Hassium (₁₀₈Hs) is asynthetic element, and thus astandard atomic weight cannot be given. Like all synthetic elements, it has nostable isotopes. The firstisotope to be synthesized was²⁶⁵Hs in 1984. There are 13 known isotopes from²⁶³Hs to²⁷⁷Hs and up to sixisomers. The most stable known isotope is²⁷¹Hs, 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 isotopes²⁶⁹Hs and²⁷⁰Hs respectively have half-lives of about 13 seconds and 7.6 seconds. It is also possible that the isomer^277mHs is more stable than these, but only one event of the decay of this isomer has been registered as of 2020.^[1][2]

Superheavy elements such as hassium are produced by bombarding lighter elements inparticle accelerators that inducefusion 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 higheratomic numbers.^[10]

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.^[11] 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 theground state, they require emission of only one or two neutrons, and thus, allows for the generation of more neutron-rich products.^[10] The latter is a distinct concept from that of where nuclear fusion claimed to be achieved at room temperature conditions (seecold fusion).^[12]

Before the first successful synthesis of hassium in 1984 by the GSI team, scientists at theJoint Institute for Nuclear Research (JINR) inDubna,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 aspontaneous fission activity assigned to²⁶⁰Sg, thedaughter of²⁶⁴Hs.^[13] Later that year, they tried the experiment again, and tried to chemically identify thedecay products of hassium to provide support to their synthesis of element 108. They were able to detect severalalpha decays of²⁵³Es and²⁵³Fm, decay products of²⁶⁵Hs.^[14]

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 of²⁶⁵Hs.^[15] After an upgrade of their facilities in 1993, the team repeated the experiment in 1994 and detected 75 atoms of²⁶⁵Hs and 2 atoms of²⁶⁴Hs, during the measurement of a partial excitation function for the 1n neutron evaporation channel.^[16] A further run of the reaction was conducted in late 1997 in which a further 20 atoms were detected.^[17] This discovery experiment was successfully repeated in 2002 atRIKEN (10 atoms) and in 2003 atGANIL (7 atoms). The team at RIKEN further studied the reaction in 2008 in order to conduct the first spectroscopic studies of theeven-even nucleus²⁶⁴Hs. They were also able to detect a further 29 atoms of²⁶⁵Hs.

The team at Dubna also conducted the analogous reaction with alead-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 to²⁶⁰Sg, daughter of²⁶⁴Hs.^[14] The team atGSI first studied the reaction in 1986 using the method of genetic correlation of alpha decays and identified a single atom of²⁶⁴Hs with a cross section of 3.2 pb.^[18] The reaction was repeated in 1994 and the team were able to measure bothalpha decay andspontaneous fission for²⁶⁴Hs. This reaction was also studied in 2008 at RIKEN in order to conduct the first spectroscopic studies of theeven-even nucleus²⁶⁴Hs. The team detected 11 atoms of²⁶⁴Hs.

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

²⁰⁶
₈₂Pb +⁵⁸
₂₆Fe →²⁶³
₁₀₈Hs +n

They were able to identify 8 atoms of the new isotope²⁶³Hs.^[19]

In 2008, the team at theLawrence Berkeley National Laboratory (LBNL) studied the analogous reaction withiron-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 isotope²⁶³Hs.^[20] A few months later, the RIKEN team also published their results on the same reaction.^[21]

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

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

They were able to detect a spontaneous fission activity assigned to²⁵⁵Rf, a product of the²⁶³Hs decay chain. Identical results were measured in a repeat run in 1984.^[14] 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 fromfermium isotopes, assigned as descendants of the decay of²⁶²Hs. This reaction has not been tried since and²⁶²Hs is currently unconfirmed.^[14]

Under the leadership ofYuri Oganessian, the team at the Joint Institute for Nuclear Research studied the hot fusion reaction betweencalcium-48 projectiles andradium-226 targets in 1978:

²²⁶
₈₈Ra +⁴⁸
₂₀Ca →²⁷⁰
₁₀₈Hs + 4n

However, results are not available in the literature.^[14] The reaction was repeated at the JINR in June 2008 and 4 atoms of the isotope²⁷⁰Hs were detected.^[22] In January 2009, the team repeated the experiment and a further 2 atoms of²⁷⁰Hs were detected.^[23]

The team at Dubna studied the reaction betweencalifornium-249 targets andneon-22 projectiles in 1983 by detectingspontaneous fission activities:

²⁴⁹
₉₈Cf +²²
₁₀Ne →^271−x
₁₀₈Hs + xn

Several short spontaneous fission activities were found, indicating the formation of nuclei of hassium.^[14]

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

²³⁸
₉₂U +³⁶
₁₆S →²⁷⁰
₁₀₈Hs + 4n

Preliminary results show that a single atom of²⁷⁰Hs was detected. This experiment confirmed the decay properties of the isotopes²⁷⁰Hs and²⁶⁶Sg.^[24]

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

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

They announced the detection of 3 atoms of²⁶⁷Hs from the 5n neutron evaporation channel.^[25] The decay properties were confirmed by the team at GSI in their simultaneous study ofdarmstadtium. The reaction was repeated at the GSI in January–February 2009 in order to search for the new isotope²⁶⁸Hs. The team, led by Prof. Nishio, detected a single atom each of both²⁶⁸Hs and²⁶⁷Hs. The new isotope²⁶⁸Hs underwent alpha decay to the previously known isotope²⁶⁴Sg.

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

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

The team studied the excitation function of the 3n, 4n, and 5n evaporation channels leading to the isotopes²⁶⁹Hs,²⁷⁰Hs, and²⁷¹Hs.^[26][27] The synthesis of thedoubly magic isotope²⁷⁰Hs was published in December 2006 by the team of scientists from theTechnical University of Munich.^[28] 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.^[29]

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 withmass 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 ofcopernicium,flerovium, orlivermorium.^[29] 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:^[39]

²⁹³
₁₁₆Lv →²⁸⁹
₁₁₄Fl +⁴
₂He

²⁸⁹
₁₁₄Fl →²⁸⁵
₁₁₂Cn +⁴
₂He

²⁸⁵
₁₁₂Cn →²⁸¹
₁₁₀Ds +⁴
₂He

²⁸¹
₁₁₀Ds →²⁷⁷
₁₀₈Hs +⁴
₂He

^277mHs

An isotope assigned to²⁷⁷Hs has been observed on one occasion decaying by SF with a long half-life of ~11 minutes.^[40] The isotope is not observed in the decay of the ground state of²⁸¹Ds 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 in²⁷⁷Hs. It has also been suggested that this activity actually comes from²⁷⁸Bh, formed as the great-great-granddaughter of²⁹⁰Fl through one electron capture to²⁹⁰Nh and three further alpha decays. Furthermore, in 2009, the team at the GSI observed a small alpha decay branch for²⁸¹Ds producing the nuclide²⁷⁷Hs 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.

²⁷³Hs

In 1999, American scientists at the University of California, Berkeley, announced that they had succeeded in synthesizing three atoms of²⁹³118.^[41] 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.^[42] The isotope, however, was produced in 2010 by the same team. The new data matched the previous (fabricated)^[43] data.^[35]

According to macroscopic-microscopic (MM) theory,Z = 108 is a deformed proton magic number, in combination with the neutron shell atN = 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 nucleus²⁹⁸Fl, 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 nucleus²⁷⁰Hs has promise as a deformed doubly magic nucleus. Experimental data from the decay ofZ = 110 isotopes²⁷¹Ds and²⁷³Ds, provides strong evidence for the magic nature of theN = 162 sub-shell. The recent synthesis of²⁶⁹Hs,²⁷⁰Hs, and²⁷¹Hs also fully support the assignment ofN = 162 as a magic closed shell. In particular, the low decay energy for²⁷⁰Hs is in complete agreement with calculations.^[44]

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

1. the variation in the partialspontaneous fission half-lives for isotones
2. the large gap in Q_α forisotonic pairs betweenZ = 108 andZ = 110.

For SF, it is necessary to measure the half-lives for the isotonic nuclei²⁶⁸Sg,²⁷⁰Hs and²⁷²Ds. Since fission of²⁷⁰Hs has not been measured, detailed data of²⁶⁸Sg fission is not yet available,^[5] and²⁷²Ds is still unknown, this method cannot be used to date to confirm the stabilizing nature of theZ = 108 shell.However, good evidence for the magicity ofZ = 108 can be deemed from the large differences in the alpha decay energies measured for²⁷⁰Hs,²⁷¹Ds and²⁷³Ds. More conclusive evidence would come from the determination of the decay energy of the yet-unknown nuclide²⁷²Ds.

²⁷⁷Hs

An isotope assigned to²⁷⁷Hs has been observed on one occasion decaying by spontaneous fission with a long half-life of ~11 minutes.^[45] The isotope is not observed in the decay of the most commonisomer of²⁸¹Ds 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 in²⁷⁷Hs. Furthermore, in 2009, the team at the GSI observed a small alpha decay branch for²⁸¹Ds producing an isotope of²⁷⁷Hs 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.^[37] A more recent study suggests that this observed activity may actually be from²⁷⁸Bh.^[46]

²⁶⁹Hs

The direct synthesis of²⁶⁹Hs has resulted in the observation of three alpha particles with energies 9.21, 9.10, and 8.94 MeV emitted from²⁶⁹Hs atoms. However, when this isotope is indirectly synthesized from the decay of²⁷⁷Cn, 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.^[26][34]

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 of²⁷¹Ds 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.^[25]

²⁶⁵Hs

The synthesis of²⁶⁵Hs 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.^[15]

Future experiments

Scientists at the GSI are planning to search for isomers of²⁷⁰Hs using the reaction²²⁶Ra(⁴⁸Ca,4n) in 2010 using the new TASCA facility at the GSI.^[47] In addition, they also hope to study the spectroscopy of²⁶⁹Hs,²⁶⁵Sg and²⁶¹Rf, using the reaction²⁴⁸Cm(²⁶Mg,5n) or²²⁶Ra(⁴⁸Ca,5n). This will allow them to determine the level structure in²⁶⁵Sg and²⁶¹Rf and attempt to give spin and parity assignments to the various proposed isomers.^[48]

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

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

• National Nuclear Data Center, Brookhaven National LaboratoryArchived 2016-01-12 at theWayback Machine

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

• Audi, G.; Kondev, F. G.; Wang, M.; et al. (2017)."The NUBASE2016 evaluation of nuclear properties"(PDF).Chinese Physics C.41 (3) 030001.Bibcode:2017ChPhC..41c0001A.doi:10.1088/1674-1137/41/3/030001.S2CID 126750783. Archived fromthe original(PDF) on 2020-08-01.
• Audi, G.; Kondev, F. G.; Wang, M.; Pfeiffer, B.; Sun, X.; Blachot, J.; MacCormick, M. (2012)."The NUBASE2012 evaluation of nuclear properties"(PDF).Chinese Physics C.36 (12): 1157–1286.Bibcode:2012ChPhC..36....1A.doi:10.1088/1674-1137/36/12/001. Archived fromthe original(PDF) on 2014-02-22.
• Audi, Georges; Bersillon, Olivier; Blachot, Jean;Wapstra, Aaldert Hendrik (2003),"The NUBASE evaluation of nuclear and decay properties",Nuclear Physics A,729:3–128,Bibcode:2003NuPhA.729....3A,doi:10.1016/j.nuclphysa.2003.11.001
• National Nuclear Data Center."NuDat 3.0 database".Brookhaven National Laboratory.
• Holden, Norman E. (2004). "11. Table of the Isotopes". In Lide, David R. (ed.).CRC Handbook of Chemistry and Physics (85th ed.).Boca Raton, Florida:CRC Press.ISBN 978-0-8493-0485-9.
• GSI (2011)."Superheavy Element Research at GSI"(PDF). GSI. Retrieved20 August 2012.

• Isotopes of hassium
• Hassium
• Lists of isotopes by element

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