• they/them

plural system in Seattle, WA (b. 1974)
lots of fictives from lots of media, some horses, some dragons, I dunno. the Pnictogen Wing is poorly mapped.

host: Mx. Kris Dreemurr (they/them)

chief messenger and usual front: Mx. Chara or Χαρά (they/them)

other members:
Mx. Frisk, historian (they/them)
Monophylos Fortikos, unicorn (he/him)
Kel the Purple, smol derg (xe/xem)
Pim the Dragon, Kel's sister (she/her)


Dr. Louis Slotin, the Canadian chemist (no, really!) and nuclear scientist, is undoubtedly most famous for killing himself in the infamous "Demon Core" criticality accident, and for exhibiting an astonishing lack of concern for his personal safety in general. Slotin once conducted underwater repairs on an experiment he was conducting with the help of the Oak Ridge X-10 Graphite Reactor—at night, alone, while the reactor was still active, exposing himself to perhaps 100 röntgens (we can't be sure because he didn't wear his film-badge dosimeter.)

Enrico Fermi once told Slotin he'd not live a year if he kept carrying on the way he did, and Slotin proved Fermi right. Why was Slotin so careless with himself? I don't suppose we'll ever know for sure. Fools might sneer about "Darwin Awards" when confronted with such behavior, but I myself feel a spiritual quality to such recklessness, a self-forgetfulness and willingness to put science before one's own life. I am reminded of the "fluorine martyrs" and everyone else who perished in the early days of systematic chemistry, exposing themselves to strange new poisons and corrosive substances so that humanity might better understand the nature of matter.

I think it's a shame that people remember Slotin chiefly for dying in an accident. There are many interesting details in his career, like studying reaction mechanisms with radiolabeled carbon dioxide. Most intriguing, to me, is that Dr. Slotin is responsible for coining the terminology of dollars and cents used to estimate the reactivity of a fissile system, i.e. its nearness to criticality. So far I haven't found any explanation of why Slotin chose to apply these monetary units to nuclear fission...although I do have some ideas.

Let me attempt to explain nuclear "dollars", though I'm not a physicist and I'm lacking confidence in my lay understanding of these things.


Nuclear reactivity is a function of neutron flux: the more neutrons that are bombarding nuclei in the fissile system, the greater the reactivity. When a neutron induces fission in a nucleus, the reaction immediately produces more neutrons, as in this sample reaction:

235U + 1n = 141Ba + 92Kr + 3 1n

These neutrons immediately generated by a fission event are called prompt neutrons. But the fissile fragments, the 141Ba and 92Kr in the above example, are themselves unstable and tend to fragment further, generating a cascade of more neutrons released after the initial event. These are called delayed neutrons, and the time lag between the release of prompt and delayed neutrons is what permits fissile systems to be controlled.

Exact criticality occurs when the neutron flux into the fissile system, the neutrons that successfully collide with nuclei and induce fissions, is balanced by the neutrons flying out of the fission events. If every neutron that induces fission is balanced by a neutron produced by fission, the system is said to have an "effective neutron multiplication factor" or k of unity, and we say the system is critical. If there's too few neutrons produced, k < 1 and the system is subcritical; if there's excess neutrons, leading to an exponentially rising number of fission events, the system is supercritical.

But there's two levels of criticality, because of the time lag between prompt and delayed neutrons. If perfect balance is achieved solely with the prompt neutrons, the ones immediately released by fission, the system is at prompt criticality—but meanwhile, delayed fission events are releasing more neutrons, so a system at prompt criticality is not stable. The delayed neutrons add excess reactivity over time and push the system towards supercriticality. At a slightly lower level of overall reactivity, the critical balance is maintained with both prompt and delayed neutrons, and this lower threshold is called delayed criticality. Nuclear reactors operate between these two thresholds; the relatively slow release of delayed neutrons allows for reactivity to be controlled through mechanical feedback, adding and subtracting reactivity as necessary (with nuclear poisons &c.) to keep the reactor within this interval between delayed and prompt criticality.

And for some reason, Dr. Louis Slotin gave this interval—this difference in reactivity between delayed and prompt criticality—a special name. He called it a dollar, and there's a hundred cents in the nuclear dollar, just like with monetary dollars. The size of the "dollar" is highly dependent upon the physical configuration of the physical system, and it must be determined experimentally, because it's extremely sensitive to small physical changes, as nuclear scientists have occasionally found out to their grief. When the criticality thresholds for a given system have been determined, we can then speak of adding or subtracting reactivity in terms of dollars and cents: we can say a system is "17¢ below prompt criticality" or speak of "adding reactivity at the rate of two dollars per second" and other such things.

Which is roughly what Slotin was doing when he killed himself: he was adding and subtracting reactivity from a simple fissile system, a plutonium sphere surrounded by beryllium neutron "reflectors" that scatter some of the neutrons flying out of the system back towards the sphere. Slotin was attempting to tune the reactivity of his system by controlling the physical separation of a beryllium reflector from the "Demon Core" sphere (or "Rufus", as it had been nicknamed) with the blade of a screwdriver, looking for just the right amount of separation to bring the system near to criticality...and he, uh, overshot the mark a bit. I guess he spent a bit too much nuclear money at once.

I am dead certain that Dr. Slotin instinctively grasped that there was some underlying kinship between these two concepts that might seem completely unrelated: nuclear reactivity and money. Money, like nuclear fissile, is an exponential thing: people who make money through passive means, through the accumulation of interest, expect that they'll be able to make exponentially increasing money. Furthermore, money amplifies itself, like fission does: get a heap of money through some exponential-growth scam and you'll have money to throw at more such scams. But it's like expecting that you'll be able to get exponentially increasing power out of a nuclear reactor: it'll operate in that regime only for a short time before it simply melts or explodes.

I think it's reasonable to guess that the same thing happens with money: the exponential hoarding of money must come to a messy end, because hoarding and spending a massive quantity of money is physically disruptive, even if capitalist culture seems to regard profit-seeking as a free action, something that can be mixed into any activity to make it more fun and lucrative—rather in the way that gamblers talk about making ordinary activities "more interesting" by betting on them.

I feel obscurely like I've missed whatever deeper point I wanted to make with this chatter but I think I'm out of words.

~Chara of Pnictogen
Dr. L. Slotin of Pnictogen


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