Roentgenium (Rg) is a synthetic element, and thus a standard atomic mass cannot be given. Like all synthetic elements, it has no stable isotopes. The first isotope to be synthesized was 272Rg in 1994, which is also the only directly synthesized isotope, all others are decay products of nihonium, moscovium, and tennessine. There are 7 known radioisotopes from 272Rg to 282Rg. The longest-lived isotope is 282Rg with a half-life of 2.1 minutes.
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Notes
Nucleosynthesis
Super-heavy elements such as roentgenium are produced by bombarding lighter elements in particle accelerators that induce fusion reactions. Whereas the lightest isotope of roentgenium, roentgenium-272, can be synthesized directly this way, all the heavier roentgenium isotopes have only been observed as decay products of elements with higher atomic numbers.
Depending on the energies involved, fusion reactions can be categorized as "hot" or "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. The latter is a distinct concept from that of where nuclear fusion claimed to be achieved at room temperature conditions (see cold fusion).
Cold fusion
Before the first successful synthesis of roentgenium in 1994 by the GSI team, a team at the Joint Institute for Nuclear Research in Dubna, Russia, also tried to synthesize roentgenium by bombarding bismuth-209 with nickel-64 in 1986. No roentgenium atoms were identified. After an upgrade of their facilities, the team at GSI successfully detected 3 atoms of 272Rg in their discovery experiment. A further 3 atoms were synthesized in 2002. The discovery of roentgenium was confirmed in 2003 when a team at RIKEN measured the decays of 14 atoms of 272Rg.
The same roentgenium isotope was also observed by an American team at the Lawrence Berkeley National Laboratory (LBNL) from the reaction:
20882Pb
+ 65
29Cu
→ 272
111Rg
+
n
This reaction was conducted as part of their study of projectiles with odd atomic number in cold fusion reactions.
As decay product
All the isotopes of roentgenium except roentgenium-272 have been detected only in the decay chains of elements with a higher atomic number, such as nihonium. Nihonium currently has six known isotopes; all of them undergo alpha decays to become roentgenium nuclei, with mass numbers between 274 and 282. Parent nihonium nuclei can be themselves decay products of moscovium or tennessine. To date, no other elements have been known to decay to roentgenium. For example, in January 2010, the Dubna team (JINR) identified roentgenium-281 as a final product in the decay of tennessine via an alpha decay sequence:
293117Ts
→ 289
115Mc
+ 4
2He
289
115Mc
→ 285
113Nh
+ 4
2He
285
113Nh
→ 281
111Rg
+ 4
2He
Nuclear isomerism
Two atoms of 274Rg have been observed in the decay chain of 278Nh. They decay by alpha emission, emitting alpha particles with different energies, and have different lifetimes. In addition, the two entire decay chains appear to be different. This suggests the presence of two nuclear isomers but further research is required.
Four alpha particles emitted from 272Rg with energies of 11.37, 11.03, 10.82, and 10.40 MeV have been detected. The GSI measured 272Rg to have a half-life of 1.6 ms while recent data from RIKEN have given a half-life of 3.8 ms. The conflicting data may be due to nuclear isomers but the current data are insufficient to come to any firm assignments.
Cold fusion
The table below provides cross-sections and excitation energies for cold fusion reactions producing roentgenium isotopes directly. Data in bold represent maxima derived from excitation function measurements. + represents an observed exit channel.