Unbihexium, Ubh, is the temporary name for element 126. Isotopes are predicted within the bands Ubh 428 to Ubh 349 and Ubh 317 to Ubh 287. Three long-lived Ubh isotopes are predicted: Ubh 354 is predicted to have a half-life of approximately 10^9.5 sec (100 years), Ubh 353 is predicted to have a half life on the order of 10^06 sec (12 days), and Ubh 352 is predicted to have a half-life of roughly 10^7.5 sec (1 yr). Other half-lives are all less than 1 hr, and most are under 1 sec. Sixty eight isotopes within the bands Ubh 428 to Ubh 405 (even-N only) and Ubh 404 to Ubh 349 are predicted to form. Two isotopes, Ubh 356 and Ubh 355 may persist (to the parts per mole of moles level) for up to 10 hours after the event which led to their formation. Two more, Ubh 352 and Ubh 351, may persist for up to 160 years; and an additional two, Ubh 353 and its daughter Ubh 349, may last up to 6 years. Ubh 354 and its daughter Ubh 350 are the stars, though. They are expected to persist for 16000 years. All Ubh isotopes, predicted or guessed, will last less than 1000 sec after the event which led to their formation.

Ubh has the highest atomic number of any element which has its own Wikipedia article.

It has been suggested that synthesis of Ubh is a sign that the world is about to end(1).

NUCLEAR PROPERTIES

INFORMATION SOURCES

While studies addressing specific issues have been carried out to very high N(2). and to moderate Z(3), (Z,N) or (Z,A) maps predicting half-lives and decay modes are almost completely limited to the region below Z = 130 and N = 220. There appears to be only one such map which extends beyond that region and is accessible(4).

(Z,N) maps for half-life and decay mode in Ref. 3 extend as high as Z = 175 and N = 333. Half-lives are reported as bands 3 orders of magnitude wide (0.001 - 1 sec, for example), and should be considered accurate only to within +/- orders of magnitude (presumably from band center. (A nuclide reported to be in the 0.001 - 1 sec band should be considered to have a possible half-life between 10^-4.5 sec and 10^1.5 sec.) Decay modes are limited alpha emission, beta emission, proton emission, and fission; and to the principal one for each nuclide. There are areas where two modes (or more) may be important, meaning that small uncertainties is model parameters could have produced different results. It is also possible that cluster decay may become important above the neutron shell closures at N = 228 and 308.

Ref. 4 does have two significant weakness in the way data are presented. Nuclides which are beta-stable are identified by black squares, overwriting decay mode and half-life information. In addition, nuclides having half-lives less than 10^-09 sec are not reported, which obscures the distinction between nuclides having half-lives in the 10^-09 and 10^-14 sec band and nuclear drops whose half-life is under 10^-14 sec.

An independent map of half-lives and decay modes is provided in Wikipedia(5). The source referenced, however, did not present maps extending beyond N = 190. Wikipedia's half-life chart did extend to Z = 127 and N = 230, but the scale used lumps half-lives in the range 1 sec to 86400 sec (1 day) into a single category, which blurs an important range in values. It also does not distinguish half-life ranges below 10^-06 sec and gives no indication of certainty. Its decay modes chart goes to Z = 132, but cuts off at N = 204. Cutoffs for individual elements are also different between half-life and decay mode charts. Ref. 5 is decidedly a weaker source than Ref. 4.

Wikipedia contains an article "Unbihexium", whose Section 3.1 addresses predicted properties of Ubh isotopes. That section discusses possible properties of isotopes lighter than Ubh 340 at some length. This article will not elaborate on those. One cited document does address possible isotopes up to Ubh 386. That document, however, stops short of actual predictions of either half-life or decay mode, making it insufficient to use as an additional resource for this article.

PREDICTED PROPERTIES

Even-N isotopes in the band Ubh 428 - Ubh 405 generally decay predominantly by beta emission, although some of the heavier isotopes are fission-dominant. It appears to be possible for structure to destabilize a nuclide(6), so fission-dominant isotopes appear to be realistic; however, beta decay is likely to be significant for those isotopes. Half-lives are masked by other features of the map, but the properties of beta decay indicate that half-lives close to 0.001 sec are likely(7). Odd-N drops in this band decay by neutron emission.

Isotopes in the band Ubh 404 - Ubh 395 are predicted to decay by beta emission. Half-lives are predicted to lie between 0.001 and 1 sec.

Between Ubh 394 and Ubh 382, all but one isotope are predicted to have half-lives in the 0.001 - 1 sec range; and that isotope, Ubh 386, can be expected to have a half-life close to 0.001 sec, in the context of other nuclides in its vicinity. Even-N isotopes slightly favor decay by fission (4 of 7) and odd-N isotopes favor decay by beta emission (4 of 6). This balance indicates that both decay modes are probably important for all isotopes in the band. Decay modes follow the trend of nuclides having specific neutron counts to decay the same way across a range of Z values in the vicinity of 126.

Between Ubh 381 and Ubh 366, most isotopes are predicted to be short-lived and fission decaying. The exceptions are Ubh 378 and Ubh 375, which are predicted to decay by beta emission with half-lives in the 0.001 - 1 sec range. Both of these fit a trend of nuclides having a specific neutron count to decay the same way for a range of Z values in the vicinity of 126. Odd-N isotopes which decay by fission in this band are all predicted to have half-lives in the 10^-09 - 10^-06 sec range, while even-N isotopes have half-lives < 10^-09 sec and are presumed to decay by fission.

Ubh 365 and Ubh 364 are predicted to decay by fission. Ubh 365 is predicted to have a half-life in the 10^-06 - 0.001 sec range and Ubh 364 is predicted to have a half-life in the 0.001 - 1 sec range, which is to be expected.

Isotopes in the Ubh 363 to Ubh 359 are predicted to decay by beta emission with half-lives in the 0.001 - 1 sec range.

Isotopes in the Ubh 358 to Ubh 355 range are predicted to decay by beta emission. Their reported half-lives are in the 1 - 1000 sec range, but a closer look (see p. 12 of Ref. 4) indicates that half-lives of only a few seconds are expected in reality, with Ubh 355 the maximum at around 100 sec.

Ubh 354 is predicted (p. 12 of Ref 4) to decay by alpha emission with a half-life on the order of 10^9.5 sec (100 yr). Predictions are often optimistic, and it has been suggested that the N = 228 shell closure may not be as strong as Ref. 4 expected; but Ubh 354 is likely to be very long-lived for a nuclide this large.

Ubh 353 is predicted to decay by beta emission, but also to be long lived, with a half-life on the order of 10^06 sec (p. 12 of Ref 4), which is plausible given that it is located within the zone of complete beta stability.

Ubh 352 is predicted to decay by alpha emission and to be long lived, with a half-life on the order of 10^7.5 sec (1 yr).

Decay by beta emission is also expected at Ubh 351, although a half-life on the order of 1 sec is likely.

Decay by fission with half-lives in the 10^-06 - 0.001 sec range is predicted for Ubh 350 and Ubh 349. This abrupt destabilization below N = 228, is unexpected and opposite to what is predicted for neutron shell closures at N = 184 and 308.

Between Ubh 348 and Ubh 318, Ref 4 shows a gap, which may contain short-lived isotopes or nuclear drops which decay in less than 10^-14 sec. This appears to be the instability band expected above N = 184.

Ref. 5 makes no predictions above Ubh 339.

Ref, 5 reports fission-decaying isotopes with half-lives below 10^-06 sec in the band Ubh 339 - Ubh 330, which is more or less consistent with Ref 4. But, it also shows a band of alpha-decaying isotopes ranging from Ubh 326 to Ubh 321. Of these, it predicts that Ubh 325 and Ubh 323 will have half-lives between 1 sec and 1 day, while the others have half-lives in the range 0.001 - 1 sec. Ref. 4 gives no indication of such a region. In addition, it predicts that Ubh 319 will have a half-life in the 10^-06 - 0.001 sec range, although it decays by fission. This is also different from what Ref. 4 predicts.

Ref. 4 predicts that isotopes in the band Ubh 317 to Ubh 314, no isotopes will have half-lives in excess of 10^-06 sec. This agrees (more or less) with Ref. 5's predicted half-lives. Ref. 4 does predict that Ubh 315 will decay by alpha emission, while Ref. 5 indicates decay by fission.

Ref. 5 makes not predictions below Ubh 314.

Ref 4 predicts that isotopes Ubh 313 and Ubh 312 will have half-lives in the 0.001 - 1 sec range and that both will decay by fission. These isotopes are part of a small, relatively stable zone predicted in the vicinity of Ubo. It is not clear whether or not this zone is an artifact of the model.

Isotopes in the Ubh 311 to Ubh 295 band are all predicted (Ref 4) to have 10^-09 - 10^-06 range and to decay by alpha emission.

Isotopes in the band Ubh 294 to Ubh 292 are predicted to have half-lives in the 10^-06 - 0.001 sec range, which seems questionably long. Ubh 393 is predicted to decay by proton emission and the others to decay by fission.

Isotopes in the band Ubh 291 to Ubh 289, and the isotope Ubh 287 are predicted to decay by alpha emission with half-lives in the range 10^-09 - 10^-06 sec.

OCCURRENCE

FORMATION

Nuclides close to the neutron dripline may form directly from disintegrating neutron star material, and the remainder may form via beta decay chains from lower-Z nuclides, particularly since beta+neutrons decay is quite likely in this region. Since some of these chains, though, may be terminated by short-lived, fission-decaying nuclides,

All even-N nuclear drops in the band Ubh 428 to Ubh 405 are predicted to be nuclides. All nuclear drops in the band Ubh 404 to Ubh 381, Ubh 378, odd-N drops between Ubh 379 and Ubh 367, all drops in the band Ubh 365 to Ubh 349, and nearly all drops in the band Ubh 317 to Ubh 287 are predicted by Ref. 4 to be nuclides. To them, Ref 5 would add drops in the band Ubh 326 to Ubh 321, Ubh 319, and drops in the band Ubh 317 to Ubh 314. Heavy isotopes near the neutron dripline may form directly, but most can only form via beta decay chains. It is possible to simulate the formation of nuclides via decay chains using data from Ref. 4 and assuming an initial distribution close to the neutron dripline. Details of the model are provided in "Nuclear Decay Chains at High A" in this wiki. Per that model, 68 predicted isotopes; even-N isotopes between Ubh 428 and Ubh 405. and all isotopes between Ubh 404 and Ubh 349; can form. These include the long-lived isotopes Ubh 354, Ubh 353, and Ubh 351. In addition, the short live isotopes Ubh 351, Ubh 350, and Ubh 349 are part of the decay chains of Ubs 355, Ubh 354, and Ubh 353 respectively, so they continue to form for a long time. Note that none of the isotopes predicted in Ref. 5 can form.

It is implausible that neutron capture can form any Ubh isotope.

PERSISTENCE

Two isotopes, Ubh 356 and Ubh 355 are expected to persist (to the parts per mole of moles level) for approximately an hour after the neutron star merger or other similar event which led to their formation. Ubh 354 and its daughter Ubh 350 predicted to persist for approximately 16000 years. Ubh 353 is predicted to survive for approximately 5 years, and its daughter Ubh 349 for around 5.5 years. Ubh 352 and Ubh 351 are both predicted to survive for approximately 160 years. All other isotopes of Ubh are expected to vanish within 1000 sec.

ATOMIC PROPERTIES

Ubh is expected to be an 8th period active metal (superactinide). Its electron configuration is generally predicted to be [Og] 5g2 6f3 8s2 8p11/2.

REFERENCES

1, "Saturday Morning Breakfast Cereal" (SMBC), 2011-06-11; Z. Weinersmith.

2. for example, "Nuclear Energy Density Functionals: What Do We Really Know?"; Aurel Bulgac, Michael McNeil Forbes, and Shi Jin; Researchgate publication 279633220 or arXiv: 1506.09195v1 [nucl-th] 30 Jun 2015.

3. for example "Fission Mechanism of Exotic Nuclei"; Research Group for Heavy Element Nuclear Science; http://asrc.jaea.go.jp/soshiki/gr/HENS-gr/np/research/pageFission_e.html.; 17 Sept 17.

4. "Decay Modes and a Limit of Existence of Nuclei"; H. Koura; 4th Int. Conf. on the Chemistry and Physics of Transactinide Elements; Sept. 2011.

5. "Extended Periodic Table", Section 4.3. The figures presented in that section cited "Superheavy Elements: Which Regions of the Nuclear Map are Accessible in the Nearest Studies"; Alexander Karpov, Valery Zagrebaev, and Walter Grenier; SHE-2015 conference at Texas A&M University; 04-01-2015 as their source.

6. "Magic Numbers of Ultraheavy Nuclei"; Vitali Denisov; Physics of Atomic Nuclei; researchgate.net/publications/225734594; July 2005.

7. "Nuclear Properties for Astrophysical Applications"; P. Moller & J. R. Nix; Los Alamos National Laboratory website; search by "LANL, T2", then "Nuclear Properties for Astrophysical Applications".

8. Other references are found in the wiki articles cited.

(08-29-20)

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