Jan 13, 2020, 05:55AM

Why The Bomb Gives Me a Woodie

Inventions have consequences.

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My interest in invisible stuff started when I was a kid. I was six in 1958. I had nine sisters and longed desperately for a pair of X-ray specs. But it wasn't just that. I had four brothers as well. My parents were distant, and my siblings were unpredictable. It was chaos. I longed for something I could figure out and understand. Perhaps I was looking for stability, something with rules.

I dissected batteries and bicycles. I played with matches. I took radios apart, looking for the source of the music. I made flashlights out of paper towel tubes, and toyed with magnets. How did they stick to the refrigerator, but not attract coins? I coiled wire around a spike and made an electromagnet. I read about the "field" in radio magazines but couldn't grasp the idea. This looking became a lifelong habit.

In my 30s I took a night course on astrophysics, because I wondered how stars formed. I wanted to know what caused them to ignite, and why they kept burning for so long. It was a well-spent few hundred bucks. I learned that stars are the result of fusing four hydrogen nuclei together, creating one helium nucleus and releasing what's known as the binding energy that holds the original nuclei together. This was first formalized by Hans Bethe in 1938. Every second, the sun converts 600 million tons of hydrogen into 596 million tons of helium. This is a monstrous thought. The four million missing tons are converted into radiant energy, in keeping with Einstein's formula of mass-energy equivalence. Some of that energy, approximately a billionth of it, travels 93 million miles in eight minutes to fuel life on earth. The sun, the text explained, is a continuously exploding thermonuclear bomb. A living hydrogen bomb.

What the fuck? Now I had to know about nuclear reactions. What was binding energy? Were mass and energy really two forms of the same thing? How does radiation work? And I wanted to know, considering no one’s ever seen an atom, how people figured such things out. Check out the catalog of bad 1950s monster movies. I've seen them all. Look how many monsters came to life via nuclear detonations. There's a moral tale there that I missed as a kid: inventions have consequences.

Radioactive decay was discovered accidentally, when Henri Becquerel left some uranium salts on a photographic plate in the dark. The plate was later found as fogged. This suggested that the uranium was emitting something like light, something that exposed the film. If so, the uranium atoms weren’t stable or indestructible; they at least had inner workings, moving parts. X-rays were discovered in 1895 in Germany, also by happenstance. Chadwick in England discovered the electron in 1897. The Curies refined radium from uranium in 1898. People carried samples around, at radium parties. It was a time of unparalleled investigation throughout Europe. Radiation was a stream of individual particles: alpha, beta, gamma. Ernest Rutherford discovered the nucleus of the atom in 1911. He discovered protons in the nucleus. In 1932, James Chadwick discovered the neutron inside the nucleus; having no electric charge, the neutron could be used as a bullet to explore the internals of other atoms. The atomic nucleus became the favored target. Labs all over the place, playing with atoms. Binding energy is the energy that holds the protons together in the nucleus in spite of their like charge.

I hadn't realized the working history of radiation was so short, a mere 50 years between Becquerel and Hiroshima. By comparison, the lens was in use by monks transcribing texts for 300 years before someone thought to put two of them together to make a telescope. I thought back to my astronomy days, when I admired Copernicus, Kepler, Brahe and Newton. These were some of the people who purified astronomy from the mists of astrology. Much was discovered without telescopes, just by careful observation. I had a fair understanding of, and great admiration for, their achievements, but those guys were not from my century.

Next came magnetism and electricity and the concept of the field. More of my heroes: Maxwell, Hertz, Faraday. Maxwell unified magnetism and electricity, and predicted the existence of radio waves.

In the 20th Century, the atom was the new world. The first books I bought were about the bomb they dropped on Hiroshima, books on Los Alamos and the Trinity test. I wanted to learn about the people involved. Fermi, Bohr, Oppenheimer, Teller, the Curies, Bethe. People closer to my own time. Various papers were declassified over the decades. You could get them on the Internet. The Farm Hall Tapes. The Los Alamos Primer. I bought them all. I bought video collections of mushroom clouds. Biographies of all the greats. An indecipherable paper by Bethe on the hydrodynamics of the fireball. The history of the high-speed cameras used to film nuclear explosions that occurred in a few microseconds. How could anything that powerful happen so quickly?

The Hiroshima bomb was a uranium device. The uranium nucleus is densely packed with protons and neutrons, and is easy to rupture. It’s the last naturally occurring element on the Periodic Table. In 1938, Hahn and Strassmann, working in Germany, split a uranium nucleus into two with neutron bombardment. The results, clarified by Lise Meitner and her nephew, were two lesser elements (barium and krypton), a release of binding energy, and the freeing up of some neutrons from the original uranium. Almost immediately, physicists like Leo Szilard could see those new neutrons splitting more uranium, releasing more energy, and sustaining a chain reaction. Two neutrons become four, which become eight, then 16, and so on. A simple geometric progression. This led immediately to thoughts of a bomb, because all reasonable physicists could see what might happen if the chain ran out of control. The first successful work was controlled when, on December 2nd, 1942, Enrico Fermi initiated the first sustained nuclear fission reaction in a "pile" of uranium chunks and carbon blocks.

The Los Alamos Primer (University of California Press) is a good place to start. It’s a series of lectures given at Los Alamos, the subject two-fold: Is it possible to build an atomic weapon and, if so, how could it be done? The lectures were declassified in 1965 but reclassified after 9/11. A neutron, traveling at 10 million meters per second, travels seven or eight centimeters inside the lump of uranium before colliding with a nucleus. This distance is called the mean free path. In a small uranium sample, most neutrons would escape the surface of the metal before hitting a nucleus. This sample wouldn’t "fish," or sustain a fission reaction. But if the chunk were large enough (at least two mean free paths in diameter), many of the neutrons would collide with and split other nuclei. That size sample is called the critical mass, the minimum quantity that would sustain a reaction. The trick would be to assemble two sub-critical pieces of uranium very quickly to create a critical mass and thus an uncontrolled reaction. The Hiroshima bomb weighed almost five tons, but contained only 140 pounds of uranium fuel. Of that 140 pounds, less than a kilogram fissioned, and only a few grams of uranium were converted into radiant energy in the form of gamma rays. Neutron multiplication, 2-4-8-16-32... continued for 80 generations. It took about a millionth of a second. This kills me. A thimble full of uranium, turned completely into energy, destroyed the city.

How did the physicists ever come to understand that the nucleus was glued together with energy? How did they measure it? How did they figure out that after 80 generations, the uranium metal would heat up and expand to the point where the nuclei would become too far apart, the neutrons would not find targets, and the reaction would fizzle out? There were no computers; the calculating was done on slide rules, but the ideas came from brutal imagination.

I was surprised to learn that the uranium bomb was never tested beforehand. The design was so simple as to be considered foolproof, and they didn't want to waste any uranium on a test. Less than one percent, 0.7 percent, of uranium ore is suitable as bomb material. It was costly and time-consuming to purify U-235 (the fissile isotope) from raw U-238. There are several ways to do this: the centrifuges you read about in Korea and Iran; gaseous diffusion of uranium hexafluoride; electro-magnetic separation of uranium isotopes in a cyclotron. Once you’ve collected enough U-235, you just shoot two chunks of it together with high explosives and walk away. If the bomb turned out to be a dud, it would at least destroy itself in the process, leaving little of value for the Japanese to study.

But they knew all this in New Mexico. What they spent more time on at Los Alamos was the plutonium bomb, which is also a fission device. Plutonium (discovered in 1940) is produced in a cyclotron by bombarding U-238 with deuterions. It’s much quicker to react than uranium, so the so-called "gun method" of combining two pieces would not work: it would pre-detonate and fizzle out. The solution for plutonium assembly is more than elegant. A hollow sphere of "ploot" is not critical because the center is missing. In that middle space you put a tiny beryllium initiator, a source of neutrons; this was designed by Robert Christy and was called an urchin, or a Christy core. You surround this hollow Pu ball with a high explosive shell and detonate it. These high explosive "lenses" create an inward facing shockwave that collapses the plutonium into a solid, and crushes the beryllium, which releases neutrons to ensure the start of the reaction. The first nuclear detonation, the Trinity test in July 1945 at Los Alamos, was this type of device. There were about 20 pounds of plutonium in the test bomb.

While all this was worked out at Los Alamos, there were a few renegades who already understood Bethe's work on fusion reactions in stars, including Fermi, Edward Teller and Stan Ulam. Teller considered the fission bomb a simple mechanical problem, and took some leeway at Los Alamos to think fusion over. He began working out a design for a hydrogen bomb with Ulam. They considered that the conditions prevailing in the sun might be duplicated with a fission bomb, which they correctly assumed would be shortly available. There were horrendous mathematics involved in modeling this "thermonuclear" process, to figure out if fusion would or could sustain itself. But now they had ENIAC, the new 20,000-tube electronic calculator to do the number crunching. Their design was known as the Teller-Ulam configuration.

A 65-ton experiment called Ivy-Mike used liquid deuterium, a hydrogen isotope. It was detonated in the Marshall Islands by the United States in 1952. Most of those tons were needed for cooling the deuterium; this was not a useable weapon, but a proof-of-concept device. The resulting mushroom cloud was 25 miles high and 100 wide.

A year later, the Soviets detonated their first "super" with one advantage over ours: it was small enough to be delivered by air. Lithium deuteride needs no cooling, and produces tritium in the fusion process, thereby creating more fuel as it burns. Such a bomb has no theoretical size limit. The more fuel you put in, the bigger the yield. This configuration works in reverse as well; there are hydrogen devices today that can fit in a backpack. The record detonation is held by the Russians: the 1961 Tsar Bomba was the equivalent of almost 4000 Hiroshima bombs. It weighed 60,000 pounds and unleashed the power equivalent of 50 million tons of TNT.

These numbers don’t impress me. It's the speed of the reactions inside the nucleus; it's the fact that the split nucleus just happens to release more neutrons; it's the tiny amount of binding energy released per fission. It's the horrible beauty of the fact that turning hydrogen into helium releases so much energy. Mostly, it's the fact that matter is frozen energy, just as Einstein said. And the fact that this energy can only be realized from splitting the heaviest element, uranium, or by joining atoms of the lightest one, hydrogen.

If there are gods, why did they allow us to learn how to do it?

I definitely would’ve worked on these bombs. I would’ve paid a bundle to see one go off. There’s no right or wrong about the science itself. Oppenheimer said it's better to know about something than to not know about it. As a scientist, or even just as a curious person, you’d have to look. You couldn’t avert your eyes.

The fact that I wouldn’t have dropped the bombs on Japan is a topic for another day.


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