U.S. Congress, Joint Committee on Atomic Energy: 1950s fallout hearings available as PDF files; the mean gamma ray energy of fallout for civil defence

Stanford University has now published PDF versions (linked here) of 144,000 pages of hearings of the U.S. Congressional Joint Committee on Atomic Energy (which existed from 1946-77), including source material heard by the Special Subcommittee on Radiation on the effects of blast, thermal radiation and fallout from nuclear weapons tests, presented by the project officers at the tests, which Samuel Glasstone used in compiling the 1962 revised edition of Effects of Nuclear Weapons, and which is incorporated into the 1977 edition co-edited with Philip J. Dolan:

Biological and environmental effects of nuclear war: summary-analysis of hearings, June 22-26, 1959. (8 MB PDF download, 68 Pages, linked here.) This summary-analysis contains no testimony, but is exceptionally well written and brief, setting out the case for examining the effects of nuclear weapons when such weapons could be used against us by terrorists and rogue states, and some of the effects (including a table the variation of the mean gamma ray energy with time after fallout, measured by Triffet) and some of the countermeasures known in 1959. It includes a summary of Herman Kahn's testimony at the end, explaining that civil defence is vital to back up deterrence, since Britain was intimidated by Hitler during the 1930s due to having no civil defence (a result of grossly exaggerating weapons effects). However, most of the vital nuclear effects data is given in the full 980 page volume of testimony at the hearings:

Biological and environmental effects of nuclear war. Hearings before the Special Subcommittee on Radiation of the Joint Committee on Atomic Energy, Congress of the United States, Eighty-sixth Congress, first session, June 22-26, 1959. (69 MB PDF file, 980 pages, linked here.)

This is possibly the most important single source document on the biological effects of nuclear weapons, since it includes an extensive summary of local fallout properties from megaton land and sea water surface bursts at Bikini Atoll during Operation REDWING, by the fallout characterization project officer Dr Terry Triffet who with Philip D. LaRiviere compiled the secret-restricted data classified version, weapon test report WT-1317 in 1961. See especially pages 84-98 of the printed hearings, on pp. 98-112 in the PDF file page numbering, which characterizes the fallout properties at two different distances for both water surface and land surface bursts, including the variation of the mean gamma ray energy which is only 0.25 MeV one week after the dirty (87% fission) 5 megaton land surface burst TEWA at 8 miles downwind, and 0.35 MeV at 60 miles downwind. This is due in part to the presence of low gamma emitters in fallout, since in addition to fission, some U-238 atoms from the tamper or pusher in the bomb will undergo an (n,2n) reaction when hit by the very high energy 14.1 MeV neutrons from fusion, thereby producing U-237 (which Japanese physicist K. Kimura discovered in the BRAVO fallout on the Lucky Dragon tuna trawler north of Rongelap in 1954), as well as simple capture which produces U-239 for all neutron energies, which rapidly decays into Np-239, and in the case of very high neutron fluences inside thermonuclear weapons there is some double neutron capture by U-238, producing U-240 which decays into Np-240. These emit very low energy gamma rays, reducing the mean gamma ray energy of fallout and increasing shielding effectiveness (see the earlier post linked here for a compilation of data).

See page 205 of the hearings or 219 in the PDF file pagination for Triffet's discussion of this effect on gamma ray energy and shielding from fallout dangers:

"... it operates to reduce the average energy in this period and shielding is immensely more effective."

RELATIONSHIP BETWEEN STABLE ELECTRON STRUCTURE OF VOLATILE NOBEL GASES AND GENERALLY STABLE NUCLEAR STRUCTURE DUE TO "EVEN" ATOMIC NUMBERS, AND THE ADDITIONAL STABILITY GIVEN BY EVEN NUCLEON (I.E., MASS) NUMBERS

Apart from the presence of low gamma ray energy emitters formed by non-fission neutron capture in the U-238 pusher, tamper or casing of a nuclear weapon, there is also a fractionation effect on the mean gamma ray energy from fission products. Local fallout consists of particles which fall out of the fireball before all of the volatile gaseous fission fragments of xenon and krypton have had a chance to condense on to those rapidly falling fallout particles (either by cooling or by decaying into elements with higher boiling points), so the fallout is "fractionated", like any mixture of substances with different boiling points when heated.

Fractionation is used to separate all of the different components of oil according to their volatility, since evaporation rates increase as boiling point is approached; heavy oils have a higher boiling point and evaporate more slowly than petrol. Distillation of alcohol from water is a simple form of fractionation: ethanol (grain alcohol) boils at 78.5 °C whereas water boils at 100 °C, so alcohol evaporates fastest. Salt in water is not volatile, so distillation can be used to separate or "fractionate" water from salt: it is simply the process of evaporation. Amongst the many fission products in the cooling fireball of a nuclear explosion, xenon and krypton are particularly volatile ans subjected to fractionation since they have low boiling points and thus are normally vapours; they are nobel gases since they both have very stable outer electron shells with 8 electrons (their electron shell structures are respectively 2, 8, 18, 18, 8 and 2, 8, 18, 8). Their nuclei also have shell structures, composed of nucleons instead of electrons, and the "excited" state of these nuclei formed by fission leads them to emit gamma rays in line spectra when nucleons fall back to a more stable configuration or "ground state", just as orbital electrons in an excited state emit line spectra when falling back to the ground state. Nobel gases have a high ionization energy due to the stability of their ground state; more energy is needed to shift electrons from the ground state, and by the conservation of energy, more energy is released when excited nobel gases return to their ground state.

The same applies in general to the nuclear structure and the gamma ray emission from excited nobel gas fission fragment nuclei in bomb fallout: they emit gamma rays with higher than average energy when falling to the ground state. Beta decay, which increases the atomic number, and changes the element, but keeps the mass number unchanged, leaves certain nucleons structures in an excited state, so gamma rays are soon emitted as they fall back to the ground state. The shell structure of the nucleus is more complex than that of the electron shells around it because of the presence of neutrons: as far as the strong nuclear force is concerned (this binds the nucleus together), neutrons and protons are similar, but the electromagnetic force only acts to produce repulsion between the protons. So while the atomic electron shell structure is determined purely by the number of electric charges, the nuclear shell structure is also influenced by the total number of nucleons.

The Pauli exclusion principle, which in the nucleus tends to pairs up protons with opposite spins and neutrons with opposite spins, basically creates orbital alpha particles in the outer shells of heavy even number nuclei, and this effect makes even-numbers of protons and nucleons more stable in the nucleus than odd numbers, thus explaining why U-233, U-235 and Pu-239 (all odd numbers of nucleons, i.e. 233, 235, and 239) are more easily fissioned i.e. are more unstable than U-238 and other istotopes with even numbers of nucleons in their nuclei, like 238.

This nucleon shell structure nuclear model helped Niels Bohr, the founder of the electron structure model of the atom, to correctly explain the initially perplexing nuclear fission cross-section data as a function of neutron energy for natural uranium in 1939, simply by guessing that the low energy neutron fission was occurring in the U-235 impurity, not in the more abundant U-238 which should be less likely to be split by a low energy neutron just because it has an even number of nucleons! Hence, volatile nobel gas fission fragments are associated with stable ground states composed of even numbers of protons and in cases like xenon-138 (which decays into rubidium 88 and cesium 138, predominating in the fission fragment gamma dose rate contributions at 1 hour after detonation) even numbers of nucleons (e.g., 138), so the stability means that there is a big gap in energy level between the excited and the ground state.

This big gap in energy levels between excited and ground state generally implies that a lot of gamma ray energy is emitted in the fall of nucleons from the excited to the ground state, so volatile fission fragments and their decay chains tend to emit higher than average gamma ray energies for the fission product mixture; thus their loss from local fallout due to fractionation tends to lower the mean gamma energy from the most intense and hazardous close-in fallout from dirty U-238 cased bombs, making the gamma radiation easier to shield.

This, in combination with the low gamma ray energy contributions from Np-239, U-237, Np-240 and U-240 (for their contributions to bomb fallout, see for instance the data compiled in the earlier blog post linked here), shows that shielding can be done more efficiently by simple, quick improvised civil defence countermeasures than suggested by published standard shielding experiments using Co-60 with 1.25 MeV mean energy gamma rays, or by standard computer calculations based on pure, unfractionated fission products which have hard spectrum with a mean energy generally of 0.7-1 MeV.

For a full analysis of the gamma ray energy for fallout samples at various distances from four tests of different thermonuclear weapon designs in 1956 at REDWING with fission fractions ranging all the way from 5% to 87%, NAVAJO, TEWA, ZUNI and FLATHEAD, see Triffet's 1961 report WT-1317, Table B.21, as well as Tables 1, 2 and 3 in W. E. Thompson's 1957 report Spectrometric Analysis of Gamma Radiation from Fallout from Operation Redwing, USNRDL-TR-146 / ADA410894, and the direct measurement of the variation in protection factor by a fixed shield of steel as a function of time after burst for fallout for each test on instrumented fallout collection ships YAG-39 and YAG-40, in Heinz R. Rinnert's 1959 report Ship Shielding Studies WT-1321, the key data from which is available in piecemeal format online in the preliminary test reports linked here (ADA410937, FLATHEAD 365 kt 73% fission lagoon water surface burst), here (ADA410940, TEWA 5.01 Mt 87% fission coral reef surface burst, which has graphs comparing the gamma ray shielding as a function of time for local fallout from tests FLATHEAD, TEWA and ZUNI, a 3.53 Mt, 15% fission coral island surface burst.) There is also data from the 1954 CASTLE fallout gamma ray shielding as a function of time after burst on page 82 of WT-934 and in WT-927, and a discussion of some of the data in DNA-1240H-2 linked here.

Pages 10 and 20 of the 1971 report NOLTR-71-103 by Leland R. Bunney and Daniel Sam, Gamma Ray Spectra of Fractionated Fission Products show, for instance, that the fractionated fission product Cs-138 which predominates (with the largest contribution to the gamma ray dose rate from unfractionated fallout) at 1 hour after detonation, is emitting extremely "hard" or high energy gamma ray line spectra, which include lines at 1.02, 1.45, 2.25, and 2.65 MeV. Fractionation depletes most of this hard gamma emitter Cs-138 from the local fallout, leaving predominantly lower-energy or "softer" unfractionated gamma emitters, thus softening the gamma spectrum of fallout by allowing the lower energy nuclides to predominate. Hence, the average gamma ray energy in fractionated fallout falls below that from unfractionated fission products, quite apart from the effect of important neutron induced activities U/Np-239, U-237 and U/Np-240 (all of which have high boiling points and thus are refractory, i.e. don't fractionate significantly) in U-238 cased "dirty" thermonuclear weapons. Cs-138 predominates as the maximum contributor to the gamma dose rate from unfractionated fission products for times of up to 1.3 hours. Such fission products are volatile and so are depleted from land surface burst local fallout, as explained quantitatively by Triffet in WT-1317. See also Glenn R. Crocker's report Radiation Properties of Fractionated Fallout; Predictions of Activities, Exposure Rates and Gamma Spectra for Selected Situations, U.S. Naval Radiological Defense Laboratory, USNRDL-TR-68/134, 1968.

This report on predictions of the fractionated fallout gamma ray spectra by Crocker, a milestone in understanding the effects of fractionation based on analysis of immense amounts of nuclear test data, which was issued shortly before the U.S. Naval Radiological Defense Laboratory was closed down in 1969 (no thanks to the immense U.S. Department of Defense budget diversion to the war against Vietcong insurgency) is not available online at present, but some useful extracts from it were published in the excellent 1,000 page data compilation by Drs Lewis Van Clief Spencer (b. 1924), A. B. Chilton and C. M. Eisenhauer, Structure Shielding Against Fallout Gamma Rays from Nuclear Detonations, U.S. National Bureau of Standards, NBS Special Publication 570, September 1980, U.S. Government Printing Office (this is an A4 size hardback book which is held by the British Libary; it was litho printed directly from the double-spaced manuscript rather than typeset, but it is a very extensive compilation, e.g. - despite the title's focus on gamma rays - it also contains tabular and graphical summaries of the beta ray spectrum of fallout and dose predictions from the beta radiation skin-contact fallout hazard).

Effect of fractionation on the gamma ray spectrum of fallout

Glenn R. Crocker's 287 pages long report Radiation Properties of Fractionated Fallout; Predictions of Activities, Exposure Rates and Gamma Spectra for Selected Situations, U.S. Naval Radiological Defense Laboratory, USNRDL-TR-68-134, 27 June 1968 does not appear to be listed in any online database, although it is cited in the experimental report linked here, so we have created a PDF file which tabulates some of Crocker's most important gamma spectra data, linked here. This shows that for the fission of U-238 in a H-bomb by thermonuclear neutrons, the mean gamma ray energy for unfractionated fission products is 0.81 MeV at one hour and 0.48 MeV at 1 week after detonation, while for fission products in which 90% of the Sr-89 is depleted (i.e. where only 10% of the Sr-89 expected - from the abundance of unfractionated Nb-95 - is present), the mean gamma ray energy is just 0.71 MeV at 1 hour and 0.44 MeV at one week after detonation.

Hence, the depletion of volatile fission products due to fractionation does cause a shift in the spectrum to lower gamma ray energy. As explained above, this shift is due to the fact that the most highly volatile fission products are shell structures for both electrons and nuclear properties via the exclusion principle which result in higher than average gamma ray energy emissions. The loss of these high gamma ray energies from fallout due to fractionation results in a downward shift in the mean gamma ray energy. This is quite apart from the additional effect of very low energy gamma ray contributions from non-fission neutron captures in U-238 which produce large quantities of Np-239, U-237, U-240, etc., in the fallout, causing an additional massive reduction in the mean gamma ray energy and making shielding against fallout (at least for low to moderate protection factors, where the low energy gamma rays are easily filtered out, leaving only the smaller proportion of higher energy gamma rays to continue).

Another source of data on the low mean gamma ray energy and its effects on emergency improvised protection from fallout in the period of a few hours to a few weeks after detonation is the U.K. Government Home Office research at nuclear tests. As mentioned in an earlier post, George R. Stanbury, civil defence project officer at the HURRICANE Monte Bello, Australian nuclear test in 1952 (a simulated terrorist attack by a nuclear bomb smuggled inside the hull of a ship in a shallow harbour), in November 1959 wrote the Confidential Home Office report A12/SA/RM 75, The Contribution of U239 and Np239 to the Radiation from Fallout, U.K. National Archives document HO 226/75, based on British TOTEM nuclear fallout data from 1953 for pure fission, plutonium core bombs with a thick, neutron-absorbing U-238 tamper. His calculation for U-239 contains a trivial calculation error (this is unimportant, since U-239 contributes a maximum percentage to fallout radiation at 40 minutes after detonation), but for Np-239 he correctly found that in such thick U-238 tamper bombs, Np-239 can easily contribute 40% of the gamma dose rate at 4 days after detonation (the time of maximum percentage contribution from any nuclide in fallout decaying as a whole at the rate t^-1.2 is the half life of the nuclide multiplied by 1.2/ln2 or 1.73, hence for a 56 hours half life of Np-239 the time of maximum contribution is 1.73x56 = 97 hours or 4 days after burst).

This is confirmed by the first Chinese fission bomb tower test, a 22 kt bomb based on Russian bomb JOE-1 employing a U-238 tamper, on 16 October 1964. Japanese physicists led by Tetsuo Mamuro analyzed a sample of the fallout from that test on Japan in their report "Radionuclide Fractionation in Debris from a Land Surface Burst" published in Health Physics, vol. 12 (1966), pp. 757-63. They found that 74% of the gamma ray emission rate emitted by a fallout particle at 3 days after detonation came from Np-239, which (allowing for the relative gamma ray energy) implies that 42% of the gamma dose rate (which is approximately proportional to the product of the gamma ray emission rate and the mean gamma ray energy per gamma ray emitted) came from Np-239 at that time, corresponding to a capture-to-fission ratio of approximately 1.6 atoms of Np-239 per fission, and a mean gamma ray energy of 0.3 MeV at 3 days after burst.

Britain also conducted an extensive series of fallout measurements on the protection factor at the ANTLER tests in Maralinga in 1957, where the bombs lacked a U-238 reflector and thus produced negligible Np-239 (beryllium was used instead) and included a "salted" nuclear weapon incorporating cobalt-59 in order to produce cobalt-60 in the fallout by neutron capture (we have explained in the previous post why cobalt-60 isn't a threat in thermonuclear weapon jackets: it emits only 2.5 MeV of gamma rays per neutron used, spread over an average time of many years which allows natural weathering and decontamination before getting an appreciable dose, whereas each neutron fissioning U-238 in a hydrogen bomb produces 200 MeV of energy, including far more fallout gamma ray energy than cobalt-60). Home Office scientists at the test, A. M. Western and H. H. Collin, issued a report, Operation ANTLER: the attenuation of residual radiation by structures, which was published in the June 1967 issue of Fission Fragments No. 10 (originally classified Restricted, now in the National Archives as document HO 229/10) giving a summary of nuclear test fallout shielding results for dosimeters in drums of varying thickness, for U.K. civil defence use (also reported in HO 227/114): 61 kg/m² of concrete shielding allowed 64% of the total integrated gamma dose to penetrate, 156 kg/m² permitted 39% through, 312 kg/m² permitted 20% through, and 781 kg/m² permitted 3.4% through. (The shielding of fallout gamma rays is mainly due to the Compton scattering effect, so it depends almost solely on the abundance of electrons per unit volume of material, which is generally directly proportional to the density of most common materials, so these mass thicknesses can be applied not just to concrete but also to soil or water, with little error.)

This is the worst case fallout shielding scenario, because Co-60 emits relatively hard line spectra gamma rays, 1.17 and 1.33 MeV, thus increasing the overall penetrating power of the fallout gamma ray spectrum above that from fission products at late times, when most of the fission products have decayed. The U.K. Home Office in fact used Co-60 in the U.K. to simulate fallout on and around houses when experimentally developing the Protect and Survive improvised fallout shelter booklet countermeasures, thereby maximising the assumed shielding problem; see A. D. Perryman's 1964 Home Office Scientific Advisory Branch report CD/SA 117, Experimental determination of protective factors in a semi detached house with or without core shelters, National Archives document HO 225/117.

The nature of radioactive fallout and its effects on man. Hearings before the Special Subcommittee on Radiation of the Joint Committee on Atomic Energy, Congress of the United States, Eighty-fifth Congress, first session. (102 MB PDF file, 1,030 pages, 1957, volume 1 of 2 volumes: this is the most important single document summarizing early fallout prediction methods used at Nevada and Pacific tests, the physics of the calculation of the beta skin dose to the beta burned Marshallese islanders using measurements of their skin contamination at evacuation time, which allowed the ratio of the contamination density by skin retention to the deposit on the ground to be computed - beta burns occurred where coconut hair oil and moist skin retained 100% of the ground deposit contamination density - and the measurements of gamma dose rate versus airborne contamination during fallout at numerous Nevada tests, as well as the controversy over the effects of low level effects of radiation. Radium dial painters irradiated at low dose rates over decades needed a massive threshold dose over 1000 R before getting an increased cancer risk, suggesting that DNA repair enzymes can repair damage at low dose rates; but initial radiation from Hiroshima or X-ray exposures over a few seconds were more likely to overload the DNA repair mechanisms and thereby lower the threshold dose for cancer, typically just a few R. In the hearings, the DNA repair mechanism was unknown but arguments using empirical data were made for each case, with the misleading linear no-threshold theory being adopted due to politically-biased anti-nuclear testing based arguments by Nobel Laureate geneticist Herman Muller in the second volume of these hearings, with terrible consequences for the nuclear industry. Contains a vital set of U.S. Naval Radiological Defense Laboratory summary reports by Dr Carl F. Miller and others.)

Fallout from nuclear weapons tests. Hearings before the Special Subcommittee on Radiation of the Joint Committee on Atomic Energy, Congress of the United States, Eighty-sixth Congress, first session. (41 MB PDF file, 516 pages, volume 2 of 3 volumes, 1959: despite its title, this is mainly concerned with global fallout and contains much less important early fallout prediction and measurement data than the two hearings already listed above.)

Fallout from nuclear weapons tests. Hearings before the Special Subcommittee on Radiation of the Joint Committee on Atomic Energy, Congress of the United States, Eighty-sixth Congress, first session. (36 MB PDF file, 736 pages, volume 3 of 3 volumes, 1959: mainly concerned with global fallout and contains much less important early fallout prediction and measurement data than the two hearings already listed above.)