Oppenheimer

I recently read American Prometheus, the Triumph and Tragedy of J. Robert Oppenheimer. It won a Pulitzer, so there's that. Moreover, it is exhaustively researched, often spending substantial effort to unravel ancient questions, or at least to indicate the problematic nature of historical investigations. Often, even the principals involved do not agree on what actually took place, a phenomenon we're all familiar with.

The book is huge, and I'm going to focus on only a tiny sliver of it, but one that has some relevance to current questions and some recent commentary on this here blog.

The history of the creation and evolution of the Cold War is complex, and many people have various "woulda, coulda, shouldas" in their heads about the entire matter, just as many people believe that the use of nuclear weapons on Hiroshima and Nagasaki was a contingent decision that could have been made in other ways. My own sense of it all has long been that these things were as close to inevitable as anything in human history. Given that nuclear weapons had been created, their use in an ongoing war was a forgone conclusion. There are details that could have been different (and I am hugely grateful that Kyoto was not a target), but the events were inevitable.

There is also some discussion in AP about the theory that the Japanese attacks were really meant as a threat to the Soviet Union, the opening salvo in the Cold War. Certainly there were those who argued for them on that basis, but there are always many players in the game, and I see no reason to believe that this reasoning was decisive. Truman also worked to get the Soviets to enter the war against Japan; it always looked to me like he was simply using everything in his arsenal to end the war.

But after the war, events developed a momentum of their own. The Strategic Air Command, part of the newly formed Air Force and under the direction of Curtis "Strategic Bombing will make Battlefields obsolete" LeMay, worked hard for a nuclear monopoly. It failed because the other services wanted them some nuke macho too.

Then there are the scientists. Let's look at Oppenheimer, Teller, and John von Neumann.

Teller is well-known as the "Father of the Hydrogen Bomb," originally just called The Super. His testimony in Oppenheimer's security clearance hearing resulted in his, Teller's ostracism by many in the scientific community for years afterwards. On the other hand, he had a national lab built for him (Livermore).

Teller's motivations for working on the Super are not as clear cut as one might think. I suspect he just thought it a very cool gadget and rationalized the rest. However, he was Hungarian, and not the most friendly guy towards the Russians, and said so, many times. He wanted the U.S. to have the Super before the Russians got it. And I will give Teller this due: his bombs have never been used in warfare, at least so far.

Von Neumann, another Hungarian, was even more bellicose than Teller. He advocated pre-emptive nuclear strikes on the Soviet Union in order to forestall their ever achieving nuclear weapons. This would also have had the effect of toppling the Soviet government and requiring a U.S. occupation of Russia, which he was also fine with. Von Neumann was also on the Japan nuclear targeting committee and was one of those pushing for nuking Kyoto, a city of practically no military significance.

Finally, there is Oppenheimer, who opposed the Super, and paid dearly for it, eventually losing his connections to the halls of power. But lest we get all teary-eyed, realize that he acceded to the Japanese strikes, albeit with some later breast beating.

Moreover, and this is the interesting part, Oppenheimer argued that, rather than building bigger weapons, such as the Super, which he thought was purely genocidal, the country's stockpile of fissile material should be used to build smaller, tactical, battlefield nuclear weapons. Subsequent thinking has been that such weapons blur the line between conventional and nuclear weapons, making escalation to full-scale nuclear war easier.

Truly large bombs, the multi-megaton behemoths have largely faded from the scene. Current nuclear arsenals contain mostly sub-megaton weapons, albeit small and MIRVed, and still several multiples of the Hiroshima sized yield. Part of that is the first part of Oppenheimer's logic: there just aren't many uses for huge bombs. Even genocide has its limits.

What I take away from the exercise is this: Oppenheimer, Teller, von Neumann, these were three of the smartest guys on the planet, each committed to "rationality" in his own mind. But each one of them managed to argue himself into a position that seems simply crazy on the face of it, unless nuclear genocide is sane, and those of us who find it horrible are somehow the ones who have lost our minds.

Firestorms

[crossposted from WAAGNFNP]

If you try to light a match under micro-gravity conditions (we all got used to “zero-g” so some smarty pants had to go and call it “micro-gravity”) and just hold it in one place, it will self-extinguish. The match will use up enough of the oxygen in its surrounding volume of air to extinguish the flame. It doesn’t have to use up all the oxygen, either; most flames go out in air that still has enough O2 in it for people to breathe—barely.

Depending on the fuel, (e.g. hydrogen needs less oxygen to burn than methane does), the usual figure given is that 14%-16% oxygen is needed to sustain a fire. People can manage on a bit less; Biosphere II dropped below 14% before they pumped in some additional O2, but they didn’t have to contend with elevated CO2 levels; in fact, what they’d been losing was CO2, by absorption into their nice new concrete structure, with bacteria converting soil organics and O2 into CO2. They’d had a bit of a “slow burn.”

Your basic candle flame is fed fresh air by gravity, specifically, the air coming in to replace the hot gases that have become lighter than air in the hot flame. That’s called the “fire draft” and fireplaces exist to direct the fire draft upwards, so the smoke doesn’t choke the people warming themselves by the fire. The chimney/flue of the fireplace also accelerates the fire draft if you build it right, and both Ben Franklin and Benjamin Thompson, (Count Rumford), invented some tricks that are still in use.

So fires always produce an updraft. In truly big fires, the question becomes how the updraft interacts with the local weather. If the local winds are stronger than the updraft, and the fire is big, uncontrolled, and uncontained, you have a conflagration. If the fire creates its own winds, you have a firestorm.

Neither is anything you or I want to be near. A running wildfire can exceed 70 mile per hour under upslope flow conditions, where the fire draft adds to the natural winds. Firestorms generate their own weather, their own winds, and can create small tornadoes, “dust devils” made out of flaming gases that light everything they touch.



The heat in the interior of a firestorm pyrolizes everything within its boundaries, but the fuel produced exceeds the air available. So the hot mass rises as a fireball, sucking more air into it, maintaining its heat even as it expands, because there is still plenty of fuel gas left to burn. A firestorm spreads as much by thermal radiation as by flaming contact, sometimes triggering fires at a distance, like across a valley.

Kurt Vonnegut lived through the firebombing of Dresden, and wrote about it, so more people know about the 35,000 people who died there than in Operation Gomorrah, which killed a larger number (50,000 est.) in Hamburg, or the 120,000 who died in the Tokyo fire raids. I’d never even heard of the raids on Kassel, Braunschweig, Darmstadt, Heilbronn, Pforzheim, and Würzburg until I looked them up for this essay.



But fires are tricky to set with conventional incendiaries. Most of the WWII fire raids were duds, or semi-duds, producing some fires, but nothing like a real firestorm. There were four attempts on Hamburg before they hit the jackpot.



But Hiroshima was a jackpot; what the first blast didn’t do, the subsequent firestorm did, and 4 square miles of the city just went away, nothing left, not even steel, much less teeth and bones.



The Nagasaki bomb was bigger, but the targeting wasn’t as good, and the city had the good fortune of having a lot of hills, which shielded some from the blast, the heat, the radiation. Moreover, the hills altered the wind field, and the resulting fires are only classed as a conflagration. Still, 40,000 people died quick, and maybe four times that number died slow, from injuries, from radiation, from the long term illnesses that go with radiation, from trauma, and grief.

In 1944, the most powerful bomb used in warfare was the British Grand Slam on the order of 10 tons of TNT, although the U.S. developed (but never used) one that was twice as big. In 1945, of course, nuclear weapons increased bomb yields by three orders of magnitude.

It’s hard to develop a sense of scale once you start dealing in factors of a thousand. People think “nuke” and think “Hiroshima and Nagasaki.” But those bombs were measured in kilotons.

Thermonuclear weapons are measured in megatons, another factor of a thousand, so we got a factor of one million increase in about half a decade. Go look for pictures of atmospheric bomb tests. See that one near the mountain? You can see the billows in the clouds and dust that it shakes up the near field.



That’s a bomb that’s in the kiloton range. There have been industrial accidents that can be measured in kilotons, like the Pacific Engineering Company plant in Henderson, Nevada, where over a thousand tons of ammonium perchlorate blew up. That was a kiloton explosion.



Now go find some photos of the Pacific island H-Bomb tests. Google on “Castle Bravo” for example, the biggest miscalculation in the history of nuclear weapons. It was supposed to produce 6 megatons; instead they got 15. “Castle Romeo” was part of the same mistake. They expected 4, but got 11.



See those shapes in the sky above the mushroom cloud? That’s the stratosphere.

The thermal radiation effects of nuclear devices loom larger as the energy release increases. In Hiroshima, the firestorm was likely caused by the blast itself, in the same way that an earthquake causes fires, by turning building into kindling, by releasing natural gas, by rupturing fuel tanks. The heat from the bomb itself probably lit only a few of the fires.

But megaton blasts, perhaps over grasslands, forests, farms? The experiment has yet to be preformed. And calculations, simulations, and estimates are so very, very unsatisfying, aren’t they?

Fallout

[crossposted from WAAGNFNP]

Hot gingerbread and dynamite
Boy, I drink nothing but that each night
Back in Nagasaki where the fellers chew tobaccy
And the women wicky wacky woo!

It was a lovely morning In Hiroshima town,
One summer morn in nineteen five and forty.
And the sun, how bright it shone
From a sky without a cloud,
One summer morn in nineteen five and forty.

Turn around, go back down, back the way you came.
Can't you see that flash of fire ten times brighter than the day?
And behold the mighty city broken in the dust again,
Oh God, the pride of man, broken in the dust again.

Hail the day so long expected,
Hail the year of full release.
Zion's walls are now erected,
And her watchmen publish peace.
Through our Shiloh's wide dominion,
Hear the trumpet loudly roar,
Babylon is fallen to rise no more.

And there were many children
Yet lying in their beds,
For this was still an early morning hour,
And the dew lay on the meadow
In the lovely slanting sunlight,
And the crowns had barely opened on the flowers.



Turn around, go back down, back the way you came
Terror is on every side, though the leaders are dismayed
Those who put their faith in fire, in fire their faith shall be repaid
Oh God, the pride of man, broken in the dust again

Hiroshima Nagasaki
Nagasaki Hiroshima
arigato Vanunu
ko n nichiwa Mossadegh
arigato Mordechai
ko n nichiwa Mohammad
Hiroshima Nagasaki

All her merchants stand with wonder,
What is this that comes to pass:
Murm'ring like the distant thunder,
Crying, "Oh alas, alas."
Swell the sound, ye kings and nobles,
Priest and people, rich and poor;
Babylon is fallen to rise no more.

Aw, man, how they entertain,
I mean, they hurry a hurricane.
Back in Nagasaki where the fellows chew tobaccky
And the women wicky-wacky-woo!

I come and stand at every door
Though none can hear my silent tread
I come and knock yet remain unheard
For I am dead, for I am dead

I'm only seven though I died
In Hiroshima long ago
I'm seven now as I was then
When children die they do not grow

Turn around, go back down, back the way you came
Shout a warning to the nations that the sword of god is raised
On Babylon that mighty city, rich in treasure, wide in fame
It shall cause thy tower to fall and make it be a pyre of flame
Oh God, the pride of man, broken in the dust again

You are my angel
Come from way above
To bring me love

Her eyes
She's on the dark side
Neutralize
Every man in sight

To love you, love you, love you ...



Hiroshima Nagasaki
ko n nichiwa arigato
Nagasaki Hiroshima
Nagasaki Hiroshima
arigato Vanunu
ko n nichiwa Mordechai
ko n nichiwa Mohammad
Hiroshima Nagasaki
ko n nichiwa arigato

Blow the trumpet in Mount Zion,
Christ shall come a second time;
Ruling with a rod of iron
All who now as foes combine.
Babel's garments we've rejected,
And our fellowship is o'er,
Babylon is fallen to rise no more.

Before we die, let’s dig that high, that frees us from our binds…
That blows all cool, that ego drool, and burns us from our minds…
That last big flash, mankind’s last gasp, the trip we can’t take twice…
But before I did let me make that trip, before the nothing comes…
The last big flash, to light my sky…and zap! the world is done…

My hair was scorched by swirling flame
My eyes grew dim, my eyes grew blind
Death came and turned my bones to dust
And they were scattered on the ground

Fujiyama, got a mama,
Then your troubles increase, boy!
It's a bottle in a, bottle in a, bottle in a, bottle in a, bottle in a
Nagasaki!



They hug and kiss each night,
By jingo, boys, worth that price!
Back in Nagasaki where the fellows chew tobaccky
And the women wicky-wacky-woo!

Oh thou that dwell on many waters, rich in treasure, wide in fame
Bow unto a god of gold, thy pride of might shall be thy shame
Oh God, the pride of man, broken in the dust again

And only God can lead the people back into the earth again
Thy holy mountain be restored, thy mercy on thy people Lord

------------------------------------------------------

The Ballad of Hiroshima Town, Jens Bjørneboe, “Vise om byen Hiroshima.” Samlede Dikt, ©1977, 1995 by Gyldendal Norsk Forlag A/S. English translation ©1997 by Esther Greenleaf Mürer
Nagasaki, performed by the Don Redman Orchestra, 1932, lyrics by Mort Dixon, 1928
Pride of Man by Hamilton Camp
Babylon is Fallen, Sacred Harp, W. E. Chute, 1878
Foreign Accents, by Robert Wyatt
Hiroshima, Baez version of the Grey Silkie
The Big Flash, from short story of the same name, Norman Spinrad
Angel by Massive Attack

N Moderation

There are reasons to suspect that science and engineering took a very different path over there: their limited understanding of nuclear weapons—they seem to think that nukes are roughly as easy to build as bottle rockets—suggests that nuclear fission may never have been developed on their timeline. – Twilight Zone by Gregory Cochran, on evidence that members of the Bush Administration are from a parallel universe.

Just how hard is it to build a nuke? And what is the smallest amount of plutonium needed to build one?

The smallest nuclear weapon ever designed was the Davy Crockett, aka the W54 warhead, weighing 51 pounds with a variable yield supposedly from 10 to 250 tons of TNT equivalent. It was the last weapon ever atmospheric-tested by the U.S. and in its two tests, (Little Feller I and II) it yielded 22 and 18 tons of explosive power. At those yields, however, the explosive power was pretty much unimportant compared to the radiation the blast produced, lethal to 50% of unshielded personnel at 400 meters, 100% lethal at 300 meters.

There’s not a lot of unclassified information about the actual design of the W54, but some conjectures can be made about it just from the nature of the nuclear chemistry involved. A “bare critical” mass of plutonium, for example, weighs roughly 10 kg, but a neutron reflector reduces this by maybe a factor of two. A uranium reflector/tamper can also increase yield because some fast fission will take place in the reflector itself (at the cost of a time delay in the return of the neutrons to the explosive core). Beryllium also multiplies neutrons, undergoing “light fission” on exposure to high-energy particles of any kind, including neutrons, to produce, well, more neutrons. This is also at the expense of slowing the neutrons and thus retarding the rapid increase in neutron population that make a bomb go ka-boom.

But slowing neutrons is called “moderation” and slower neutrons tend to react more easily with nuclei (have a higher capture cross section) than fast neutrons. This is a consequence of quantum mechanics, where fast particles have a more certain position than do slow ones. Think of the slow neutrons as being more “fuzzy,” virtually bigger, if you will. So if there is a nearby nucleus that is “sticky” for neutrons, a slow neutron is more likely to glom onto it.

That is pretty much the principle of nuclear reactors, where neutrons are slowed down to better react with the fissile elements in the reactor. A mass that is sub-critical for fast neutrons can be more than critical for slow neutrons.

The result is that, with a thick beryllium reflector, the critical mass of normal plutonium can be reduced to less than 20% of its “bare” critical number. The thickness of the reflector in the Davy Crockett was probably dictated by the limit that is reached when adding more reflector increases the overall mass of the design rather than reducing it.

The variable yield of the W54 looks like a signature of a variable fusion boost, but I’ve seen statements to the effect that D-T fusion doesn’t get going until you reach the 100 ton range, so the W54 may have had multiple fission core compositions. Still, the upper limit of the W54 is within the fusion boosting range, so a design modification could possibly have boosted its potential yield to a full kiloton.

A reasonable question arises, is the implied 2 kg core the minimum amount of plutonium (or U233, which has a bare critical mass of about 16 kg) that can be used to make a nuclear weapon, even one of such a low yield as the W54?

Advanced implosion techniques can produce such high core densities that the critical mass for plutonium can be reduced to as little as 1 kilogram, but the tradeoff is a much more complicated design. Besides, if we’re talking terrorists or a small belligerent state with limited technical resources, we’re much more concerned with basement bomb makers, aren’t we? What’s the least amount of bomb grade stuff necessary to be dangerous?

In one sense, the answer is…none. Nuclear reactors can be made from materials that are not considered bomb grade material. There was a nuclear accident in Japan a few years ago that occurred when workers added water (a moderator) to some highly enriched uranium and accidentally produced a critical excursion. The HEU was only 20% U-235, which is considered far below bomb grade, and there was only 35 kilograms of material involved. Nevertheless, the radiation release killed several workers and put an entire town into panic mode.

Ordinary reactor fuel, on the order of 7% U-235 could also serve as a terror weapon, especially if it were moderated by heavy water, which, unlike light water, does not absorb neutrons very effectively. However, hundreds of kilograms of such fuel would be needed.

Suppose, however, that a hypothetical bad guy had some amount of plutonium, just not enough to build a “conventional” nuclear bomb. How much would he need to cause some havoc?

Based on various published figures, plus some conjectures from reactor design principles, I guesstimate that a “prompt critical” device could be built from as little as 50 grams of plutonium, though you’d also need on the order of several hundred kilograms of natural uranium for a reflector/tamper, and a substantial amount of heavy water. Both of those components, however, are relatively easy to procure, although you might need a cover story to get them (maybe a potter with a hobby of trying to build a cold fusion device). In any event, the tamper/casing of the bomb could be produced from materials that one can obtain within the United States; no smuggling would be required. The explosive yield of such a device could be anywhere from a few pounds of TNT up to something approaching the Davy Crockett. In all cases the local radiation would be lethal to some distance, with significant fission product contamination. A full Davy Crockett yield could almost certainly bring down a building or two; the Oklahoma City bomb was about 2 tons in yield, 10% of the Davy Crockett.

Obtaining plutonium, of course, is a difficult matter, but it’s sobering to realize how much MOX (mixed oxide) fuel is around and about, not to mention the fact that waste nuclear fuel rods become less dangerous with each passing year. We’ve already gone through almost two half lives of the most dangerous intermediate isotopes (cesium and strontium). The rule of thumb is that ten half-lives is sufficient for a radiation source to become safe. That reduces the radiation by a factor of 1000. For intermediate fission products, that is about 300 years; 240 now that we’ve passed the first two half-lives, and the spent fuel is now only ¼ as reactive as it was in 1950.

Some fuel rods have been or will be buried in what is called “geological storage” or, as I like to call them, future plutonium mines.

The Neutron Dance

[Crossposted from We Are All Giant Nuclear Fireball Now Party]

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.



Update: Pronoun Problems Leading to Further Speculations


"Aha! Pronoun problems. It's not `shoot you, shoot you', it's `shoot me, shoot me'. So, go ahead, shoot ME, shoot ME (BLAM)... You're Despicable" -- Daffy Duck

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. – From the ex post facto Forward to the Neutron Dance, by James Killus

What I’d meant to say was that a fanboy such as myself might speculate as to whether he himself could survive a three-way with Blaise and Buffy. But we had ourselves an antecedent problem, in that it’s trivially easy to think that the “he” in the above quote referred to The Hulk. This led to some confusion in a conversation with The Wife, then enlightenment. We pick up the conversation in mid-stream, during enlightenment:

“But The Hulk could survive a Modesty and Buffy three-way?”

“Oh, sure. Even when Peter David had him back to being the Gray-Skinned Hulk for a while. He wasn’t as strong as the green-skinned guy, but he was still plenty strong. He also probably had more interest in sex during that phase of it, though from what I’ve heard, he’s lately gotten smarter and he had an alien lover who got killed, and well, that exhausts my vague knowledge of the matter. I haven’t been following the Marvel Universe for a long time.”

“How about Superman?”

“Survival wouldn’t be an issue. Getting him into a three-way would be the issue. He’s brave, strong, and pure, and he’s married to Lois still, I think. Fan boys like to imagine him with Wonder Woman, as I understand it. If Wonder Woman kept to Marston’s original conception, she’d be the one for the three-way, but they don’t writer her much like that anymore. She did have a fling with Batman, as I recall. Or maybe it was with Bruce Wayne. They’re sometimes hard to keep separate.”

“So could Batman survive Modesty and Buffy? Or Wonder Woman and Buffy?”

“I think it depends on whether or not he gets to keep his utility belt.”

“That makes sense. Like Iron Man could survive, but Tony Stark wouldn’t.”

“That’s sad, isn’t it?”

“How about Spiderman?”

“I’m pretty sure Spiderman could survive the three-way. It’s the explaining to Mary Jane afterwards that would do him in.”

"Ah, that’s Peter Parker in a nutshell, isn’t it?”

[note: some comments interpolated to the point of invention]

Dead Sheep

I have another story about SDI, if I can manage to figure out how to tell it. Until then, to the song Angel, by Massive Attack:

The Gamma Laser

Originally written July 27, 2006

I saw Sharon Weinberger on the Daily Show, last night, touting her book, Imaginary Weapons. The book is her expose of weird DOD projects involving fringe science, etc. Amid the talk about psychic espionage and mind control rays, she mentioned the “atomic hand grenade” and hafnium. I know a lot of the background of that one, so there’s an excuse.

In the early 60s, when I was barely a teenager, there was an article in Scientific American about the gamma ray laser, graser, gaser, call it what you will. I read the article, talked about it with my science buddies, then put it in the back of my mind for a while. Then I went to RPI and joined the Rensselaer Engineer, the school’s student engineering magazine, and wrote a lot of articles, so many that some had to be under pseudonyms. One of them was on the gamma laser.

RPI’s library at the time was under fire for being inadequate, but it was good enough to get me a copy of the paper by Lev Rivlin describing the gamma laser, and I was young and cocky and indulged in a bit of speculation of my own in the article. So let me give some technical background.

Lasers work by a quantum trick. Light is typically emitted from an atom that is in an “excited state,” i.e. one or more of its electrons is not in its lowest possible energy level. The situation is symmetric, in that an atom in its lowest energy state (the ground state) will also absorb a photon to put it up into the excited state. The probability of the atom emitting a photon is related (actually, with caveats, it’s identical) to its likelihood of absorbing the photon in the reverse reaction.

It turns out (this was an Einstein thing), that you can get the atom to emit its photon “prematurely” if you hit it with a photon of exactly the same energy as the one it will emit. Thus the “stimulated” part of the light amplification through stimulated emission of radiation, and since you now have two identical photons, you also get the “amplification.”

In order to get real amplification, you need what is called an “inverted population,” where the number of atoms in the excited state is greater than the number of atoms in the ground state, otherwise the ground state atoms absorb all the photons you can make. Inversion is usually done by “pumping” the ground state into much higher energy states, which then decay into a “metastable state,” one that hangs around for a much longer time. Pumping can be done optically, chemically, or electrically, and all three are used in lasers.

The gamma ray laser does all this with energy shells in the atomic nucleus rather than electrons in the outer atomic shell. Also, because gamma rays are more energetic than regular light, you get a problem called “dynamic line broadening.” What happens there is that, because gamma rays pack a lot of energy, they have a “kick” that causes the emitting nucleus to recoil. But that recoil lowers the energy of the emitted photon, so it’s no longer at the right energy to stimulate the emission of the next atom. So the lasing action becomes very inefficient.

What Rivlin proposed was to make use of the Mossbauer effect. In the ME, the atom is embedded in a crystal matrix, and said crystal matrix allows the atom to vibrate only at certain fixed energies, so-called “phonon resonances.” That’s another quantum effect. If the “kick” from the gamma emission doesn’t match one of these resonances, then the entire crystal matrix is what rebounds. Well, the difference in masses between a macrocystal and a single atom is so great that all the energy goes into the photon and essentially none is lost to the matrix.

That left two problems for the gamma laser. The first is how to get the inverted population. The second is how to make an “infinite medium” i.e. get a long enough path in the lasing medium to obtain a lot of amplification. In most lasers, you put mirrors at both ends to create a “long path,” for the photons and lasing medium to do their thing.

Rivlin suggested that with a properly metastable isotope of high purity, only a few centimeters would constitute an effectively long path and no mirrors would be necessary. Others have suggested specially created crystal diffraction mirrors (which can reflect even low energy gamma rays if they are properly tuned to the correct frequency). In my little article, I suggested that low angel reflection might be sufficient, so you’d have maybe dozens of rods arranged in a polygon, each only a couple of degrees off the next, with a low angle metal surface in between. A similar trick is used for x-ray astronomy.

Pumping was something else again. I don’t think that either Rivlin or the Scientific American article suggested nuclear transmutation via neutrons, but that was something that I also speculated about.

Nothing much happened on the nuclear laser for another decade or more, but it became a hot topic for a little while during the SDI (“Star Wars”) period. There was even an underground bomb test that was briefly touted as having achieved amplification. Later that result was said to be a measurement error by some, while others hinted darkly at fraud. It appeared like the design was an attempt to “brute force” the matter (and there’s no brute like a thermonuclear bomb), but I could never figure out how they were going to solve the line broadening problem, and, by all reports, they didn’t.

In 1987 there was an experiment reported involving a metastable isotope of tantalum (Ta180m) exposed to high energy x-rays, with the result being a fluorescence that seemed to indicate some quantum stimulation was occurring. For a variety of practical and theoretical reasons, a lot of attention was then given to hafnium-178m2, the second metastable isotope of hafnium, having a half-life of about 30 years.

In 1999, a University of Texas group announced a stimulated emission result from hf178m2, triggered by a dental x-ray machine. It seemed like the Holy Grail was coming into view.
But then the criticisms began, the worst of which being that no researcher has ever replicated the original result, not even one of the UT group. Also, the original experiment did not have a control, so WTF?

Then came theoretical calculations that said that the process shouldn’t have actually reached breakeven, but a practical consideration was more important. The isotope does not occur in nature and is the product of an accelerator, which makes it hugely expensive. The idea of making a weapon out of it is ludicrous.

Well, so much for that.

But, as I say, I follow the field generally, and there is another story out in the hinterlands:

United States Patent 4,939,742 Bowman July 3, 1990 Neutron-driven gamma-ray laser

Abstract
A lasing cylinder emits laser radiation at a gamma-ray wavelength of 0.87 .ANG. when subjected to an intense neutron flux of about 400 eV neutrons. A 250 .ANG. thick layer of Be is provided between two layers of 100 .ANG. thick layer of .sup.57 Co and these layers are supported on a foil substrate. The coated foil is coiled to form the lasing cylinder. Under the neutron flux .sup.57 Co becomes .sup.58 Co by neutron absorption. The .sup.58 Co then decays to .sup.57 Fe by 1.6 MeV proton emission. .sup.57 Fe then transitions by mesne decay to a population inversion for lasing action at 14.4 keV. Recoil from the proton emission separates the .sup.57 Fe from the .sup.57 Co and into the Be, where Mossbauer emission occurs at a gamma-ray wavelength.

This if very similar to some of the speculations I had back in 1969, but it gets around a big problem I noticed. I thought that the target nucleus would have to be of low atomic weight (low z) because otherwise the Compton effect would be too parasitic to allow amplification. This patent suggests that the production of the excited nucleus can be made to eject the atom from its normal substrate into a medium (in this case, beryllium foil) that is very low z, where the actual lasing would occur. I was thinking thermal neutrons, but the patent uses higher energy neutrons, so that’s how the recoil would occur. I also suspect that there may be some small angle gamma/x-ray reflection occurring in the device as well.

The patent holder, Charles Bowman, is someone I’ve noticed before; he was on of he scientists analyzing the Yucca Mountain nuclear waste site and who described a scenario where there might be a (low yield) nuclear explosion from the nuclear waste. That particular bit of work fit in with some other speculations that I’ve had, about highly moderated nuclear supercritical reactions, something that’s about half-way between a so-called “dirty bomb” and a real nuke. But that’s a topic for another time.

SDI

A buddy of mine from the air biz used to work at Lawrence Livermore Labs, and he was once at a luncheon where Edward Teller was holding forth. Since there were several atmospheric scientists at that particular lunch, at one point Teller speculated on whether it would be possible to set up a series of nuclear explosions that would cause atmospheric particulates to precipitate out of the air.

My friend was a little nonplussed, because this was a truly loony idea. But after thinking about it for a while, he chalked it up to Teller having a little fun with his own reputation. He had, after all, basically invented the thermonuclear bomb, and had then spent much of his remaining career overseeing its refinement, and looking for some place to use it. From proposed massive canal building projects to attempts to get more natural gas out of geological formations, Teller always had that single tool that he was trying to use: the H-bomb.

Later, when we all heard about the Teller’s backing of the Strategic Defense Initiative (called “Star Wars” in the popular press), some of us immediately wondered, “Where’s the bomb?”

We learned soon enough about the proposed X-ray (or gamma ray) laser, which was supposed to be pumped by a thermonuclear explosion, so there you are and bob’s your uncle. As I've noted in another essay, I didn’t expect that to work, for technical reasons, and it didn’t.

SDI did not die with the gamma laser failure, however. We’ve had various debates about the feasibility of “hitting a bullet with a bullet” vs “smart rocks” or “brilliant pebbles,” (or “sentient sand” for all I know). In any case, there’s really no idea so lame that a DOD bureaucracy won’t champion it, but there are some things that generally don’t get said, so I’m going to say them here.

The fact is that there are certain paths of least resistance in engineering. Some ideas, no matter their soundness or unsoundness, will never happen, because something else that is technically easier will happen first. It’s important to know what it is that will happen first.

A ballistic missile’s brief career is divided into several important phases: launch, boost, ballistic, re-entry, target, then boom. There were actually some studies in the early 1970s, during the ABM (anti-ballistic missile) debate, that suggested that it might barely be possible to stop a single bomb in the near-target phase, using what is essentially massive anti-aircraft fire, putting a more or less continuous shroud of shrapnel as an umbrella near the target. No one really thought of this as a good solution, for several thousand reasons, including the fact that it would only work on one bomb, and an early trigger would then blow all your anti-aircraft weapons to hell and gone.

Similarly, despite the PR graphics of SDI as a “shield,” there was never much intention to try to get at ballistic weapons in the re-entry phase, not least being that a single thermonuclear explosion at the edge of the atmosphere creates a good sized EMP pulse that will then blind subsequent defense radar.

That argument also applies to defensive measures during the entire ballistic phase, when the warheads are outside of the atmosphere in free-fall. But there’s actually a worse problem in the ballistic phase, camouflage.

In the absence of an atmosphere, anything, no matter how lightweight, follows a ballistic trajectory. It is very easy, therefore, to create decoys, simple balloons with the same radar signature as the warhead. In fact, you can put a balloon around the warhead and make it look exactly like the balloon. Since the balloon/decoys weigh only a few ounces, you can put hundreds of them in the same ballistic trajectory as your warhead, turning the problem from “hitting a bullet with a bullet” to “shooting a needle in a haystack.”

So no one really expects to take out a warhead during the ballistic phase. That leaves us with launch and boost. Launch is over in a few seconds, so the real development work is on stopping missiles in the boost phase, when they are conveniently located far away from the target (us) and near the launch site (them).

But how? First you have to sense the launch, then find the missile, then target it, then put something near to it, then kill it. That implies a really good sensor network, plus the ability to put your kill vehicle near the target very quickly.

The sensor network is easy, or, more accurately, it’s so difficult that there’s really only one way to do it, and that must be space-based. You need orbiting infrared sensors to see the launch, then something akin to radar to track it. The radar will need to be close to the boost, and that too is almost necessarily space-based. There have been arguments about “pop up” systems, but those are mostly red herrings; it’s a lot easier to do it from space.

Likewise, it’s a lot easier to target a high velocity vehicle with something that starts off at high velocity. If your initial sensor is in space, and your radar net is in space, the same arguments tell you that your kill vehicle needs to be from space.

Along this development pathway, as your identification and tracking systems get better and better, there will come a time when only the most effective type of kill vehicle will work. You can talk all you want about “brilliant pebbles,” and “kinetic kill” vehicles, but nothing beats a nuke for destruction at a distance. At high altitudes, the energy from a nuclear weapon is primarily in the form of hard X-rays, with an attendant electromagnetic pulse. The hard X-rays can melt or crack a warhead by uneven heating, and a nuke doesn’t really care how many decoys you put up, it’s going to blow them all away. The electromagnetic pulse will probably even wreck any putative missile guidance system from a much greater range.

So let me be very blunt here. There’s nothing secret about any of this. It is the inevitable result of any feasibility analysis. SDI is about putting nuclear weapons in orbit. It always has been.