Where are positrons in an atom

Positron spectroscopy

The antiparticle of the electron - the positron - is suitable as a nanoscopic probe particle with which even individual missing atoms in a crystal can be detected. Christoph Hugenschmidt from the Research Neutron Source Heinz Maier-Leibnitz, FRM II for short, explained how this works in our podcast. Here you can find the article for reading.

We and everything around us are made up of atoms, and these in turn are made up of protons, neutrons and electrons. It has been known for around eighty years that there is a counterpart to these particles, or more generally to matter - antimatter. For example, the positron resembles the electron in all aspects - except for the electrical charge, because this is positive in the positron. In the meantime, antimatter can not only be created and researched artificially, but scientists even use the antiparticles to gain knowledge about ordinary matter.

Christoph Hugenschmidt

Christoph Hugenschmidt: “The main interest in positron research lies in examining imperfect, ideal solids, as we know them from textbooks. The interest here is rather to detect the smallest material and crystal defects, which for example consist in the fact that individual atoms are missing, i.e. there are holes in the lattice. "

The neutrons are generated here in a research reactor and then used, among other things, to produce positrons. However, several steps are necessary to extract the antimatter. First, the researchers direct the neutrons onto a sheet made of cadmium.

“In neutron research, cadmium is known as the perfect neutron absorber. This means that it acts like a black hole and traps all thermal neutrons. This process leads to the fact that one can generate high-energy gamma radiation. "

The scientists direct the high-energy gamma radiation onto a platinum foil around a hundred micrometers thick. The gamma photons interact with the atoms in them, converting their energy into matter or antimatter. This so-called pair creation can be understood with a famous formula.

“So according to Einstein's relation E = mc2 Particles and their antiparticles can always be produced in equal proportions from pure radiation energy. In the lowest energy process this is exactly a pair of an electron and a positron ”.

Around a billion positrons per second

E = mc2 describes the equivalence of mass and energy. This means: If a photon is to be converted into an electron and a positron, the energy of the photon must at least correspond to the rest energy "E" of these two particles. This rest energy results from the mass "m" of the particles multiplied by the square of the speed of light - c2. The gamma radiation that is used at the FRM II to generate pairs has an energy of several megaelectron volts. That is enough to generate an electron and a positron, because together they have a rest energy of around one megaelectron volt. The remaining energy of the gamma radiation is converted into kinetic energy.

“The spectrum of positrons produced is very broad, which means that high-energy positrons are also produced. In our case, the platinum foil structure has the property of cooling positrons very efficiently. In this way you get very low energies and manage to extract a monoenergetic positron beam. "

Experiment hall around positron source NEPOMUC

In this way, around a billion positrons per second can be generated with the same energy. Finally, a magnetic field directs this antiparticle beam a few meters through an ultra-high vacuum and onto the sample to be examined. If the positrons meet the electrons in the material there, they are destroyed again. Annihilation, or pair annihilation, is the opposite of pair creation. One positron and one electron each are converted into gamma radiation. Measuring this radiation provides researchers with a great deal of information about the material from which it originates. How far a positron penetrates the sample before it is destroyed depends on its kinetic energy.

“The main advantage of using a monoenergetic positron beam is that the surface can be examined in a targeted manner or, if the beam energy is increased, layers close to the surface or thin layers can be examined. You can even get volume information if the positron beam is high-energy. "

In a perfect crystal lattice, it takes around a hundred picoseconds, i.e. fractions of a billionth of a second, before the positron hits an electron. However, if the crystal lattice of the sample has defects - i.e. if individual atoms are missing - this affects the life of the positrons.

“The positron as a probe particle in the crystal lattice is so sensitive to defects because a missing atom there represents an attractive potential for positrons. This means that the positron falls into a kind of hole and lives there longer than in the undisturbed crystal lattice. This longer lifespan of the positron can be determined in experiments using time-resolved spectroscopy. "

This so-called positron annihilation spectroscopy can actually detect individual defects in a sample.

High sensitivity

“The sensitivity of positron spectroscopy is extremely high. If you imagine that ten million atoms are arranged in a regular crystal lattice, then has the positron with the smallest effect - that is, an atom is missing - the sensitivity to be able to detect precisely this missing atom. "

In addition, the sample is not destroyed during the experiment. Because although some electrons annihilate together with the positrons, this has little effect on the material itself.

“A billion positrons that we implant at most in a sample material is still comparatively small compared to the roughly 1023 Atoms that you have per cubic centimeter. "

Positron source NEPOMUC

The nature of a material on the microscopic scale influences its macroscopic properties. Therefore, even small defects in the material can affect its function; for example, they can influence the electrical conductivity of a material.

“In addition to the pure basic experiments that we carry out with the positron beam, it is also used for application-oriented research. By this we mean defect spectroscopy, especially in innovative materials. This can be new solid alloys, metal alloys or even the smallest material defects in semiconductor materials bethat play a prominent role in the whole of computer technology or cellular technology. "

Christoph Hugenschmidt and his colleagues are currently working on a further improvement of the positron source - its intensity is to be increased threefold.