The Nancay Decameter Array For many years, the origin of non-thermal cosmic radiation is a very stimulating field of researches. Recent results at the Pierre Auger Observatory have maybe opened a new window in the domain of Ultra High Energy Cosmic Rays: the arrival directions of the charged cosmic particles (above 10^19 eV) can be correlated to the locations of active galactic nuclei (The Pierre Auger collaboration, Science 318, November 2007). These objects could be the source of the radiation but definite conclusions can only be made when more statistic and better detection performances can be obtained. To gain fairly quickly this necessary vault, improvement of the Extensive Air Showers (EAS) detection in terms of surface, duty cycle and cost could be required.

High-energy cosmic rays are detected indirectly by the observation of the giant shower of secondary particles they induce in the atmosphere, with particle detectors on the ground or fluorescence/Cherenkov light detectors. Their characterization requires the simultaneous measurement of several parameters and high statistics. Radio detection of electric field pulses, with or without the associated use of other methods of detection, may provide a major and decisive contribution to high-energy cosmic ray research. The potentialities of this detection technique are the possibility to determine the direction, energy and nature of the primary, together with a high duty cycle and a relatively low cost.

A CODALEMA antenna and its LNA chip In 2003, the Astroparticle group in Subatech was the promoter of the CODALEMA project in association with a few people from the LESIA (Observatoire de Meudon) and from the radio decametric array at the Nançay Radioastronomy Station.

In 2005, a grant from the French ANR (Agence Nationale de la Recherche) allowed to extend the former experimental array of few antennas and particle detectors up to the current 24-dipole antennas and 17 scintillator arrays. Several laboratories were involved in this ANR project (in alphabetical order): ESEO (Angers), LAL (Orsay), LESIA (Observatoire de Meudon), LPCE (Orléans), LPSC (Grenoble), Observatoire de Besançon, Station de radioastronomie de Nançay.

Besides, a subsequent effort has been made in testing the technique at the Auger Cosmic Ray Observatory (Malargüe, Argentina). Those tests had led to the setting of a prototype of self contained, self triggered and autonomous radio station which prefigures the future of cosmic ray radio-detection for deployment over very large areas.

Several main results have been obtained from the experimental data such as:

  • the correlation between the arrival directions and times measured by the antenna array and by the scintillator arrays,
  • the detection efficiency and the angular distributions depending on the Earth magnetic field,
  • the polarization and the polarity of the radio signals also influenced by the geomagnetic field,
  • the lateral distributions of the radio signals usually with an exponential shape but with a more complex form in some cases,
  • the clear correlation between the primary cosmic ray energy determined from the particle detectors and the estimate of this energy deduced from the radio signals.
References to the articles and conferences presenting these results are given in the Main Publications section

The activities of the Astroparticle group during the 2 past years thus cover several aspects, which are discussed with some details in the following reports (from the 2006-2008 Subatech's scientific report) :

First prototype of the RAuger autonomous station CODALEMA is currently offering a calibration of the method around 10^17 eV. For subsequent years, the plan is to set-up an extended array of autonomous radio stations in Nançay, spanning a large area together with an extension of the ground particle detectors array.

In parallel, most of the groups of the CODALEMA collaboration are involved in the AERA (Auger Engineering Radio Array) international project, aiming to set up a 20 km² 'super hybrid' detector on the Auger Southern site, which will combine information from ground, fluorescence and radio detectors. Those parallel developments would open the way to the extension of the radio detection method to energies above 10^18 eV.