With antiparticles on troubleshooting

Measurement technology indicates defects at the atomic level

In the sample chamber of the positron source NEPOMUC at the research neutron source Heinz Maier-Leibnitz (FRM II) of the Technical University of Munich, the positron beam is focused on the surface of a sample in ultra-high vacuum © Photo: Jakob Mayer / TU München
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The mysterious antimatter is not only exotic accessory in movies like "Illuminati", but also a fascinating field of science. Munich researchers gain the antiparticles of electrons, the so-called positrons - in the world's highest intensity. The almost one billion positrons per second are used in nano-material research: They detect defects in the atomic lattice and can thereby precisely distinguish individual elements.

While Tom Hanks has to hunt all over Rome in search of antimatter from the particle physics laboratory CERN in Switzerland, Christoph Hugenschmidt from the research neutron source Heinz Maier-Leibnitz (FRM II) of the Technical University of Munich has one billion antiparticles per second available. The positron is as harmless as its counterpart, the electron. NEPOMUC (NEutron-induced POsitron source MUniCh), the TUM physicist called the neutron-induced positron source - the most intense in the world.

The special feature of the positron source in Garching is that the particles can be guided in the ultra-high vacuum by magnetic and electric fields almost loss-free up to the five different experimental stations. "The scientists who perform their experiments at the positron source of the FRM II are thus up to 1, 000 times more positrons per second available than in any other laboratory in the world, " says Hugenschmidt. This saves valuable experimentation time. Experiments with positrons, which otherwise take weeks, can be performed on the FRM II within a few minutes or hours.

Three-body problem in the visor

"At the same time, we have increased the sensitivity, and it is therefore possible to answer completely new questions in fundamental physics, " Hugenschmidt points out further advantages. For example, the negatively charged positronium, a particle consisting of two electrons and a positron, is currently under investigation. The three particles that revolve around each other are of particular interest in the three-body problem that Kepler and Copernicus already raised: how do the orbits of three bodies run under the influence of their mutual attraction?

The positrons are generated indirectly from neutrons of the reactor. The heart of the positron source consists of a structure of cadmium and platinum foils. The cadmium captures the neutrons and releases high-energy gamma radiation. The energy of this electromagnetic radiation is converted into platinum in accordance with the Einstein equivalency of mass and energy E = mc2 in mass. At the same time, matter and antimatter are created: electrons and positrons. In order to use the positrons as long as possible for experimentation, one must keep them away from matter. Because when they come into contact with an electron, they immediately disperse. display

Positrons for materials research

Apart from basic experiments, positrons are used primarily in materials research because they not only detect defects in the atomic lattice, but can also distinguish atomic species. Depending on the element, the positrons radiate differently when touched by the electrons. The measurable gamma radiation is like a fingerprint specific to an element. The sensitivities of the positrons have now been proven by Hugenschmidt's researchers in an experiment with aluminum and tin. A single layer of tin atoms was embedded under a layer of 500 layers of aluminum atoms only 200 nanometers thick. Nevertheless, the positrons were able to detect the tin layer.

This measurement technique is not only intended to show defects at the atomic level, but will in future be applied to doped semiconductors and metallic materials in order to make smallest impurities visible in them. Hugenschmidt is currently developing new measuring equipment at the positron source of the FRM II and undertaking experiments in cooperation with the University of Munich, the University of Munich and the Max Planck Institute for Research Nuclear physics in Heidelberg.

(idw - Technical University Munich, 01.10.2009 - DLO)