Isotopes of meitnerium

Meitnerium (₁₀₉Mt) is a synthetic element, and thus a standard atomic weight cannot be given. Like all synthetic elements, it has no stable isotopes. The first isotope to be synthesized was ²⁶⁶Mt in 1982, and this is also the only isotope directly synthesized; all other isotopes are only known as decay products of heavier elements. There are eight known isotopes, from ²⁶⁶Mt to ²⁷⁸Mt. There may also be three isomers. The longest-lived of the known isotopes is ²⁷⁸Mt with a half-life of 4.5 seconds. The unconfirmed heavier ²⁸²Mt appears to have an even longer half-life of 67 seconds.

Super-heavy elements such as meitnerium are produced by bombarding lighter elements in particle accelerators that induce fusion reactions. Whereas the lightest isotope of meitnerium, meitnerium-266, can be synthesized directly this way, all the heavier meitnerium isotopes have only been observed as decay products of elements with higher atomic numbers.

Depending on the energies involved, the former are separated into "hot" and "cold". In hot fusion reactions, very light, high-energy projectiles are accelerated toward very heavy targets (actinides), giving rise to compound nuclei at high excitation energy (~40–50 MeV) that may either fission or evaporate several (3 to 5) neutrons. In cold fusion reactions, the produced fused nuclei have a relatively low excitation energy (~10–20 MeV), which decreases the probability that these products will undergo fission reactions. As the fused nuclei cool to the ground state, they require emission of only one or two neutrons, and thus, allows for the generation of more neutron-rich products. Nevertheless, the products of hot fusion tend to still have more neutrons overall. The latter is a distinct concept from that of where nuclear fusion claimed to be achieved at room temperature conditions (see cold fusion).

The table below contains various combinations of targets and projectiles which could be used to form compound nuclei with Z = 109.

After the first successful synthesis of meitnerium in 1982 by the GSI team, a team at the Joint Institute for Nuclear Research in Dubna, Russia, also tried to observe the new element by bombarding bismuth-209 with iron-58. In 1985 they managed to identity alpha decays from the descendant isotope ²⁴⁶Cf indicating the formation of meitnerium. The observation of a further two atoms of ²⁶⁶Mt from the same reaction was reported in 1988 and of another 12 in 1997 by the German team at GSI.

The same meitnerium isotope was also observed by the Russian team at Dubna in 1985 from the reaction:

by detecting the alpha decay of the descendant ²⁴⁶Cf nuclei. In 2007, an American team at the Lawrence Berkeley National Laboratory (LBNL) confirmed the decay chain of the ²⁶⁶Mt isotope from this reaction.

In 2002–2003, the team at LBNL attempted to generate the isotope ²⁷¹Mt to study its chemical properties by bombarding uranium-238 with chlorine-37, but without success. Another possible reaction that would form this isotope would be the fusion of berkelium-249 with magnesium-26; however, the yield for this reaction is expected to be very low due to the high radioactivity of the berkelium-249 target. Other potentially longer-lived isotopes were unsuccessfully targeted by a team at Lawrence Livermore National Laboratory (LLNL) in 1988 by bombarding einsteinium-254 with neon-22.