Isotopes of bohrium

Bohrium (Bh) has no stable isotopes. A standard atomic mass cannot be given, though the atomic mass of the most stable isotope is approx. 264.12.

Nucleosynthesis

Cold fusion

This section deals with the synthesis of nuclei of bohrium by so-called "cold" fusion reactions. These are processes which create compound nuclei at low excitation energy (~10-20 MeV, hence "cold"), leading to a higher probability of survival from fission. The excited nucleus then decays to the ground state via the emission of one or two neutrons only.

²⁰⁹Bi(⁵⁴Cr,xn)^263-xBh (x=1,2)

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. Analysis was by detection of spontaneous fission (SF). They discovered two SF activities, one with a 1-2 ms half-life and one with a 5 s activity. 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 for²⁶²Bh and the daughter ²⁵⁸Db. The 1n excitation function was remeasured in 2005 by the team at 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.

²⁰⁹Bi(⁵³Cr,xn)^262-xBh

The team at Dubna studied this reaction in 1976 in order to assist in their assignments of the SF activities from their experiments with a Cr-54 beam. They were unable to detect any such activity, indicating the formation of different isotopes decaying primarily by alpha decay.

²⁰⁹Bi(⁵²Cr,xn)^261-xBh (x=1)

This reaction was studied for the first time in 2007 by the team at LBNL to search for the lightest bohrium isotope ²⁶⁰Bh. The team successfully detected 8 atoms of ²⁶⁰Bh decaying by correlated 10.16 MeV alpha particle emission to ²⁵⁶Db. The alpha decay energy indicates the continued stabilising effect of the N=152 closed shell.

²⁰⁸Pb(⁵⁵Mn,xn)^263-xBh (x=1)

The team at Dubna also studied this reaction in 1976 as part of their newly established cold fusion approach to new elements. As for the reaction using a Bi-209 target, they observed the same SF activities and assigned them to ²⁶¹107 and ²⁵⁷105. Later evidence indicated that these should be reassigned to²⁵⁸105 and ²⁵⁸104 (see above). In 1983, they repeated the experiment using a new technique: measurement of alpha decay from a descendant using chemical separation. The team were able to detect the alpha decay from a descendant of the 1n evaporation channel, providing some evidence for the formation of element 107 nuclei. This reaction was later studied in detail using modern techniques by the team at LBNL. In 2005 they measured 33 decays of ²⁶²Bh and 2 atoms of²⁶¹Bh, providing a 1n excitation function and some spectroscopic data of both ²⁶²Bh isomers. The 2n excitation function was further studied in a 2006 repeat of the reaction. The team found that this reaction had a higher 1n cross section than the corresponding reaction with a Bi-209 target, contrary to expectations. Further research is required to understand the reasons.

Hot fusion

This section deals with the synthesis of nuclei of bohrium by so-called "hot" fusion reactions. These are processes which create compound nuclei at high excitation energy (~40-50 MeV, hence "hot"), leading to a reduced probability of survival from fission and quasi-fission. The excited nucleus then decays to the ground state via the emission of 3-5 neutrons.

²³⁸Am(³¹P,xn)^269-xBh (x=5?)

This reaction was first studied in 2006 at the LBNL as part of their systematic study of fusion reactions using ²³⁸U targets. Results have not been published but preliminary results appear to indicate the observation of spontaneous fission, possibly from²⁶⁴Bh.

²⁴³Am(²⁶Mg,xn)^269-xBh (x=3,4,5)

Recently, the team at the Institute of Modern Physics (IMP), Lanzhou, have studied the nuclear reaction between americium-243 and magnesium-26 ions in order to synthesise the new isotope ²⁶⁵Bh and gather more data on ²⁶⁶Bh. In two series of experiments, the team has measured partial excitation functions of the 3n,4n and 5n evaporation channels.

²⁴⁸Cm(²³Na,xn)^271-xBh (x=4,5)

This reaction was studied for the first time in 2008 by the team at RIKEN, Japan, in order to study the decay properties of ²⁶⁶Bh, which is a decay product in their claimed decay chains of ununtrium. The decay of ²⁶⁶Bh by the emission of 9.04 MeV alpha particles was confirmed, although lines at 9.29 MeV (see below) and 9.77 MeV (see ununtrium) were not.

²⁴⁹Bk(²²Ne,xn)^271-xBh (x=4)

The first attempts to synthesize bohrium by hot fusion pathways were performed in 1979 by the team at Dubna. The reaction was repeated in 1983. In both cases, they were unable to detect any spontaneous fission from nuclei of bohrium. More recently, hot fusions pathways to bohrium have been re-investigated in order to allow for the synthesis of more long-lived, neutron rich isotopes to allow a first chemical study of bohrium. In 1999, the team at LBNL claimed the discovery of long-lived ²⁶⁷Bh (5 atoms) and ²⁶⁶Bh (1 atom). In the following year, the same team attempted to confirm the synthesis and decay of ²⁶⁶Bh, but were unable to do so. Current research has been unable to confirm the 9.29 MeV decay claimed and the synthesis of ²⁶⁶Bh by this reaction is not accepted at this moment in time. The team at the Paul Scherrer Institute (PSI) in Bern, Switzerland later synthesized 6 atoms of ²⁶⁷Bh in the first definitive study of the chemistry of bohrium (see below).

²⁵⁴Es(¹⁶O,xn)^270-xBh

As an alternative means of producing long-lived bohrium isotopes suitable for a chemical study, the synthesis of ²⁶⁷Bh and ²⁶⁶Bh were attempted in 1995 by the team at GSI using the highly asymmetric reaction using an einsteinium-254 target. They were unable to detect any product atoms.

As decay products

Isotopes of bohrium have also been detected in the decay of heavier elements. Observations to date are shown in the table below:

Evaporation Residue	Observed Bh isotope
294Uus	274Bh
288Uup	272Bh
287Uup	271Bh
282Uut	270Bh
278Uut	266Bh
272Rg	264Bh
266Mt	262Bh

List of discovered isotopes

Isotope	Year discovered	discovery reaction
260Bh	2007	209Bi(52Cr,n)
261Bh	1989	209Bi(54Cr,2n)
262Bhg,m	1981	209Bi(54Cr,n)
263Bh	unknown
264Bh	1994	209Bi(64Ni,n)
265Bh	2004	243Am(26Mg,4n)
266Bh	2004	209Bi(70Zn,n)
267Bh	2000	249Bk(22Ne,4n)
268Bh	unknown
269Bh	unknown
270Bh	2006	237Np(48Ca,3n)
271Bh	unknown
272Bh	2003	243Am(48Ca,3n)
273Bh	unknown
274Bh	2009	249Bk(48Ca,3n)

Nuclear isomerism

²⁶²Bh

The only confirmed example of isomerism in bohrium is for the isotope ²⁶²Bh. Direct production populates two states, a ground state and an isomeric state. The ground state is confirmed as decaying by alpha emission with alpha lines at 10.08,9.82 and 9.76 MeV with a revised half life of 84 ms. The excited state decays by alpha emission with lines at 10.37 and 10.24 MeV with a revised half-life of 9.6 ms.

Chemical yields of isotopes

Cold Fusion

The table below provides cross-sections and excitation energies for cold fusion reactions producing bohrium isotopes directly. Data in bold represents maxima derived from excitation function measurements. + represents an observed exit channel.

Projectile	Target	CN	1n	2n	3n
55Mn	208Pb	263Bh	590 pb , 14.1 MeV	~35 pb
54Cr	209Bi	263Bh	510 pb , 15.8 MeV	~50 pb
52Cr	209Bi	261Bh	59 pb , 15.0 MeV

Hot Fusion

The table below provides cross-sections and excitation energies for hot fusion reactions producing bohrium isotopes directly. Data in bold represents maxima derived from excitation function measurements. + represents an observed exit channel.

Projectile	Target	CN	3n	4n	5n
26Mg	243Am	271Bh	+	+	+
22Ne	249Bk	271Bh		~96 pb	+

Table

nuclide symbol	Z(p)	N(n)	isotopic mass (u)	half-life	nuclear spin	representative isotopic composition (mole fraction)	range of natural variation (mole fraction)
	excitation energy
260Bh	107	153	260.12197(62)#	0.3# ms
261Bh	107	154	261.12166(25)#	13(4) ms [12(+5-3) ms]
262Bh	107	155	262.12289(37)#	290(160) ms
262mBh	300(60) keV			14(4) ms
263Bh	107	156	263.12304(39)#	200# ms
264Bh	107	157	264.1246(3)#	1.3(5) s [0.44(+60-16) s]
265Bh	107	158	265.12515(41)#	0.9(+7-3) s
266Bh	107	159	266.12694(22)#	5(3) s
267Bh	107	160	267.12765(28)#	22(10) s [17(+14-6) s]
268Bh	107	161	268.12976(41)#	25# s
269Bh	107	162	269.13069(44)#	25# s
270Bh	107	163	270.13362(50)#	30# s
271Bh	107	164	271.13518(60)#	40# s
272Bh	107	165	272.13803(65)#	10(+12-4) s
273Bh	107	166	273.13962(89)#	90# min
274Bh	107	167	274.14244(84)#	90# min
275Bh	107	168	275.14425(70)#	40# min

Notes

• Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses.
• Uncertainties are given in concise form in parentheses after the corresponding last digits. Uncertainty values denote one standard deviation, except isotopic composition and standard atomic mass from IUPAC which use expanded uncertainties.

References

• Isotope masses from:
  • G. Audi, A. H. Wapstra, C. Thibault, J. Blachot and O. Bersillon (2003). "The NUBASE evaluation of nuclear and decay properties" (PDF). Nuclear Physics A. 729: 3–128. doi:10.1016/j.nuclphysa.2003.11.001.{{cite journal}}: CS1 maint: multiple names: authors list (link)
• Isotopic compositions and standard atomic masses from:
  • J. R. de Laeter, J. K. Böhlke, P. De Bièvre, H. Hidaka, H. S. Peiser, K. J. R. Rosman and P. D. P. Taylor (2003). "Atomic weights of the elements. Review 2000 (IUPAC Technical Report)". Pure and Applied Chemistry. 75 (6): 683–800. doi:10.1351/pac200375060683.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  • M. E. Wieser (2006). "Atomic weights of the elements 2005 (IUPAC Technical Report)". Pure and Applied Chemistry. 78 (11): 2051–2066. doi:10.1351/pac200678112051. {{cite journal}}: Unknown parameter |laysummary= ignored (help)
• Half-life, spin, and isomer data selected from the following sources. See editing notes on this article's talk page.
  • G. Audi, A. H. Wapstra, C. Thibault, J. Blachot and O. Bersillon (2003). "The NUBASE evaluation of nuclear and decay properties" (PDF). Nuclear Physics A. 729: 3–128. doi:10.1016/j.nuclphysa.2003.11.001.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  • National Nuclear Data Center. "NuDat 2.1 database". Brookhaven National Laboratory. Retrieved September 2005. {{cite web}}: Check date values in: |accessdate= (help)
  • N. E. Holden (2004). "Table of the Isotopes". In D. R. Lide (ed.). CRC Handbook of Chemistry and Physics (85th ed.). CRC Press. Section 11. ISBN 978-0849304859. {{cite book}}: Unknown parameter |nopp= ignored (|no-pp= suggested) (help)