Unhexunium

Unhexunium, Uhu, is the temporary name for element 161. It is expected to be a transient element, one without long-lived isotopes or long-lived precoursers. Uhu may form during neutron star mergers.

NUCLEAR PROPERTIES

At least one model exists predicting the half-lives and decay modes for nuclides up to Z = 175 and N = 333⁽¹⁾, which includes isotopes Uhu 494 and lighter.  It is helpful to view p 18 of Ref. 1, which maps predicted half-lives of nuclides in this region, and p 15, which maps principal decay modes.  The properties of a given element are easier to understand in the context of all the nuclides in proximity to it. These maps are the result of large extrapolations from what can be tested, though.  Half-lives are accurate only to within three orders of magnitude and decay modes give no information about competing minor decay modes.  Results given in this article should be regarded as tentative.

Ref. 1 predicts a band of nuclides ranging from Uhu 434 to Uhu 470.  As neutron count increases in this band, half-life increases, reaching a maximum exceeding a second.  Principal decay mode shifts from fission, to alpha emission, then to beta emission.  This pattern is expected, given the widely predicted neutron shell closure at N = 308.  However, there is also a band of alpha-decaying isotopes with millisecond-scale half-lives predicted to lie between Uhu 471 and Uhu 481, a zone which should be strongly destabilized by the N = 308 closure.  A neutron shell closure has also been predicted to occur at N = 318⁽²⁾ and a proton shell closure has been predicted at Z = 154⁽³⁾. The long lives reported for Uhu 471 to Uhu 481 may indicate effects of those two closures. Between Uhu 482 and Uhu 494, the predicted short-lived, fission-decaying isotopes and nuclear drops too short-lived to be nuclei are expected.

Beyond Uhu 494, predicting nuclear properties is largely guesswork, since this region has received very little study.  At least two sets of predictions do exist for location of the neutron dripline up to Z = 175^(3),(4).  These two indicate that the dripline occurs between Uhu 551 and Uhu 581.  Neutron shell closures have been predicted to occur at N = 370⁽³⁾ and 406⁽⁵⁾, and  Ref. 5 also includes predicted structural correction energies provided by shell closures at N = 406 and Z = 164.  Increasing the computed liquid-drop fission barrier by the correction energies as predicted in Ref. 5 results in fission barriers that exceed 3.2 MeV (meaning fission half-lives exceeding 0.001 sec) for all of Uhu 457 through Uhu 494, which is inconsistent with the much higher-quality results of Ref. 1.  Multiplying the Z = 164 corrective energy by 0.7 makes the two sets of data consistent with each other.  With this adjustment, beta-decay can be expected to predominate in isotopes between Uhu 548 and Uhu 581, with a significant beta-decay branch occurring as low as Uhu 540.  The band from Uhu 548 to Uhu 551 is particularly important, since it lies below the lower neutron dripline prediction, strongly implying that these isotopes will beta decay. Millisecond-scale half-lives are expected in all cases.

Although it cannot be quantified,  the N = 370 closure should produce a second band of beta-decaying isotopes which may have a span as wide as Uhu 515 to Uhu 535.

FORMATION

Polar jets emanating from young neutron stars and black holes function like giant, sloppy particle accelerators.  It is possible for fusion or multinucleon transfer reactions to produce all isotopes of Uhu, but only in atoms-per star quantities.  This section addresses possible isotopes which can form in quantity.

Material originally found 800-1000 m beneath the surface is expected to be ejected from a neutron star when it disintegrates during a merger.  This material will consist of nuclides at or near the neutron dripline and having a proton count which may go as high as Z = 170.  A zone of beta- decaying nuclides extending from the dripline to Z = 160 extends as low as A = 547. If b+x*n decay is taken into account, this implies that the isotopes between Uhu 544 and Uhu 551 are able to form in quantity, and that isotopes between Uhu 552 and Uhu 581 are possible, depending on where the dripline actually occurs. These nuclei will have millisecond-scale half-lives.

Neutron capture is unlikely to produce more than an atom or two per star of nuclides heavier than A = 350.

ATOMIC PROPERTIES

The last (1s) ionization energy appears to be in the range 660 - 765 keV, which means that conventional stationary-state orbitals can describe the electrons of Uhu. It also means that bare nuclei are abundant only at temperatures above 5 gK.

Several predictions (see "Extended Periodic Table, Wikipedia) agree that conventional stationary-state orbitals are descriptive of Uhu, and that it is a transition metal (d block).

REFERENCES 

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

2. “The Highest Limiting Z in the Extended Periodic Table”; Y.K. Gambhir, A. Bhagwat, and M. Gupta; Journal of Physics G: Nuclear and Particle Physics.42 (12): 125105. DOI:10.1088/0954 3899/42/12/ 125105.

3.  “Single Particle Levels of Spherical Nuclei in the Superheavy and Extremely Superheavy Mass Region”; H. Koura and S. Chiba; Journal of the Physical Society of Japan; DOI 10.7566/JPSJ.82.014201; Jan. 2013.

4. "Neutron and Proton Drip Lines Using the Modified Bethe-Weizsacker Mass Formula; D.N. Basu et al; Int.J.Mod.Phys.; arXiv:nucl-th/0306061; url: https://arxiv.org/abs/nucl-th/0306061

5.  "Magic Numbers of Ultraheavy Nuclei"; V. Yu Denisov; Physics of Atomic Nuclei, v.68, no. 7, pp 1133-1137; 2005.

(02-14-20)