The Tunka Radio Extension (Tunka-Rex) is an array of 25 radio antenna stations at the Tunka-133 experiment, covering an area of approximately 1 km². While Tunka-133 measures the Cherenkov light of air showers in the energy range up to 1 EeV, Tunka-Rex measures the radio emission of the same air showers. Tunka-Rex started operation on 8 October 2012 with 18 antenna stations and was extended to 25 stations in October 2013. Thus, Tunka is currently the world-wide unique place for performing Chrenkov-Radio hybrid measurements of ultra-high energy cosmic rays.

In 2014, Tunka-Rex will be extended by approx. 20 additional radio antennas triggered by the scintillator extension of Tunka detecting the secondary particles of the air shower. This enables radio measurements around the clock, independent of the light conditions.

The goals of Tunka-Rex are:

• A cross-calibration of the radio and the Chrenkov signal emitted by air showers. This allows to determine which precision radio measurements have for the energy and mass of the primary cosmic ray particles.
• Demonstrating that radio antenna arrays allow to cover relatively large areas with a cost-effective and efficient technology for ultra-high energy cosmic rays.
• Enhancing the duty cycle of Tunka by an order of magnitude, since radio measurements are possible during almost any weather conditions, while Chrenkov light measurements of air showers can only be performed during clear and moonless nights. Although no stand-alone mode is foreseen for the operation of Tunka-Rex, this goal can be achieved by combining Tunka-Rex with the planned Tunka enhancement of scintillaton detectors.

Consequently, Tunka-Rex is a path-finder experiment for a new and promising detection technology for ultra-high energy cosmic rays. It contributes to the still unsolved question, from where the highest energy particles ever measured in the universe do originate.

More information at: Tunka-Rex Homepage

Tunka-Rex antenna. Left in the image: Tunka photomultiplyer detector for the Cherekov light generated by air showers, and white electronics box for data-acquisition of the antenna and the photomultipliers.