Isotopes of hassium

Hassium (Hs) is an artificial element, and thus a standard atomic mass cannot be given. Like all artificial elements, it has no stable isotopes. The first isotope to be synthesized was ²⁶⁵Hs in 1984. There are 12 known isotopes from ²⁶³Hs to ²⁷⁷Hs and 1-4 isomers. The longest-lived isotope is ²⁶⁹Hs with a half-life of 9.6 seconds.

Table

nuclide symbol	Z(p)	N(n)	isotopic mass (u)	half-life	decay mode(s)	daughter isotope(s)	nuclear spin
	excitation energy
263Hs	108	155	263.12856(37)#	1# ms	α	259Sg	7/2+#
264Hs	108	156	264.12839(5)	540(300) µs	α (50%)	260Sg	0+
					SF (50%)	(various)
265Hs	108	157	265.13009(15)#	2.1(3) ms [2.0(+3-2) ms]	α	261Sg	9/2+#
265mHs	300(70) keV			0.780(0.15) ms [0.75(+17-12) ms]	α	261Sg	3/2+#
266Hs	108	158	266.13010(31)#	2.7(10) ms [2.3(+13-6) ms]	α	262Sg	0+
267Hs	108	159	267.13179(11)#	52(+13-8) s ms	α	263Sg	3/2+#
267mHs					α	263Sg
268Hs	108	160	268.13216(44)#	2# s	α	264Sg	0+
269Hs	108	161	269.13406(13)#	9.7(+99-33) s	α	265Sg
269mHs					α	265Sg
270Hs	108	162	270.13465(31)#	3.6(+8-14) s	α	266Sg	0+
271Hs	108	163	271.13766(36)#	40# s	α	267Sg
273Hs	108	165	273.14199(89)#	240 ms	α	269Sg	(3/2+)#
275Hs	108	167	275.14595(77)#	0.15(+27-6) s	α	271Sg
277Hs	108	169	277.14984(78)#		SF	(various)	3/2+#
277mHs				40(30) min	SF	(various)

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.

Isotopes and nuclear properties

Nucleosynthesis

Cold fusion

This section deals with the synthesis of nuclei of hassium 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.

¹³⁶Xe(¹³⁶Xe,xn)^272−xHs

Important future experiments will involve the attempted synthesis of hassium isotopes in this symmetric reaction using the fission fragments. This reaction was carried out at Dubna in 2007 but no atoms were detected, leading to a cross section limit of 1 pb. If confirmed, this would indicate that such symmetric fusion reactions should be modelled as 'hot fusion' reactions rather than 'cold fusion' ones, as first suggested. This would indicate that such reactions will unfortunately have limited use in the synthesis of superheavy elements.

¹⁹⁸Pt(⁷⁰Zn,xn)^268−xHs

This reaction was performed in May 2002 at the GSI. Unfortunately, the experiment was cut short due to a failure of the zinc-70 beam.

²⁰⁸Pb(⁵⁸Fe,xn)^266−xHs (x=1,2)

This reaction was first reported in 1978 by the team at Dubna. In a later experiment in 1984, using the rotating drum technique, they were able to detect a spontaneous fission activity assigned to ²⁶⁰Sg, daughter of ²⁶⁴Hs. In a repeat experiment in the same year, they applied the method of chemical identification of a descendant to provide support to the synthesis of element 108. They were able to detect several alpha decays of ²⁵³Es and ²⁵³Fm, descendants of ²⁶⁵Hs.

In the official discovery of the element in 1984, the team at GSI studied the reaction using the alpha decay genetic correlation method. They were able to positively identify 3 atoms of ²⁶⁵Hs. 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. The maximum of the 1n channel was measured as 69 pb in a further run in late 1997 in which a further 20 atoms were detected.

The 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 first spectroscopic studies of the even-even nucleus ²⁶⁴Hs. They were also able to detect a further 29 atoms of ²⁶⁵Hs.

²⁰⁷Pb(⁵⁸Fe,xn)^265−xHs (x=1)

The use of a Pb-207 target was first used in 1984 at Dubna. They were able to detect the same SF activity as observed in the Pb-208 run and once again assigned it to²⁶⁰Sg, daughter of ²⁶⁴Hs. The team at GSI first studied the reaction in 1986 using the method of correlation of genetic alpha decays and identified a single atom of ²⁶⁴Hs with a cross section of 3.2 pb. The reaction was repeated in 1994 and the team were able to measure both alpha decay and spontaneous fission for ²⁶⁴Hs.

This reaction was studied in 2008 at RIKEN in order to conduct first spectrscopic studies of the even-even nucleus ²⁶⁴Hs. The team detected 11 atoms of the isotope.

²⁰⁸Pb(⁵⁶Fe,xn)^264−xHs (x=1)

This reaction was studied for the first time in 2008 by the team at LBNL. They were able to produce and identify 6 atoms of the new isotope²⁶³Hs. A few months later, the RIKEN team also published their results on the same reaction.

²⁰⁶Pb(⁵⁸Fe,xn)^264−xHs (x=1)

This reaction was studied for the first time in 2008 by the team at RIKEN. They were able to identify 8 atoms of the new isotope²⁶³Hs.

²⁰⁹Bi(⁵⁵Mn,xn)^264−xHs

First attempts to synthesise nuclei of hassium were performed using this reaction by the team at Dubna in 1983. Using the rotating drum technique, they were able to detect a spontaneous fission activity assigned to ²⁵⁵Rf, descendant of the ²⁶³Hs decay chain. Identical results were measured in a repeat run in 1984. 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.

Hot fusion

This section deals with the synthesis of nuclei of hassium 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.

²²⁶Ra(⁴⁸Ca,xn)^274−xHs (x=4)

This reaction was reportedly first studied in 1978 by the team at the Flerov Laboratory of Nuclear Reactions (FLNR) under the leadership of Yuri Oganessian. However, results are not available in the literature. The reaction was repeated at the FLNR in June 2008 and results show that the 4 atoms of the isotope ²⁷⁰Hs were detected with a yield of 9 pb. The decay data for the recently discovered isotope was confirmed, although the alpha energy was slightly higher. In Jan 2009, the team repeated the experiment and a further 2 atoms of ²⁷⁰Hs were detected.

²³²Th(⁴⁰Ar,xn)^272−xHs

This reaction was first studied at Dubna in 1987. Detection was by spontaneous fission and no activities were found leading to a calculated cross section limit of 2 pb.

²³⁸U(³⁶S,xn)^274−xHs (x=4)

This reaction with the rare and expensive ³⁶S isotope was conducted at the GSI in April–May 2008. Preliminary results show that a single atom of²⁷⁰Hs was detected with a yield of 0.8 pb. The data confirms the decay properties of ²⁷⁰Hs and²⁶⁶Sg.

²³⁸U(³⁴S,xn)^272−xHs (x=4,5)

In March 1994, the team at Dubna led by the late Yuri Lazerev announced the detection of 3 atoms of ²⁶⁷Hs from the 5n neutron evaporation channel. The decay properties were confirmed by the team at GSI in their simultaneous study of darmstadtium.

The reaction was repeated at the GSI in Jan-Feb 2009 in order to search for the new isotope ²⁶⁸Hs. The team, led by Prof. Nishio, detected a single atom of both ²⁶⁸Hs and ²⁶⁷Hs. The new isotope underwent alpha-decay to the previously known isotope ²⁶⁴Sg.

²⁴⁸Cm(²⁶Mg,xn)^274−xHs (x=3,4,5)

Most recently, a GSI-PSI collaboration has studied the nuclear reaction of curium-248 with magnesium-26 ions. Between May 2001 and August 2005, the team has studied the excitation function of the 3n, 4n, and 5n evaporation channels leading to ²⁶⁹Hs, ²⁷⁰Hs, and²⁷¹Hs. The synthesis of the important isotope²⁷⁰Hs was published in December 2006 by the team of scientists from the Technical University of Munich. It was reported that this isotope decayed by emission of an alpha-particle with an energy of 8.83 MeV and a projected half-life of ~22 s, assuming a 0⁺ to 0⁺ ground state decay to ²⁶⁶Sg using the Viola-Seaborg equation.

²⁴⁸Cm(²⁵Mg,xn)^273−xHs

This new reaction was studied at the GSI in July–August 2006 in a search for the new isotope ²⁶⁸Hs. They were unable to detect any atoms from neutron evaporation and calculated a cross section limit of 1 pb.

²⁴⁹Cf(²²Ne,xn)^271−xHs

The team at Dubna studied this reaction in 1983 using detection by spontaneous fission (SF). Several short SF activities were found indicating the formation of nuclei of hassium.

Chronology of isotope discovery

Isotope	Year discovered	Discovery reaction
263Hs	2008	208Pb(56Fe,n)
264Hs	1986	207Pb(58Fe,n)
265Hs	1984	208Pb(58Fe,n)
266Hs	2000	207Pb(64Ni,n)
267Hs	1995	238U(34S,5n)
268Hs	2009	238U(34S,4n)
269Hs	1996	208Pb(70Zn,n)
270Hs	2004	248Cm(26Mg,4n)
271Hs	2004	248Cm(26Mg,3n)[citation needed]
272Hs	unknown
273Hs	2010	242Pu(48Ca,5n)
274Hs	unknown
275Hs	2003	242Pu(48Ca,3n)
276Hs	unknown
277aHs	2009	244Pu(48Ca,3n)
277bHs?	1999	244Pu(48Ca,3n)

Unconfirmed isotopes

^277bHs

An isotope assigned to ²⁷⁷Hs has been observed on one occasion decaying by SF with a long half-life of ~11 minutes. The isotope is not observed in the decay of the most common isotope of²⁸¹Ds but is observed in the decay from a rare, as yet unconfirmed isomeric level, namely ^281bDs . 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^281aDs producing an isotope of ²⁷⁷Hs decaying by SF in a short lifetime. The measured half-life is close to the expected value for ground state isomer, ^277aHs. Further research is required to confirm the production of the isomer.

Retracted isotopes

²⁷³Hs

The claimed synthesis of element 118 by LBNL in 1999 involved the intermediate ²⁷³Hs. This isotope was claimed to decay by 9.78 and 9.47 MeV alpha emission with a half-life of 1.2 s. The claim to discovery of ²⁹³118 was retracted in 2001. This isotope was finally created in 2010 and the data confirmed the fabrication of previous data.

²⁷⁰Hs: prospects for a deformed doubly magic nucleus

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 nucleus²⁹⁸114, 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.

Evidence for the Z=108 deformed proton shell

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 Q_α 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 the seaborgium anddarmstadtium isotopes are not known at this time, and fission of ²⁷⁰Hs has not been measured, this method can be used to date to confirm the stabilizing nature of the Z=108 shell. However, good evidence for the magicity of the 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 for the nucleus ²⁷²Ds.

Nuclear isomerism

²⁶⁹Hs

The direct synthesis of ²⁶⁹Hs has resulted in three alpha lines at 9.21, 9.10, and 8.94 MeV. In the decay of ²⁷⁷112, only 9.21 MeV²⁶⁹Hs alpha decays have been observed indicating that this decay occurs from an isomeric level. Further research is required to confirm this.

²⁶⁷Hs

The decay of ²⁶⁷Hs is known to occur by alpha decay with three alpha lines at 9.88, 9.83, and 9.75 MeV and a half-life of 52 ms. In the recent syntheses of^271m,gDs additional activities have been observed. A .94ms activity decaying by 9.83 MeV alpha emission has been observed in addition to longer lived ~.8 s and ~6.0 s activities. Each of these is currently not assigned and confirmed and further research is required to positively identify them.

²⁶⁵Hs

The synthesis of ²⁶⁵Hs has also provided evidence for two levels. The ground state decays by 10.30 MeV alpha emission with a half-life of 2.0 ms. The isomeric state is placed at 300 keV above the ground state and decays by 10.57 MeV alpha emission with a half-life of .75 ms.

Physical production yields

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

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

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

Evaporation residue cross sections

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
238U	34S	272Hs	4n (268Hs)	10 pb	DNS

References

• National Nuclear Data Center, Brookhaven National Laboratory
• Isotope masses from:
  • G. Audi, A. H. Wapstra, C. Thibault, J. Blachot and O. Bersillon (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. http://www.nndc.bnl.gov/amdc/nubase/Nubase2003.pdf. 
• 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. http://www.iupac.org/publications/pac/75/6/0683/pdf/. 
  • 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. http://iupac.org/publications/pac/78/11/2051/pdf/. Lay summary. 
• 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". Nuclear Physics A 729: 3–128. Bibcode 2003NuPhA.729....3A. doi:10.1016/j.nuclphysa.2003.11.001. http://www.nndc.bnl.gov/amdc/nubase/Nubase2003.pdf. 
  • National Nuclear Data Center. "NuDat 2.1 database". Brookhaven National Laboratory. http://www.nndc.bnl.gov/nudat2/. Retrieved September 2005. 
  • N. E. Holden (2004). "Table of the Isotopes". In D. R. Lide. CRC Handbook of Chemistry and Physics (85th ed.). CRC Press. Section 11. ISBN 978-0849304859. 

Isotopes of bohrium	Isotopes of hassium	Isotopes of meitnerium
Index to isotope pages · Table of nuclides