HEAVY METALS -- LIGHT FANTASTIC
by
Michael D. Winkle
In the days of my misspent youth, whenever a science fiction story made mention of some valuable, unstable, super-hard or otherwise newly-invented element, I razzed the author with the arrogance of a well-read pre-teen. After all, every element above bismuth (number 83) on the periodic table is radioactive. Protons repel one another in the atom; only the power of nuclear attraction, the "strong" force, holds us all together. This nuclear force, however, works over a distance of only about 10-13 cm; as protons and neutrons pile up in heavier atoms, they exceed the reach of the nuclear force. The nuclei become unstable, eventually flying apart.
Man-made (transuranium) elements are even less stable than natural fissionable isotopes. For instance, Lawrencium-259, first created in a linear accelerator in 1961, has a half-life of only eight seconds.
Any new elements would have to have even higher atomic numbers than these ephemeral transuranics, and presumably even shorter life spans, so we can forget about Vibranium, Kryptonite, Dilithium, and other SF staples. Right?
Maybe not. My ambles through the library bring me to Science, volume 193, 16 July 1976: "Nuclear Science: X-ray Evidence for Superheavy Elements," by Arthur L. Robinson. "Although elements with atomic numbers much greater than 100 are highly unstable, numerous calculations have indicated the possibility of long-lived nuclei with from 110 to 114 protons and 184 neutrons." This theoretical sequence of atoms is "the so-called island of stability of superheavy nuclei." [1]
Columbuses of physics have stepped onto the shores of this island in recent years, as reported by Yuri Oganessian, et al., in Scientific American. "Since 1994, research groups in Germany, the U.S., and Russia have added six new elements to the periodic table, with atomic numbers as high as 118. The most important synthesis has been the production of isotopes of element 114, which has conclusively demonstrated the existence of the island of stability." [2]
How long do these new elements last? "The lifetime of our element 114 isotope with 175 neutrons is more than 1,000 times longer than the lifetime of the 174-neutron isotope." (Calculations suggest that a 184-neutron atom, as yet not detected, would make element 114 even more stable.) Also: "Our isotope of element 112 with 173 neutrons is more than a million times longer-lived than the isotope with 165 neutrons, discovered in Darmstadt in 1996." [3]
A. L. Robinson described a possible mystery concerning superheavy elements back in 1976 which enterprising SF writers might tackle. Superheavy elements might explain large halos sometimes found around "thorium-rich monazite inclusions in biotites." "The size of a halo increases with the energy of the alpha particle emitted, but the giant halos (with radii from 50 to 100 micrometers) were too large to be explained by alpha decay of any known element."
Models of supposed element 126 indicate that it might have "half-lives from a few nanoseconds to about a thousand years, according to the number of neutrons. . . But the geologic age of the earth is 4.5 x 109 years." Thus a process that created any element 126 atoms had to occur relatively recently.
What sort of event might this be? There seems to be some doubt that even a supernova could create these heavy elements during its process of nucleosynthesis, yet, just to make things more amazing: "[T]he details of the giant halos are such that it is possible that they were caused by alpha decay of even heavier elements than those apparently now residing in the monazite inclusions, say the experimenters, and thus would be much harder to produce." [4] Of the superheavy elements created in laboratories, "only the ‘lightest’ two -- neptunium and plutonium -- exist at all in nature." [5] If such massive elements are ever found, their origins will remain a mystery.
While superheavy elements cover the "more is better" area of the periodic table, a monograph entitled Positronium and Muonium Chemistry (Advances in Chemistry Series Vol. 175) pushes us sideways.
Positronium (abbreviated Ps) "can be visualized as an analog of the hydrogen atom, in which the proton is substituted by a positron [anti-electron], and thus can be considered as the lightest isotope of hydrogen," according to Hans J. Ache’s "Positronium Chemistry." This can’t be considered an element, can it? Ache writes, "chemists remained rather indifferent to this new atom and it was only during the past five to seven years that chemists of all persuasions have become more and more involved in the chemistry of this exotic atom." [6]
If one speaks of the chemical properties and atoms of something, presumably it is an element.
John Archibald Wheeler, who invented or expanded upon such concepts as geons, wormholes, and black holes, was fascinated early in his career by the possible existence of positronium. In a paper written near the end of World War II, he theorized the existence of "atoms and molecules that could be constructed from electrons and positrons alone, and calculated their properties." He named the simple electron/positron pairing a "bi-electron", but later accepted the term positronium.
Wheeler continues: "Later, I went further, calculating how a large amount of positronium atoms might behave. Liquid positronium should be superconducting. But will we ever see a drop of it? It would be worth striving to demonstrate its existence, for it would be a new extreme form of matter." [7]
Besides positronium, there is muonium. Donald Fleming, et al., write in "Muonium Chemistry -- A Review": "The muonium atom can properly be regarded as an ultra-light isotope of hydrogen, in which the proton nucleus is replaced by a positive muon of only one-ninth its mass." A muon has .1126 the mass of a proton and is about 207 times the size of an electron. A positive muon circled by an electron comes closer to our concept of an atom than a positron-electron pairing. "From a chemical point of view, then, muonium differs from hydrogen only in mass and in small corrections for nuclear motion." [8]
Neither positronium nor muonium last very long. There are two types of Ps, one of which (the "singlet") exists for 1.25 x 10-10 seconds, and the other (the "triplet") for 1.4 x 10-7 seconds. A muon self-annihilates after 2.2 x 10-9 seconds.
John Wheeler wondered if there were anyway to stabilize positronium atoms. When he started working with Richard Feynman (his own protégé) at Princeton, "we found the answer to this question to be yes. We found a self-consistent formulation in which forces (actions) propagate through space without any need for electric or magnetic fields, either static or in the form of radiation. This was a satisfying result, given that it came at a time when the thinking of all physicists was embedded in fields." [9]
Suppose it were possible to have stable atoms with nuclei composed of something other than protons and neutrons? Suppose elements and compounds could be built of these atoms? We could construct whole periodic tables parallel to the one we know, filled with "alternate" elements. These para-elements might possess attributes quite different from ordinary materials. At the very least they would be much lighter than their mundane counterparts.
Perhaps SF writers can once again rationalize newly-created "-onium"s for their futuristic epics.
I just hope they’re careful what they put in their Explosive Space Modulators.
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