“Cosi” is designed to detect annihilation radiation

The telescopes that astrophysicists use to survey the universe can provide very different insights - depending on the wavelength range in which they register radiation from space.

Telescopes capture visible light that can also be seen by the human eye. Galileo Galilei was the first to look with a self-made optical instrument and discovered, among other things, the four so-called Galilean moons of Jupiter and craters on the moon.

For one thing, visible-light telescopes have become increasingly powerful since then. The Hubble Space Telescope has been delivering spectacular images from space for years. On the other hand, the researchers are gradually using measuring instruments with which they can also explore space at wavelengths that are inaccessible to the human eye.

For example, the James Webb telescope, which has been stationed in space since 2022, captures infrared light from the depths of the universe and can thus see through cosmic dust clouds. With UV light, microwave radiation or X-rays, the researchers have gradually opened further windows of knowledge about space.

However, there is comparatively little astronomical research at particularly short wavelengths – i.e. in the gamma-ray range. Gamma radiation is a particularly high-energy electromagnetic radiation that is also emitted by radioactive substances.

As early as the 1950s, physicists predicted that various processes in space would produce gamma radiation - for example a supernova explosion. This radiation is difficult to detect on Earth because most of it is absorbed by the Earth's atmosphere - fortunately, because the ionizing gamma radiation would seriously endanger the health of Earth's inhabitants.

The age of gamma-ray astronomy began in 1961 with the Explorer-11 satellite, which had a telescope for this wavelength range on board. With the fewer than 100 gamma photons that the measuring instrument captured at the time, not much could be done scientifically. But the proof was there: There is gamma radiation that comes from space and hits the earth.

The NASA telescope "Comptel" and the European gamma-ray instrument "Integral" were already much more powerful, but could only see the brightest gamma sources in their sky surveys. The gamma-ray telescope "Cosi" (Compton Spectrometer and Imager), whose launch into space is planned for 2027, is intended to bring gamma-ray astronomy into a new era with a sensitivity that has not been reached before.

The researchers involved hope that "Cosi" will provide important insights into the history of the formation and death of stars in our Milky Way - in particular an answer to the question of where which chemical elements were formed. This is also central to the formation of planet Earth and the possibility of biological evolution.

And it can work like this: When a radioactive atom decays, it emits a characteristic gamma radiation, from which the chemical element can be read. For example, if you walk through the Bavarian Forest with a detector for gamma radiation, you will still be able to register the gamma radiation emitted by cesium-137. This pollution has existed there since the Chernobyl reactor accident.

Similarly, chemical elements can also be identified in space based on the gamma radiation they emit – and here from a great distance. These radioactive elements are formed in stars and supernovae.

"Since the radioactive elements emit characteristic gamma rays, which can be distinguished from one another thanks to Cosi's high spectral resolution, we can learn why the distribution of the elements in the Milky Way is as it is," says Thomas Siegert from the Chair of Astronomy at the Julius- Maximilian University of Würzburg. The NASA project "Cosi" is managed by the Space Sciences Laboratory at the University of California Berkeley, while physicists from the Universities of Würzburg and Mainz are involved in the program.

Measuring the radiation from radioactive elements is just one of many tasks planned for "Cosi". The gamma-ray telescope will also analyze particle jets emitted by binary stars with a neutron star or a black hole. The question here is whether the jets also release larger amounts of antimatter. Researchers suspect that the jets are positron sources. The positron is the antiparticle of the electron. It is as heavy as an electron but has a positive charge.

The existence of positrons could be indirectly proven with the help of "Cosi". When a positron hits an electron, both particles annihilate. They cease to exist and their mass is gone. In this process, matter is converted into radiant energy – gamma rays of a very specific wavelength. The more radiation "Cosi" registers at this wavelength, the more positrons must have been present at the observed location.

"The search for galactic positrons is one of Cosi's tasks," says Professor Uwe Oberlack from the University of Mainz, "another is the search for dark matter." So far, researchers do not know what dark matter consists of. There are a number of possible candidates and hypothetical particles - including those whose masses correspond to the energy of gamma rays as detected by Cosi. "Cosi will open up a new observation window for the search for dark matter with gamma rays," says Oberlack.

The measuring range of "Cosi" is between 200,000 and five million electron volts. This indescriptive unit of energy is useful for physicists. An electron volt is defined as the energy gained by an electron after passing through a voltage of one volt.

When an electron and a positron undergo what is known as "pair annihilation," two gamma photons are emitted, each with 511,000 electron volts. In technical jargon, physicists also speak of "annihilation radiation" - a word that can be misunderstood. And the aforementioned radioactive cesium-137 isotope emits gamma rays of 662,000 electron volts as it decays. All of this is within the measuring range of "Cosi".

The cost of the "Cosi" project amounts to around 150 million dollars. However, this does not include the rocket launch.

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