Although much of the focus of the community goes to the extreme energy end of the cosmic-ray spectrum, the so-called “transition region” between Galactic and extragalactic sources, at energies 1016.5-18.5 eV, is equally mysterious. Theoretical studies tell us that a discrimination between the different source scenarii will require a precise measurement of the energy spectra of individual mass groups at the transition region. It will further require to solve the so-called muon number discrepancy observed between measured and simulated air-showers, which is a paramount hadronic physics issue per se.
The first autonomous radio detector!
GRANDProto300 will be an array of 300 antennas deployed over 200 km2. It will operate in the 50-200 MHz band and will trigger, from radio data alone, on nearly horizontal air showers, separate the signal from the background, and reconstruct the properties of the primary particles with a precision rivaling standard cosmic-ray detection techniques. Demonstrating that autonomous detection can succeed is an exciting challenge. This dedicated experiment, with adequate electronics capable of treating signals at high frequency rates, installed in an unprecedented clean radio environment, should be in the best situation to undertake in this challenge.
A hierarchical density array to cover the full Galactic to extragalactic transition energy range.
The preliminary layout of GRANDProto300 covers 200 km2 with ~200 detection units to enable large statistics at EeV energies, complemented with two denser levels of infills. This density hierarchy enables to reach good accuracy and large statistics from energies as low as 1016.5 up to 1018.5 eV, with a single system. The exact layout of GP300 will be defined by dedicated simulations, taking also into account the physical properties of the selected site. With 105 detected cosmic-ray events above 1016.5 eV already in the first year, GRANDProto300 will be in a privileged position to study the Galactic to extragalactic transition.
A hybrid array for an exquisite accuracy on the cosmic-ray mass composition.
The accuracy of the mass composition measurement can be improved by measuring simultaneously and independently several components of the air-shower. An ideal setup, as in GRANDProto300, is thus a hybrid array of radio antennas and muon detector, to have a handle on both the electromagnetic and muonic contents of the shower. For inclined showers as will be observed in GRANDProto300, the electrons are absorbed in the atmosphere, leaving only muons on the ground. Autonomous radio detection allows to measure the shower energy mostly independently from hadronic models.
A pathfinder for GRAND.
GRANDProto300 will act as a pathfinder for future large-scale radio experiments in general. In particular, it will be the first prototype phase of the GRAND experiment.
The radio experimental setup of GRANDProto300 is mostly ready.
The radio hardware has been conceived and is being produced: the antennas, specifically designed at SUBATECH Nantes, to observed horizontal air showers, are being tested and finalized, and will be calibrated and produced at Xidian University in China. The Data Acquisition systems (DAQ) have been designed at Radboud University in Nijmegen, and are being currently produced in the Netherlands, before entering massive production in China. The consumption of one detection unit is estimated at 10 W. Thus, a 100-W solar panel, coupled to a battery, should allow for its continuous operation. The mechanical integration of the detection unit is under study. Given the antenna size and weight, we can adopt standard solutions used in electric power distribution and deploy the units on 5 m-tall wooden utility poles buried 1 m underground. In addition, by placing the electronics atop a pole, we reduce the ecological footprint of the detector and the risk of damage by wild fauna and cattle.
Deployment site and political support in China. A dozen of site surveys have been conducted in the past 2 years in mountainous and desertic areas in China and an excellent 300 km2 site with clean radio background properties and good accessibility has been selected in the Qinghai Province, on the verge of the Tibetan plateau. The project has strong political support at local and Province level, and deployment is planned for 2021.
Funding sources. The total cost of the project is estimated to be 1.3 Million euros for the radio hardware aspects: this funding is fully secured through Chinese grants. The plan is to have the muon detector array funded collectively by non-Chinese grants.