Isotopes of bohrium

Bohrium (₁₀₇Bh) is an artificial element. Like all artificial elements, it has no stable isotopes, and a standard atomic weight cannot be given. The first isotope to be synthesized was ²⁶²Bh in 1981. There are 11 known isotopes ranging from ²⁶⁰Bh to ²⁷⁴Bh, and 1 isomer, ^262mBh. The longest-lived isotope is ²⁷⁰Bh with a half-life of 2.4 minutes, although the unconfirmed ²⁷⁸Bh may have an even longer half-life of about 11.5 minutes.

Superheavy elements such as bohrium are produced by bombarding lighter elements in particle accelerators that induce fusion reactions. Whereas most of the isotopes of bohrium can be synthesized directly this way, some heavier ones 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. As the fused nuclei cool to the ground state, they require emission of only one or two neutrons, thus allowing 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).

The table below contains various combinations of targets and projectiles which could be used to form compound nuclei with Z = 107.

Before the first successful synthesis of hassium in 1981 by the GSI team, the synthesis of bohrium was first attempted in 1976 by scientists at the Joint Institute for Nuclear Research at Dubna using this cold fusion reaction. They detected two spontaneous fission activities, one with a half-life of 1–2 ms and one with a half-life of 5 s. Based on the results of other cold fusion reactions, they concluded that they were due to ²⁶¹Bh and ²⁵⁷Db respectively. However, later evidence gave a much lower SF branching for ²⁶¹Bh reducing confidence in this assignment. The assignment of the dubnium activity was later changed to ²⁵⁸Db, presuming that the decay of bohrium was missed. The 2 ms SF activity was assigned to ²⁵⁸Rf resulting from the 33% EC branch. The GSI team studied the reaction in 1981 in their discovery experiments. Five atoms of ²⁶²Bh were detected using the method of correlation of genetic parent-daughter decays. In 1987, an internal report from Dubna indicated that the team had been able to detect the spontaneous fission of ²⁶¹Bh directly. The GSI team further studied the reaction in 1989 and discovered the new isotope ²⁶¹Bh during the measurement of the 1n and 2n excitation functions but were unable to detect an SF branching for ²⁶¹Bh. They continued their study in 2003 using newly developed bismuth(III) fluoride (BiF₃) targets, used to provide further data on the decay data for ²⁶²Bh and the daughter ²⁵⁸Db. The 1n excitation function was remeasured in 2005 by the team at the Lawrence Berkeley National Laboratory (LBNL) after some doubt about the accuracy of previous data. They observed 18 atoms of ²⁶²Bh and 3 atoms of ²⁶¹Bh and confirmed the two isomers of ²⁶²Bh.

In 2007, the team at LBNL studied the analogous reaction with chromium-52 projectiles for the first time to search for the lightest bohrium isotope ²⁶⁰Bh:

The team successfully detected 8 atoms of ²⁶⁰Bh decaying by alpha decay to ²⁵⁶Db, emitting alpha particles with energy 10.16 MeV. The alpha decay energy indicates the continued stabilizing effect of the N=152 closed shell.

The team at Dubna also studied the reaction between lead-208 targets and manganese-55 projectiles in 1976 as part of their newly established cold fusion approach to new elements: