Mercator of the Nuclear World

The medieval alchemists tried in vain to create new elements in their crucible-based experiments out of just a few ingredients such as lead and mercury and some common acids. In the 20th century nuclear physicists not only finally succeeded in transmuting one element into another but were able to create new elements.

A new experiment at the Gesellschaft fur Schwerionenforschung (GSI) in Darmstadt does not create new elements (although in previous experiments GSI discovered 6 elements: 107-112) but it has created and analyzed the largest number of elements (from nitrogen up to uranium) and the largest number of subsidiary isotopes (1400) ever seen in a single nuclear research effort. The only ingredients: uranium and hydrogen. The crucible in which the elements were warmed up: a particle accelerator.

The GSI physicists did not, as you might guess, smash a beam of protons (bare hydrogen nuclei) into a stationary uranium target but rather the other way around. The reason for slamming energetic U-238 nuclei into a stationary liquid-hydrogen target is that fragment nuclei of all sizes, flying away from the collision point, don't glom together (as they might if emerging from a uranium target) and, furthermore, can be more accurately identified since they are free of bound electrons whose electrical charge might confuse the task of measuring the number of protons in the detected particle.

What comes out of this meticulous and comprehensive of nuclear experiment is a set of cross sections---each a measure of the likelihood for creating that particular nuclide (that is, each stable element and its complement of isotopes, variations on the same nucleus but containing differing numbers of neutrons). The GSI work, in other words, not only enumerates a chart of the nuclides (the sort of thing on the wall of every nuclear lab in the world) but produces a chart of cross sections for producing those nuclides in a collision (see figure at http://www.aip.org/png/2004/228.htm).

This information is valuable for a number of reasons: for planning a future accelerator of rare isotopes, for studying how to break down nuclear waste in sub-critical reactors, and for studying fundamental aspects of nuclear fission and nuclear viscosity. (Armbruster et al., Physical Review Letters, 19 Nov 2004; lab website at www-w2k.gsi.de/charms/; contact Karl-Heinz Schmidt, k.h.schmidt@gsi.de)