page_title: Mendelevium
section_title: Discovery
hierarchical_section_title: Mendelevium / Discovery
caption_attribution_description: English: Photograph shows the 60-inch cyclotron at the University of California Lawrence Radiation Laboratory, Berkeley, in August, 1939. The metal frame at left is the machine's huge electromagnet. The flat vacuum chamber in which the particles are accelerated is located in the narrow space between the magnet's 60 inch (152 centimeter, 5 foot) pole pieces. The beamline which analyses the particles resulting from the collisions is at right. The machine was the most powerful particle accelerator in the world at the time. It had started operating early in the year. During the period of the photograph Dr. Edwin M. McMillan was doing the work which led to the discovery of neptunium (element 93) a year later. The instrument was used later by Dr. Glenn T. Seaborg and his colleagues for the discovery of element 94 (plutonium) early in 1941. Subsequently, other transuranium elements were discovered with the machine, as well as many radioisotopes, including carbon-14. For their work, Drs. Seaborg and McMillan shared the Nobel Prize in 1951. The machine was used for the "long bombardments" which produced the first weighable and visible quantities of plutonium, which was used at Chicago by Seaborg and his colleagues to work out the method for separating plutonium on an industrial scale at the Hanford, Washington, plutonium pro... Русский: Фотография показывает 60-дюймовый циклотрон в университете Лаборатории California Lawrence Radiation, Беркли, в августе 1939. Машина была самым сильным ускорителем частиц в мире в то время.
context_page_description: Mendelevium is a synthetic element with the symbol Md and atomic number 101. A metallic radioactive transuranic element in the actinide series, it is the first element by atomic number that currently cannot be produced in macroscopic quantities through neutron bombardment of lighter elements. It is the third-to-last actinide and the ninth transuranic element. It can only be produced in particle accelerators by bombarding lighter elements with charged particles. A total of seventeen mendelevium isotopes are known, the most stable being ²⁵⁸Md with a half-life of 51 days; nevertheless, the shorter-lived ²⁵⁶Md is most commonly used in chemistry because it can be produced on a larger scale.
  Mendelevium was discovered by bombarding einsteinium with alpha particles in 1955, the same method still used to produce it today. It was named after Dmitri Mendeleev, father of the periodic table of the chemical elements. Using available microgram quantities of the isotope einsteinium-253, over a million mendelevium atoms may be produced each hour.
context_section_description: Mendelevium was the ninth transuranic element to be synthesized. It was first synthesized by Albert Ghiorso, Glenn T. Seaborg, Gregory Robert Choppin, Bernard G. Harvey, and team leader Stanley G. Thompson in early 1955 at the University of California, Berkeley. The team produced ²⁵⁶Md (half-life of 77 minutes) when they bombarded an ²⁵³Es target consisting of only a billion (10⁹) einsteinium atoms with alpha particles (helium nuclei) in the Berkeley Radiation Laboratory's 60-inch cyclotron, thus increasing the target's atomic number by two. ²⁵⁶Md thus became the first isotope of any element to be synthesized one atom at a time. In total, seventeen mendelevium atoms were produced. This discovery was part of a program, begun in 1952, that irradiated plutonium with neutrons to transmute it into heavier actinides. This method was necessary as the previous method used to synthesize transuranic elements, neutron capture, could not work because of a lack of known beta decaying isotopes of fermium that would produce isotopes of the next element, mendelevium, and also due to the very short half-life to spontaneous fission of ²⁵⁸Fm that thus constituted a hard limit to the success of the neutron capture process.
  To predict if the production of mendelevium would be possible, the team made use of a rough calculation. The number of atoms that would be produced would be approximately equal to the product of the number of atoms of target material, the target's cross section, the ion beam intensity, and the time of bombardment; this last factor was related to the half-life of the product when bombarding for a time on the order of its half-life. This gave one atom per experiment. Thus under optimum conditions, the preparation of only one atom of element 101 per experiment could be expected. This calculation demonstrated that it was feasible to go ahead with the experiment. The target material, einsteinium-253, could be produced readily from irradiating plutonium: one year of irradiation would give a billion atoms, and its three-week half-life meant that the element 101 experiments could be conducted in one week after the produced einsteinium was separated and purified to make the target. However, it was necessary to upgrade the cyclotron to obtain the needed intensity of 10¹⁴ alpha particles per second; Seaborg applied for the necessary funds.
  While Seaborg applied for funding, Harvey worked on the einsteinium target, while Thomson and Choppin focused on methods for chemical isolation. Choppin suggested using α-hydroxyisobutyric acid to separate the mendelevium atoms from those of the lighter actinides. The actual synthesis was done by a recoil technique, introduced by Albert Ghiorso. In this technique, the einsteinium was placed on the opposite side of the target from the beam, so that the recoiling mendelevium atoms would get enough momentum to leave the target and be caught on a catcher foil made of gold. This recoil target was made by an electroplating technique, developed by Alfred Chetham-Strode. This technique gave a very high yield, which was absolutely necessary when working with such a rare and valuable product as the einsteinium target material. The recoil target consisted of 10⁹ atoms of ²⁵³Es which were deposited electrolytically on a thin gold foil. It was bombarded by 41 MeV alpha particles in the Berkeley cyclotron with a very high beam density of 6×10¹³ particles per second over an area of 0.05 cm². The target was cooled by water or liquid helium, and the foil could be replaced.
  Initial experiments were carried out in September 1954. No alpha decay was seen from mendelevium atoms; thus, Ghiorso suggested that the mendelevium had all decayed by electron capture to fermium and that the experiment should be repeated to search instead for spontaneous fission events. The repetition of the experiment happened in February 1955.
  On the day of discovery, 19 February, alpha irradiation of the einsteinium target occurred in three three-hour sessions. The cyclotron was in the University of California campus, while the Radiation Lab
caption: The 60-inch cyclotron at the Lawrence Radiation Laboratory, University of California, Berkeley, in August 1939
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