Initial gamma radiation from high-yield tests
Left: 6.9 megaton Union test radiation pulse due to "hydro-dynamic enhancement" (the easily-penetrated low density air behind the compressed air of the blast wave, once the shock front has passed by the target location), some 2,186 metres from ground zero. The fission product gamma dose rate is also affected by fireball expansion, radioactive decay and cloud rise. Fireball rise terminates the initial radiation after 20 seconds or so. The earth's full atmosphere is equivalent to a 10 metre thick water shield, and just 1 metre of water equivalent will stop most of the radiation from a nuclear explosion (it is less penetrating than cosmic rays).
'Initial gamma rate versus time was measured at fixed distances from ground zero. In particular, measurements were made of the effect on initial gamma rate caused by the passage of the shock front from ground zero through the detector station. ... Data obtained indicate that the expanding fireball and the passage of the shock front from ground zero through the detector station had a marked effect on the initial gamma rate - hence on the integrated exposure. The initial gamma rate reached its first peak immediately after the detonation, decreased slowly, began to rise slowly, and then increased rapidly to a second peak (which was about the same value as the first peak). After reaching the second peak value, the initial gamma rate decreased rapidly to zero.
'The slow decrease in initial gamma rate was attributed to the natural decay of the fission products; the slow rise, to the expanding of the fireball and approach of the shock front; and the rapid increase, to the passage of the shock front through [past or around] the detector station. These effects were also evidenced in the integrated exposure both prior and subsequent to the arrival of the shock front.' - Peter Brown and Gerald Carp, Gamma Rate Versus Time, Operation Castle, project 2.2, U.S. Army Signal Engineering Labs., Fort Monmouth, N.J., Weapon Test report WT-913, originally classified Secret - Restricted Data, p5.
In reviewing American nuclear test data, some interesting superlatives emerge. Because of the difficulty in making gamma radiation measuring instruments survive the blast within the fireball, the highest dose measurement was made in 1962 at the tiny, 0.5 kt, Johnnie Boy Nevada test: the measured gamma dose was 785,000 R at a distance of just 27.4 m.
The highest initial gamma radiation dose in the atmosphere from a megaton-range device was 520,000 R on Ruchi, 2,286 metres from ground zero in the 10.4 Mt Mike test. This was only because there was a measuring station in the concrete blockhouse that close, within the actual fireball, which had a maximum radius at final thermal maximum power of 2,615 m. (The reading has been corrected for the shielding factor of the concrete blockhouse shelter where the instrument was located.) The 1.69 Mt Nectar shot produced 397,000 R at 1,134 metres range, while the third highest initial gamma dose was 277,000 R at 2,060 metres from the 5.01 Mt Tewa shot.
However, the highest measured initial gamma radiation doses were produced by the 11 Mt Romeo test, but the measurements only exist for very great distances, but still exceed those of Mike at corresponding distances. Romeo gave an initial gamma dose of 13 R at the colossal distance of 5,514 m, which exceeded the dose given by Mike at the same distance. Obviously, the Bravo and Yankee shots with higher yields (14.8 Mt and 13.5 Mt, respectively) than Romeo and Mike, would have produced larger doses, but no measurements were made.
See page 20 of WT-913: Bravo was measured at 2,196 m from ground zero and the peak dose rate was 60,000,000 R/hr (at 0.9 second) giving an integrated dose of 9,900 R up to 0.9 second after detonation time, when the shock wave arrived and destroyed the instrument.
Above: as explained in a previous post, in the blast wave the air mass is concentrated in a thin shell called the shock front. The bomb releases a lot of energy, but not a lot of mass. So the air compressed at the shock front has to come from somewhere: it causes a low-density volume near the fireball. Nuclear radiation emitted during the first minute is enhanced if air density falls. So a bigger bomb has disproportionately higher fission product gamma radiation, as shown above.
The 10 megaton Mike test produces initial gamma radiation doses at 2-3 km which are seen to be more than 10,000 times those of a 100 kt bomb; ignoring hydrodynamic enhancement you would expect a bomb 100 times more powerful (10 Mt/100 kt = 100) to give radiation doses 100 times bigger. Instead, they are 10,000 times bigger. So hydrodynamic enhancement can increase initial gamma radiation doses by 100 times, simply by reducing the amount of air shielding the radiation. If the mean free path of the gamma rays in normal air is 300 metres, then the normal air over a distance of 2.5 km will attenuate the radiation by a factor of e2500/300 = 4,160. So it is quite conceivable that the near vacuum near the fireball in a big bomb test will enhance the initial radiation by factor such as 100.
On the graph above, initial gamma dose data for ten tests above 100 kt in yield are plotted. Most of the tests were detonated in barges, close to the surface of the lagoons of Bikini or Eniwetok Atolls, or on reefs or islands. Mike, Koon and Zuni were coral island surface bursts, but the islands were very small, so the radiation went above water to the recording stations on other islands. Tewa was a burst on a barge in very shallow water above the coral reef at Bikini Atoll, King was a pure fission air burst, and all remaining tests were barge surface shots in the lagoons. The two ‘clean’ tests are Navajo (5 % fission) and Zuni (15 % fission). Because of the reduced fission yield of 3.53 Mt (total yield) Zuni, its data for high doses is similar to the high dose data from 1.69 Mt Nectar shot (80 % fission). At distances beyond 2 km, however, Zuni doses predominate because the greater total yield of Zuni created a stronger blast wave, with greater hydrodynamic enhancement of doses, which is more prominent beyond 2 km.
Errata: my illustration will need to be updated. There is data for the Dakota initial gamma radiation (this report, page 35), but that is not very useful without knowing the fission yield of that test. Dakota (1.1 Mt barge burst, 20 inches in diameter, 58 inches long, and weighed 1797 lb, W-28 small diameter, light weight thermonuclear weapon design) test at Bikini Atoll, initial gamma radiation data:
1,348 m: 117,000 R
1,676 m: 24,700 R
2,013 m: 5,135 R
2,027 m: 4,485 R
2,201 m: 705 R
The fission yields of all the tests shown in the diagram above are in the public domain (if we ignore confusion over Koon and Mike fission yields), but Dakota fission yield is still secret. The normal sea-level air density before each test shown (ref NDL-TR-53) was worked out from the measured ambient pressure and temperature at shot time: in units of kg/m3 the air density was 1.18 for Koon, Romeo, Zuni, Flathead and Navajo, 1.17 for Union, Nectar, Tewa, Dakota and Mike, and 1.14 for the King shot (which was an air burst at 451 m altitude; NDL-TR-53 page 100 adds the information that the core of King was surrounded by a layer of high explosive 44 cm thick).
The list of references on the diagram is also incomplete. Additional citations to be added:
J. S. Malik, Gamma Radiation Versus Time, Projects 5.1 and 5.2, Operation Ivy, WT-634, 1952; Los Alamos Scientific Laboratory, Los Alamos, New Mexico; originally Secret Restricted Data.
Peter Brown and G. Carp, Gamma Rate Versus Time, Project 2.2, Operation Castle, WT-913, 1959; Signal Corps Engineering Laboratories, U. S. Army, Fort Monmouth, New Jersey; originally Secret Restricted Data. Additional data for Castle including good quality diagrams can be found in report WT-934.
Peter Brown et al., Gamma Exposure Versus Distance, Project 2.1, Operation Redwing, WT-1310, 1960; U. S. Army Signal Research and Development Laboratory, Fort Monmouth, New Jersey; originally Secret Restricted Data.
Peter Brown et al., Gamma Exposure Rate Versus Time, Project 2.2, Operation Redwing, WT-1311, 1960; U. S. Army Signal Research and Development Laboratory, Fort Monmouth, New Jersey; originally Secret Restricted Data. Another version is linked here.
Further information on clean tests Zuni and Navajo: http://glasstone.blogspot.com/2006/03/clean-nuclear-weapon-tests-navajo-and.html
For more nuclear test data, see also the declassified (originally secret-restricted data) manual, The Nuclear Radiation Handbook, AFSWP-1100, 25 March 1957, Nuclear Development Corporation of America, White Plains, New York (PDF linked here).