Wikipedia

Isotopes of hassium

Hassium (108Hs) 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 265Hs in 1984. There are 13 known isotopes from 263Hs to 277Hs and up to six isomers. The most stable known isotope is 271Hs, with a half-life of about 46 seconds, though this assignment is not definite due to uncertainty arising from a low number of measurements. The isotopes 269Hs and 270Hs respectively have half-lives of about 12 seconds and 7.6 seconds. It is also possible that the isomer 277mHs is more stable than these, with a reported half-life 130±100 seconds, but only one event of decay of this isotope has been registered as of 2016.[2][3]

Isotopes of hassium (108Hs)
Main isotopes[1] Decay
abun­dance half-life (t1/2) mode pro­duct
269Hs synth 12 s α 265Sg
270Hs synth 7.6 s α 266Sg
271Hs synth 46 s α 267Sg
277mHs synth 130 s SF –

List of isotopes

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Nuclide
[n 1] 		Z N Isotopic mass (Da)[4]
[n 2][n 3] 		Half-life
 Decay
mode
[n 4] 		Daughter
isotope
 Spin and
parity
[n 5]
Excitation energy
263Hs 108 155 263.12848(21)# 760(40) μs α 259Sg 3/2+#
264Hs[5] 108 156 264.12836(3) 0.7(3) ms α (70%) 260Sg 0+
SF (30%) (various)
265Hs[6] 108 157 265.129792(26) 1.96(16) ms α 261Sg 9/2+#
265mHs 229(22) keV 360(150) μs α 261Sg 3/2+#
266Hs[n 6][7] 108 158 266.130049(29) 2.97+0.78
−0.51 ms 		α (76%) 262Sg 0+
SF (24%) (various)
266mHs 1100(70) keV 280(220) ms
[74+354
−34 ms] 		α 262Sg 9-#
267Hs 108 159 267.13168(10)# 55(11) ms α 263Sg 5/2+#
267mHs[n 7] 39(24) keV 990(90) μs α 263Sg
268Hs 108 160 268.13201(32)# 1.42(1.13) s
[0.38+1.8
−0.17 s] 		α 264Sg 0+
269Hs 108 161 269.13365(14)# 13+10
−4 s[8] 		α 265Sg 9/2+#
269mHs[8] 20 keV# 2.8+13.6
−1.3 s 		α 265mSg 1/2#
IT 269Hs
270Hs[9] 108 162 270.13431(27)# 7.6+4.9
−2.2 s 		α 266Sg 0+
SF (<50%) (various)
271Hs 108 163 271.13708(30)# 46+56
−16 s[8] 		α 267Sg 11/2#
271mHs[8] 20 keV# 7.1+8.4
−2.5 s 		α 267mSg 3/2#
IT 271Hs
272Hs[n 8][10] 108 164 272.13849(55)# 160+190
−60 ms 		α 268Sg 0+
273Hs[n 9] 108 165 273.14146(40)# 510+300
−140 ms[11] 		α 269Sg 3/2+#
275Hs[n 10][12] 108 167 275.14653(64)# 600+230
−130 ms 		α 271Sg
SF (<11%) (various)
277Hs[n 11] 108 169 277.15177(48)# 18+25
−7 ms[13] 		SF (various) 3/2+#
277mHs[n 7][n 11][1] 100(100) keV# 130(100) s SF (various)
This table header & footer:
  1. ^ mHs – 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. ^ # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
  6. ^ Not directly synthesized, occurs as decay product of 270Ds
  7. ^ a b Existence of this isomer is unconfirmed
  8. ^ Not directly synthesized, occurs in decay chain of 276Ds
  9. ^ Not directly synthesized, occurs in decay chain of 285Fl
  10. ^ Not directly synthesized, occurs in decay chain of 287Fl
  11. ^ a b Not directly synthesized, occurs in decay chain of 289Fl

Isotopes and nuclear properties

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Target-projectile combinations leading to Z=108 compound nuclei

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Target Projectile CN Attempt result
136Xe 136Xe 272Hs Failure to date
198Pt 70Zn 268Hs Failure to date[14]
208Pb 58Fe 266Hs Successful reaction
207Pb 58Fe 265Hs Successful reaction
208Pb 56Fe 264Hs Successful reaction
207Pb 56Fe 263Hs Reaction yet to be attempted
206Pb 58Fe 264Hs Successful reaction
209Bi 55Mn 264Hs Failure to date
226Ra 48Ca 274Hs Successful reaction
232Th 40Ar 272Hs Reaction yet to be attempted
238U 36S 274Hs Successful reaction
238U 34S 272Hs Successful reaction
244Pu 30Si 274Hs Reaction yet to be attempted
248Cm 26Mg 274Hs Successful reaction
248Cm 25Mg 273Hs Failure to date
250Cm 26Mg 276Hs Reaction yet to be attempted
249Cf 22Ne 271Hs Successful reaction

Nucleosynthesis

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Superheavy elements such as hassium are produced by bombarding lighter elements in particle accelerators that induce fusion reactions. Whereas most of the isotopes of hassium 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 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.[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]

Cold fusion

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Before the first successful synthesis of hassium in 1984 by the GSI team, scientists at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia also tried to synthesize hassium by bombarding lead-208 with iron-58 in 1978. No hassium atoms were identified. They repeated the experiment in 1984 and were able to detect a spontaneous fission activity assigned to 260Sg, the daughter of 264Hs.[18] Later that year, they tried the experiment again, and tried to chemically identify the decay products of hassium to provide support to their synthesis of element 108. They were able to detect several alpha decays of 253Es and 253Fm, decay products of 265Hs.[19]

In the official discovery of the element in 1984, the team at GSI studied the same reaction using the alpha decay genetic correlation method and were able to positively identify 3 atoms of 265Hs.[20] After an upgrade of their facilities in 1993, the team repeated the experiment in 1994 and detected 75 atoms of 265Hs and 2 atoms of 264Hs, during the measurement of a partial excitation function for the 1n neutron evaporation channel.[21] A further run of the reaction was conducted in late 1997 in which a further 20 atoms were detected.[22] This discovery experiment was successfully repeated in 2002 at RIKEN (10 atoms) and in 2003 at GANIL (7 atoms). The team at RIKEN further studied the reaction in 2008 in order to conduct the first spectroscopic studies of the even-even nucleus 264Hs. They were also able to detect a further 29 atoms of 265Hs.

The team at Dubna also conducted the analogous reaction with a lead-207 target instead of a lead-208 target in 1984:

207
82Pb
 + 58
26Fe
 → 264
108Hs
 +
n

They were able to detect the same spontaneous fission activity as observed in the reaction with a lead-208 target and once again assigned it to 260Sg, daughter of 264Hs.[19] The team at GSI first studied the reaction in 1986 using the method of genetic correlation of alpha decays and identified a single atom of 264Hs with a cross section of 3.2 pb.[23] The reaction was repeated in 1994 and the team were able to measure both alpha decay and spontaneous fission for 264Hs. This reaction was also studied in 2008 at RIKEN in order to conduct the first spectroscopic studies of the even-even nucleus 264Hs. The team detected 11 atoms of 264Hs.

In 2008, the team at RIKEN conducted the analogous reaction with a lead-206 target for the first time:

206
82Pb
 + 58
26Fe
 → 263
108Hs
 +
n

They were able to identify 8 atoms of the new isotope 263Hs.[24]

In 2008, the team at the Lawrence Berkeley National Laboratory (LBNL) studied the analogous reaction with iron-56 projectiles for the first time:

208
82Pb
 + 56
26Fe
 → 263
108Hs
 +
n

They were able to produce and identify six atoms of the new isotope 263Hs.[25] A few months later, the RIKEN team also published their results on the same reaction.[26]

Further attempts to synthesise nuclei of hassium were performed the team at Dubna in 1983 using the cold fusion reaction between a bismuth-209 target and manganese-55 projectiles:

209
83Bi
 + 55
25Mn
 → 264−x
108Hs
 + x
n
 (x = 1 or 2)

They were able to detect a spontaneous fission activity assigned to 255Rf, a product of the 263Hs decay chain. Identical results were measured in a repeat run in 1984.[19] In a subsequent experiment in 1983, they applied the method of chemical identification of a descendant to provide support to the synthesis of hassium. They were able to detect alpha decays from fermium isotopes, assigned as descendants of the decay of 262Hs. This reaction has not been tried since and 262Hs is currently unconfirmed.[19]

Hot fusion

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Under the leadership of Yuri Oganessian, the team at the Joint Institute for Nuclear Research studied the hot fusion reaction between calcium-48 projectiles and radium-226 targets in 1978:

226
88Ra
 + 48
20Ca
 → 270
108Hs
 + 4
n

However, results are not available in the literature.[19] The reaction was repeated at the JINR in June 2008 and 4 atoms of the isotope 270Hs were detected.[27] In January 2009, the team repeated the experiment and a further 2 atoms of 270Hs were detected.[28]

The team at Dubna studied the reaction between californium-249 targets and neon-22 projectiles in 1983 by detecting spontaneous fission activities:

249
98Cf
 + 22
10Ne
 → 271−x
108Hs
 + x
n

Several short spontaneous fission activities were found, indicating the formation of nuclei of hassium.[19]

The hot fusion reaction between uranium-238 targets and projectiles of the rare and expensive isotope sulfur-36 was conducted at the GSI in April–May 2008:

238
92U
 + 36
16S
 → 270
108Hs
 + 4
n

Preliminary results show that a single atom of 270Hs was detected. This experiment confirmed the decay properties of the isotopes 270Hs and 266Sg.[29]

In March 1994, the team at Dubna led by the late Yuri Lazarev attempted the analogous reaction with sulfur-34 projectiles:

238
92U
 + 34
16S
 → 272−x
108Hs
 + x
n
 (x = 4 or 5)

They announced the detection of 3 atoms of 267Hs from the 5n neutron evaporation channel.[30] The decay properties were confirmed by the team at GSI in their simultaneous study of darmstadtium. The reaction was repeated at the GSI in January–February 2009 in order to search for the new isotope 268Hs. The team, led by Prof. Nishio, detected a single atom each of both 268Hs and 267Hs. The new isotope 268Hs underwent alpha decay to the previously known isotope 264Sg.

Between May 2001 and August 2005, a GSI–PSI (Paul Scherrer Institute) collaboration studied the nuclear reaction between curium-248 targets and magnesium-26 projectiles:

248
96Cm
 + 26
12Mg
 → 274−x
108Hs
 + x
n
 (x = 3, 4, or 5)

The team studied the excitation function of the 3n, 4n, and 5n evaporation channels leading to the isotopes 269Hs, 270Hs, and 271Hs.[31][32] The synthesis of the doubly magic isotope 270Hs was published in December 2006 by the team of scientists from the Technical University of Munich.[33] It was reported that this isotope decayed by emission of an alpha particle with an energy of 8.83 MeV and a half-life of ~22 s. This figure has since been revised to 3.6 s.[34]

As decay product

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List of hassium isotopes observed by decay
Evaporation residue Observed hassium isotope
267Ds 263Hs[35]
269Ds 265Hs[36]
270Ds 266Hs[37]
271Ds 267Hs[38]
277Cn, 273Ds 269Hs[39]
276Ds 272Hs[10]
285Fl, 281Cn, 277Ds 273Hs[40]
291Lv, 287Fl, 283Cn, 279Ds 275Hs[41]
293Lv, 289Fl, 285Cn, 281Ds 277Hs[42][43][44]

Hassium isotopes have been observed as decay products of darmstadtium. Darmstadtium currently has ten known isotopes, all but one of which have been shown to undergo alpha decays to become hassium nuclei with mass numbers between 263 and 277. Hassium isotopes with mass numbers 266, 272, 273, 275, and 277 to date have only been produced by decay of darmstadtium nuclei. Parent darmstadtium nuclei can be themselves decay products of copernicium, flerovium, or livermorium.[34] For example, in 2004, the Dubna team identified hassium-277 as a final product in the decay of livermorium-293 via an alpha decay sequence:[44]

293
116Lv
 → 289
114Fl
 + 4
2He
289
114Fl
 → 285
112Cn
 + 4
2He
285
112Cn
 → 281
110Ds
 + 4
2He
281
110Ds
 → 277
108Hs
 + 4
2He

Unconfirmed isotopes

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277mHs

An isotope assigned to 277Hs has been observed on one occasion decaying by SF with a long half-life of ~11 minutes.[45] The isotope is not observed in the decay of the ground state of 281Ds but is observed in the decay from a rare, as yet unconfirmed isomeric level, namely 281mDs. The half-life is very long for the ground state and it is possible that it belongs to an isomeric level in 277Hs. It has also been suggested that this activity actually comes from 278Bh, formed as the great-great-granddaughter of 290Fl through one electron capture to 290Nh and three further alpha decays. Furthermore, in 2009, the team at the GSI observed a small alpha decay branch for 281Ds producing the nuclide 277Hs decaying by SF in a short lifetime. The measured half-life is close to the expected value for ground state isomer, 277Hs. Further research is required to confirm the production of the isomer.

Retracted isotopes

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273Hs

In 1999, American scientists at the University of California, Berkeley, announced that they had succeeded in synthesizing three atoms of 293118.[46] These parent nuclei were reported to have successively emitted three alpha particles to form hassium-273 nuclei, which were claimed to have undergone an alpha decay, emitting alpha particles with decay energies of 9.78 and 9.47 MeV and half-life 1.2 s, but their claim was retracted in 2001.[47] The isotope, however, was produced in 2010 by the same team. The new data matched the previous (fabricated)[48] data.[40]

270Hs: prospects for a deformed doubly magic nucleus

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According to macroscopic-microscopic (MM) theory, Z = 108 is a deformed proton magic number, in combination with the neutron shell at N = 162. This means that such nuclei are permanently deformed in their ground state but have high, narrow fission barriers to further deformation and hence relatively long SF partial half-lives. The SF half-lives in this region are typically reduced by a factor of 109 in comparison with those in the vicinity of the spherical doubly magic nucleus 298Fl, caused by an increase in the probability of barrier penetration by quantum tunnelling, due to the narrower fission barrier. In addition, N = 162 has been calculated as a deformed neutron magic number and hence the nucleus 270Hs has promise as a deformed doubly magic nucleus. Experimental data from the decay of Z = 110 isotopes 271Ds and 273Ds, provides strong evidence for the magic nature of the N = 162 sub-shell. The recent synthesis of 269Hs, 270Hs, and 271Hs also fully support the assignment of N = 162 as a magic closed shell. In particular, the low decay energy for 270Hs is in complete agreement with calculations.[49]

Evidence for the Z = 108 deformed proton shell

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Evidence for the magicity of the Z = 108 proton shell can be deemed from two sources:

  1. the variation in the partial spontaneous fission half-lives for isotones
  2. the large gap in Qα for isotonic pairs between Z = 108 and Z = 110.

For SF, it is necessary to measure the half-lives for the isotonic nuclei 268Sg, 270Hs and 272Ds. Since fission of 270Hs has not been measured, detailed data of 268Sg fission is not yet available,[10] and 272Ds is still unknown, this method cannot be used to date to confirm the stabilizing nature of the Z = 108 shell. However, good evidence for the magicity of Z = 108 can be deemed from the large differences in the alpha decay energies measured for 270Hs, 271Ds and 273Ds. More conclusive evidence would come from the determination of the decay energy of the yet-unknown nuclide 272Ds.

Nuclear isomerism

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277Hs

An isotope assigned to 277Hs has been observed on one occasion decaying by spontaneous fission with a long half-life of ~11 minutes.[50] The isotope is not observed in the decay of the most common isomer of 281Ds but is observed in the decay from a rare, as yet unconfirmed isomeric level, namely 281mDs. The half-life is very long for the ground state and it is possible that it belongs to an isomeric level in 277Hs. Furthermore, in 2009, the team at the GSI observed a small alpha decay branch for 281Ds producing an isotope of 277Hs decaying by spontaneous fission with a short lifetime. The measured half-life is close to the expected value for ground state isomer, 277Hs. Further research is required to confirm the production of the isomer.[42] A more recent study suggests that this observed activity may actually be from 278Bh.[51]

269Hs

The direct synthesis of 269Hs has resulted in the observation of three alpha particles with energies 9.21, 9.10, and 8.94 MeV emitted from 269Hs atoms. However, when this isotope is indirectly synthesized from the decay of 277Cn, only alpha particles with energy 9.21 MeV have been observed, indicating that this decay occurs from an isomeric level. Further research is required to confirm this.[31][39]

267Hs

267Hs is known to decay by alpha decay, emitting alpha particles with energies of 9.88, 9.83, and 9.75 MeV. It has a half-life of 52 ms. In the recent syntheses of 271Ds and 271mDs, additional activities have been observed. A 0.94 ms activity emitting alpha particles with energy 9.83 MeV has been observed in addition to longer lived ~0.8 s and ~6.0 s activities. Currently, none of these are assigned and confirmed and further research is required to positively identify them.[30]

265Hs

The synthesis of 265Hs has also provided evidence for two isomeric levels. The ground state decays by emission of an alpha particle with energy 10.30 MeV and has a half-life of 2.0 ms. The isomeric state has 300 keV of excess energy and decays by the emission of an alpha particle with energy 10.57 MeV and has a half-life of 0.75 ms.[20]

Future experiments

Scientists at the GSI are planning to search for isomers of 270Hs using the reaction 226Ra(48Ca,4n) in 2010 using the new TASCA facility at the GSI.[52] In addition, they also hope to study the spectroscopy of 269Hs, 265Sg and 261Rf, using the reaction 248Cm(26Mg,5n) or 226Ra(48Ca,5n). This will allow them to determine the level structure in 265Sg and 261Rf and attempt to give spin and parity assignments to the various proposed isomers.[53]

Physical production yields

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The tables below provides cross-sections and excitation energies for nuclear reactions that produce isotopes of hassium directly. Data in bold represent maxima derived from excitation function measurements. + represents an observed exit channel.

Cold fusion

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Projectile Target CN 1n 2n 3n
58Fe 208Pb 266Hs 69 pb, 13.9 MeV 4.5 pb
58Fe 207Pb 265Hs 3.2 pb

Hot fusion

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Projectile Target CN 3n 4n 5n
48Ca 226Ra 274Hs 9.0 pb
36S 238U 274Hs 0.8 pb
34S 238U 272Hs 2.5 pb, 50.0 MeV
26Mg 248Cm 274Hs 2.5 pb 3.0 pb 7.0 pb

Theoretical calculations

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Evaporation residue cross sections

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The below table contains various targets-projectile combinations for which calculations have provided estimates for cross section yields from various neutron evaporation channels. The channel with the highest expected yield is given.

DNS = Di-nuclear system ; σ = cross section

Target Projectile CN Channel (product) σ max Model Ref
136Xe 136Xe 272Hs 1–4n (271–268Hs) 10−6 pb DNS [54]
238U 34S 272Hs 4n (268Hs) 10 pb DNS [54]
238U 36S 274Hs 4n (270Hs) 42.97 pb DNS [55]
244Pu 30Si 274Hs 4n (270Hs) 185.1 pb DNS [55]
248Cm 26Mg 274Hs 4n (270Hs) 719.1 pb DNS [55]
250Cm 26Mg 276Hs 4n (272Hs) 185.2 pb DNS [55]

References

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Half-life, spin, and isomer data selected from:

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