Astatine

Astatine. At, is the name of element 85. Its nuclear properties are rather strange. Its atomic properties are more straightforward, Getting enough At to make a 1 cm diameter sphere of the element would require the services of a high-grade magical laboratory, such as can be found in the High Energy Magic department of Unseen University. When released from time stasis, the sphere would become a ball of plasma in less than a millisecond, a tiny nuclear fireball. Fortunately, more mundane labs are not capable of isolating such large quantities of the element.

Radioactivity

Nuclear properties

Around 85 isotopes, ranging from the neutron dripline near ²⁷⁰At down to at least ¹⁸⁴At, have been predicted. Of these, 34 have been observed (plus 24 isomers), ranging from ²²⁴At down to ¹⁹¹At. ²²¹Fr and all heavier isotopes decay exclusively by beta emission, Half-lives rise as A declines from around 0.001 sec at the neutron dripline, reaching 1 sec around ²³⁴At, and peaking at 150 sec in ²²⁴At. Below this, there is a band from ²²⁰At to ²¹⁶At in which both beta emission and alpha emission are active decay modes. The mixture changes, as A falls, from largely beta to largely alpha emission. Half-lives reported for this band peak at 3.71 min in ²²⁰At and decline steadily to 0.0003 sec in ²¹⁶At. This decline in half-life is driven by dramatically shortening alpha decay partial half-lives, mainly due to structural instability in nuclides with somewhat more than 126 neutrons. Since At is rather close to a proton shell closure at Pb, alpha decay occurring in relatively neutron-rich isotopes may be partly due to proton instability at Z slightly above 82. Between ²¹⁵At and ²¹³At only alpha decay is reported and half-lives are under 1 microsec, which is consistent with instability above N = 126.

²¹²At is sandwiched between ²¹²Rn and ²¹²Po, both of which are beta-stable. Its neutron count is 127, which means it can't decay to a nuclide with N = 126. As a result, its half-life is 0.314 sec, much longer than the half-lives of slightly heavier isotopes. It also decays by both alpha emission and positron emission, with a very weak beta emission branch also reported. The latter doesn't produce any significant amount of ²¹²Rn; but does demonstrate a phenomenon often encountered in smaller nuclides where an odd-Z, odd-N nuclide located between a pair of even-Z, beta-stable nuclides will decay to both.

²¹¹At has a neutron count of 126. Its half-life is 7.21 hours, far greater than any heavier At isotope. Equally impressive is the fact that its branch ratio for alpha decay is only 0.42. Partial half-lives against alpha decay rise significantly at and below N = 126. Note that electron capture, rather than positron emission occurs at ²¹¹At, since decay energy is not great enough to create a positron (0.5 MeV).

Between ²¹⁰At and ¹⁹⁵At, decay is by a mixture of alpha emission and positive beta decay. For heavier nuclides of this band, alpha emission is a rare mode, but it becomes dominant by ¹⁹⁵At. Below that, alpha emission dominates and conventional positive beta decay disappears, although beta-delayed fission is reported at ¹⁹²At. Proton emission is predicted to replace alpha at ¹⁸⁴At, ¹⁸⁵At and ¹⁸⁷At. Half-lives in this band peak at 8.1 +/- 0.4 hrs in ²¹⁰At, then fall, dropping below 1 hr at ²⁰⁶At, 1 min at ²⁰⁰At, 1 sec at ¹⁹⁶At, and ending up with half-lives in the microsecond range at the band's low end.

Astatine isotopes ²¹⁶At and ²¹⁴At down to ²¹⁰Po decay exclusively by alpha emission. Astatine isotopes ²¹⁵At and ²¹⁷At and above can form when high-A, neutron-rich material is produced in a supernova or neutron star merger then evolves through beta-decay chains to At. Only secondary reactions can produce lighter isotopes, and the short half-lives against alpha decay occurring in the ²¹⁵At - ²¹³At band mean that the amounts produced will be lost in the noise of charged-particle reactions occurring in electromagnetic storms.

Those isotopes which can form in abundance will quickly disappear, leaving only a few isotopes which are generated continuously because they are in decay chains of long-lived nuclides. Concentration of each of these, compared to concentration of its chain head, [^AZ]/[^A0Z₀] can be determined. The overall branch ratio (BR) between ^A0Z₀ and ^AZ along an allowable path between the two is the product of branch ratio at each decay leading to ^AZ. Once equilibrium has been established, the activity of a nuclide in a decay chain, the rate at which it decays; is equal to the rate at which it forms. The total rate at which ^AZ forms is equal to the sum over all paths of the overall branch ratio of each path. Since the activity (rate of decay) of a nuclide is given by activity(^AZ) = ln(2)*(t/t12(^AZ) [^AZ], concentration ratio is

[^AZ]/[^A0Z₀] = sum(BR_{path j}) * t12(AZ)/t12(^A0Z₀)

where t12 denotes half-life. ²¹⁹At and ²¹⁵At both descend from ²³⁵U. Only one path leads to ²¹⁹At, and its overall branch ratio is 8.28E-07. Using t12(²¹⁹At) = 56 +/-3 sec gives [²¹⁹At]/[²³⁵U] = 2.1E-21 (+/- 0.1E-21). Although there are four paths linking ²³⁵U and ²¹⁵Po, all come together at that isotope. Effectively, BR(²¹⁵Po) is 1. 215Po has a rare beta decay branch (BR = 2.3E-06) leading to ²¹⁵At. Since t12(²¹⁵At) = 0.0001 +/- 0.00003 sec, [²¹⁵At]/[²³⁵U] < 1.24E-26. (An inequalty is used here because error band for this computation is around 1 order of magnitude.) ²¹⁸At descends from 238U via a single pathway whose BR = 0.0002. Given a half-life of 1.5 +/- 0.3 sec, 1.7E-21 < [²¹⁸At]/[²³⁸U] < 2.6E-21. (This wide range in value results from uncertainty in t12(²¹⁸At).) In principle, ²¹⁷At is in the ²³⁷Np decay chain, but ²³⁷Np itself is produced only by neutron capture by ²³⁵U and neutron capture by ²³⁶U using neutrons generated by spontaneous fission of ²³⁸U. The quantity produced is negligible. Finally, it should be noted that the ²³²Th decay chain does not produce any At isotopes.

Uranium has been estimated to have a molar concentration [U] = 2E-09 in the earth as a whole. From estimated abundances of the elements in earth as a whole, molar masses of each element, and earth's total overall mass of 5.972E24 kg, it is possible to determine that earth contains 2.296E26 mol and that an average mole has a mass of 0.026 kg. Since the mole fraction of ²³⁵U in natural uranium is 0.007, the total number of moles of 235U in the earth is n(²³⁵U) = 3.31E15 mol. This gives n(²¹⁹At) = 6.9E-06 mol or 0.00018 g. Similarly, n(²¹⁵At) = 4.1E-11 mol or 1.1E-09 g. The most abundant isotope, ²¹⁸At, has n(²¹⁸At) = 0.00119 mol or 0.031 g.

There is a second means by which At isotopes can form - physicist catalyzed reactions. Astatine's most stable isotopes, ²¹¹At, ²¹⁰At, and ²⁰⁹At can all be synthesized easily using the reaction ²⁰⁹Bi + ⁴He --> ^(213-x)At + x n where x = 2, 3, or 4. Quantities which can be obtained are in the 100E-09 g range, meaning synthetic At can be made in quantities large with respect to natural ²¹⁵At but small with respect to ²¹⁸At.

Since ²¹¹At - ²⁰⁹At have no long-lived precursers, they will go extinct within a month or so of the supernova or merger which produced them, while the less-stable 215At, 218At, and 219At do have long-lived precursers, so remain as traces. Laboratory produced At and naturally-occurring At includes two completely different sets of isotopes.

Atomic properties

Astatine's position on the periodic table is Period 6, Group 17. As such, its chemistry resembles the other halogens in many ways. There are a few differences, though, Astatine displays some metallic properties, for instance, and is capable of existing as At¹⁺ simple ions. It should be noted, though, that the chemistry of At in the laboratory does not indicate its behavior outside the lab. No isotopes of At are long-lived, which means that any At which forms will remain in its initial environment, acting as a trace impurity.

(02-02-20)

Gallery

Autunite, a mineral that contains uranium which can decay into astatine.

Photographic card for astatine by Theodore W. Gray.

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9-Period Periodic Table of Elements
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