Doin' the Neutron Dance

“The object of the project is to produce a practical military weapon in the form of a bomb in which the energy is released by a fast neutron chain reaction in one or more of the materials known to show nuclear fission.”

Thus begins one of the most unusual artifacts of the Second World War. The book at hand is The Los Alamos Primer by Robert Serber, and the topic is how to build an atomic bomb.

If the material sounds familiar, it may have something to do with last summer’s “blockbuster” movie, Oppenheimer, and John Adams’s opera, Doctor Atomic, both of which traverse similar ground, although to much different purpose. The movie was long on angst, short on physics; and the opera proves that John Adams is the finest living composer of one-act operas…regrettably, he is drawn to the two-act form. The Los Alamos Primer is a wholly different beast: it was a summation of what was then known about nuclear engineering, delivered to the physicists and engineers who were going to build the first atomic bomb. Or The Bomb, as it was known in the ‘50s and ‘60s. Ed Condon, himself a theoretical physicist, took notes as Serber lectured; these were typed up and mimeographed copies (google it) used to orient additional scientific staff as they came on board, hence their "primer." For the book, Serber has reconstructed his original lectures, adding commentary interleaved with Condon’s notes. As such, it is a true, “you are there” moment in history…as close as you and I are likely to get to attending a classified briefing. (All the material in the book, and thereby in this review, has been declassified. Eagle-eyed readers will notice the declassification marks on the book’s cover.)

Fair warning: there is math in the book, some of it advanced. However, one can read the narrative around the equations and still follow the thrust of Serber’s arguments, without working through the derivations. The discussion that follows is sans math. (Oh, quit cheering and sit down.)

To understand why developing the bomb mattered, you do need one number: 170,000,000.

An atomic reaction releases 170,000,000 times as much energy as a chemical reaction. Chemical reaction includes conventional explosives like TNT or plastic explosive…all the things that make the phone booth/bus/grain silo blow up in a James Bond movie. It would take 170,000,000 of those to equal one nuclear explosion. If your average, satisfying, action movie explosion takes, say, 90 seconds onscreen, that would translate into watching onscreen explosions for 485 years nonstop (plus the following January) to log as much energy as one onscreen nuclear explosion. Whichever nation developed an atomic weapon first held a near-unbeatable military position. For that reason, the U.S. assembled a team of physicists in Los Alamos, New Mexico in March of 1943 to figure out how to build an atomic bomb.

First they needed fuel, the radioactive material that actually explodes. Two elements were considered, uranium and plutonium. Uranium is naturally occurring, and as you may recall from school, it exists in two isotopes, U-238 and U-235; these vary in the number of neutrons in their nucleus, their atomic weight. Only 0.7% of natural uranium is U-235, and only U-235 is fissionable…that is, can be used to fuel a bomb. The other element under consideration as a fuel was plutonium. Plutonium is man-made, created when you bombard the nucleus of U-238 atoms with neutrons. A neutron is absorbed, and after a couple of intermediate products, produces plutonium, with an atomic weight of 239, Pu-239. Plutonium is highly radioactive, and therefore an even better fuel for an atomic bomb than uranium…in theory. The problem with both U-235 and Pu-239 lies in creating sufficient quantities: when Los Alamos opened, plutonium existed only in grams. They needed pounds.

Neutrons figure prominently in our discussion, as the material above demonstrates. Neutrons, specifically “fast” neutrons, are needed to start and sustain the chain reaction required for a bomb. (There are slow neutrons as well, but as Serber demonstrates mathematically, they arrive too late and are too easily captured to be meaningful in a chain reaction.) We therefore need a vessel of some sort to contain the nuclear material and its fast neutrons. And since neutrons are whizzing around in all directions, ideally the container would reflect as many fast neutrons as possible back into the device. This is the point where the ominous phrase, “practical military device” rears its ugly head. It was one thing for Enrico Fermi and his staff to construct a building-size “pile” in Chicago and sustain a reaction therein; but the bomb had to fit inside an airplane and get dropped on somebody…truly, size mattered. Serber calculates the minimum amount of nuclear fuel needed to ignite a chain reaction (roughly 60 kilograms of U-235), the so-called “critical mass,” and finds that to account for inefficient explosion and allow a margin of error, approximately four times critical mass would be needed. The fuel, at least, was of manageable weight.

That left the housing and whatever material surrounded the fuel, the “tamper.” (Tamper is a term taken from ballistic ordinance.) It is the tamper that is responsible for reflecting neutrons back into the fissionable mass, and all sorts of materials were considered. Gold, for example. It was a government project, a war was on, and vaporizing a few pounds of precious metal was the least of anybody’s concern…they even got a bowling-ball size gold sphere delivered to the project. In the end, uranium itself offered the best tradeoff of neutron reflectivity and weight. (Again, you can follow the calculations in the book.) Choosing uranium as the tamper also knocked plutonium out of the running as a fuel: plutonium is easily triggered, and the uranium tamper emits enough neutrons that the plutonium would begin reacting before critical mass was attained. The result would be a small, unsustained reaction (as the explosion begins, the fuel is blown apart, reducing it below critical mass), known as a “fizzle.”

A surprising amount of math goes into calculating the force of a fizzle…it was important to ensure that the device would at least destroy itself, so that the enemy could not capture it. Serious stuff.

Finally, they had just the teeniest little problem of how to transport all this radioactive material to and in an airplane without exploding…and then make it explode while falling through the sky. (Impact detonation, accomplished when a bomb hits the ground, would have wasted the energy that makes a crater; exploding mid-air maximized damage…which is the purpose of a bomb, after all.) To make it work, one has to rely on our old friend critical mass: you can keep two sub-critical hunks of radioactive material separate, then jam them together to make a larger, critical mass that chain reacts. Brilliant.

But easier said than done…it’s not a job you can do by hand.

Two approaches proved to be workable, and it turns out you’ve heard of them both: Little Boy and Fat Man. (Get over it, this discussion has nothing to do with body image.) Little Boy was the bomb dropped on Hiroshima, and it was an example of a gun assembly: you shoot a small piece of radioactive fuel into a larger target (think of a bullet penetrating an orange, say) to increase it to critical mass and initiate a chain reaction. This method has relative simplicity (you only need one explosive charge, and the range of motion is constrained) going for it, but with size limitations. The other approach, Fat Man, was the first device exploded, in test, from a gantry in White Sands, NM, in the apocalyptically-named Trinity test. As the name implies, the bomb is spherical. The radioactive material is machined into a hollow ball, then numerous detonators surrounding the ball fire, compressing it to critical mass. (Later work proved that the approach works with a solid ball of fuel as well.) This method allows for larger sizes but introduces a great deal of complexity in timing the detonations across the ball’s surface. Despite its complexity, the Fat Man design carried the day: every bomb other than Little Boy itself has been a form of the Fat Man design. The bomb dropped on Nagasaki was a Fat Man.

Those hoping for a big finish, a grand summation at the end of the book, will be disappointed. These are physicists, not novelists, and it was government work. There is a Conclusion, which blandly points to measurement of different materials’ neutron properties and “the ordinance problem” as immediate areas of concern. Now go build a bomb. The book, however, has more impact: Serber is a good writer, and his Preface is biographical, filling in much of the human aspects that are rehearsed (or tortured, depending on one’s taste) in the popular works. There are also appendices, one containing two memos concerning the bomb, the Frisch-Peierls memoranda; these were the documents that led Roosevelt to establish the Los Alamos project. The book, then, is greater than just the enormously estimable lectures themselves, and makes for a more satisfying read than just the lectures alone.

We focus here on technical matters, for the good and simple reason that it was the technical that consumed most waking cognition at Los Alamos. These were enormously difficult problems, the time was short, and the need pressing. Moreover, the physics and engineering advances that the Los Alamos group bequeathed us count as their greatest accomplishments and their most fitting monument, easily more so than their transient emotional responses.

“All autocatalytic schemes that have been thought of so far require large amounts of active material, are low in efficiency unless very large quantities are used, and are dangerous to handle. Some bright ideas are needed.”

That sentence is from the penultimate section of the lectures. While it speaks to an ignition method that was abandoned, it carries a sense of the difficulties the Los Alamos scientists encountered.

Some bright ideas were indeed needed. The world is insanely lucky that they came.