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 –

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List of isotopes

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: view

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. 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. Not directly synthesized, occurs in decay chain of ²⁸⁹Fl

Isotopes and nuclear properties

Target-projectile combinations leading to Z=108 compound nuclei

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

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

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

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

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

^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

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

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

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 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

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

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	[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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