Meditations

Szilard’s chain reaction: visionary or crank?

by Alex Wellerstein, published May 16th, 2014

Leo Szilard is one of the most fascinating characters of the nuclear age. He was colorful, principled, clever, and often genuinely ahead of his time. And he always shows up early in the story.

Leo Szilard at the University of Chicago in 1954. Source.

Leo Szilard at the University of Chicago in 1954. Source.

Richard Rhodes starts off his The Making of the Atomic Bomb with Szilard’s famous 1933 epiphany:

In London, where Southampton Row passes Russell Square, across from the British Museum in Bloomsbury, Leo Szilard waited irritably one gray Depression morning for the stoplight to change. A trace of rain had fallen during the night; Tuesday, September 12, 1933, dawned cool, humid and dull. Drizzling rain would begin again in early afternoon. When Szilard told the story later he never mentioned his destination that morning. He may have had none; he often walked to think. In any case another destination intervened. The stoplight changed to green. Szilard stepped off the curb. As he crossed the street time cracked open before him and he saw a way to the future, death into the world and all our woe, the shape of things to come. […]

“As the light changed to green and I crossed the street,” Szilard recalls, “it … suddenly occurred to me that if we could find an element which is split by neutrons and which would emit two neutrons when it absorbs one neutron, such an element, if assembled in sufficiently large mass, could sustain a nuclear chain reaction. “I didn’t see at the moment just how one would go about finding such an element, or what experiments would be needed, but the idea never left me. In certain circumstances it might be possible to set up a nuclear chain reaction, liberate energy on an industrial scale, and construct atomic bombs.” Leo Szilard stepped up onto the sidewalk. Behind him the light changed to red.1

It makes for a good read, though there are disputes about the exact timing of this apparent epiphany. But the basic fact seems to remain: Leo Szilard thought up the nuclear chain reaction over five years before fission was discovered. But he wasn’t taken seriously.

But what did he really propose at the time, though, and not just in retrospect? And should he have been taken more seriously? This is what I want to discuss at some length here, because it is a point of common confusion in a lot of writing on nuclear history.

Szilard had a really interesting idea in the fall of 1933. He took out a patent on it in the United Kingdom, which he required to be made secret. Was Szilard’s idea really an atomic bomb? Was it even a nuclear reactor?  The reason to suspect it was not, on the face of it, is that nuclear fission hadn’t been discovered in 1933. That didn’t happen until late 1938, and it wasn’t announced until early 1939. So what, really, was Szilard’s idea? And why did he file a (secret) patent on it? Was Szilard ahead of his time, or just a crank?

Szilard patent GB630726

Szilard’s 1934 patent is easily available these days, and is worth looking at carefully with an eye to what it both says and doesn’t says. The patent in question is GB630,726: “Improvements in or relating to the Transmutation of Chemical Elements.”2 He filed the application first in late June 1934, updated it in early July, and finalized it by April 1935. The UK Patent Office accepted it as valid in late March 1936, but it was “withheld from publication” at Szilard’s request under Section 30 of the Patent and Designs Act. It was eventually published in late September 1949, 15 years after it had been originally applied for.

The basic summary of the patent is straightforward:

This invention has for its object the production of radio active bodies[,] the storage of energy through the production of such bodies, and the liberation of nuclear energy for power production and other purposes through nuclear transmutation.

In accordance with the present invention nuclear transmutation leading to the liberation of neutrons and of energy may be brought about by maintaining a chain reaction in which particles which carry no positive charge and the mass of which is approximately equal to the proton mass or a multiple thereof form the links of the chain.

This sounds awfully promising, especially when you know what you are looking for. It looks like he’s got the right idea, for a reactor at least: it is patent for creating a neutron-based chain reaction. The reason that neutrons matter is because they lack an electrical charge, and so are not repelled by either the protons or the electrons in atoms. This allows them to penetrate into the nucleus. If they can be linked up so that one reaction produces more reaction, they become a chain reaction. Sounds good, especially if we assume that he means an exponential chain reaction (i.e. each reaction produces more than one subsequent reaction).

But once you get beyond the heading, the details of the patent are, frankly, kind of a confused mess.

Szilard doesn’t actually even state that the chain reaction is going to be produced by neutrons. He hedges his bets there — he describes a neutron, essentially, but generalizes the claim for anything that might behave like a neutron. He calls these “efficient particles” (terrible name), and they have to basically be proton-like in mass but lacking a positive charge. OK, fine. The neutron had just been discovered in 1932, so Szilard is probably thinking that there might be other possible particles out there that acted the same way.

The really weird stuff comes in when he tries to explain how this really works. He defines a chain reaction as when “two efficient particles of different mass number alternate a ‘doublet chain.'” Wait, what? He gives an example:

C(12) + n(2) = C(13) + n(1)
Be(9) + n(1) = “Be(8)” + n(2)

Let’s unpack this. C-12 is Carbon-12, C-13 is Carbon-13, Be-9 is Beryllium-9, “Be(8)” is Beryllium-8, put in quotes here because Szilard know it is pretty unstable (it has an extremely short half-life before it alpha decays). The weird parts are the neutrons — n(1) is just a regular neutron. n(2) seems to be a dineutron, a particle which does exist but was only discovered in 2012, and is certainly not something you can count on. (Szilard never says it is a dineutron, but he implies that you might be able separate n(2) into n(1)+n(1) with another reaction, so it seems to be just that.)

Leo Szilard

So the idea here is that the Carbon-12 absorbs a dineutron, emits a neutron, which is then absorbed by the Beryllium-9, which emits another dineutron. It’s essentially a linear chain reaction, which is not nearly as impressive or fast as an exponential chain reaction. But it would generate some significant energy: calculating the mass deficit of these equations shows that together the net energy release would be around 3.3 MeV, about 100X less than a fission reaction, but is some 330,000X more powerful than the combustion of a single molecule of TNT (~10 eV).3 You’d also maybe get some alpha particles (from the Be(8) decay), but it isn’t going to generate a lot of neutrons or dineutrons (they are going to be eaten up by the reaction itself).

Szilard then notes that maybe there are exponential ways to do this. He suggests that maybe some elements will create multiple neutrons when irradiated with neutrons, e.g.

 Be(9) n(1) = “Be(8)” + n(1) + n(1)

This is a much more exciting possibility, because if every reaction creates the possibility of two more reactions, now we are talking about a reaction that can grow really dramatically. The only problem here is that this reaction seems to be endothermic; if you use E=mc2 to calculate the mass deficit, it comes out as -1.67 MeV. Which ought to be a hint that it isn’t going to work.

The final specification of Szilard’s reactor chamber, which is much more simple in operation than it at first appears.

Szilard then continues by saying that he could make this work well if only he knew what elements might behave this wayWhich is really the crux of it, of course. Szilard has no evidence that any element behaves this way. He has no a priori reason to think any of them do. It’s just a pie-in-the-sky idea: what if there were elements that, when they absorbed one neutron, released two? But Szilard doesn’t dwell on this lack of knowledge. He immediately moves on to how he would design a simple reactor if an element was found. It is nothing terribly interesting: he describes a way to create neutrons and aims them at the reacting substance, then siphons off heat with a heat exchanger and uses it to run a turbine.

In July 1934, Szilard filed an “additional specification” — another patent claim attached to his original patent application. It is an elaboration on the reactor idea. Since he still doesn’t know what fuel would make it run, it’s still not very interesting, other than the fact that he’s put a lot of evident work into figuring out some of the basic properties of the reactor despite not having any clue how its core would actually work. Interestingly he does discuss uranium, but not as a fuel (he thinks it would maybe emit X-rays if he shot high energy electrons at it).

Finally, in April 1935 he filed the last, “Complete” specification. This is more or less identical to a combination of the previous two, except he further makes explicit that he thinks there are going to be “explicit particles” other than neutrons that might work. Basically he asserts that there are probably “heavier isotopes of the neutron”4 and that “It is essential that two isotopes of the neutron should take part in the reaction in order to obtain a chain” (my emphasis). The latter instance shows that he is still not thinking of this quite right — it is not essential that there are multiple isotopes of neutrons.

In his examples, he believes that a “tetraneutron” (i.e. n(4)) exists and can play a role in the reactions. (I know nothing of tetraneutrons, but Wikipedia says that they were claimed to be discovered in 2002 but that the experiments could not be replicated.) Szilard seems to be basing his patent claims here on experiments, but it’s not clear whether he did them or someone else did them, but it seems likely he’s misinterpreting the data. It’s a very odd argument, and he rests quite a lot on it — he seems to think it is far more likely that a nuclear reaction will release bunches of bound neutrons (dineutrons, tetraneutrons) instead of multiples of free neutrons (i.e. as fission does). And then the whole thing was kept secret until 1949 — an awful long time for something that actually reveals nothing of any practical utility, much less military applications.

According to The Collected Works of Leo Szilard, there was an additional claim in his patent application of March 1934 that Szilard had removed from the final specification:

(a) Pure neutron chains, in which the links of the chain are formed by neutrons of the mass number 1, alone. Such chains are only possible in the presence of a metastable element. A metastable element is an element the mass of which (packing fraction) is sufficiently high to allow its disintegration into its parts under liberation of energy. Elements like uranium and thorium are examples of such metastable elements; these two elements reveal their metastable nature by emitting alpha particles. Other elements may be metastable without revealing their nature in this way.5

This is much, much closer to the truth, although it is still somewhat unclear what Szilard really thinks about this. It’s not clear whether he’s describing radioactive decay in the traditional sense, nuclear metastability (which is something different altogether), or something different. Uranium and thorium are radioactive and undergo alpha decay — that, by itself, doesn’t actually indicate that they are good candidates for the kinds of reactions Szilard is thinking about. Szilard doesn’t think they are going to split, he thinks they are going to become artificially radioactive. Not the same thing at all. Still, this is a lot closer to the correct formulation, but we have to read it in the context of everything else he put in the patent.

Anyway, so what’s the verdict? Does the patent describe a bomb? Does it even describe a reactor? Definitely not a bomb, and not really a reactor. Most of Szilard’s energies on the patent are describing something that would, at best, take an input amount of energy and magnify it a bit: you’d use a cathode ray to generate high energy electrons, which would generate high energy neutrons, which would stimulate linear chain reactions that would create radioactive byproducts and release a little energy. Maybe you could keep it self-sustaining but it seems like kind of a long-shot to me.6

An animated version of the above "reactor" operating in a pulsed fashion.

A crudely animated version of the 1934 “reactor” operating in a pulsed fashion, just in case you are having trouble visualizing it.

If you read the patent today with the benefit of hindsight, it’s easy to see where Szilard was right and where he was wrong. There is a germ of rightness in the patent, but it is clouded by a fog of wrongness, or at least confusion. I’m not blaming Szilard for this, of course. Like almost everyone else, he didn’t predict fission. He was ahead of his time, in the sense of anticipating that neutrons in particular were going to be important particles for creating nuclear chain reactions. But he didn’t really understand how it would work. As a result, most of the patent involves describing a device that wouldn’t work. To guess even something right about the future is a large task, even if one gets a few things wrong.

So was Szilard a visionary or a crank? To someone in 1934 or 1935, it would have been completely reasonable to dismiss Szilard’s patent as being too speculative and potentially too wrong (dineutrons, tetraneutrons, etc.) to be worth spending time worrying about. It also isn’t clear it has any real military implications — it isn’t even clear it would work as a power source, much less a weapon. To dismiss Szilard as something of a crank prior to the discovery of fission wouldn’t have been wrong. Szilard’s point of reference here isn’t fission, it’s artificial (induced) radioactivity, which had been discovered by the Joliot-Curies just prior to Szilard’s patent filing. But you can’t make artificial radioactivity work the way Szilard wants it to. I don’t fault anyone for not taking him very seriously at the time — because Szilard’s scheme was missing an absolutely essential component, and in its place there were a lot of incorrect assumptions.

After the discovery of fission in late 1938/early 1939, suddenly it is easy to pick out the visionary aspects of Szilard’s work. It suddenly becomes clear that Szilard was, in fact, a little ahead of the game. That if instead of his plans for beryllium-carbon reactions with neutrons and dineutrons, that a simple, neutron-based, exponential chain reaction would be possible with nuclear fission, and that furthermore it would release a lot more energy a lot quicker than what Szilard had dreamed up in the early 1930s.

Which is a conclusion that complicates the simple visionary/crank dichotomy. Szilard wasn’t really either in my mind. He had a germ of a good idea, but not the whole picture. But when the missing element came along, he was uniquely ready to see how it would complete his original idea. That’s the real story here, the real accomplishment: Szilard didn’t have to play catch-up when fission was announced, because he’d already thought a lot of this through. But that shouldn’t lead us to over-estimate the importance of the original patent work — it wasn’t a bomb, it wasn’t really even a reactor. But it did become a useful framework for thinking about fission, when fission came along.

Notes
  1. Richard Rhodes, The Making of the Atomic Bomb (Simon and Schuster, 1986), 13 and 28. []
  2. It should not be confused with another patent he filed for at the same time with an identical name (GB440,023) which has nothing to do with chain reactions at all.  GB440,023 is basically a patent for producing artificially radioactive elements. The device it describes involves using a cathode tube to generate X-rays, then using the X-rays to stimulate neutron emission in beryllium, and using those neutrons to make artificially radioactive elements through induced radioactivity. It’s not a bad idea — it is now known as the Szilard-Chalmers method and it works. But it’s not a chain reaction at all . Szilard filed a patent for the same idea in the US as well. That Szilard considered it something quite different is also evidenced by the fact that he doesn’t seem to have tried to keep it secret. He references the basic method in GB630,726 as the driver of the reactions in question. []
  3. The beryllium reaction is endothermic but the carbon one is not. []
  4. “I have reason to believe that apart from neutrons which carry no charge and have a mass approximately equal to the proton mass heavier isotopes of the neutron exist which particles carry no charge and has a mass number approximately equal to a multiple of the proton mass.” []
  5. Quoted in Julius Tabin, Introduction, Part V: Patents, Patent Applications, and Disclosures (1923-1959), The Collected Works of Leo Szilard: Scientific Papers (MIT Press, 1972), on 529. []
  6. The neutron multiplication factor, to use modern reactor terminology, seems to me like it is going to be 1 at best, and probably less than that given inefficiencies, losses, etc. One question unasked and unanswered in the patent is how many neutrons he thinks he is going to produce per blast. I think it is easy to overestimate how effective this would be from that point of view. The neutron initiator used in the Fat Man bomb, as an aside, produced only around 100 neutrons on average. This isn’t the same process at all, but in terms of orders of magnitude this is probably not inaccurate when it comes to imagining how many neutrons can be easily stimulated. It is nothing like what a fission chain reaction can generate with its exponential growth. []

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18 Responses to “Szilard’s chain reaction: visionary or crank?”

  1. Howard Morland says:

    Was he in touch with Fermi in 1934? That was about the time Fermi’s neutron experiments produced fission, without being recognized as such.

    • That’s a good question. He was definitely in touch with Fermi in 1936. In The Collected Works of Leo Szilard for example there is a reproduction of a letter from Szilard to Fermi from March 1936. He does not seem to be corresponding about Fermi’s experiments though, just about Szilard’s attempts to patent things (he is telling Fermi that he is only patenting them so he can assign the patents to a third-party if they prove useful, to make sure money is re-invested in the work), and he is wondering if Fermi is going to do similar things with his “slow-neutron” patent. He wrote a similar letter to Segrè.

      I think that he, like most people at the time (Ida Noddack excepted), had no clue of the importance of Fermi’s work in this respect.

  2. Michael G.arbour.III Felbeck says:

    Interesting article, much to follow up on. A 1933 patent in England; did it perhaps make its way to the Unified Prussia and then our double agents heard rumor of it?!

    Anyways, speaking as the grandson and great-grandson of two MED men, I’m appreciative that you continue this work Alex. Sadly too many personal affects in our family were lost in 1962/1970, but as a fourth generation removed, third male generation, I am appreciative of all the Army men who have written their life stories as much as they could in the past 75 years. I still have to read Red Sun Rising cover to cover; a lot more dense than Mr. Rhodes “Farm”!

    • I don’t think anyone who looked at this prior to the discovery of fission would have thought it was very likely to be important. Even on its own terms, it didn’t claim to be useful for military purposes.

  3. Gene Dannen says:

    Alex, I think that Szilard deserves much more credit for these patents than you realize. Understanding them requires knowing the nebulous state of nuclear physics in 1934, and also the arcane nature of the patent system. There is much less “crank” and much more genius here than you’re seeing.

    As you might expect, I know quite a bit about Szilard’s chain reaction patent. I’ll have much more to say about this later today. For now, I’ll say only the following:

    First, Rutherford proposed the existence of the neutron in 1920. In the same talk, he proposed the existence of di-neutrons and higher multiples. So that was hardly a “crank” idea of Szilard’s.

    You wrote that Szilard’s public patent 440,023 was for producing artificial radioactive elements. Actually, it covered 3 things:

    1) artificial radioactivity produced by neutrons 2) the Szilard-Chalmers reaction 3) nuclear fusion.

    The Joliot-Curies had discovered that artificial radioactivity could be produced by alpha-particles. It was Szilard who realized that it could be done much better with neutrons.

    As for nuclear fusion, Szilard proposed to heat a small space to an extreme temperature “for a fraction of a second.” He proposed using deuterium and lithium hydride. That is exactly right.

    More later.

    • Hi Gene —

      Thanks so much for writing — I knew you would chime in with something provocative.

      I’m not trying to slight Szilard at all. The patents are clever, but I think they are much more clever in retrospect when we throw out the wrong stuff and focus on the parts that are more correct. Which is common when trying to reevaluate work that was slightly ahead of its time. I think they played a key role in Szilard’s thinking and put him ahead of the curve when fission was discovered. But I think people were correct to dismiss them at the time as not quite right and certainly not of military importance.

      On multiple-isotope neutrons, I have no problem saying they were in the realm of theoretical speculation, but it’s another thing to try and get a patent monopoly on processes that you think require hypothetical particles to function. I think that is somewhat odd given that they had not at all been demonstrated.

      As for 440,023 — none of those save the Szilard-Chalmers reaction are things I would give to Szilard. I am not even sure if Szilard is talking about nuclear fusion in the case of deuterium; he speaks of it as collisions that emit neutrons, but he does not speak of it in a way that is different from any of his other reactions. He was not the first to probe neutron-based artificial radioactivity by a long shot — even Chadwick knew about this in 1932; see Alberto G. Gregorio, “Neutron physics in the early 1930s,” Historical Studies in the Physical and Biological Sciences, 35, No. 2 (March 2005), 293-340 (I can send you a copy if you don’t have access). Szilard’s discussion of it is a bit wild and woolly compared to the scientific articles others were publishing as well; I think it is clear he is guessing on a lot of things.

      I think giving Szilard priority for these is a bit unwarranted. What Szilard is trying to do — and the others weren’t — is to take these discoveries, observations, and theories and come up with practical technologies based on them. That is a big difference from what most of the others were doing on the whole. It is also probably why he felt the need to send so many explanatory and somewhat apologetic letters to these sorts of people explaining why he was doing this, though he was not alone. Simone Turchetti’s “The invisible businessman: Nuclear physics, patenting practices, and trading activities in the 1930s,” Historical Studies in the Physical and Biological Sciences, 37, No. 1 (September 2006), 153-172, provides great context for Fermi, Szilard, and Lawrence’s patenting practices around the same time. Szilard’s patents seem to me to be of a very different character than Lawrence’s and Fermi’s, which were less industrial and ambitious (e.g. Fermi patented his “slow neutron” method of irradiation, but he didn’t try to claim it was going to allow him to make new power sources).

      Again, I’m not saying Szilard wasn’t clever or creative. I just don’t think he was as justified as he thought he was in these assumptions. He was using the patent system not entirely as it was intended (not for achieved, reduced-to-practice inventions, but as a place to think out loud about big ideas). I don’t say this to criticize him, just to point out that seeing him as slightly crankish in the early-to-mid 1930s seems an understandable response. He ended up being correct on a significant number of things, which is more than most people (crankish or not) end up with, so I think he certainly deserves a lot of respect!

  4. Charles Day says:

    Also of note: Enrico Fermi discovered that bombarding stable elements with neutrons could induce them to become radioactive. According to Wikipedia, he reported the discovery in La Ricerca Scientifica on 25 March 1934.

  5. Gene Dannen says:

    Alex, I don’t think that Szilard’s fellow physicists doubted his ideas because of any specifics of his proposals (after all, they hadn’t read his patent applications), but because they fundamentally believed that atomic energy would prove to be impossible. As Henry Tizard is reported to have said, “Do you really think that the universe was made in this way?” Most didn’t. Szilard did.

    Szilard DID propose three specific elements as likely candidates to sustain a chain reaction. He named beryllium, bromine, and uranium. Uranium is the right answer.

    Based on the published masses of helium and beryllium at that time, the beryllium reaction should have been exothermic. It was Aston and Bainbridge, who had determined the masses of helium and beryllium, who were wrong — not Szilard.

    In suspecting uranium, Szilard was right — and Fermi who was wrong. When Fermi bombarded uranium with neutrons, he reported that uranium was transmuted into heavier elements, which he investigated in detail. He even named two of them in his Nobel Prize speech: Ausenium and Hesperium. Hahn, Strassmann, and Meitner extensively investigated such “transuranic” elements. All wrong. What they were seeing were fission fragments.

    Szilard, who knew what good physicists these colleagues were, therefore made his real mistake of assuming that they were right, and didn’t perform the experiments with uranium himself. If he had, he would have discovered the neutron emission from fission, because neutron emission was what he was looking for.

    I disagree with the statements that there was no bomb, and no military application, in Szilard’s secret patent. Szilard assigned the patent to the British Admiralty to keep it secret because he correctly saw that there was. “If the thickness is larger than the critical value… I can produce an explosion.” That is how a nuclear weapon works. That is a bomb.

    The British Patent Office was aware, of course, of Fermi’s publication and its date. But Szilard’s patent application was submitted earlier, on March 10. Szilard had priority, and he GOT the patent.

    • Hi Gene,

      But the thing is that the universe did not work the way Szilard thought it did. You are picking out the elements of rightness from the swamp of wrongness and lining them up with what we now know is the case. This is a bad methodology for the history of science. You are holding Szilard to a different standard than Fermi et al. — everything Szilard has somewhat right you mark in his favor, far above the parts he has wrong, whether his rightness is by logic or coincidence. I consider his invocation of uranium as coincidental — he did not think it a candidate for the right reasons (uranium being an alpha emitter does not indicate it is fissile), and it turns out to work in a totally different way than he imagined (fission versus his artificial radioactivity model).

      I am not saying Fermi et al. were any more right here. I am just trying to come up with a truly balanced assessment of what is in Szilard’s patents and what is not. I do not think it is even clear from his patents that you could make a bomb with this process at all, even if the universe did work the way he thought it did. To know that he would need to know a) whether it worked (it didn’t), b) whether the reactions were exothermic (not all of them were), and c) whether the reaction times were fast enough to generate significant energy before the system broke down (I doubt it, since he is basing a lot of this on delayed decays). Even with uranium fission it took careful measurement to figure out these sorts of things. I see no bomb in the patent as written — of course Szilard himself might have thought at the time that there was a bomb possibility. But I don’t think he had strong reasons for thinking so.

      As for priority etc., my impression from the Gregorio article is that this was in the air for everyone working on nuclear physics at the time. Szilard may have beat Fermi to the patent office (which as you probably know is what matters in the UK, as it is a first-to-file system, not a first-to-invent system like the US was until quite recently) but he does not appear to have beat him to the idea. (Fermi’s specific innovation was not neutron-induced radioactivity anyway, but the improvements that could be made by slowing the neutrons down first.) What makes Szilard unique to me is not that he was thinking about how to use neutrons, but that he was trying to do it in an immediately technological, industrial, even military way, as opposed to an approach more rooted in purely research concerns.

      My overall point, again, is not to denigrate Szilard in any way. I just think that if we are going to look at what he did seriously, we have to take a very balanced, symmetrical approach to it, looking at the right and the wrong. I would do the same thing with Fermi, Rutherford, Einstein, you-name-it. Again, I think Szilard’s ultimate contribution here is that he was trying to think ahead of what was known, and it is no surprise that he got a few things wrong. I don’t think he was a crank (I can understand why some of his contemporaries might have wondered whether he was or not), but I also am not so sure I would call him a visionary. I would call him forward-thinking in a way that allowed him to recognize opportunities immediately as they appeared, which is non-trivially difficult to do as well as he did them!

      • Gene Dannen says:

        Alex, I’m trying to make sure we understand what Szilard got right before we pass judgment. You pronounced a negative verdict on many points that you didn’t fully understand.

        To continue: A published patent isn’t as straightforward a document as it might appear to be. It actually may bear little resemblance to the application originally filed by the inventor. Much of the original may have been deleted or rewritten as the insistence of the patent examiners. This seems to be the case with Szilard’s secret patent.

        You noted that neutron chains of mass 1 weren’t included in the secret patent. One must read Szilard’s original application as filed — which does indeed include them. The last version of Szilard’s application (the complete specification filed April 9, 1935) is reproduced in facsimile in Szilard’s Collected Scientific Papers pp 605-621. Neutron chains of mass 1 are mentioned on page 615.

        Why doesn’t this claim appear in the secret patent? Most likely because the examiner ruled that too much time had passed since Szilard’s first original application that mentioned it.

        You criticized Szilard’s use of the words “efficient particles,” but this is actually good patent writing. Much of what you characterized as “confusion” may be the same.

        Szilard had considerable experience in patenting by this time, mostly in Germany, but also in other countries. He also had learned much about patent intricacies from the famous former patent clerk Albert Einstein.

        A wise inventor draws the application as broadly as possible, uses the most inclusive terms that he can, and includes as many variations of his idea as he can imagine. If he doesn’t, a competitor will file a claim just different enough to slide through the loophole. And new discoveries my change the landscape. When choosing his terms and claims, Szilard undoubtedly had all that in mind.

        • Hi Gene,

          I am aware that applications and granted patents can vary. I included the excerpt from the Collected Scientific Papers in the post, and agreed that it is much closer to the right idea (though still not quite there). As for why it wasn’t included in the final specification, the CSP says that it was Szilard himself who kept it out. I don’t know one way or the other. I very much doubt it is because “too much time had passed” — that is not generally how patent priority claims work in any of the different systems. But I do not know the UK patent system as well as the American system (which as I think you know I have studied pretty closely, especially with regards to secret applications).

          I was not criticizing the use of the term “efficient particles” — I think it is a bad name from an aesthetic/descriptive point of view, but that’s not a criticism of the idea itself. Defining a term and then suggesting it might be broader than the initial definition is indeed standard patent practice.

          I fully understand that patents are generally meant to be broad. My point is that Szilard’s broadness also reflects his uncertainties, unknowns, and confusions. That there is confusion cannot really be denied — the question is whether the few somewhat correct things that are in the maze of completely incorrect things really justify concluding in retrospect that he had the right idea all along.

          Here’s my ultimate verdict: in retrospect, it is easy to pick out the right aspects of Szilard’s thinking on this. He certainly was able to once fission provided the missing ingredient (a chain-able nuclear reaction started by neutrons that released neutrons as a by-product). However, viewed from the standpoint of 1934 or so, Szilard’s thinking on this had many incorrect aspects, many huge assertions without justification, and the patent as granted actually lacks some of the key, important ideas (which does not mean that Szilard did not himself have them). I think Szilard’s version of the story, and the versions of the story which make Szilard the “visionary,” typically privilege the retrospective analysis, which makes it completely unclear as to why others did not take him seriously at the time. But viewed through an historical lens that takes the past on its own terms, it is clear why this would not have been taken very seriously: it is hugely speculative, it is based on several unproven premises, and, ultimately, it is not correct. It has the germ of a very important idea in it though, and once fission was discovered Szilard was able to jump swifter than most to the possible applications of fission. But the device proposed by Szilard would not work. One could imagine making a lot of modifications to it so that it would work with fissile material, but these are the kinds of modifications that would reveal exactly the degree to which Szilard’s original understanding was (understandably) incorrect.

          I am not “blaming” Szilard for getting things wrong, though I can understand why more careful experimenters who were trying to figure out how nature worked before trying to capitalize on it or patent it would have thought Szilard’s approach to patenting was unusual.

          • Gene Dannen says:

            No, Alex, the Szilard CSP doesn’t say that at all. It says that Szilard deleted information from the AMERICAN version of his patent.

            I’m really getting the feeling that you don’t realize how well I know this subject.

          • This is what I was referring to, from CSP, Part V, page 529:

            “Subsequently, because of his conviction that if a nuclear chain reaction could be made to work it might be used as an instrument of war to set up violent explosions, Szilard separated that part of the application which related to the nuclear chain reaction and incorporated it in modified form into a later filed application, No. 19157, which he assigned to the British Admiralty in order to prevent its publication. It is worth noting that the following prophetic passage was included in Patent Application 7840 as filed in March 1934: Pure neutron chains…[etc.]

            It then notes that he removed this from the American one as well, in a footnote. The overall implication seems to be that Szilard is the one to have removed this passage. I don’t find plausible that the patent office removed it against Szilard’s intent.

  6. Daniel Olive says:

    Might the patent not have been held back until 1949 because to release it would make clear it was considered a dead end?

    Looked at from the perspective of western allies who think the Russian programme is way behind the US one, it seems to make sense not to let the Russians know that this is a dead end, which is what declassification/release implies.

    • I think we would have to get further into the paper trail to know why it was held back as long as it was and why it was released when it was. E.g. it could have been meant to be released earlier but the granting process might have been delayed for some other (non-secrecy) reason. In any event it was unlikely to have been purposefully released after the Soviet test (which was only announced on September 23, 1949, only five days prior), because no patent bureaucracy works that fast.

  7. Gene Dannen says:

    Alex, if you want to know the history of the secret patent, you’ll have to find the official file. Years ago, I tried to do that. The British government was unable to locate it, despite apparently sincere attempts.

    Thanks for letting me offer my viewpoint. And many congratulations on your new job at the Stevens Institute of Technology. They’ll be lucky to have you.

    • Well, thank you for offering your viewpoint! Even though I think we have agreed to disagree, I learned much from the exchange and it forced me to sharpen my analysis.

  8. […] critical “radius,” “size,” and “condition.” Leo Szilard’s pre-fission, 1935 patent on chain reacting systems uses the terms critical “thickness,” and “value,” not mass. This is not to […]