Motivation

Recently, the EDGES experiment announced discovery of a new spectral feature — an absorption profile centered at 78 MHz and 18 MHz wide. The shape of the profile can be described by certain theoretical models but the estimated amplitude was 0.5 K which is two times higher than the expected value. This discrepancy could not be explained in the existing models with “standard” astrophysics. Thus, almost immediately, a large number of extended models, featuring e.g. Dark Matter with non-trivial properties which help it to cool the gas, was suggested.

In Tunka-21cm we approach to the problem from the side of air-shower detection with radio antennas. Modern radio detectors are equipped with antennas and electronics operating in the frequency band of 30-80 MHz, which brings the potential of application of air-shower detectors to the problems of EoR signal detection

The main challenges in these case are the following:

  • Data acquisition in air-shower detectors differs from one using in radio telescopes. Instead of the integration of the signal, air-shower detectors record the entire uncorrelated traces from the different antennas. On the one hand it decreases the exposure of the detector, on the other it can improve the quality of the data, since the data contain more information.
  • The systematic uncertainties are crucial for the detection of EoR signal, meanwhile for the air-shower detection the uncertainties in order of 10% are sufficient.

Antennas and electronics

The array is co-located with the Tunka Advanced Instrument for cosmic rays and Gamma Astronomy (TAIGA) and is installed in northern part of facility. The array consists of four antenna stations located in the square grid 10×10 m, each antenna station is screened from below by a 4×4 m grounded metal net. All antenna stations are equipped with the same electronics and plugged to the ADC in the cluster center. Digitized data is transmitted to the central DAQ of TAIGA via optical fibers.

Antenna station design is taken from the Tunka-Rex detector.
Single station consists of two perpendicular short aperiodic loaded loop antennas (SALLA). The signals from the antennas are pre-amplified with Low Noise Amplifier (LNA) and are transmitted via 15 m coaxial cables to the analog filter-amplifier, which cuts the frequency band to 30-80 MHz.
The signal circuit (LNAs and filter-amplifiers) was calibrated under laboratory conditions.

Sketch of Tunka-21cm array

From the experience of Tunka-Rex we can set the upper limits for the following uncertainties:

  • Antenna production and alignment can bring up to 2%. This value was average over the entire Tunka-Rex array during its live cycle, however for the Tunka-21cm the status of mechanics can be controlled for each run, so we can decrease this uncertainty significantly.
  • Environmental temperature can bring up to 4%. With climate-control at the cluster center we can fix the temperatures for the ADC and filter-amplifiers, however the gain of the LNA is affected by the outside temperature. The influence of the temperature on the gain is measured and known and can be taking into account to cancel this uncertainty.
  • The influence of crosstalk between channels is measured in order of 2%, this uncertainty is by design and most likely cannot be decreased without significant improvement of the electronics. To protect signal chain against external background we developed and installed additional screens on the ADC and filter-amplifiers.

Data acquisition

For the data acquisition we use ADC board recently developed by SINP MSU. The board digitizes radio with the sampling rate of 200 MS/s and dynamic range of 12 bits same as for Tunka-Rex, however the length of the traces is about 56 microseconds, what gives a spectral resolution of about 20 kHz, which is almost an order of magnitude better than for Tunka-Rex. The order is equipped with the modern Ethernet interface allowing remote control and having a bandwidth of about 30 Mbit/s, what converts to a maximal trigger rate of about 15 Hz and data flow of about 10 GiB/h. This way, our initial DAQ conditions are similar to the modern and future self-trigger radio detectors.

The sketch of the Tunka-21cm data acquisition. One can see, that the chain consists of three nodes: cluster center of Tunka-21cm array with ADC and filter-amplifiers, then link to the Tunka cluster 20 and then existing optic fiber connection between Tunka cluster 20 and central DAQ of TAIGA facility with Tunka-21cm central server and data storage. This structure was chosen in order to use the existing TAIGA infrastructure with maximum efficiency.