Posts Tagged ‘Little Boy’

Meditations

The Mysterious Design of Little Boy

Tuesday, November 8th, 2011

On August 11, 1945 — just two days after the bombing of Nagasaki — the U.S. government issued a technical history of the Manhattan Project, written by Princeton physicist Henry DeWolf Smyth.1 The Smyth Report, as it came to be known (its official title was unpleasantly long), was meant to serve as the authoritative guide for what could be publicly said by Manhattan Project participants about the atomic bomb.

One of the areas that the Report was most sheepish about is how the actual charges of the atomic bombs — now called the "physics packages" — are designed. Implosion, the method used on the Trinity "Gadget" and the Nagasaki bomb ("Fat Man"), was ignored completely (and not declassified until 1951). Even the simple "gun-type" design used in the Hiroshima bomb, "Little Boy," was treated only obliquely:

Since estimates had been made of the speed that would bring together subcritical masses of U-235 rapidly enough to avoid predetonation, a good deal of thought had been given to practical methods of doing this. The obvious method of very rapidly assembling an atomic bomb was to shoot one part as a projectile in a gun against a second part as a target.2

In the early days, most people assumed that meant shooting two halves of a critical mass together, or, in more "real-looking" depictions, such as this very early one from the Austrian physicist Hans Thirring's Die Geschichte der Atombombe (1946), a small "projectile" being shot into a dense "target":

"One of the possible constructions of the atomic bomb." Click to see the full page.

On Thirring's diagram,3 a "Phantasie" of "Details der Bombenkonstruktion" (you have to love the German here) based on the description in the Smyth Report, you can see that there is a projectile (P) which gets shot down an artillery barrel (R) by conventional explosives into the target (S), which is a larger amount of fissile material embedded in a tamper (T). The role of the tamper (which is discussed in the Smyth Report) is to reflect neutrons and hold together the fissioning mass a few milliseconds longer than it might otherwise be inclined. This allows for more fission reactions and more of an explosion.

So this is more or less how we've been talking about gun-type designs since 1945... until very recently. John Coster-Mullen, a trucker/photographer/bomb geek (and a friend of mine), dubbed "Atomic John," by the New Yorker in 2008, found, through some painstaking research, that this old story was wrong on one important detail.

The actual "Little Boy" bomb was not a small "projectile" being shot into the larger "target." It's a large "projectile" being shot into a smaller "target." That is, as John puts it, "Little Boy" was in fact a "girl":

A Little Boy diagram from Wikipedia based on John Coster-Mullen's description.

Now half of you are saying "so what," the other half are saying "I already know this, I'm an atomic wonk," and the two of you who are not in that category (and are left out of the halves by rounding errors) are saying, "Cooooool."

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Notes
  1. The paranoid pedant in me wants to point out that the date, August 11, is correct for the distribution date, whereas it is often quoted as August 12. In order to avoid any one newspaper getting the "scoop," the government requested that none report on it until the morning of the 12th, however. So either date is technically fine. Don't you feel better, knowing that? []
  2. See §12.19, "Method of Assembly," in Chapter 12, "The Work on the Atomic Bomb." []
  3. Those who are very into this bomb thing may recognize that this is the same image as the supposed "Nazi nuke" that made the rounds in 2005. Needless to say I am not super impressed with the claims that this was an actually working bomb and not just a visualization based on Thirring's book, which itself was clearly based on the Smyth Report. The fact that the "Nazi nuke" refers to the fissile material as "Plutonium," a name given to it in secret by Americans and only released after the bomb project was made project, makes it patently clear this is very much a postwar construction. []