The Linear Hypothesis

One of the many problems with using "animal models" for estimating human health effects of exposure to various toxins, radiation, etc., is that most animals are short-lived, so the effects of chronic exposure to low levels of the toxin of interest does not become apparent during the animal's lifetime. The method generally used to try to get around this problem is to boost the dosage, which is assumed to shorten the "induction period" of disease progression in a more-or-less linear fashion. The idea is that doubling the dosage will halve the time it takes to get a response, more or less.

That is a pretty kludgy method, of course, but all the methods are pretty kludgy. Using longer-lived animals also has problems, not least being that you have to wait much longer for results. Moreover, the lower the general exposure, the fewer responses you are likely to get, statistically speaking, so the use of realistic exposures and exposure times becomes prohibitively expensive. Also, longer-lived animals tend to be more "charismatic" in the sense that people like them more and animal rights activists pay them more attention, sometimes to the detriment of the researchers.

For the purposes of this little essay, I'm going to use radiation as the example, mostly because there are so many places to get information on the radiation/cancer debate, but also because the chemical/cancer debate gets even more arcane in spots, and I'm doing a once-over-lightly here.

The main alternative to the "linear hypothesis" is the "threshold hypothesis," the idea that a toxin or radiation does not overwhelm the body's cellular defenses until it gets above a certain level, or threshold. There are clearly many, many cases where such thresholds exist; "the dose makes the poison" as Paracelsus claimed, and there are few things that don't become poisonous at a high enough concentration.

There is a variation of the threshold hypothesis, which is called the "hormesis model." This is a somewhat more extreme version of the threshold model, and postulates that low doses of radiation are good for you. This isn't an entirely loopy suggestion; after all, radiation is used to treat some kinds of cancer, because cancer cells are more susceptible to dying from radiation than most body cells. It does, however, contain echoes of the early days of radiation, when things like radium were used as "invigorating" tonics, and that didn't work out well.

There are a variety of arguments and observations made to support both alternatives to the linear model. One of my favorite involves studies that create biological systems that lack the naturally occurring radioisotope potassium-40 and include only potassium-39. This apparently leads to birth defects. However, a high concentration of deuterium in the body (i.e. biological systems using heavy water) also produces severe-to-lethal effects, with no radiation involvement whatsoever. It's not out of the question to suggest that our bodies' enzymes are "tuned" to a particular isotopic weight of the elements involved, and that even relatively small changes in these elements can cause problems. To the best of my knowledge, the potassium isotope experiment has never been performed with some external source substituting for the missing radiation. The result would need to be normal development, obviously, for hormesis to be validated.

Other observations that seem to support various versions of threshold or hormesis include epidemiology in areas of high natural background radiation, which seem to show no excess cancers. Again, matters of the adaptation of local populations, questions of whether or not differences in infant mortality create a "harvesting effect" (where susceptible individuals die before they reach the age where cancers would present), or even simple things like actually getting the exposure levels correctly measured, become important. I've seen claims that the linear hypothesis cannot explain the epidemiology of the Japanese atom bomb survivors, for example, but I know for a fact that the actual radiation exposure to these individuals is a matter of estimation and guesswork, so the "failure" may simply be a matter of not knowing what the true exposure was.

Then there is the fact that when we talk of "radiation" we're not talking about a unitary subject. There are many different kinds of radiation, and many different ways of being exposed to it. These "hypotheses" and "models" that we are talking about are just that: models. A lot of different phenomena are being compressed onto a single, seemingly authoritative graph, but the real, underlying situation is complex and complicated. It's entirely possible that some forms of radiation show some hormesis effect, while others are linear, with no safe levels. The science necessary to make these distinctions is lacking.

Ultimately, however, these models are not some abstract scientific question, but rather, they are used to sort out issues of regulatory policy. And there is where the rubber meets the road. Advocates of threshold and hormesis models are invariably proponents of nuclear power (the reverse is not necessarily true, since there are nuclear power advocates to have no problem with the linearity regulations). There are claims that the public is "radiophobic," which may be true, but then again, that is the public's right. It is not as if there has been a consistent policy of telling the public the truth about these matters, and people tend to get a bit antsy when they know they've been lied to.

Ultimately, the linear hypothesis is the easiest to administer and produces the most clear-cut regulatory framework. It is conservative. Threshold standards tend to create situations where pollutant releases go right up to the threshold and bump against the standard, usually exceeding it from time to time. Linear standards say, "Reduce your impact to the lowest possible level." I find this to be a useful first (and usually second and third) approximation to regulation. But then I am what used to be called a conservative.

The Big Mo

I think I need to get the Ayn Rand Institute cooties out of my head, so how about a cosmic ray tangent?

Cosmic rays are pretty interesting, highly energetic charged particles from space. Most are protons, but there's about a 9% component of helium nuclei, and there are even some heavy nuclei like iron in the mix. Prior to the creation of really big particle accelerators, cosmic rays were the only way to study particles having energies of above a GeV. The energy spectrum decreases with increasing energy, but some cosmic rays are way above a mere billion electron volts.

On October 15, 1991 a cosmic ray event was observed with an energy of 3 x 10^20 eletron volts, i.e. 300 billion, billion electron volts, or about 50 joules. There have been a number of similar observations since, confirming the existence of particles so energetic that they must be of recent origin (in the astronomical sense). Otherwise, they would lose energy by interacting with the cosmic microwave background left over from the big bang. The first such particle discovered was dubbed the "Oh-My-God" particle, a joking reference to the nickname of the Higgs particle as "The God Particle."

All very cool, but that's not precisely what this essay is about. No, this essay is about energy and momentum.

Every article that I've ever seen compares the energy of Ultra High Energy Cosmic Rays (as they are called) to some macroscopic object, traveling at a fairly low velocities. Science magazine writers are particularly fond of comparing UHECRs to a strongly hit golf ball. The Wikipedia article on them compares the energy to a baseball thrown at 60 miles per hour.

The thing is, people do not interact with macroscopic objects via their energy content. An object's momentum is what produces force when it encounters another object. If you are hit by a golf ball, and it bounces off you elastically, there need not be a lot of energy transfer, but the momentum transfer (and damage) can be substantial.

Once, on the inevitable science fiction convention panel, during the Q&A, we were asked what we considered to be the greatest scientific error common among the general public. My answer was "the confusion of energy and momentum."

Think of a movie like the Schwarzenegger vehicle Eraser, where the MacGuffin is an ultra-high velocity rail-gun rifle. The gun is shown as knocking people backwards, using the old "stunt wire" trick that movies love so much. The problem is that such a weapon would transfer very little momentum to the target (and would have very little recoil). What would happen is that the projectile would basically explode on contact with an interacting mass. To a lesser degree, that is what happens with high velocity rifle shells. Similarly, Hollywood used the stunt wire for practically all gun shots, often giving the impression that a handgun is really a "momentum pistol," like the one seen in Fritz Lieber's The Wanderer.

Meteor Crater in Arizona was originally thought to be of volcanic origin, in part because it was circular, and it was believed that a meteor would almost certainly come down at an angle, producing an elliptical crater. Eventually, experiments with high velocity projectiles confirmed that they produce circular craters from almost any angle.

A fellow on the old Compuserve Science Forum explained it by analogy to throwing a hand grenade. The grenade carries so much explosive energy that it overwhelms the momentum, so if you throw one into a sand box, it will create a circular crater, no matter what the angle it hits. In fact, the ratio of energy to momentum for a meteor is considerably higher than for a thrown grenade.

If you really want to get a feel for the energy of an Oh-My-God Particle, you should compare it to things in ordinary experience where the energy is important. Thus, the energy from such a particle would light a 50 watt bulb for one second. Or it would power a single flash from a mid-size photoflash, perhaps ten from a small flash attachment.

Fifty joules will raise the temperature of one gram of water about 12 degree C, about 22 degrees F.

Or, if you want to stay with the moving mass analogy, how about propelling a mid-size automobile at a speed of a quarter of a mile per hour, assuming you have a friction free environment and some perfect method of converting cosmic ray energy into the motion of a motor vehicle, and, these days, who doesn't have those lying around?

The Philosopher's Stone

My favorite lab course at RPI was Radiochemistry, which was the second part of a two semester sequence. The first course was on the theoretical underpinnings of nuclear and radiochemistry. I didn't take it, but talked my way into the second course after an interview with the two lead professors that convinced them that I knew the material well enough. The two were Overman and Clark, who had written our textbook, called simply Radiochemistry, and a little googling tells me that, yes, it was a widely used text.

My favorite lab work in the course was neutron activation analysis, which is very, very cool. After exposure to neutrons, many elements become radioactive, having at least one isotope that does so after neutron absorption. This is especially true of the heavier elements, which tend to have a lot of isotopes and a lot of energy shells in their nuclei. Most of the artificially created radioisotopes have a gamma spectrum associated with them, so if you measure their decay using a multichannel gamma spectrometer, you get a "fingerprint" of that isotope. Repeated measurements also give you decay rate information, and, putting all that together, you can usually get a pretty good quantitative trace element analysis of the original sample. Also, because only a few atoms absorb neutrons, and the product isotopes are usually fairly short lived, it's a non-destructive technique that can be repeated many times using the same sample.

The neutron source we used was called a "bomb" as I recall, not because it could explode, but because it was big and heavy and looked like laboratory "bombs" that contained compressed gas. What our N source contained was, if memory serves, beryllium and cobalt-60, the latter explaining the heaviness of the device. It was very thickly shielded.

Under high energy radiation, beryllium becomes a neutron source. Our source exploited the gamma-neutron reaction, which splits off a neutron from Be-9; the resulting Be-8 is unstable, and goes to two alpha particles. There is also an alpha-neutron reaction that gives a neutron and carbon-12. The early "initiators" on nuclear bombs used this reaction, with polonium as the alpha source, because it doesn't emit any gamma radiation, so it only worked after the Po and Be were brought close together, reducing the likelihood of pre-detonation.

Both boron-11 and deuterium also undergo the gamma-neutron reaction, though not as efficiently as beryllium. Californium-252 is also sometimes used these days as a neutron source, as it undergoes a fairly rapid rate of spontaneous fission.

There are commercial neutron sources that use nuclear fusion as the neutron source. Many of these are basically small particle accelerators that aim a beam of deuterons at a target that has tritium adsorbed onto it. A sufficient number of fusion events occur to make this a viable source of high energy neutrons for industrial applications. The D/T reaction has a characteristic energy (around 14 MEV), which is useful for deep imaging applications. Some of these type of neutron sources are quite small.

Larger installations often use full bore particle accelerators to create fusion, and sometimes the deuterium-deuterium reaction is used. This gives a lower energy to the resulting neutron, but deuterium is much cheaper and easier to work with than tritium, tritium being both radioactive and a proliferation hazard.

Another interesting neutron source is the Farnsworth Fusor, or rather, its descendants. These are "inertial electrostatic confinement" fusion generators, and examples of them have even shown up in high school science fairs. Usually, the rate of fusion is quite low; I've seen numbers for quasi-amateur builds that produce maybe a million fusion events per second. The radiation hazard from attendant X-rays is greater than the neutron hazard. However, I've also seen some advanced laboratory results suggesting output as high as 10^14 events per second which is respectable and dangerous. Once you get to this level of output, radiation damage to the electrical components becomes important, an indication of just how difficult the fusion problem is, since you're still orders of magnitude from practical power production.

If you want a really high neutron flux, the usual method of production is a nuclear reactor. Reactor-based high energy neutrons are usually obtained in a reactor with a fast core but a moderated outer shell that achieves criticality, thereby sidestepping the safety issues that come into play with fast reactors. However, there are "pulsed" reactors that rely on changes in neutron cross section with temperature to create "inherently safe" designs. One such design was described by Freeman Dyson in his book Disturbing the Universe.

Reactors only produce "fission spectrum" neutrons, of course, although for fast reactors the spectrum is shifted toward higher energies. If you really want a big flux of high energy neutrons, failing some major breakthrough in fusion technology, you want a "spallation source."

Spallation is based on the fact that, if you hit a heavy nucleus with a very high energy proton, you get a neutron "splash" effect (this would be the "liquid drop" model of the nucleus, we're using here). The Spallation Neutron Source in Oak Ridge, Tennessee runs a high power (over a megawatt) proton beam at about 1 Gev (1000 Mev) and gets about 30 neutrons per proton in the beam. The neutron spectrum of the output peaks at around 10 Mev, but some of the neutrons have much higher energies.

The Oak Ridge facility uses mercury as the spallation target, with the indication that liquid targets are more robust to the sort of shocks that a pulsed accelerator beam produces. There were some designs from an old USSR program that used a eutectic mix of lead and bismuth for a similar purpose, heated to liquefy the metal.

Remember that I noted above that reactors are sometimes used as neutron sources. One reactor design is to use a "subcritical" fast reactor and drive it to power production via a spallation beam. If, for example, the criticality of the reactor is 0.95, meaning that it is only 95% of the way to self-sustaining, then any neutron introduced into the reactor core will induce a reacton chain of about 20 more neutrons. Thus, a spallation source plus a sub-critical fast reactor can be a copious source of fast neutrons. Moreover, it's overall power balance will be positive; it will generate considerably more power than it consumes.

I've noted before that fast neutrons are a modern equivalent of the "Philosopher's Stone," able to transmute elements, and able to convey, well, not eternal life, but eternal death to those exposed. It's the transmutation aspects that have caught some interest. Fast neutrons will fission all transuranic elements, so, properly run, there is no plutonium et al. remaining after an accelerator driven reactor fuel cycle has run its course. In fact, an accelerator-driven reactor system can be designed to run on nuclear waste remaining from other reactors. It's also been suggested that other long-lived waste products, like technetium-99 and iodine-129 be transmuted to shorter-lived isotopes, taking the nuclear waste disposal problem from a time scale of millennia to a matter of years, or perhaps centuries if you don't want to transmute the cesium and strontium waste isotopes.

It sounds great, doesn't it? Nuclear energy without the waste disposal problem? So what's the catch?

Well, there are a couple of technical catches, such as the fact that even the Oak Ridge facility doesn't have the power to drive a full closed-cycle system. But that's a technical matter, and I have no doubt it's solvable. There's also the fact that such a system requires on-site fuel reprocessing, to extract the transuranics and other long-lived isotopes from the waste stream. That's a chemical engineering problem, and we don't have much experience with designing chemical processes that are totally closed cycle. More accurately, trying to do so has always resulted in some leakage, plus the occasional outright accident.

Still, it might be possible to get the thing to work well enough, technically speaking.

But there's a deeper problem, and that has to do with social and economic systems and ideology. As I've said before, nuclear energy is inherently "socialistic" in the sense that it requires government level planning and operation at every step of the way. Yes, an accelerator-driven nuclear power system would produce a substantial power surplus—at enormous initial capital cost. A government can pick up that tab and take that kind of risk; corporations could raise the money (the estimated price tag for a ADS is on the order of $20 billion, but would probably be notably higher, given NIMBY concerns, etc.) but are simply not trustworthy when it comes to high public risk endeavors. Corporations take risks to enhance profits. It is up to government to regulate corporations' risk taking, but the ascendance of Conservative Movement ideology in this country has degraded the regulatory process to such an extent that one simply can't trust the regulatory function of government. The NIMBY folks are not being mere obstructionists. They are being realistic.

It's said that there was a time, in the early days of explosives manufacturing in Europe, when the owners of an explosives company were required to live on site. Ask yourself how many corporate executives would situate themselves and their families next door to any nuclear reactor site.

ADS systems are well-suited for thorium fuel cycle nuclear power, and India has a lot of thorium, but not much uranium. And if China ever decides to curb its greenhouse gas emissions, ADS systems would look very attractive. Naturally socialistic, remember? Very much in the Chinese tradition. The two countries look like natural competitors in this particular game.

The U.S. does not. The oil and gas men are still in charge, willing to expend trillions for neo-colonial wars and the perpetuation of various sorts of privilege. The idea of spending government money on the creation of actual industrial capital is ultimately foreign to them. So here, as in so many other endeavors, the U.S. will not be on the cutting edge of technology. It is no longer up to the job.

Pyramid Scheme

Zinc sulfide forms the basis of many scintillation detectors, dating from practically the beginning of the science of radioactivity. The old radium dial clocks mixed radium with the zinc sulfide, to provide that "glow in the dark" wonderfulness. Few modern scintillation detectors use zinc sulfide, however, because other crystalline compounds such as NaI are better suited to electronic detection.

When doped with various elements, the color emitted by zinc sulfide scintillation varies, with silver (blue emission), manganese (reddish orange), and copper (green) being most common.

Suppose you were to mix a zinc sulfide powder, properly doped, with nuclear waste material. It would, of course, glow in the dark. Actually, it would glow all the time, but it would be most noticeable in the dark.

One way to do this would be to do the mixing at an intermediate step in a process of nuclear waste vitrification. One typical way of doing this is to convert the liquid waste into a silica gel, which is then and dried, followed by heating to melt the gel into a glassy substance. If zinc sulfide were added to the mix at the dry gel stage, the resultant glassy substance should scintillate with the radiation of the waste.

Now let's imagine surrounding the glassy material with fused quartz, a clear, hard substance. I'm figuring on getting a block that's maybe two meters on a side, which would weight around 24 tons, but one could easily create larger or smaller blocks if there were practical reasons to do so.

It's said that nuclear waste needs to be stored for hundreds of thousands of years, but that's waiting for the long-lived actinides to decay. Without the actinides, several centuries would do. Either way, stone pyramids in the middle of a desert have been shown to last for thousands of years to date, which is a good start. And it's always a good idea to keep things where you can keep track of them.

Imagine glowing pyramids in the middle of the Nevada desert. Any breach in containment would be easy to detect; the radiation has its own glowing tracer that would follow it. The radiation penetration through the blocks could be engineered to be minimal; if the quartz isn't enough, put a few layers of leaded glass around the center. God knows, we have plenty of leaded glass around from old CRT screens.

I figure it would be a tourist attraction. Properly managed, you might be able to build a casino or two nearby, where the gamblers could sit at night and watch our nuclear legacy glower in the dark. Just a little reminder of some of the other ways there are to gamble.

Rocket Boys Meet the Radioactive Boy Scout

[Crossposted from WAAGNFP]

Until I began to build and launch rockets, I didn't know that my hometown was at war with itself over its children, and that my parents were locked in a kind of bloodless combat over how my brother and I would live our lives. I didn't know that if a girl broke your heart, another girl, virtuous at least in spirit, cold mend it on the same night. And I didn't know that the enthalpy decrease in a converging passage could be transformed into jet kinetic energy if a divergent passage was added. The other boys discovered their own truths when we built our rockets, but those were mine.

-- Rocket Boys by Homer Hickam

Rocket Boys was made into a movie, “October Sky,” the title being an anagram of Rocket Boys, and I’m still charmed by it. I’ve found that the film is much beloved in some quarters, but I found it to be a disappointment, as so many such films are, because the book had the texture of truth, while the film had the texture of Hollywood. Relationships were generified, characters were stereotyped, you know the drill.

There have been a number of historical paths whereby the bright kid gets out and up in the world. Rocket Boys is a description of a new path: Rocket Scientist, exemplified by Hickam himself, but also, to my reading, the more important character, Quentin, the hard scrabble kid who uses his brain and big words to protect himself from his circumstances, and who decides that Hickam, the son of the mine superintendent, has access to the resources they would need to start a rocketry club.

In 1957, the town of Coalwood, in West Virginia, is cut off from the world in ways that are simply unfathomable today. For example, a major point in the book is when their science teacher, through considerable effort, manages to procure for them a book on rocketry. One. Single. Book. Is it possible to picture such a time today, when Amazon.com and Abebooks.com are universally available? I’ve lived in towns nearly as removed as Coalwood, but I have to work very hard at imagining (or remembering) what it was like. It’s simply another world.

One running joke through Rocket Boys are the crazy ideas that Quentin gets, like when he and the rest of the club are talking about making out with girls, considering the wonders of the female undergarment, and Quentin begins to speculate that it might be an efficient thing to combine stockings with panties into a single garment. Or when he’s considering orange juice and instant coffee and wondering if it would be possible to produce some product like instant orange juice.

In the epilog that follows where the rocketry club boys wound up, a goodly number of them became engineers. One can only speculate how many of them became science fiction fans.

The dark side of the teenage geek can be seen in the story of the Radioactive Boy Scout, the story of David Hahn, who, as a teenager in suburban Detroit, managed to accumulate a large collection of radioactive materials, plus build a homemade neutron source that he used to irradiate thorium and uranium, in hopes of building a breeder reactor. What he got was a decontamination team from the NRC, who hauled away the shed in which he’d kept his material and a tour in the Navy, where he wasn’t allowed to work near nuclear reactors, because he’d already substantially surpassed the allowed lifetime exposure to radiation.

The book is based on an article from Harper’s magazine.

I’ll note this warning about the book, which often speaks of how “advanced” was David’s knowledge of radiation chemistry. In reality, Hahn’s knowledge was pretty spotty, which is what you’d expect from an autodidact. At one point he’s shown to be baffled as to why he doesn’t get a Geiger counter reading from polonium (it’s a pure alpha emitter and alpha radiation cannot penetrate the counting tube). He seems to have only the vaguest understanding about neutron moderation and implications on fissile fuel breeding, and, needless to say, the concept of radiation health safety is pretty much beyond him.

To be fair, I don’t know how much of this ignorance is Hahn’s and how much of it is the author’s, or, more specifically, how much of the author’s obvious ignorance is also the case for Hahn.

I’m given to muse a bit on both the upside and the downside of the geek effect. Is the difference in outcome between Rocket Boys, and The Radioactive Boy Scout merely one of luck? After all, the rockets were far from safe, and did nearly cause damage a time or two (though it must be observed that the fatalities in Coalwood were invariably from the coal mine, not the rockets). More importantly, I think, comes the observation that, if you’re going to go off into the wild blue yonder, figuratively or literally, it helps to have some friends in it with you, just to keep you grounded.