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Unpentunium, Upu, is the temporary name for element 151. It is expected to be a transient element, one without long-lived isotopes or long-lived precoursers. Upu may form during neutron star mergers.

NUCLEAR PROPERTIES

Between Z = 175 and Z near 130, one set of predictions for half-life and principal decay mode has been published(1). Ref. 1 is publicly available and can be found via a search by paper title. Anyone interested in this element should study pp 15 and 18, which allow a given element to be understood in the context of adjacent nuclides.

These data are limited to nuclides for which N <= 333. Half-lives are presented in bands covering 3 orders of magnitude (0.001 sec to 1 sec, for instance) and are accurate to within +/- 3 orders of magnitude, which seems rather crude until the enormous extrapolation from what is known is taken into account, Minimum half-life is set at 10^-09 sec, rather than 10^-14 sec; which introduces a little uncertainty, but not a great deal because fission half-lives tend to transition quickly from values well above 10^-09 sec to values well below 10^-14 sec; and, while alpha-decay half-lives change more slowly, alpha emission is rarely dominant except where fission is suppressed. Significantly, beta-decay half-lives do not decline far below 10^-03 sec, even for highly energetic decays, so there is little uncertainty about neutron-rich nuclides.

Ref. 1 does have one significant weakness. Nuclides which are beta-stable are identified by black squares, overwriting decay mode and half-life information. In many cases, these data can be estimated from adjacent nuclides.

No predictions exist for N > 333. The liquid-drop sketch developed in "The Final Element" (this wiki) for Z = 176 and above can be used to guess at where nuclides with Z < 175 and N > 333 may exist. Probability criteria for this purpose were set in "Nuclear Guesswork" (this wiki). Below Z = 171, it is necessary to look only at nuclear drops which are not expected to decay by neutron emission and require only normal amounts of structural correction energy in order to suppress spontaneous fission.

PREDICTED PROPERTIES

Ref. 1 predicts isotopes ranging from Upu 479 to Upu 418. Format used to display isotope properties is: isotope(s); half-life in seconds; dominant decay mode; comments.

Upu 479 - Upu 478; 10^-09 - 10^-06; fission.

Upu 477 - Upu 475; <10^-09; fission. Decay mode is estimated, but highly probable.

Upu 474; 10^-06 - 0.001; fission. A fission half-life this long is plausible only if there is a shell closure at N = 318(2). It is too far above N = 308 for stabilization against fission.

Upu 473; 10^-09 - 10^-06; fission.

Upu 472 - Upu 471; 10^-06 - 0.001; fission. A fission half-life this long is plausible only if there is a shell closure at N = 318. It is too far above N = 308 for that closure to stabilize it against fission. If N = 308 is the only closure, this nuclide should either decay quickly by fission or decay in a millisecond time frame by beta emission.

Upu 470 - Upu 469; 0.001 - 1; fission. A fission half-life this long is plausible only under the conditions described for Upu 472 and Upu 471,

Upu 468; 1 - 1000; fission. A fission half-life this long is plausible only under the conditions described for Upu 472 and Upu 471.

Upu 467 - Upu 465; 0.001 - 1; beta.

Upu 464; 1 - 1000; alpha. Stabilization against fission by the N = 308 shell closure is probable, so decay by alpha emission is likely. However, shell closures do not appear to significantly increase beta-decay half-lives(3). This far from beta stability, half-lives of a second or more seem unlikely.

Upu 463; 0.001 - 1; beta.

Upu 462; 1 - 1000; alpha. Stabilization against fission by the N = 308 shell closure is probable, so decay by alpha emission is likely. However, shell closures do not appear to significantly increase beta-decay half-lives. This far from beta stability, half-lives of a second or more seem unlikely.

Upu 461 - Upu 445; 0.001 - 1; beta.

Upu 444; 1 - 1000; alpha. Beta emission is likely to be an important secondary decay mode.

Upu 443 - Upu 440; 1 - 1000; fission. Ref 1 predicts a band of fission-decaying nuclides with N between 285 and 295. It appears to be possible for structure to destabilize a nuclide(2), so isotopes between Upu 444 and Upu 437 which decay in the manner predicted are not implausible.

Upu 439; 1 - 1000; alpha.  Predicted mode and half-life are plausible, as stated for Upu 443 above. Fission and beta emission are likely to be important secondary decay modes.

Upu 438 - Upu 437; 1 - 1000; fission. Predicted mode and half-life are plausible, as stated for Upu 443 above. Alpha and beta emission are likely to be important secondary decay modes.

Upu 436 - Upu 432; 1 - 1000; alpha. Beta emission and fission are likely to be important secondary decay modes.

Upu 431; 0.001 - 1; fission. Predicted mode and half-life are plausible, as stated for Upu 443 above.

Upu 430 - Upu 431; 0.001 - 1; alpha. Fission is likely to be an important secondary decay mode.

Upu 429; 0.001 - 1; fission. Alpha emission is likely to be an important secondary decay mode.

Upu 428 - Upu 425; 0.001 - 1; alpha.

Upu 424; 0.001 - 1; fission.

Upu 423 - Upu 421; 10^-06 - 0.001; fission. Upu 423 is beta-stable, so its properties are estimated.

Upu 420 - Upu 418; 10^-09 - 10^-06; fission.

Below N = 308, this pattern is generally to be expected, given a neutron shell closure at N = 308. Above this point, predictions become more confusing. Presence of relatively long-lived, fission-decaying isotopes of Upu indicates a more sophisticated structure than implied by a simple liquid-drop picture.

GUESSED PROPERTIES

Drops in the bands Upu 542 to Upu 523 and Upu 461 to Upu 449 are unlikely to decay by neutron emission and are stable against fission. Nuclides in these bands are likely. Drops in the bands Upu 522 to Upu 462, Upu 448 to Upu 343, and Upu 337 to Upu 333 are unlikely to decay by neutron emission and require a moderate amount of structural correction energy. Drops in these bands are unlikely.

COMPARISON

In the region where predictions and guesses overlap, the estimating technique lists only Upu 461 to Upu 449 as "likely". Ref. 1 predicts that Upu 479 to Upu 418 will exist, a much broader range.

FORMATION

Since a disintegrating neutron star can supply neutron-rich pieces of nuclear matter of the correct size (see "Neutron Star", this wiki), Upu isotopes can form where inhibition of fission allows beta decay from the neutron dripline.

Isotopes in the band Upu 542 to Upu 523 can form by a series of beta decays from the neutron dripline. Formation of isotopes in this band is likely. It is improbable that other isotopes in the band Upu 522 - Upu 485 can form.

Beta decay from the neutron dripline also forms nuclei in the region described in Ref. 1. Although only one decay mode is reported for each nuclide, branching decay can be expected unless the partial half-life against one mode of decay is much shorter than any of the others. In practice, that means beta decay series will extend either to nuclides which are stable against beta decay, nuclides lighter than beta-stable nuclides (which can decay by positron emission), or whose spontaneous fission partial half-lives are under 1 us.

Under this assumption, Upu 455 to Upu 431, Upu 429, Upu 427, Upu 425, and Upu 423 can form. It is improbable that other isotopes in the band Upu 484 to Upu 418 can form.

ATOMIC PROPERTIES

Electron structure of Upu has been predicted by several sources (see "Extended Periodic Table" in Wikipedia). However, these predictions should be used with caution. Upu is large enough that nuclear shape may have an effect on electron structure, which might cause different isotopes of Upu to have different electronic structures. (That means it is no longer an element in the chemical sense.)

If this effect is small, Upu will be an active (superactinide) metal of the 8th period. Its electron configuration has been predicted(4) to be [Og] 5g18 6f8 7d3 8s2 8p21/2.

REFERENCES

1. "Decay Modes and a Limit of Existence of Nuclei"; H. Koura; 4th 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. "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".

4. "Extended Periodic Table", Wikipedia.

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

(06-20-20)

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