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By James Killus

So I had this little essay entitled, “The Neutron Dance,” because I’m a fan of both neutrons and The Pointer Sisters (June Pointer RIP, 11 April, 2006) and I sent it to the Minister of Justice as part of the We Are All Giant Nuclear Fireball Now Party’s ongoing campaign for a Free Nuclear Zone.

Or something like that. And there’s the rub. Because the Minister of Justice responded by asking me to make some changes, give some context perhaps, add some background and “say a little something about where you’re going with it and why we should care.”

Fair enough, albeit with a soupçon of “are you really sure you want to get me started?” Because I can go meta in six different directions before breakfast and twelve after lunch, to say nothing of übernerd posturing, name dropping, and doing my little Smartest Guy in the Room dance at the drop of a hat.

One tempting tangent is the fact that when I was a lad, the universe was protons, neutrons, and electrons to make stuff with, and photons to make it glow. Sure, there were these cool things called “neutrinos” that had been predicted in 1930 and not actually seen until 1955 and the discoverers were lucky they were young and long-lived, because they didn’t get their Nobels until 40 years later, a full 7 years after the later discovery of the mu neutrino, there’s no justice in the world, I’m just sayin’.

There were also, when I was a lad, these things called “mesons” which are pronounced meh-son, mee-son, or even may-son, provided you want to make puns like “meson jar” or “Meson-Dixon Line.” But those were primarily good for getting funding for particle accelerators and shooting down giant birds from outer space.

But soon the particle accelerator guys got enough money to create something called The Standard Model which they insist is close to a Theory of Everything, (ToE) if by “everything” you mean “a few dozen particles and physical constants.” I mean, I’ve checked, and there is not one word in String Theory, or any of the other proposed ToEs that explains who put the bop in the bop she bop, or even where babies come from.

That is how Fundamentalism works in science, but that is a different rant, and besides, not having a Fundamentalist explanation for where babies come from is a plus, not a minus, at least in my book.

The thing is, again when I was a lad, a scientist was someone in a white lab coat staring at a bunch of beakers and test tubes. There was a periodic table on the wall, we were up to about 100 elements, and it was pretty clear that there weren’t too many more on the way, because the ones above about 95-96 were so radioactive and short-lived that you had to get them from the particle accelerator to the chem lab by motorcycle, maybe with a police escort or something, and that was all very cool, too. And the whole damn periodic table was just protons, neutrons and electrons, as I said before. You also had your three kinds of nuclear radiation, alpha, beta, and gamma (the latter being good for turning your skin green and making you very strong when angry), though being precocious, I learned about weird things like k-capture, spontaneous fission, and positron emission before I was even a teenager, little did I know.

So scientific fundamentalism moved past the “merely” subatomic particles, but the big three, the p, n, and e, are still the basis for both chemistry and nuclear chemistry, and those are, in my estimation, a much bigger deal than quarks, gluons, color, charm, and super-symmetry. And for the nuclear stuff, it’s really all about the neutron, first created in the laboratory in 1930, then they had three years thinking it was some weird sort of gamma ray. Then in 1934 Enrico Fermi

whammed some of them into uranium and nobody figured out what that did until 1938, when, on the run from the Nazis, Lise Meitner

convinced her nephew Otto Robert Frisch that the damn uranium was splitting into lighter elements, and releasing one godawful amount of energy in the process.So there’s that. The sheer romance of the thing. Plus the whole tech thing is so wet dreamy; Freeman Dyson called the hydrogen bomb, the Super, “technically sweet,” but the fact is that the whole magilla is technically sweet, from the film badges to the nuclear power subs carrying nuclear tipped MIRVs. And just look at the last few minutes of Dr. Strangelove sometime and try to deny that the nukes aren’t beautiful. The Giant Nuclear Fireball is one mother set of headlights and you can’t blame any deer that’s caught in the tracks.

So I write about neutrons for the same reason any fan boy writes about whether The Hulk could beat Superman or whether he could survive a three-way with Modesty Blaise and Buffy the Vampire Slayer. It’s just what we do.

The Neutron Dance

There are two main natural sources of neutrons in the terrestrial environment, spallation by cosmic rays, and spontaneous fission, primarily of uranium238. In the former, a cosmic ray of sufficient energy kicks a neutron out of some atom it encounters, while with the latter, a U238 nucleus splits, rather than just emitting an alpha particle.

There are two main sources of the universe’s supply of neutrons. One is the proton-proton fusion reaction, a very slow reaction, since it is basically the inverse of beta decay, and is mediated by the weak force:

P + P -> D + positron + neutrino

This reaction takes place in the center of the sun; the deuterium produced fuses rapidly to helium through some intermediary reactions that sometimes have neutrons as products. However, any neutrons that are produced remain at the center of the sun, since they almost immediately combine with protons to form more deuterium (D). Besides, the core of the sun is too dense for anything but neutrinos to escape (what happens at the center of the sun stays at the center of the sun).

Neutrons are also produced in older stars by the Carbon/Nitrogen/Oxygen (CNO) cycle:

12C + 1H -> 13N

13N -> 13C + positron + neutrino

13C + 1H -> 14N

14N + 1H -> 15O

15O -> 15N + positron + neutrino

15N + 1H -> 12C + 4He

The neutrons so produced are always bound and never exist as free particles.

The Big Bang produced a certain amount of D and He (plus very small quantities of Li and Be), which implies that there is also a cosmic background of neutrinos, but the implied energy of those particles (about 2 Kelvin) is undetectable by current methods. The neutrons in all elements other than those formed in the Big Bang are created in stars, and all elements heavier than iron are formed in supernovae explosions. Nuclear power from fission is in essence a fossil fuel; it’s just that it’s a remnant of a supernova blast.

Neutrons have different effects on matter depending upon their energy. Most of the neutrons we have to work with are from nuclear fission, and start with a “fission spectrum” of energy. For uranium235, the fission spectrum median is about 1.5 Mev, and the mean is about 2 Mev, reflecting the skewed nature of the spectrum. (For plutonium239, these numbers are slightly higher). The spectrum peaks at about half an Mev (500 Kev) for both isotopes, and the highest energy neutrons are about 10 Mev.

An Mev is 1.6 millionth of an erg, and an erg is 1 ten millionth of a joule (a watt-sec). So an Mev is an extremely small packet of energy, except that in this case it’s associated with an even smaller amount of matter. Matter whose atoms have an average energy of 1 Mev has a temperature of 11 billion degrees.

The term “fast neutron” is pretty loose; often it is used to simply distinguish between slow, “thermal neutrons” and those that haven’t been thermalized (moderated to thermal energies). Even for thermal neutrons, however, there are plenty of quibbles and distinctions, since there are “cold” and “hot” thermal neutrons, and those with energies between 1 Kev and 1 Mev are sometimes called “intermediate.”

Even within the fission spectrum there are distinctions, since isotopes like U238 will fission if the neutron hitting it is fast enough. In fact, fast neutron fission has been observed all the way down into the stable isotope range (e.g. bismuth), albeit with _very_ fast neutrons (>100 Mev). A certain amount of power reactor fission is, in fact, fast fission of U238. However, U238 itself cannot sustain a chain reaction, because inelastic scattering by U238 slows neutrons, in competition with fast fission. The slowing (moderation) of neutrons puts them into resonance regions of the U238 capture spectrum, and they then get absorbed, forming U239, which decays to Np239, then to Pu239. This represents “breeding” and a significant portion of normal reactor power production does come from fission of the internally bred Pu239.

The easiest fusion reaction to initiate is the tritium-deuterium reaction, which produces a neutron of 14.6 Mev. A neutron of that energy will fission U238 at an almost 100% efficiency, leading to a fission event having an energy of around 200 Mev, an order of magnitude increase. Moreover, such fission events produce an enhancement of almost a factor of 2 in fission neutron production when compared to normal fission spectrum neutron fission, leading to a longer fission chain.

Any fusion technology will invariably work first on the T-D reaction, and such fusion will always have a higher energy output if used in a “fusion/fission” reactor, where the fast neutrons then are used to fission natural uranium. Moreover, the F/F reactor can be made sub-critical, since the fusion reactions supply the control factor that is usually accomplished by the delayed neutrons from fission. Such reactors can also be run at a higher breeding efficiency, because some of the control factors (such as the use of oxide fuel to assist the “Doppler broadening” of neutron resonance capture), could be dispensed with.

Similar arguments can be made for “accelerator driven” reactor technology, where a high-current, high-energy proton beam is used to spallate fast neutrons from lead or bismuth, also serving as a controlled neutron source.

Finally, most thermonuclear bombs use the fusion/fission effect to amplify yield. Since most of the energy in a thermonuclear fusion burn comes off as fast neutrons, the yield can be significantly boosted if one uses a uranium tamper and bomb casing. The amplification isn’t the order-of-magnitude increase implied by the above calculation, because some moderation occurs from the scattering that is enhanced by the extreme compression of a thermonuclear detonation, and also because 100% capture of the fast neutrons would require a prohibitively thick bomb casing.

The most powerful bomb ever detonated was roughly 50 megatons, testing in Siberia in 1961.

It was tested with a non-fissile tamper and bomb casing, so it did not use fissile materials to increase the yield; this made it one of the “cleanest” bombs ever tested.

If fissile materials had been used (and the bomb was designed for those as well), it would have exceeded 100 megatons in yield, with an enormous amount of fission product fallout.