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Radiation Source Teas in Atmospheric Testing A. INTRODUCTION Ionizing radiation emitted as a consequence of a nuclear explosion includes photons, neutrons, beta particles, and alpha particles. (Photons refer to high- energy electromagnetic radiation that includes both x and gamma rays, which physically are the same kind of radiation. Historically, the term x ray was given to those high energy photons originating from energy transitions in the orbital electrons outside of the atomic nucleus and the term gamma ray was given to those high-energy photons originating from energy transitions occurring within the atomic nucleus. Photons with the same energy, however, are indistinguish- able regardless of their origin). Of these, most of the neutrons and a portion of the gamma rays are emitted simultaneously with the explosion. During subsequent nuclear processes, beta particles and other gamma and x rays are emitted. Alpha particles are emitted by unfissioned uranium or plutonium, by certain activation products produced during the explosion, and directly by fusion reactions (Glasstone and Dolan 1977~. In addition, x rays are emitted both as a direct result of the fission process as well as from the various radioactive species associated with a nuclear explosion. Initial radiation will be defined as ionizing radiation emitted within the first minute after the detonation. The selection of this demarcation is somewhat arbitrary, and was originally based on the approximately one minute required for the radioactive cloud to rise to a height of two miles following the explosion. This appeared to be independent of the yield of the explosion (Glasstone and Dolan 24
3 RADIATION SOURCE TERMS 25 1977~. Ionizing radiation emitted after the first minute following the detonation is classified as residual radiation. B. INITIAL RADIATION Initial radiation includes neutrons, gamma and x rays, alpha particles and beta particles which are emitted almost instantaneously with the explosion, and gamma rays emitted by fission products and activation products present in the rising cloud. Both neutrons and gamma rays, as well as x-rays, can travel considerable distances in air due to their low probabilities of interaction. Alpha particles and beta particles, on the other hand, have very short ranges in air, typically a few centimeters to a few meters, respectively. Therefore, of the initial radiation, only neutrons, gamma rays, and x rays can travel far enough from a detonation to present a significant hazard to living organisms surviving other weapons effects (e.g., heat and blast). Although the total energy of initial neutrons, gamma rays and x rays is only a few percent of the total energy released during detonation of a fission device, greater penetrating ability and the nature of interactions with matter by these radiations makes them a significant aspect of a nuclear explosion (Glasstone and Dolan 1977~. Although only a small fraction of initial neutrons, gamma rays, and x rays emitted during a weapon detonation escapes from the explosion region, these radiations present a significant hazard even at large distances from the explosion. Most of the neutrons from a nuclear explosion are emitted within a fraction of a second and are released in either the fission or fusion process. Both prompt and delayed neutrons are emitted as initial radiation, with delayed neutrons being emitted throughout the initial time period. Although high-energy neutrons are emitted during the explosion, their interactions as they emerge from the region of the explosion create a spectrum of neutron energies. This energy spectrum continues to decrease in mean energy within the first few hundred meters from the point of detonation as the neutrons pass through air, after which an equilibrium neutron spectrum is achieved (Glasstone and Dolan 1977~. At greater distances from the explosion, the neutron energy spectrum does not change appreciably, although there is a rapid reduction in neutron dose rate with distance due to geometric effects and neutron absorption. Gamma rays which are present in the initial radiation are from several distinct sources. These include: a. gamma rays accompanying fission, b. gamma rays emitted as a consequence of capture of fission neutrons by nonfissionable nuclei (both weapons components and surrounding materi- als),
26 FILM BADGE DOSIMEI RY TV ATMOSPlIERIC NUCLEAR TESTS c. gamma rays emitted following inelastic scattering of fission neutrons, and d. gamma rays emitted from decay of short-lived radionuclides formed in the explosion. The calculated time dependence of the gamma-ray energy output from a hypothetical explosion is shown in Figure 3-1. Variations in this relationship occur as a consequence of differences in weapons type, type of burst, and a number of other factors. X rays are also present in initial radiation as a conse- quence of fission processes and decay of isomers formed in the explosion. Initial radiation from fusion devices also includes x rays from several sources. Further details of the initial nuclear radiation are found in The Elects of Nuclear Weapons by Glasstone and Dolan (1977~. C. RESIDUAL RADIATION Ionizing radiation emitted after the first minute of a nuclear explosion is referred to as residual radiation. Residual radiation is emitted from the fallout following the detonation and from radioactivity induced in nearby materials by neutrons emitted during the detonation. Both of these sources of radiation may continue to emit radiation for many years. The induced radiation field decreases more rapidly with time than the fallout radiation field. Alpha particles, beta particles, and gamma rays are the principal components of residual radiation, because neutrons are emitted primarily as initial radiation during the explosion. Of the components of the residual radiation, only ionizing electromagnetic radiation is penetrating radiation. These include gamma rays and other electro- magnetic radiations present after the initial explosion, such as x rays from fission products and activation products, photons from positron annihilation, and bremsstrahlung from interactions of beta particles. Weakly penetrating radiations include alpha particles, beta particles, conver- sion electrons, and Auger electrons, and are generally termed non-penetrating radiations. Because nearly all radioactive decay of fission products and activation products includes beta-particle emission, residual radiation fields include a sig- nificant beta-particle component. The spectral nature of beta particle fields, the short range of beta particles in matter, and unpredictable field exposure conditions cause the calculation and measurement of beta-radiation doses to be highly unreliable. Beta radiation dose is of concern for skin and eye irradiation, but external exposure to beta particles does not contribute to the radiation dose to deeper radiosensitive organs in the body. Neutron activation can occur in virtually any material. The soil, building materials, steel, and other materials in naval vessels or other transportation ve- hicles, and sea water are but a few examples (Hashizume et al. 1969~. Although
3 RADIATION SOURCE TERMS 1 o3o 1 o29 1 o28 1 o27 1 o26 - ~, - 1 o25 1 o24 o23 1 o22 1021 1 o2o 1019 18 17 _ Prompt \\ Inelastic Scattering of Neutrons | ~\by Nuclei of Air Atoms Decays \ \ Neutron Capture \in Nitrogen _ \ \ Fission Product \ 27 \ ~. ~ ~I 1 1 -8 10-7 10-6 10-5 104 10-3 10-2 10-1 1 10 102 TIME (see) FIGURE 3-1 Calculated Time Dependence of the Ganma-ray Energy Output Per Kiloton Energy Yield from a Hypothetical NucJ ear E'cplo~ion (the dashed line refers to an explosion at very high altitude).
28 FILM BADGE DOSIMETRY IN ATMOSPHERIC NUCLEAR TESTS the elemental content of these materials varies greatly, all contain trace amounts or more of elements which have a high probability for neutron activation, such as iron, manganese, silicon, aluminum, sodium, chlorine, and cobalt. The radioac- tive isotopes of these elements present as induced residual activity are relatively few. The most important among these radionuclides are aluminum 28, manga- nese 56, sodium 24, chlorine 38, scandium 46, cobalt 60, and cesium 134. Induced activity can be present in fallout and in materials exposed to the initial radiation and not entrained into the radioactive debris cloud. Selected properties of these radionuclides are presented in the next section. As is well known, there are significant differences in initial radiation produced in fusion and fission device detonations. However, these detonations resulted in residual radiation fields that were quite similar. With a few exceptions, as discussed in subsequent sections, the residual radiation field following detonation of either fusion or fission weapons is due to the same radionuclides, with differing relative abundances. Fallout Radioactive materials that appear in fallout include fission products, unfis- sioned uranium or plutonium, and activation products. (Cook 1957; Cook 1959; Glasstone and Dolan 1977~. More than 200 radionuclides are produced in the detonation of a fission or fusion weapon. Nearly all emit beta particles, and many also emit gamma rays and x rays as they decay. The total initial activity of fission products is extremely large but decreases rapidly because half-lives of most of the radionuclides are very short. There is more than a 2000-fold decrease in residual radiation due to fission products from the one-minute point to the end of the first 24 hours after detonation. Despite such a rapid decrease, the very large quantities of fission products that may be con- tained in fallout can produce a considerable amount of fission-product fallout activity after the first day following the explosion (Glasstone and Dolan 1977~. Activation products produced by neutron interactions with weapon compo nents during and after the detonation include quantities of radioactive isotopes of iron, chromium, manganese, nickel, molybdenum, copper, cobalt, and vanadium from the weapon components. Although many of the radionuclides produced in this way have very short half-lives, there are several with half-lives exceeding several weeks. An important activation product present in fallout is cobalt 60 with a half-life of 5.3 years. Other activation products include uranium 237, uranium 239, neptunium 239, neptunium 240, plutonium 239, and plutonium 240. Of these, the most prominent
3 RADIATION SOURCE TERMS 29 is neptunium 239 which is produced by beta decay of uranium 239 following radiative capture of neutrons by uranium 238. With a half-life of approximately 2.4 days, measurable levels of neptunium 239 are present in fallout for several weeks following a nuclear explosion (Cook 1960~. Materials (other than weapons components) in the vicinity of a nuclear detona- tion may become activated by neutrons from the explosion and subsequently entrained in the radioactive debris cloud. These materials include soil and other small pariiculates, vaporized structures, vaporized metallic objects, and water vapor. Elements in these materials which undergo neutron activation include sodium, manganese, silicon, iron, aluminum, chlorine, and potassium. Induced Activity Other Than Fallout Activity can be induced in materials in the vicinity of a nuclear explosion and not become entrained in the radioactive debris cloud. For a low-altitude detona- tion of a nuclear weapon, this activity can be significant. It includes many of the same induced radionuclides found in fallout. The location and concentrations of induced activity depend on several factors, including: a. type of weapon b. weapon yield c. type of burst d. distance from point of detonation environmental conditions f. elemental composition of materials in the vicinity of the detonation g. time since detonation These factors will determine the relative contributions that fallout and non- fallout induced activity make to the overall residual radiation. When there is little or no local fission-product fallout' neutron-induced activity is of primary concern for external dosimetry purposes. D. PHOTON FIELDS FROM RESIDUAL RADIOACTIVITY As discussed in the preceding section, the residual radiation following a nuclear explosion arises from fallout and induced radioactivity. The photon field from these radiation sources consists of bremsstrahlung, other x rays, and gamma rays. The field from gamma rays is composed of direct, unscattered photons as well as photons which have undergone one or more scattering interactions with surrounding materials.
30 Bremsstrahlung FILM BADGE DOSIMEI RY TV ATMOSPHERIC NUCLEAR TESTS As stated earlier, beta radiation from fallout is of concern for protection of personnel from exposures to the skin and the lenses of the eyes. Because beta particles have a short range in matter, external exposure to beta particles does not contribute to radiation doses to deeper radiosensitive organs. On the other hand, as beta particles are stopped in matter, bremsstrahlung is produced and subse- quently contributes to the photon field. The energy distribution and intensity of bremsstrahlung depend primarily on the maximum energy of the beta particles and on the properties of the material with which beta particles interact. The intensity of bremsstrahlung produced has been shown to be proportional to the energy of the beta particle and the atomic number of the material. Low-energy beta particles interacting with low atomic-number atoms do not produce appre- ciable levels of bremsstrahlung. For fission products and activation products produced as a consequence of the detonation of a fission or fusion device, the overall beta-pariicle spectrum is composed of numerous individual beta spectra of each radionuclide. The overall spectrum is dominated by beta particles with energies less than 1 MeV. In general, materials with which fallout beta particles can interact have low atomic numbers. These include nitrogen, oxygen, carbon, sodium, hydrogen, silicon, etc., which are components of air, water and soil. Thus bremsstrahlung production in the vicinity of residual radioactivity does not contribute signifi- cantly to the photon field. This is confirmed by photon-spectrum measurements of the residual radiation fields. X Rays Characteristic x rays are emitted as a consequence of radioactive decay of many fission products and activation products. Few of these x rays have energies exceeding 100 keV and emission intensities (x ray per decay) are much lower than gamma-ray emission intensities. Gamma Rays A summary of gamma-ray energies for the selected radionuclides described previously is given in Table 3-1. Although many more radionuclides constituting residual radioactivity have been investigated in both theoretical and experimental studies (Cook 1959; Hashizume et al. 1969; Sandmeier and Battat 1982; NCRP 1982), the list of principal gamma-ray emitters producing the residual radiation field after a few hours can be reduced to the activation products neptunium 239, sodium 24, manganese 56, and fission products with half-lives exceeding am proximately one minute.
3 RADIATION SOURCE TERMS TABLE 3-1 Significant Contributors to Residual Photon Fields 31 Half Production Radionuclide Iife Gamma-Ray ** Intensity Energy (keV) (%) Activation Np 239 2.36 days Products Na 24 15.0 hr. Mn 56 2.58 hr. a 38 37.2 min. Al 28 2.24 min. Sc 46 83.8 days Cs 134 2.06 yr. Co 60 5.27 yr. F. . lSS10~1 Products Numerous loo 117 210 228 278 1369 2754 847 1811 2113 1642 2168 1779 889 1121 569 605 796 1 173 1332 Range of Energies 61 11 3 11 14 100 100 99 27 14 33 44 100 100 100 15 98 85 100 100 *(Kocher 1981) **Principal emissions
32 FILM BADGE DOSIMETRY ~1 ATMOSPHERIC NUCLEAR TESTS Photon energies listed in Table 3-1 range from 100 keV to 2.754 MeV. The photon energy spectra for radionuclides listed in Table 3-1 are time-dependent. Because half-lives of the individual radionuclides are different and because quan- tities that are produced during the explosion are related, the short-lived radionu- clides dominate the photon energy spectra in the first few hours after detonation. Because there are a large number of fission products in fallout which emit gamma rays with a wide range of energies, it is not practical to list every gamma- ray emitter produced as a fission product. The range of half-lives of these fission products is also very great (ranging over several orders of magnitude). The tabulation of photon emitters is further complicated by the chain of decay of initial fission fragments. The photon spectrum due to fission-product activity has been reported for selected times following detonation (Nelms and Cooper 1959~. The photon intensity as a function of energy, taken from the referenced report, is shown in Figure 3-2. In this energy spectrum, the dominance of gamma rays between 100 keV and 2 MeV is apparent. Photon energy spectra for fallout have been measured for times ranging from two hours to 3000 hours following detonation (Cook 1960~. Measured spectra indicate that between 65 and 85 percent of the photon intensity is from photons with energies between 100 keV and 1600 keV. Gamma rays with energies above 1600 keV contribute approximately 15 percent of the total photon intensity at three hours following detonation (Cook 1960~. The contribution of lower-energy photons to the photon intensity from direct radiation is a few percent. The overall photon intensity from fallout includes a contribution from scattered photons which can be significant for selected exposure geometries. The mean energy per photon in the fallout field has been shown to vary with time after detonation. This energy has been determined by calculation and measurement to decrease from approximately 1 MeV/photon at 2 hours following detonation to approximately 0.7 MeV/photon between 10 and 3000 hours after detonation. The concept of the mean photon energy is presented with the impor- tant caveat that it should not be used for shielding calculations or other physical processes (Cook 1960~. Although the mean photon energy is as stated above, there is a significant photon intensity for energies below 300 keV until several days following detona- tion. This low-energy contribution is probably from the presence of large quanti- ties of neptunium-239 for the first few days after detonation (Cook 1960~. Measurement results reported by other authors are in good agreement with these data (DeVries 1964; Sondhaus and Bond 1955; Ferguson et al. 1958; Webb et al. 1956; Thompson 1957~.
3 RADIATION SOURCE TERMS 10 > a) ~1.0 - z o o I 0.1 in An lll o to > cr: 0.001 0.01 0.0001 An, L \ \ \ \ \ I 1 1 1 1 0 1 2 3 4 5 \ PHOTON ENERGY, MeV FIGURE 3-2 Expenrnental Photon Spectnnn (t = 25.8 min.) (Helms and Cooper 1959). 33 7
34 E. CONCLUSION FILM BADGE DOSIMETRY IN ATMOSPHERIC NUCLEAR TESTS The residual radiation field following detonation of nuclear weapons consists of radiations from fission products, activation products, and unfissioned uranium or plutonium. During atmospheric testing of fission devices and fusion devices, differences in residual photon fields of residual radioactivity from detonations were observed. The nature of these differences has been determined to be caused by the relative abundance of a few radionuclides which were produced in each atmospheric test. For example, a low-altitude detonation of a fusion weapon induces large quantities of activation products emitting high-energy gamma rays which dominate the residual radiation spectrum for the first few days following the detonation. Conversely, a low-altitude detonation of a fission weapon pro- duces large quantities of fission products which emit a very wide range of photon energies. In either type of weapon, depending on the design of the device, there can be a large amount of activity from the neptunium 239 produced, which can dominate the spectrum for several days. Although the residual radiation intensity depends on a number of factors which may vary from shot to shot, there are relatively few radionuclides, common to all shots, which contribute to the major part of the photon spectrum. The relative abundance of each of these radionuclides determines the photon energy spectrum. In all cases, the photon field is from photons with energies between approxi- mately 100 keV and 2 MeV. There is very little contribution from photons with energies less than 100 keV except for scattering from large area sources. In those cases, this scattered radiation was determined to have an energy of approximately 75 keV and to have contributed up to 10 percent of the overall photon spectrum. In conclusion, atmospheric nuclear meting which included underwater, sur- face, and atmospheric shots at the Pacific Proving Ground and surface and atmospheric shots on the continent produced residual radiation which had photon fields with energies from approximately 100 keV to 2 MeV. Although photon spectra varied considerably from shot to shot, the range of photon energies was relatively constant.