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one hundred years after their discovery the properties of uhecr , the most energetic particles in the universe , remain still uncertain .
their sources , nature and acceleration and propagation mechanisms are nowadays some of the major open questions in astrophysics .
a great effort has being carried out with experiments such as hires  @xcite and agasa  @xcite and at present the pierre auger observatory  @xcite to acumulate statistics in a energy region ( well above 1 eev ) where the rate of events is extremely low due to the suppression of the flux , recently observed and confirmed  @xcite , when uhecr interact with the cosmic microwave radiation .    when an uhecr interacts with an atmospheric molecule it developes an extensive air shower ( eas ) .
the particles in the shower deposit part of the energy carried by the primary in the atmosphere through ionization , resulting in a free electron plasma .
microwave emission due to the molecular bremsstrahlung of free - electron collisions with the neutral molecules of the atmosphere has been measured in a particle accelerator and could be used to detect air showers @xcite .
a cosmic ray detector based on this technique would provide calorimetric measurement of the primary energy and a direct measurement of the maximum development   @xmath0 , main advantages of the fluorescence detectors used at present in cosmic ray experiments .
this technique has also a nearly 100% duty cycle , as surface detectors sampling the shower at ground level , and very little atmospheric attenuation .
the midas experiment , a prototype detector to characterize this microwave emission , has been recently commissioned at the university of chicago .
design , specifications of the electronic components , a description of the implemented trigger logic , and preliminary test results are discussed below .
the main goal of the midas experiment is to detect and characterize the microwave emission from eas . for this purpose ,
we have built a wide field of view telescope with a camera of 53 channels of 0.8 ghz bandwidth in the 4 ghz microwave band .
the center of the camera is located at the focus of a 4.5 m diameter parabolic antenna .
each channel is equipped with a c - band commercial satellite television receiver , i.e. a combination of feed horn , low noise amplifier , and frequency down converter .
the signal from each of these receivers is then transformed to a voltage level using a fast power detector with logarithmic response to the input rf power . the dc output signal ( ranging from 0.5 - 2.1 v ) is continuously digitized in custom 14-bit 20 mhz analog - to - digital converters ( adc ) modules .
digital samples are processed locally with on - board field programmable gate arrays ( fpgas ) , where a trigger logic allows discrimination of events .
a trigger master board is used to synchronize all adc boards , and to generate a global trigger .
it provides a system clock for simultaneous sampling and receives the local trigger from each adc board via front lvds - panel connections . after a trigger ,
a 100 microsecond time stream of data ( 25 @xmath1s corresponding to data before the trigger ) is packed and buffered for readout via a vme back panel and stored for later analysis . a vme gps module ( hytec 2092 )
is also used for precise timing information .
the 53 receiver channels are serviced by 4 identical adc boards . on - board fpgas
allow local data analysis , based on a trigger level system developed to select candidate events .
the trigger system is based on the same principles as the one used for the fluorescence detector of the pierre auger observatory  @xcite .
a first level trigger ( flt ) at the channel level , has been designed to identify peaks in the fadc trace .
the flt information of all channels is then sent to the trigger master board to perform a second level trigger ( slt ) at the level of the whole camera .
if the slt condition is satisfied , the system trigger pulse is send to the 4 adc boards for the readout .
these first and second level trigger conditions are implemented as follows :    flt : a channel is marked as triggered if the running sum of 20 consecutive bins exceeds an adjustable threshold .
this status is extended for 10 @xmath1s to allow coincidences between channels in subsequent trigger requirements .
the threshold is continuously regulated to compensate the changing background conditions and to ensure a constant trigger rate per channel of 100 hz .
the procedure is illustrated in figure [ fig : flt ] .
slt : the flt information is sent to the master trigger board where another on - board fpga is used to evaluate trigger conditions at the level of the whole camera . the slt block is designed to search for 4-fold patterns of flt channels compatible with an uhecr shower track .
a slt rate of much less than 1 hz is expected from random coincidences .
high level trigger conditions have been also developed to deal with the different background conditions .
for example , the threshold regulation and data acquisition are inhibited when slt rate is higher than a certain limit .
the daq system is continuosly monitoring the data conditions by recording each second an average baseline ( over 10 ms ) for each channel , the instantaneous threshold and flt rate . figure [ fig : sun ] shows the temporal evolution of the baseline for the central channel of the camera with the adjustable threshold for the same period .
as it can be observed , the threshold reproduces accurately the baseline profile .
the large peak shown in the plot is originated by the sun passing through the field of view of the feed .
the fast peaks in the figure are noise bursts , possible induced by aircrafts , which reduce significantly the duty cycle of the experiment .
an absolute calibration of the antenna is provided by the sun , whose emission in the frequency of interest is measured every day by several radiotelescopes around the world .
figure [ fig : sun ] shows the signal in adc counts generated by the sun crossing the field of view of the central channel of the camera .
the system temperature can then be derived from the difference in adc counts between the peak induced by the sun and the baseline . a patch antenna located at the center of the 4.5 m dish
is used to generate rf signals of known intensity , thus performing a relative calibration of the individual feed channels .
also , it allows the study of the relative timing , which was found to be well below the 50 ns sampling .      up to now
, the telescope has been taking data in stable conditions during 40 days with an average event rate of 0.5 hz .
a prelimary analysis suggests that data are dominated by noise , both the expected uncorrelated random noise and correlated noise due to fast transient events of unknown origin .
all channels present a good long term stability with baseline fluctuations smaller than 1 db . physics event search is still in progress .
events simulations have been also carried out including the measured values of the detector calibration , the noise level and the microwave emission scaled from the laboratory measurements .
the midas experiment has been assembled and commissioned between october 2009 and february 2010 at the university of chicago .
the first period of data taking has been focused on the understanding of the detector and of the noise conditions .
the sensitivity of the prototype was found to be adequate for detection of uhecr .
some improvements are being implemented to increase the duty cycle to close to 100% . in order to crosscheck candidate events with already established detection techniques ,
we foresee the operation of midas at the southern auger observatory .
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recent measurements suggest that extensive air showers initiated by ultra - high energy cosmic rays ( uhecr ) emit signals in the microwave band of the electromagnetic spectrum caused by the collisions of the free - electrons with the atmospheric neutral molecules in the plasma produced by the passage of the shower . 
 such emission is isotropic and could allow the detection of air showers with 100% duty cycle and a calorimetric - like energy measurement , a significant improvement over current detection techniques . 
 we have built midas ( microwave detection of air showers ) , a prototype of microwave detector , which consists of a 4.5 m diameter antenna with a cluster of 53 feed - horns in the 4 ghz range . 
 the details of the prototype and first results will be presented .