Posted on behalf of Brett Thornton and Shawn Burdette. This blog post is an epilogue to the In Your Element (IYE) article on fermium.

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When exactly was fermium first created by humans? The date for fermium’s initial production is given in some sources as October 1952, while others claim November — both dates are given for the Ivy Mike nuclear weapons test, the first time humans created elements 99 and 100. The discrepancy apparently is because Enewetak Atoll, the site of the test, lies on the other side of the International Date Line from the United States. The Ivy Mike nuclear weapons test there occurred on 1 November 1952 local time, but 31 October 1952 in the U.S. mainland.

The Ivy Mike weapons test was the first thermonuclear device — or ‘hydrogen’ bomb — and this explosion injected large amounts of radioactive debris high into the atmosphere. In the stratosphere, this debris spread over the entire globe as fallout, which was no different than any other above-ground nuclear weapons test. (Concerns about fallout was a major impetus for the the Partial Test Ban Treaty in 1963, which banned all above-ground or above-water nuclear tests). Arkansas was one of the many places the fallout landed and, in 1952, someone there was watching the sky.

After the Ivy Mike test, initial studies had revealed that the fallout contained ²⁴⁴Pu, a previously unknown plutonium isotope with a relatively large number of neutrons compared to ²³⁸U, its likely source. Glenn Seaborg at the University of California Radiation Laboratory (UCRL) received a somewhat cryptic telegram informing him that the existence of 244Pu was classified, even if it was produced by unclassified means. The UCRL group had not produced ²⁴⁴Pu yet, but they knew it was possible now. Seaborg and his UCRL group were well-known as leaders in transuranium element research, having already helped with the discoveries of plutonium, americium, curium, berkelium, and californium. If anyone stumbled across or created ²⁴⁴Pu besides the people analyzing nuclear weapon test fallout, the ‘top men’ at UCRL would be the first suspects on the list.

The knowledge that ²⁴⁴Pu existed, even if classified, was enough to get the UCRL group thinking. This probably meant that six neutrons had been almost instantaneously fused into the nucleus of ²³⁸U, much faster than was possible in a high-neutron flux reactor. This prompted UCRL researcher Albert Ghiorso to request samples of the Ivy Mike debris. Ghiorso wondered exactly how many neutrons might have been added. Was ²⁴⁴Pu the heaviest isotope in the debris or were much heavier isotopes also present? (ref. 1). Seaborg was skeptical that so many neutrons could be added to a uranium nucleus, but supported the work. This eventually led to the UCRL group finding elements 99 (ref. 2) and 100 in that fallout debris, as we describe in the fermium IYE essay.

Back in Arkansas, Paul (née Kazuo) Kuroda was interested in nuclear fallout. Kuroda was a Japanese radiochemist who had studied natural radioactive soruces in Japan. He emigrated to the United States in 1949, and worked as a postdoctoral researcher at the University of Minnesota in analytical chemistry until 1952 when he received a faculty appointment at the University of Arkansas. At Arkansas, he returned to his previous interest in radioactivity by studying the local hot springs³. Soon after starting his independent career, Kuroda came across a quote from Edward Teller, one of the principal developers of the hydrogen bomb, stating that the ”radioactive and non-radioactive elements” (the fallout) left behind by a nuclear explosion could be studied to ”learn much about the bomb”.

The Teller quotes are found in Harold Urey’s 1952 book The Planets: Their Origin and Development. Teller was paralleling the isotopic signature in bomb fallout with the isotopic signature left by the creation of the solar system and Earth. Kuroda was puzzled that Urey’s book contained no follow-up on these ideas. Kuroda later wrote ”I therefore decided to initiate my own research project on radioactive fallout from nuclear weapons tests.” (ref. 4). In 1952, Kuroda only knew about the American atomic weapons tests in the Nevada desert, and assumed that any fallout in Arkansas came from these tests. Kuroda realized that the radioactive debris from large nuclear explosions would disperse over the entire planet after being injected into the stratosphere.

In the summer of 1953, Kuroda and his co-worker Paul Damon noticed high concentrations of fission products in the Arkansas rain. They published their results quickly⁵. Their publication ”On the artificial radioactivity of rainfall”, did not go unnoticed. In the autumn of 1953, they were ordered to stop studying fallout, because ”the study of radioactive fallout by non-authorized scientists was strictly forbidden by the U.S. government as a classified military secret” (ref. 4). Damon and Kuroda apparently were mostly silent about the order to stop, but in 1954, they published a report titled ”On the natural radioactivity of rainfall”, which included the pithy statement about artificial radioactivity in rainfall: ”Presumably, considerable work is underway, but has not yet been published.” (ref. 6).

The concentrations of Es and Fm in the fallout reaching Arkansas in 1953 must have been vanishingly small, so Damon and Kuroda would almost certainly not have been able to detect the new elements. They also lacked the huge hint the UCRL group received about the ²⁴⁴Pu produced in the Ivy Mike test, which was the key insight that inspired Ghiorso’s search for elements 99 and 100 (ref. 1). On the other hand, Kuroda and Damon might have noticed ²⁴⁴Pu on their own. We like to envision Kuroda and Damon as characters in a movie asking government agents ”exactly who is investigating the fallout?” Then, like at the end of 1981 film Raiders of the Lost Ark, when Indiana Jones is assured that ”top men” are studying the Ark of the Covenant, Kuroda was being told that ”top men” were looking into it. Unlike Raiders though, the ”top men” were actually looking at the fallout samples⁷, instead of packing them in a crate and then hiding the crate in a warehouse.

Why would studying radioactive fallout be classified? Likely because, as Ghiorso and Seaborg discovered, the existence of ²⁴⁴Pu was classified. ²⁴⁴Pu is the longest-lived isotope of plutonium, and is not useful for building a nuclear weapon, but as the quote from Teller plainly said, knowledge of the fallout could reveal ”much about the bomb”. In this case, the existance of a neutron-rich isotope like ²⁴⁴Pu found from the fallout analysis might reveal something about the large neutron flux of the weapon. So ²⁴⁴Pu’s existence suggested a high neutron flux — which was key to Ghiorso’s search for elements 99 and 100. In 1953, this was definitely information best kept secret. Of course, the American government could only stop American scientists from studying fallout in rain. Papers began appearing in other countries, especially once knowledge of the hydrogen bomb tests became widely known, and the long range at which fallout could be transported was realized^8,9.

Befitting its numerologically significant position on the periodic table, fermium represents the heaviest element which has been forged in a nuclear reactor. The “fermium wall” prevents production of elements heavier than fermium by neutron absorption due to the short half-life (i.e., spontaneous fission) of ²⁵⁸Fm. To go beyond element 100, nuclear scientists had to turn to the same atom-at-a-time techniques — and the same heavy ion beams which were used to produce the first unclassified ”discovery” of fermium (see the IYE article).

In the 1950s though, you didn’t need an nuclear reactor or a convenient hydrogen bomb to find fermium — it fell from the sky.

Brett F. Thornton is in the Department of Geological Sciences (IGV) and Bolin Centre for Climate Research, Stockholm University, 106 91 Stockholm, Sweden. Shawn C. Burdette is in the Department of Chemistry and Biochemistry, Worcester Polytechnic Institute, Worcester, Massachusetts 01609-2280, USA. e-mail: brett.thornton@geo.su.se; scburdette@WPI.EDU

References

1. Ghiorso, A. Chem. Eng. News 81, 174–175 (2003). [LINK]

2. Redfern, J. Nat. Chem. 8, 1168-1168 (2016). [LINK]

3. Kuroda, P. K., Damon, P. E. & Hyde, H. Am. J. Sci. 252, 76-86 (1954). [LINK]

4. Kuroda, P. K. J. Radioanal. Nucl. Chem. 203, 591-599 (1996). [LINK]

5. Damon, P. & Kuroda, P. Nucleonics 11, 59 (1953).

6. Damon, P. & Kuroda, P. Eos, Transactions American Geophysical Union 35, 208-216 (1954). [LINK]

7. Ghiorso, A. et al. Phys. Rev. 99, 1048-1049 (1955). [LINK]

8. Miyake, Y. Papers in Meteorology and Geophysics 5, 173-177 (1954). [LINK]

9. Miyake, Y. Papers in Meteorology and Geophysics 6, 26-37 (1955). [LINK]