Alternate View Column AV-17
Keywords: supercritical, electric field, positrons, GSI, uranium acceleration
Published in the Mid-December-1986 issue of Analog Science Fiction & Fact Magazine;
This column was written and submitted 6/7/86 and is copyrighted © 1986, John G. Cramer. All rights reserved.
No part may be reproduced in any form without the explicit permission of the author.
The path to a new discovery in physics is often a very twisted one. The subject of this Alternate View column is an example of this process. A major accelerator, built with with the prospect of discovering super-heavy elements, is now being used in an experiment to produce "super-atoms" with very large electric fields, and this work has quite unexpectedly revealed what looks like a new and mysterious particle. It is reminiscent of the SF of the 1930's where one of the standard science gimmicks was the discovery of a new element with amazing properties. It also sounds a bit like the Paul Preuss novel Broken Symmetries, where the plot revolves around the discovery at a large accelerator laboratory of a mysterious new particle. But this is real science, folks. Honest!
In the 1970's it looked as if we were due for a major expansion of the periodic table of chemical elements. The periodic table lists 92 natural elements ranging from hydrogen with Z, the atomic (or proton) number, of one to uranium with 92 protons (Z=92). In addition to the natural elements there are a dozen or so "transuranic" elements, all heavier than uranium. The transuranics, with latinized names like berkelium, californium, and americium, do not occur in nature. They were made and identified in physics laboratories, principally, as the names suggest, at the Lawrence Berkeley Laboratory of the University of California. Just above Z=106 the periodic table ends for even these synthetic additions. But there is a limit to the number of protons that can be crammed into one nucleus. Proton, carriers of positive electrical charge, strongly repel one another. Nuclei with more than that 106 or so protons are destabilized by this repulsion, so that they cannot exist for even a microsecond. The periodic table stops here. Or does it?
In the early 1970's there was great excitement over the possibility of an entirely new group of stable "super-heavy" elements. Theoretical calculations of nuclear stability showed that there might be an "island of stability" around Z=118 to 126. In all nuclei the protons and neutrons are organized in "shells" of similar motion. Calculations suggested that in the Z=118-126 region the shell structure gave increased stability to nuclei. Thus there might be a whole new unexplored region of the periodic table, new elements waiting to be discovered and equally important, to be given names used by all future generations of humanity. Beside the great fundamental interest in discovering the properties of a brand new set of chemical elements, there was also another predicted payoff. These "superheavies", even if as stable as predicted, would be fissionable. And unlike uranium and plutonium, which require a "critical mass" of many kilograms to produce even a small fission reactor or bomb, the superheavies were expected to have critical masses measured in milligrams. The possibility of pocket size nuclear power sources (and other devices) was quite tantalizing.
These optimistic predictions produced a sort of international gold rush to produce and identify the first super-heavy elements. The principal participants were physics groups in the USA, USSR, France, and West Germany. The West German effort clearly had the most class. Instead of trying to make super-heavy elements "on the cheap" with aging and somewhat inappropriate accelerators, as was the pattern in the USA and USSR, the government of West Germany decided to go first class by constructing a whole new accelerator laboratory, the Gesellschaft Schwerionen or GSI, near the city of Darmstadt. The GSI accelerator, the Unilac, was designed specifically to allow the bombardment of stationary uranium atoms with a beam of fast moving uranium nuclei with energy enough to fuse, forming much heavier composite systems (and perhaps a few stable superheavies). The other national groups in the race did not have uranium beams and had to explore other possibilities, leaving the GSI group with the inside track.
But this "race" has turned out to be a very slow one indeed. You may have noticed that there have been no news items heralding the discovery of superheavy elements. The GSI group has discovered a few new elements, but they are not superheavies. They are elements with numbers 106, 107, and possibly 109, all quite far from the fabled Island of Stability. If stable superheavies do exist, as now seems less likely, their production is far more difficult than expected. But the GSI laboratory has not been idle, even though the voyage to the Island of Stability has been frustrated. It has supported a varied program of experimental physics, including the "super-atom" work referred to in the title of this column.
There is considerable similarity between electrons in orbit around a nucleus and planets in orbit around a sun, but there are also important differences between these two physical systems. Unlike planetary orbits, the orbits of electrons are tightly restricted by the laws of quantum mechanics. For example, the innermost electrons of all uranium atoms have exactly the same orbital velocity and other characteristics. In the simple atomic theory of Bohr (which is incorrect because it ignores special relativity and other effects) an inner electron has an orbit speed of c×(Z/137), where c is the velocity of light. With Z=92 (uranium) an inner electron would have a velocity of about 67% of c, and a hypothetical atom with Z=137 would have inner electrons traveling at the speed of light!
Bohr's theory is too simple to accurately describe an atom, but it does indicate that in atoms with Z's above 137 we might expect interesting effects. Better calculations which include correct treatment of special relativity show that the critical value of Z comes not at 137 but at 173. At this Z-value the electric field near the nucleus becomes so strong that an inner electron has a binding energy which is more than twice its mass-energy. This means that it costs more energy to pull the electron loose from the atom than it would to create a brand new electron along with it's antimatter complement, a positron.
Atoms with very large Z-values may appear to be only a theoretical abstraction since the very heaviest elements have Z-values of less than 110. However, there is a way, at least temporarily, of getting into the Z~173 region where things get interesting. This is because we can collide one uranium with another so that they momentarily stick together. Then for a period of time perhaps long enough to allow a few hundred electron orbits we have an "atom" with Znet=(2×92)=184. Three different groups of physicists at GSI have been able to produce such super-atoms with Z large enough to create an electron-positron pair, making an electric field strong enough to literally suck an electron out of the vacuum. The creation of an electron by this mechanism leaves behind a "hole", a positron which is propelled away from the atom by the strong positive charge of the nucleus. The electric field near the super-atom is made so large that new electrons and positrons are created out of empty space by a field becomes so strong that it literally "sparks" the vacuum itself.
The same trick has been worked to make super-atoms with other systems: uranium+curium (Znet=188), uranium+thorium (182), thorium+thorium (180), uranium+lead (174), thorium+lead (172), and lead+lead (164). When these heavy "atoms" are created, it is found that positrons are indeed produced. Over the last five years the three GSI groups have studied these systems with increasing skill and precision, and a remarkable new result has emerged from their work.
All theories of positron production by the strong electric fields of these super-atoms predict that the positrons should emerge with a broad smear of energies. And indeed the measurements show this broad distribution. But they also show a sharp energy spike in the distribution of positron energies at about 325 keV, about 64% of an electron's mass-energy. This peak appears always at about the same energy, independent of the target-beam combination or the energy of the beam particles. The mystery of this positron peak has deepened recently because it has been discovered that the sub-critical systems (Z<173) such as thorium+lead and lead+lead produce it also. And it has also been discovered that electrons are emitted along with the positrons in the peak region. The energy distribution of these electrons shows a spike at the same energy of 325 keV. Some unknown process in these heavy nucleus collisions is producing electron-positron pairs such that both particles have about 325 keV of kinetic energy. No known process can do this!
The most popular explanation for the GSI data is this: the super-high electric fields near the super-atom are producing a new particle. This particle has a mass which is enough to create an electron-positron pair. This would require a particle, probably electrically neutral, with a mass-energy of about 1670 keV, about 3.2 times more massive than an electron.
This is very disturbing. With the recent development of quantum chromodynamics and grand unified theories there seemed to be a slot provided for every particle ever discovered. All know particles come in four categories: baryons, mesons, leptons, and mediating particles. The mystery pacticle cannot be a baryon or meson because both are made of quarks and are much too heavy for consistency with the observed mass. Likewise, it can't be a lepton because all are accounted for and cannot decay into electron-positron pairs. Mediating particles correspond to the four known forces of nature and are either very massive (gluon, W±, and Zo) or massless (photon, graviton). There is also a fifth category containing various "possible" particles that have been predicted by one theory or another but which have never been observed. Most of these could-be particles are too massive for consideration. The leading candidate among them for explaining the GSI results is the axion (see "The Dark Side of the Force of Gravity", Analog 2/85), except that recent experiments at Fermilab seem to have eliminated 1670 keV axions as a realistic explanation. The new particle from Germany just doesn't fit into the framework of our understanding.
And so in testing the limits of very large electric fields in super-atoms the physicists at GSI seem to have stumbled on something completely new and unexpected. It is a discovery which is every experimental physicist's dream: a "new phenomenon" (see my article, "New Phenomena", ANALOG 5/82). It is an experimental observation that was not anticipated by present theories and which will probably require revisions of those theories. Children often play a sort of game with their parents, probing limits by pushing against the rules to learn how much they can get away with. Physicists work the same game on Mother Nature herself, and sometimes it pays off.
Followup note: An improved followup experiment (APEX) performed at Argonne National Laboratory in the USA has failed to observed the positron lines that had previously been reported by two groups at the GSI in Germany. The consensus of the nuclear physics community is now that the GSI positron-line results were bogus.
Quantum Electrodynamics in Strong Fields, Walter Greiner, editor; Plenum, New York (1983).
Energy Peaks in Electrons & Positrons:
T. Cowan, H. Backe, K. Bethge, H. Bockmeyer, H. Folger, J. S. Greenberg, K. Sakaguchi, D. Schwalm, J. Schweppe, K. E. Steibling, and P. Vincent, Physical Review Letters 56, 444 (1986).
SF Novels by John Cramer: my two hard SF novels, Twistor and Einstein's Bridge, are newly released as eBooks by Book View Cafe and are available at : http://bookviewcafe.com/bookstore/?s=Cramer .
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