; possible revisions in maps (Kaul 2000); and reexamining the effects of various characteristics, such as height of burst, yield, shielding, and organ doses. New determinations have not been agreed on. Nevertheless, even if the fast-neutron and thermal-neutron discrepancy is resolved, it will be necessary to consider (soon) whether possible revisions in parameters (see Chapter 4) should be collectively applied in a revised DS86 and adopted for use in specifications of the dose to each survivor. The US working group will address these problems in its report.

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Status of the Dosimetry for the Radiation Effects Research Foundation (DS86) The present report reviews the present status of DS86, the gamma-ray dosimetry, and such dosimetry issues as the thermal-neutron discrepancies between measurement and calculation at various distances in Hiroshima and Nagasaki. It recommends approaches and measurements to bring those issues to closure; that is, it sets the stage for the program of the working groups. It also outlines the changes in various physical characteristics relating to DS86 in the last 14 years and encourages the incorporation of the changes into a revised dosimetry system. In the succeeding chapters, the report reexamines aspects of measurement and calculation for the most important radiation-field components of the dosimetry: gamma-ray measurements, thermal-neutron and fast-neutron measurements, data-quality assessment, improvement in parameters of DS86, some features of neutron transport, biological dosimetry at RERF, uncertainty in DS86, and the implications of the foregoing for risk assessment. Four appendixes address the dosimetry database at RERF, the uncertainty analysis of neutron-activation measurements in Hiroshima, and the cosmic-ray neutron contribution to sample activation. Appendix D includes the 1996 letter report of this committee. A glossary and a list of references complete the report.