"The Speck Of Matter God Had Not Welcomed At Creation..."

Today marks an anniversary of the Nagasaki atomic bomb, the so-called "Fat Man." Many forget that we dropped two kinds of atomic bombs in three days in 1945: The first, "Little Boy," contained all the laboriously accumulated U-235; its design was so simple that it didn't need testing: Fire one subcritical U-235 mass at another U-235 subcritical mass and blammo! --  the whole thing went nuclear.

The second was a plutonium bomb -- and its design was so radical that it had to be tested first, hence "Trinity" at Yucca Flats.

The bomb makers knew early on that U-235 would be the limiting factor: Only 1/140th of any natural uranium source was useable for a bomb -- the rest is the unusable U-238 isotope. But the eggheads (really a who's who of nuclear physicists and chemists) figured out that nuking the otherwise useless U-238 with cyclotron radiation would transmute that useless uranium isotope into heavier elements. Thus began a series of top secret experiments at Berkeley which extended the Periodic Table one element at a time.  That sort of work still continues.

First came element 93, the very first transuranic element, now known as neptunium, and synthesized by Edwin McMillan and Philip H. Abelson in 1940. That element proved unusable as a fissile material, so the search for the next heavier element continued. The work quickly became a rather a dirty job -- separating the toxic gemischt into identifiable components --and one more suited for chemists. Glenn T. Seaborg, and a graduate student, Arthur C. Wahl, did the yeoman's work. By early 1941, they knew that they had something new, but were unable to separate it from co-produced thorium.  Seaborg and Wahl pressed on, working in a cramped third floor laboratory in the chemistry department at Berkeley. Success followed and by early March 1941 Seaborg recorded:

With this final separation from thorium, it has been demonstrated that our alpha [particle] activity can be separated from all known elements and thus it is now clear that our alpha activity is due to the new element with atomic number 94. 

Within weeks, and after gathering enough material, tests showed that element 94 was fissile bomb material. They were already way ahead of anyone else. Because it was immediately apparent that chemical separation of elements was easier than isotopic separation, plutonium production became a second major project in the Manhattan Project, running in parallel to uranium isotope separation.

Richard Rhodes wrote in his incomparable "The Making Of The Atomic Bomb:"

Not until 1942 would they officially propose a name for the new element that fissioned like U-235 but could be chemically separated from uranium. But Seaborg already knew what he would call it. Consistent with Martin Klaproth's inspiration in 1789 to link his discovery of a new element [uranium] with the recent discovery of the planet Uranus and with McMillan's suggestion to extend the scheme to Neptune, Seaborg would name element 94 for Pluto, the ninth planet outward from the sun, discovered in 1930 and named for the Greek god of the underworld, a god of the earth's fertility but also the god of the dead: Plutonium. 

The Frightening News First Heard In German...

Lise Meitner with Otto Hahn. Leitner, who was born Jewish, had fled Berlin that July 1938. She first spread the news of fission to the rest of the physics community.

Splitting the atom in 1938 was something wholly different than what had gone on with radiation since its discovery in 1896. To my mind, the news must have been like expanding the notion of arithmetic from simple addition and subtraction to suddenly include the concept of division. It really blew people's minds at the time. Soddy had explained how elements incrementally transmuted downwards in atomic number (subtraction) by shedding alpha particles and how they transmuted upwards (addition) in number by losing beta particles, but nobody was looking for this:*

original

In 1938, Otto Hahn and Fritz Straßmann bombarded uranium with neutrons and fished out the products. Neutrons were all the rage after their discovery in 1932 and physicists wanted to know what they did to all types of matter. Enrico Fermi had started this sort of work in Italy, but had been interrupted. It was only a matter of time before someone figured out what was going on. Hahn and Straßmann had expected to observe slightly lighter atoms like radium, actinium, and thorium; instead they observed the much lighter elements barium, lanthanum, and cerium:

Als Chemiker müssen wir aus den kurz dargelegten Versuchen das obengebrachte Schema eigentlich umbenennen und statt Ra, Ac, Th die Symbole Ba, La und Ce einsetzen. Als der Physik in gewisser Weise nahestehende Kernchemiker können wir uns zu diesem, allen bisherigen Erfahrungen der Kernphysik widersprechenden Sprung noch nicht entschließen. Es könnte doch noch vielleicht eine Reihe seltsamer Zufälle unsere Ergebnisse vorgetäuscht haben.

As chemists, we must rename [our] scheme and insert the symbols Ba, La, Ce in place of Ra, Ac, Th. As nuclear chemists closely associated with physics, we cannot yet convince ourselves to make this leap, which contradicts all previous experience in nuclear physics. A series of strange coincidences could still prove our results false.

This was the first published account of nuclear fission. Smart people realized that the exact chemical products required that more neutrons were coming out than were going in--this was soon verified and led physicists to realize that Leo Szilárd's chain reaction was now feasable. The news spread like fallout. And because it came from 1938 Berlin, the entire rest of the physics community panicked...and then they organized.**

Read more about the discovery of fission here.

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*A German chemist, Ida Noddack, had suggested as early as 1934 that "it is conceivable that the nucleus breaks up into several large fragments, which would of course be isotopes of known elements but would not be neighbors of the irradiated element"---but no one took her seriously.  link

**Another frightening fact was that the Austrians had cut off everyone else's supply of uranium from the original Czech mine--the same place Marie Curie had obtained her original samples in 1896.

Doing the aftermath of Becquerel's discovery

[continuation in part from here]

Becquerel's discovery of uranium's radioactivity led to several immediate questions:

(1) Were other elements besides uranium radioactive?
(2) What is radioactivity? Was it it like X-rays?
(3) How to square radioactivity with the Law Of Conservation of Energy.

In the mid 1890's the Periodic Table looked like this:

Original

Note that the noble gases (which had just been discovered--were absent). Also, nickel and cobalt were incorrectly ordered under group VIII as I mentioned here.

Marie Curie found that thorium (an element known since 1828) was radioactive in 1898.  There was a priority dispute with a German chemist, Gerhard Carl Schmidt which I'm still reading about. Thorium's importance briefly eclipsed uranium's, because the latter was in short supply until more sources could be found. Curie, along with her husband Pierre, began extracting uranium from samples of impure Czech pitchblende. They discovered that the purer they got the uranium, the stronger the radioactivity was in the concentrated waste left behind. This told them that something else was in there. They discovered polonium in July and radium in December of 1898, adding two new elements to the chart shown above.  In this way, radioactivity became a tool for discovering new elements.

The man-made sun on earth...

...was first preceded by a moment of darkness. Nuclear weapons were contrived invention, but the phenomena behind them are quite natural. The Sun is our main source of energy--directly or indirectly--and at first its light was thought necessary for radioactivity.

Henri Becquerel (1852-1908)

Henri Becquerel was fascinated by phosphorescent materials--materials which glowed in the dark after exposure to sunlight--including uranium salts. In 1896, he knew of Roentgen's discovery the previous year--the discovery of X-rays--and he wondered if phosphorescent uranium would give off penetrating rays after exposure to light.* Here he describes his eureka moment:

One wraps a Lumière photographic plate with a [Ag] bromide emulsion in two sheets of very thick black paper, such that the plate does not become clouded upon being exposed to the sun for a day. One places on the sheet of paper, on the outside, a slab of the phosphorescent substance, and one exposes the whole to the sun for several hours. When one then develops the photographic plate, one recognizes that the silhouette of the phosphorescent substance appears in black on the negative. If one places between the phosphorescent substance and the paper a piece of money or a metal screen pierced with a cut-out design, one sees the image of these objects appear on the negative...

The shadow of a small copper cross is visible

One must conclude from these experiments that the phosphorescent substance in question emits rays which pass through the opaque paper and reduce silver salts.

But further experiments led him to doubt the necessity of sunlight and to abandon this hypothesis. Later, he reported:

I will insist particularly upon the following fact, which seems to me quite important and beyond the phenomena which one could expect to observe: The same crystalline crusts [of potassium uranyl sulfate], arranged the same way with respect to the photographic plates, in the same conditions and through the same screens, but sheltered from the excitation of incident rays and kept in darkness, still produce the same photographic images.  March 2, 1896

Becquerel's students, Pierre and Marie Curie, went on to discover two new elements, radium and polonium. Marie Curie coined the term "radioactivity" to describe the elements' natural property of begetting penetrating rays.

[story continues]
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*Becquerel may have been aware of Niepce de Saint-Victor's near discovery of the same phenomenon a generation earlier. link

Conversations with Henry

[Continued from here]

At Henry's suggestion, I wrote to Jacob Bigeleisen. He replied:*

[Salutation]:

How did I get into isotope chemistry? In 1943 I worked at SAM Laboratory at Columbia U (Manhattan Project). My initial assignment was to look for isotope shifts in the electronic spectra of uranium compounds (principally uranyl ion). The purpose of this research was to examine the feasibility of a photochemical separation of the uranium isotopes for military purposes. It was a small project. It was here that I became acquainted with Maria Mayer. She worked on the theory of the spectra. I did experiments and consulted with her regularly about my results and her results. The project was reviewed in July 1943 by James B. Conant and Richard C. Tolman, two high officials in the war time science effort. They brought along as an advisor E.B. Wilson, Jr., the outstanding spectroscopist and quantum chemist from Harvard. The Committee found our work very interesting, but recommended that it be discontinued. The time schedule for any practical application of a photochemical process was inconsistent with the plans for the production of a weapon. People like Urey favored a small scale continuing effort as part of scientific intelligence. Was this a path the Germans could be following?

The dozen or so people working on the project were reassigned in September. I was assigned to write up the work of the project as a final report, which was issued under the name of H.C. Urey. Maria Mayer went to the hospital in October 1943 for gall bladder surgery. In late November an assistant of Urey's (one of his former graduate students) Isidor Kirschenbaum came to see me. He said 'you know all about spectra of uranium compounds.' I said I didn't know everything; what was known, I knew. He said: 'Here is this formula. Put in the information about all uranium compounds and give me the results. We are interested in the possible chemical separation of the uranium isotopes. What we would be particularly interested in would be a volatile compound, which dissociates in the vapor. We have reason to believe that this would be very favorable.'  I asked: 'How do you know this?'  He said he could not tell me. I looked at the formula he gave me. It was the Urey-Greiff equation. I told him that I was not familiar with that whole field and I would have to study it out before I put numbers into the equation. He then said: 'Don't you know there is a war on?' He reported to Urey that I was not a very cooperative person.

Well I studied out the Urey-Greiff equation. There are a lot of factors. For uranium compounds, I could see that we did not know some of the factors; for some of the factors the number was of the order +1.001; for some of the factors the number was -1.001. With the computing facilities available then (desktop mechanical calculators) one could get any final answer from +1.00x to -1.00x from the Urey-Greiff equation. So, I decided I would look into a different approach. In chemistry and most of physics, one does not measure absolute quantities. One measures differences. Would it be possible to calculate differences directly instead of absolute quantities and then subtract the two to get the isotope effect? I started on this approach and I completed the zero point energy and the Boltzmann excitation terms.

On Monday after Thanksgiving 1943 Maria Mayer returned to work. She asked me how I was coming along with the final report. I told her I was not working on it. 'What was I doing?' she asked. I explained the problem to her and showed her my progress. This was a general type of problem she was thoroughly familiar with. In collaboration with George E. Kimball and Walter Stockmayer, she had calculated the isotope effect in the reaction HD + H₂O = H₂ + HDO. This reaction was used to produce heavy water at a plant in Trail B.C. and in Norway.) She was also familiar with the general subject since she and her husband, Joe Mayer, had just written a book on statistical mechanics. I had studied this book as a graduate student. She found my approach very interesting, very sensible and very promising. She then volunteered by asking me whether she could join me in working on this project. I said sure, that would be great. So she did and by the end of the day we completed the derivation of the Bigeleisen-Mayer equation. [1] We then made a number of predictions of systems that would be hopeless for uranium isotope separation and pointed to potential interesting avenues. An experimental program was then organized under Clyde Hutchinson. I worked on that for about a half a year.

Urey was too occupied to look into what we had done. His deputies either did not understand or did not believe that a green Ph.D. new to the field could simplify the Urey-Greiff equation to the point where meaningful calculations could be made. There was a lot of secrecy and people were not told everything they needed to know to make the best progress. In April I became involved in determining the structure of UF₆ by spectroscopic means. I did the experiments at American Cyanamid in Stamford, Conn., where they had outstanding spectroscopic equipment. I worked on the analysis of the the spectra with Maria Mayer, who had tried two years earlier in collaboration with Edward Teller to predict the vibrational structure of UF₆ from first principles! I told Maria Mayer when I started on that project that it was an experimental project, not one for calculation. She asked me whether I could do it (determine the Raman spectrum). She took out of her drawer the infra-red spectrum which had been measured by John Turkevich at Princeton. Neither she, Turkevich nor Edward Teller were able to decipher the infra-red spectrum of UF₆. While Maria Mayer and I worked on the analysis of the spectra, from which we deduced unequivocally the regular octahedral structure, in contrast to the electron diffraction results of Simon Bauer, she told me that she had written a summary report of our work on the theory of isotope effects in equilibria and our calculations relevant to uranium isotope separation.

She prepared this report at the request of Martin Kilpatrick, Urey's deputy to whom we reported. The reason for this was that Edward Teller was to make one of his regular consulting visits from Los Alamos. Kilpatrick showed Teller Maria Mayer's report of our work. Teller had also worked on this problem (there is a 1938 paper by Herzfeld and Teller). He told Kilpatrick that the work was correct and first class. Kilpatrick reported to Maria Mayer that Teller approved of the work. Fine. That made her furious. She said to me: 'They trust Edward Teller and not me.'

Regards to Henry. Pass this message on to him.

Jacob Bigeleisen

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* Bigeleisen wrote to me in longhand. I transcribed it here. I supplied the links as well in case anyone was interested.

[1] Bigeleisen later retold a reporter about this amazing moment when he was briefly overwhelmed by Maria Mayer's brilliance:

She looked at my work and asked 'why don't you finish it up by taking out the classical part?'  Without a pause, she wrote the simplified equation, saying 'Now you have it; it's all done.' I didn't immediately understand what she meant when she said to cut out the classical part. I went home. I worked on it, and eventually I got the same result.

Fluorine Gave Uranium Wings*

Fluorospar or Fluorite

Fluorine gas is wretched stuff.  Nothing can tear-out valence electrons like elemental fluorine can. Watch it corrode solid brick here: link  There's another video link along the sidebar there of fluorine eating through a dead chicken. Fluorine was too hot to handle for WW I trench warfare, and even Fritz Haber had to settle for chlorine, the next lower (and less reactive) halogen. Fluorine is superlative in a number of other ways: Uberchemist Martyn Poliakoff explains here: link
[added: an updated video here: link]

The name came from the rock in which it was found, and that mineral, fluorspar, was so named because it helped molten metal flow, a property known since the Middle Ages. Calcium fluoride is still used in welding flux. Fluorospar also glows blue when heated, and that property gave us the term fluorescence.

Fluorine and its heavier halide brethren are the polar opposites of the alkaline earths: Li, Na, K, Cs, etc. Here's some raw video of two polar extremes going at it: link  making salt (halogen means salt-forming in Greek) and a bunch of energy.

Hydrofluoric acid (HF) burns are particularly nasty: a decent amount of it burns right through flesh, overpowering the natural buffering system, and it keeps burning through flesh until it finds bone because calcium is the natural bonding partner of fluorine (as in fluorospar). I once witnessed the aftermath of a grad student who suffered an HF burn: he had to be med-evaced to Denver.

During WW II, uranium hexafluoride (or "hex" as it was so aptly nicknamed) became the vehicle of choice for the gaseous diffusion of uranium isotopes. Consider that nearly every single atom of U-235 that went into "Little Boy" was first borne aloft by six little fluoride wings (as volatile UF₆) before the Enola Gay carried them aloft en mass for Hiroshima. Teflon (polytetrafluoroethylene) was used by chemists during WW II to enable the safe handling of UF₆ during isotope separation.
UF₆ was also of early interest to the Manhattan Project, as told to me by Jacob Bigeleisen: link
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*The title is my homage to Ludwig Mond who in the words of Lord Kelvin "gave metal wings," referring to Mond's discovery of nickel tetracarbonyl, Ni(CO)₄, a volatile compound so insidiously poisonous that it packs a double whammy if inhaled: it nickel plates your lungs while poisoning you with carbon monoxide.