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Isotopes of bohrium

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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 690 seconds.

Isotopes of bohrium (107Bh)
Main isotopes[1] Decay
abun­dance half-life (t1/2) mode pro­duct
267Bh synth 17 s α 263Db
270Bh synth 2.4 min α 266Db
271Bh synth 2.9 s[2] α 267Db
272Bh synth 8.8 s α 268Db
274Bh synth 40 s[3] α 270Db
278Bh synth 11.5 min?[4] SF –

List of isotopes

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Nuclide
[n 1] 		Z N Isotopic mass (Da)
[n 2][n 3] 		Half-life
 Decay
mode
[n 4] 		Daughter
isotope
 Spin and
parity
[n 5]
Excitation energy
260Bh[5] 107 153 260.12144(21)# 41(14) ms
[35+19
−9 ms] 		α 256Db
261Bh[6] 107 154 261.121400(190)[7] 12.8(3.2) ms
[11.8+3.9
−2.4 ms] 		α 257Db (5/2−)
SF (rare) (various)
262Bh[8] 107 155 262.122650(100)[9] 135+15
−12 ms 		α (>94.9%) 258Db
β+ (<3.0%) 262Sg
SF (2.1%) (various)
262mBh 220(50) keV 13.2+1.2
−1.0 ms 		α 258Db
264Bh 107 157 264.12449(19)# 1.07(21) s α (85%) 260Db
SF(β+?) (15%)[10] (various)
265Bh[11] 107 158 265.12496(26)# 1.19(52) s
[0.94+0.70
−0.31 s] 		α 261Db
266Bh 107 159 266.12679(18)# 10.0+2.6
−1.7 s[12] 		α 262Db
β+? 266Sg
267Bh 107 160 267.12750(28)# 22(10) s
[17+14
−6 s] 		α 263Db
270Bh[n 6] 107 163 270.13337(32)# 2.4+4.4
−0.9 min[13] 		α 266Db
271Bh[n 7] 107 164 271.13512(41)# 2.9+2.2
−0.9 s[13] 		α 267Db
272Bh[n 8] 107 165 272.13826(57)# 8.8(7) s[13] α 268Db
274Bh[n 9] 107 167 274.14360(62)# 57(27) s
[44+34
−13 s][14] 		α 270Db
278Bh[n 10] 107 171 278.15499(43)# 11.5 min? SF (various)
This table header & footer:
  1. ^ mBh – Excited nuclear isomer.
  2. ^ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
  3. ^ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
  4. ^ Modes of decay:
    SF: Spontaneous fission
  5. ^ ( ) spin value – Indicates spin with weak assignment arguments.
  6. ^ Not directly synthesized, occurs in decay chain of 282Nh
  7. ^ Not directly synthesized, occurs in decay chain of 287Mc
  8. ^ Not directly synthesized, occurs in decay chain of 288Mc
  9. ^ Not directly synthesized, occurs in decay chain of 294Ts
  10. ^ Not directly synthesized, occurs in decay chain of 290Fl and 294Lv; unconfirmed

Nucleosynthesis

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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.[15]

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.[16] 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.[15] The latter is a distinct concept from that of where nuclear fusion claimed to be achieved at room temperature conditions (see cold fusion).[17]

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

Target Projectile CN Attempt result
208Pb 55Mn 263Bh Successful reaction
209Bi 54Cr 263Bh Successful reaction
209Bi 52Cr 261Bh Successful reaction
238U 31P 269Bh Successful reaction
243Am 26Mg 269Bh Successful reaction
248Cm 23Na 271Bh Successful reaction
249Bk 22Ne 271Bh Successful reaction

Cold fusion

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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.[18] 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.[19] 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.[20]

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:

209
83Bi
 + 52
24Cr
 → 260
107Bh
 +
n

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.[21]

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:

208
82Pb
 + 55
25Mn
 → 262
107Bh
 +
n

They observed the same spontaneous fission activities as those observed in the reaction between bismuth-209 and chromium-54 and again assigned them to 261Bh and 257Db. Later evidence indicated that these should be reassigned to 258Db and 258Rf (see above). In 1983, they repeated the experiment using a new technique: measurement of alpha decay from a decay product that had been separated out chemically. The team were able to detect the alpha decay from a decay product of 262Bh, providing some evidence for the formation of bohrium 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 of 261Bh, providing an excitation function for the reaction emitting one neutron and some spectroscopic data of both 262Bh isomers. The excitation function for the reaction emitting two neutrons was further studied in a 2006 repeat of the reaction. The team found that the reaction emitting one neutron had a higher cross section than the corresponding reaction with a 209Bi target, contrary to expectations. Further research is required to understand the reasons.[22][23]

Hot fusion

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The reaction between uranium-238 targets and phosphorus-31 projectiles was first studied in 2006 at the LBNL as part of their systematic study of fusion reactions using uranium-238 targets:

238
92U
 + 31
15P
 → 264
107Bh
 + 5
n

Results have not been published but preliminary results appear to indicate the observation of spontaneous fission, possibly from 264Bh.[24]

Recently, the team at the Institute of Modern Physics (IMP), Lanzhou, have studied the nuclear reaction between americium-243 targets and accelerated nuclei of magnesium-26 in order to synthesise the new isotope 265Bh and gather more data on 266Bh:

243
95Am
 + 26
12Mg
 → 269−x
107Bh
 + x
n
 (x = 3, 4, or 5)

In two series of experiments, the team measured partial excitation functions for the reactions emitting three, four, and five neutrons.[25]

The reaction between targets of curium-248 and accelerated nuclei of sodium-23 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 nihonium:[26]

248
96Cm
 + 23
11Na
 → 271−x
107Bh
 + x
n
 (x = 4 or 5)

The decay of 266Bh by the emission of alpha particles with energies of 9.05–9.23 MeV was further confirmed in 2010.[27]

The first attempts to synthesize bohrium by hot fusion pathways were performed in 1979 by the team at Dubna, using the reaction between accelerated nuclei of neon-22 and targets of berkelium-249:

249
97Bk
 + 22
10Ne
 → 271−x
107Bh
 + x
n
 (x = 4 or 5)

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).[28] Later, both of these were confirmed.[29] 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.[30]

As decay products

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List of bohrium isotopes observed by decay
Evaporation residue Observed bohrium isotope
294Lv, 290Fl, 290Nh, 286Rg, 282Mt ? 278Bh ?
294Ts, 290Mc, 286Nh, 282Rg, 278Mt 274Bh[3]
288Mc, 284Nh, 280Rg, 276Mt 272Bh[31][32]
287Mc, 283Nh, 279Rg, 275Mt 271Bh[31]
286Mc, 282Nh, 278Rg, 274Mt 270Bh[31]
278Nh, 274Rg, 270Mt 266Bh[32]
272Rg, 268Mt 264Bh[33]
266Mt 262Bh[34]

Bohrium has been detected in the decay chains of elements with a higher atomic number, such as meitnerium. Meitnerium currently has seven known isotopes; all of them undergo alpha decays to become bohrium nuclei, with mass numbers between 262 and 274. Parent meitnerium nuclei can be themselves decay products of roentgenium, nihonium, flerovium, moscovium, livermorium, or tennessine.[35] For example, in January 2010, the Dubna team (JINR) identified bohrium-274 as a product in the decay of tennessine via an alpha decay sequence:[3]

294
117Ts
 → 290
115Mc
 + 4
2He
290
115Mc
 → 286
113Nh
 + 4
2He
286
113Nh
 → 282
111Rg
 + 4
2He
282
111Rg
 → 278
109Mt
 + 4
2He
278
109Mt
 → 274
107Bh
 + 4
2He

Nuclear isomerism

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262Bh

The only confirmed example of isomerism in bohrium is in the isotope 262Bh. Direct synthesis of 262Bh results in two states, a ground state and an isomeric state. The ground state is confirmed to decay by alpha decay, emitting alpha particles with energies of 10.08, 9.82, and 9.76 MeV, and has a revised half-life of 84 ms. The excited state also decays by alpha decay, emitting alpha particles with energies of 10.37 and 10.24 MeV, and has a revised half-life of 9.6 ms.[18]

Chemical yields of isotopes

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Cold fusion

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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

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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 +

References

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