Einsteinium was first identified in December 1952 by Albert Ghiorso and co-workers at the University of California, Berkeley in collaboration with the Argonne and Los Alamos National Laboratories, in the fallout from the Ivy Mike nuclear test.
Ghiorso and co-workers analyzed filter papers which had been flown through the explosion cloud on airplanes (the same sampling technique that had been used to discover 244
94Pu). Larger amounts of radioactive material were later isolated from coral debris of the atoll, which were delivered to the U.S. The separation of suspected new elements was carried out in the presence of a citric acid/ammonium buffer solution in a weakly acidic medium (pH ≈ 3.5), using ion exchange at elevated temperatures; fewer than 200 atoms of einsteinium were recovered in the end.
Nevertheless, element 99 (einsteinium), namely its 253Es isotope, could be detected via its characteristic high-energy alpha decay at 6.6 MeV. It was produced by the capture of 15 neutrons by uranium-238 nuclei followed by seven beta-decays, and had a half-life of 20.5 days.
Later, isotopes of element 99 (as well as of new element 100, fermium) were produced in the Berkeley and Argonne laboratories, in a nuclear reaction between nitrogen-14 and uranium-238, and later by intense neutron irradiation of plutonium or californium:
These results were published in several articles in 1954 with the disclaimer that these were not the first studies that had been carried out on the elements. The Berkeley team also reported some results on the chemical properties of einsteinium and fermium.
In their discovery of the elements 99 and 100, the American teams had competed with a group at the Nobel Institute for Physics, Stockholm, Sweden. In late 1953 – early 1954, the Swedish group succeeded in the synthesis of light isotopes of element 100, in particular 250Fm, by bombarding uranium with oxygen nuclei. These results were also published in 1954. Nevertheless, the priority of the Berkeley team was generally recognized, as its publications preceded the Swedish article, and they were based on the previously undisclosed results of the 1952 thermonuclear explosion; thus the Berkeley team was given the privilege to name the new elements. The official names suggested by the Berkeley group derived from two prominent scientists, Albert Einstein and Enrico Fermi: The discovery of these new elements was announced by Albert Ghiorso at the first Geneva Atomic Conference held on 8–20 August 1955. The symbol for einsteinium was first given as "E" and later changed to "Es" by IUPAC.
Einsteinium is a synthetic, silvery-white, radioactive metal. In the periodic table, it is located to the right of the actinide californium, to the left of the actinide fermium and below the lanthanide holmium with which it shares many similarities in physical and chemical properties. Its density of 8.84 g/cm3 is lower than that of californium (15.1 g/cm3) and is nearly the same as that of holmium (8.79 g/cm3), despite atomic einsteinium being much heavier than holmium. The melting point of einsteinium (860 °C) is also relatively low – below californium (900 °C), fermium (1527 °C) and holmium (1461 °C). Einsteinium is a soft metal, with the bulk modulus of only 15 GPa, which value is one of the lowest among non-alkali metals.
The metal is divalent and has a noticeably high volatility.
Magnetic properties have been studied for einsteinium metal, its oxide and fluoride. All three materials showed Curie–Weiss paramagnetic behavior from liquid helium to room temperature. The effective magnetic moments were deduced as 10.4 ± 0.3 µB for Es2O3 and 11.4 ± 0.3 µB for the EsF3, which are the highest values among actinides, and the corresponding Curie temperatures are 53 and 37 K.
Like all actinides, einsteinium is rather reactive. Its trivalent oxidation state is most stable in solids and aqueous solution where it induced pale pink color. The existence of divalent einsteinium is firmly established, especially in solid phase; such +2 state is not observed in many other actinides, including protactinium, uranium, neptunium, plutonium, curium and berkelium. Einsteinium(II) compounds can be obtained, for example, by reducing einsteinium(III) with samarium(II) chloride. The oxidation state +4 was postulated from vapor studies and is yet uncertain.
Nineteen nuclides and three nuclear isomers are known for einsteinium with atomic weights ranging from 240 to 258. All are radioactive and the most stable nuclide, 252Es, has a half-life of 471.7 days.[ Next most stable isotopes are 254Es (half-life 275.7 days), 255Es (39.8 days) and 253Es (20.47 days). All of the remaining isotopes have half-lives shorter than 40 hours, and most of them decay within less than 30 minutes. Of the three nuclear isomers, the most stable is 254mEs with half-life of 39.3 hours.
Synthesis and extraction
Einsteinium is produced in minute quantities by bombarding lighter actinides with neutrons in dedicated high-flux nuclear reactors. The world's major irradiation sources are the 85-megawatt High Flux Isotope Reactor (HFIR) at the Oak Ridge National Laboratory in Tennessee, U.S., and the SM-2 loop reactor at the Research Institute of Atomic Reactors (NIIAR) in Dimitrovgrad, Russia,which are both dedicated to the production of transcurium (Z > 96) elements. These facilities have similar power and flux levels, and are expected to have comparable production capacities for transcurium elements, although the quantities produced at NIIAR are not widely reported. In a "typical processing campaign" at Oak Ridge, tens of grams of curium are irradiated to produce decigram quantities of californium, milligram quantities of berkelium (249Bk) and einsteinium and picogram quantities of fermium.
The first microscopic sample of 253Es sample weighing about 10 nanograms was prepared in 1961 at HFIR. Larger batches were produced later starting from several kilograms of plutonium with the einsteinium yields (mostly 253Es) of 0.48 milligrams in 1967–1970, 3.2 milligrams in 1971–1973, followed by steady production of about 3 milligrams per year between 1974 and 1978. These quantities however refer to the integral amount in the target right after irradiation. Subsequent separation procedures reduced the amount of isotopically pure einsteinium roughly tenfold.
Heavy neutron irradiation of plutonium results in four major isotopes of einsteinium: 253Es (α-emitter with half-life of 20.03 days and with a spontaneous fission half-life of 7×105 years); 254mEs (β-emitter with half-life of 38.5 hours), 254Es (α-emitter with half-life of about 276 days) and 255Es (β-emitter with half-life of 24 days). An alternative route involves bombardment of uranium-238 with high-intensity nitrogen or oxygen ion beams.
Einsteinium-247 (half-life 4.55 minutes) was produced by irradiating americium-241 with carbon or uranium-238 with nitrogen ions. The latter reaction was first realized in 1967 in Dubna, Russia, and the involved scientists were awarded the Lenin Komsomol Prize.
The isotope 248Es was produced by irradiating 249Cf with deuterium ions. It mainly decays by emission of electrons to 248Cf with a half-life of 25 (±5) minutes, but also releases α-particles of 6.87 MeV energy, with the ratio of electrons to α-particles of about 400.
The heavier isotopes 249Es, 250Es, 251Es and 252Es were obtained by bombarding 249Bk with α-particles. One to four neutrons are liberated in this process making possible the formation of four different isotopes in one reaction.
Einsteinium-253 was produced by irradiating a 0.1–0.2 milligram 252Cf target with a thermal neutron flux of (2–5)×1014 neutrons·cm−2·s−1 for 500–900 hours:
A personal account by Ghiorso