Einsteinium

• 1. Introduction

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  Einsteinium, element 99 on the periodic table, was born in the fire of history's most destructive weapon. On the 1st of November 1952, the United States detonated Ivy Mike, the first successful thermonuclear bomb, at Enewetak Atoll in the Pacific Ocean. Hidden inside the radioactive cloud drifting away from that blast were atoms of something no human had ever seen before. Scientists would later determine that the bomb's extraordinary neutron flux had forced uranium nuclei to swallow 15 extra neutrons in rapid succession, triggering a chain of nuclear transformations that produced a brand-new element. How do you study a substance that glows from its own radioactivity, destroys its own crystal structure, and exists in quantities measured in millionths of a gram? And how did a secret Cold War weapon program accidentally help explain how the universe builds its heaviest atoms inside exploding stars? Those are the questions at the heart of this story.

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• 2. Born in a Bomb

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  Albert Ghiorso and co-workers at the University of California, Berkeley identified einsteinium in December 1952, just weeks after the Ivy Mike detonation. The team worked in collaboration with Argonne and Los Alamos National Laboratories. Their first clue that something extraordinary had happened came from filter papers flown through the explosion cloud by aircraft, the same sampling technique that had previously been used to discover plutonium-244. Larger amounts of radioactive material came from coral debris collected at the atoll and shipped back to the United States.

  The separation of these suspected new elements was carried out in a weakly acidic medium with a pH of roughly 3.5, using ion exchange at elevated temperatures. After all that effort, fewer than 200 atoms of einsteinium were recovered. Yet those atoms were enough. Element 99 could be detected through its characteristic high-energy alpha decay at 6.6 MeV, with a half-life of 20.5 days. The uranium nuclei in the bomb had captured 15 neutrons, then undergone seven beta decays to climb the periodic table one step at a time.

  Some uranium atoms captured two additional neutrons, totalling 17, producing a heavier einsteinium isotope as well as atoms of yet another new element, fermium. The entire discovery was placed under military secrecy by order of the U.S. military and kept classified until 1955, a direct consequence of Cold War competition with the Soviet Union over nuclear technologies.

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• 3. The Race to Name It

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  While the Berkeley team's findings sat locked in classified files, a group at the Nobel Institute for Physics in Stockholm independently synthesized light isotopes of element 100 in late 1953 to early 1954, by bombarding uranium with oxygen nuclei. Both the American and Swedish results appeared in scientific publications in 1954. The Berkeley articles carried an unusual disclaimer acknowledging that these were not the first studies carried out on the elements, a careful acknowledgment of the secret work that predated them.

  The priority of the Berkeley team was generally recognized because their publications came first and rested on the undisclosed results of the 1952 explosion. That gave them the right to name both new elements. During the classified phase, the project that had led to Ivy Mike's design was codenamed Project PANDA, and element 99 had been jokingly nicknamed "Pandemonium" inside the program. The official proposal was more solemn. The Berkeley group wrote: "We suggest for the name for the element with the atomic number 99, einsteinium (symbol E) after Albert Einstein and for the name for the element with atomic number 100, fermium (symbol Fm), after Enrico Fermi." Both Einstein and Fermi died in the period between when the names were first proposed and when they were formally announced. Albert Ghiorso announced the discovery of these elements at the first Geneva Atomic Conference, held from the 8th to the 20th of August 1955. The symbol was initially assigned as "E" and later changed to "Es" by IUPAC.

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• 4. A Metal That Destroys Itself

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  Einsteinium is a soft, silvery metal with a density of 8.84 grams per cubic centimeter, substantially lower than californium at 15.1 g/cm3, though einsteinium atoms are far heavier. Its melting point of 860 degrees Celsius is relatively low, falling below californium at 900 degrees, fermium at 1,527 degrees, and holmium at 1,461 degrees. With a bulk modulus of only 15 GPa, it ranks among the least rigid non-alkali metals known.

  The element's radioactivity creates a fundamental problem for anyone who wants to study it in solid form. The energy released as einsteinium-253 decays runs to about 1,000 watts per gram, enough to produce a visible glow and rapidly wreck the crystal structure of the metal. In practice, measurements of solid einsteinium and its compounds are taken right after thermal annealing to minimize accumulated radiation damage. Some compounds are studied under a reducing gas atmosphere, partly to allow the sample to regrow as it decomposes.

  A second hazard is self-contamination. Einsteinium-253 decays to berkelium-249, which then decays to californium-249, at a combined rate of about 3.3% per day. Within weeks, a freshly purified sample is riddled with two other elements. Researchers have worked around this by using tunable lasers to selectively excite einsteinium ions optically, distinguishing their signals from the contaminating decay products. Because samples are available only once or twice a year in sub-milligram amounts, every measurement must count.

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• 5. Making More of It

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  The only way to produce einsteinium in any meaningful quantity is to bombard lighter actinides with neutrons inside dedicated high-flux nuclear reactors. The world's two main facilities for this purpose are the 85-megawatt High Flux Isotope Reactor at Oak Ridge National Laboratory in Tennessee and the SM-2 loop reactor at the Research Institute of Atomic Reactors in Dimitrovgrad, Russia. In a typical processing campaign at Oak Ridge, tens of grams of curium are irradiated to yield decigram quantities of californium, milligrams of berkelium and einsteinium, and only picograms of fermium.

  The first microscopic sample of einsteinium, weighing about 10 nanograms, was prepared at Oak Ridge's reactor in 1961, and a special magnetic balance had to be designed just to estimate its weight. Batches grew slowly over subsequent years; production runs between 1974 and 1978 averaged roughly 3 milligrams per year before separation, with the isotopically pure yield roughly ten times lower after processing. In 2020, scientists at Oak Ridge created about 200 nanograms of einsteinium in a single campaign, enough to study some of the element's chemical properties for the first time.

  Separation from reactor products is painstaking. The raw product is a mixture of actinide isotopes and lanthanides from fission decay. Purification requires multiple rounds of cation-exchange chromatography at elevated temperature and pressure, followed by anion-exchange steps. A critical step separates einsteinium from berkelium by exploiting the fact that berkelium readily oxidizes to a solid +4 state and precipitates, while einsteinium stays in its +3 state in solution.

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• 6. Supernovas and Neutron Capture

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  The rapid capture of 15 neutrons by uranium during the Ivy Mike explosion turned out to have implications far beyond weapons science. That process, in which heavy isotopes absorb neutrons faster than they can decay, is called the r-process, and astrophysicists had proposed it as the mechanism behind the formation of many stable heavy elements inside exploding stars. Before Ivy Mike, this idea lacked direct experimental proof. The bomb's debris provided it: a man-made neutron environment so intense that it recreated on a small scale what supernovas do on a cosmic scale.

  The neutron flux density during the detonation reached roughly 10 to the 23rd power neutrons per square centimeter within a microsecond. By comparison, the High Flux Isotope Reactor at Oak Ridge operates at a flux of 5 times 10 to the 15th neutrons per square centimeter per second. The difference helps explain why the bomb created new elements that years of reactor work had not. Newly formed heavy isotopes had so many neutrons available during the explosion that they could absorb more before disintegrating into lighter elements.

  Experiments at Enewetak Atoll continued between 1954 and 1956, with a dedicated laboratory set up on the atoll itself because some isotopes would have decayed before debris samples could reach the U.S. mainland. Despite hopes of discovering elements heavier than fermium, none were found even after a series of megaton explosions. Underground tests at the Nevada Test Site in the 1960s also fell short of that goal, though the last of nine underground tests between 1962 and 1969 produced milligrams of einsteinium within a single microsecond, a yield that would otherwise have required a full year in a high-power reactor.

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• 7. What Einsteinium Is Actually Used For

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  In 1955, einsteinium became the tool that created the next element on the periodic table. Scientists at Berkeley Laboratory irradiated a target of roughly 10 to the 13th atoms of einsteinium-253 in the laboratory's 60-inch cyclotron. The reaction between einsteinium and alpha particles yielded 17 atoms of mendelevium, element 101, the first element ever synthesized one atom at a time.

  A heavier isotope, einsteinium-254, is preferred for attempts to create superheavy elements because of its large atomic mass, a relatively long half-life of 270 days, and availability in significant amounts of several micrograms. In 1985, einsteinium-254 was used as a target in an attempt to synthesize element 119, known as ununennium, by bombarding it with calcium-48 ions at the superHILAC linear particle accelerator at Berkeley. No atoms were identified; the experiment set an upper limit on the reaction probability of 300 nanobarns.

  Einsteinium-254 has also served a practical role in space science. It was used as a calibration marker in the alpha-scattering surface analyzer carried by the Surveyor 5 lunar probe. The element's large atomic mass reduced spectral overlap between its signal and those of the lighter elements it was being used to identify on the lunar surface. Outside these applications, nearly all use of einsteinium is restricted to basic research aimed at producing heavier transuranium and superheavy elements, constrained always by the element's extreme scarcity and short-lived isotopes.

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