Quest for superheavy nuclei

Abstract

europhysics news JANUARY/FEBRUARY 2002 Historical Background The quest for superheavy nuclei began in the 1940s with the syn-thesis of atomic nuclei with a number of protons greater than uranium (Z=92). In 1940, neptunium (Z=93) and plutonium (Z=94) were discovered. This was followed by the synthesis of americium (Z=95) and curium (Z=96) in 1944, berkelium (Z=97) in 1949, californium (Z=98), einsteinium (Z=99) and fer-mium (Z=100) in 1952, and mendelevium (Z=lOl) in 1955. All these elements were produced by intense neutron irradiations or by proton, deuteron, or helium (alpha particle) bombardment in a cyclotron. Some of these isotopes have been produced in sizable amounts. Einsteinium and fermium were discovered in the debris from the thermonuclear explosion conducted at Eniwetok Atoll in the Pacific Ocean. Amazingly, the fermium nucleus was made through the capture of 17 neutrons by 23BU followed by the subsequent beta decays. In nature, a similar process (the so-called r-process, believed to be taking place in supernova) is responsible for the synthesis of heavy elements. Still heavier elements (transuraniums) were produced in heavy-ion accelerators by fusing heavy actinide targets (plutoni-um-californium) with light ions of carbon (nobelium, Z=102, 1958; ruthefordium, Z=104, 1969), boron (lawrencium, Z=103, 1961), neon (dubnium, Z=105, 1967), and oxygen (seaborgium, Z=106, 1974). In order to go heavier, to compensate the decrease of the proton-to-neutron ratio with mass, fusion of nuclei with the largest possible surplus ofneutrons had to be used. In the 70s, "cold fusion" reactions involving medium-mass projectiles with Z ~24 and lead or bismuth targets were introduced and replaced "hot fusion" reaction with actinide targets. This enabled the dis-covery of bohrium (Z=107, 1976), hassium (Z=108, 1984), meitnerium (Z=109, 1982), ununnilium (Z=1l0, 1994), unununium (Z=1l1, 1994), and ununbium (Z=1l2, 1996). The last three elements are still unnamed. So far, they carry tempo-rary names derived directly from the atomic number according to the systematic nomenclature of the International Union of Pure and Applied Chemistry (IUPAC). Most of the heaviest elements were found in three "heavy