Nehls, S

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29th International Cosmic Ray Conference Pune (2005) 8, 4548LOPES30: A digital antenna array for measuring high-energy cosmicray air showersS.Nehls , W.D. Apel , F. Badea , L. Ba¨hren, K. Bekk , A. Bercuci, M. Bertaina,P.L. Biermann, J. Blu¨mer  , H. Bozdog , I.M. Brancus, S. Buitink  , M. Bru¨ggemann	,P. Buchholz	, H. Butcher, A. Chiavassa, K. Daumiller , A.G. de Bruyn, C.M. de Vos,F. Di Pierro, P. Doll , R. Engel , H. Falcke  , H. Gemmeke, P.L. Ghia, R. Glasstetter,C. Grupen	, A. Haungs , D. Heck , J.R. Ho¨randel, A. Horneffer , T. Huege  , K.-H.Kampert, G.W. Kant, U. Klein, Y. Kolotaev	, Y. Koopman, O. Kro¨mer, J. Kuijpers  ,S. Lafebre  , G. Maier , H.J. Mathes , H.J. Mayer , J. Milke , B. Mitrica, C. Morello,G. Navarra, A. Nigl  , R. Obenland , J. Oehlschla¨ger , S. Ostapchenko , S. Over	, H.J.Pepping, M. Petcu, J. Petrovic  , T. Pierog , S. Plewnia , H. Rebel , A. Risse  , M. Roth,H. Schieler , G. SchoonderbeekO. Sima, M. Stu¨mpert, G. Toma, G.C. Trinchero,H. Ulrich , S. Valchierotti, J. van Buren , W. van Capellen, W. Walkowiak	, A. Weindl ,S. Wijnholds, J. Wochele , J. Zabierowski  , J.A. Zensusand D. Zimmermann	(a) Institut f¤ur Kernphysik, Forschungszentrum Karlsruhe, 76021 Karlsruhe, Germany(b) ASTRON, 7990 AA Dwingeloo, The Netherlands(c) National Institute of Physics and Nuclear Engineering, 7690 Bucharest, Romania(d) Dipartimento di Fisica Generale dell’Universita, 10125 Torino, Italy(e) Max-Planck-Institut f¤ur Radioastronomie, 53010 Bonn, Germany(f) Institut f¤ur Experimentelle Kernphysik, Universit¤at Karlsruhe, 76021 Karlsruhe, Germany(g) Department of Astrophysics, Radboud University Nijmegen, 6525 ED Nijmegen, The Netherlands(h) Fachbereich Physik, Universit¤at Siegen, 57068 Siegen, Germany(i) Inst. Prozessdatenverarbeitung und Elektronik, Forschungszentrum Karlsruhe, 76021 Karlsruhe, Germany(j) Istituto di Fisica dello Spazio Interplanetario, INAF, 10133 Torino, Italy(k) Fachbereich Physik, Universit¤at Wuppertal, 42097 Wuppertal, Germany(l) Radioastronomisches Institut der Universit¤at Bonn, 53020 Bonn, Germany(m) Soltan Institute for Nuclear Studies, 90950 Lodz, PolandPresenter: A. Haungs (andreas.haungs@ik.fzk.de), ger-haungs-A-abs2-he15-posterLOPES, a digital radio antenna array is designed to measure radio emission in extensive air showers (EAS)generated by high-energy cosmic rays. LOPES30 is the extension of an array of 10 antennas, LOPES10,to now 30 inverted V-shaped dipole antennas measuring in the frequency range between 40 and 80 MHz.LOPES operates in coincidence with the KASCADE-Grande air shower experiment, which provides triggerinformation and well-calibrated parameters of the air-shower properties on a single EAS basis in the energyrange from 10 PeV to 1 EeV. The extension of the new antenna field with a maximum baseline of  270 mwill allow to measure the lateral distribution of the radio signal as well as the absolute field strengths and willsignificantly improve the pointing accuracy compared to LOPES10.1. IntroductionDue to their stochastic production processes cosmic ray air showers are a complicated phenomenon. Thereforeas many observables as possible are needed to reconstruct the shower parameters correctly. There are differentapproaches to achieve this experimentally. One can detect the particles produced in many interactions forminga cascade of particles on the ground. A more direct way to study EAS is the detection of fluorescence light46 S. Nehls et al.produced during the shower development. This allows a time resolved investigation of the shower but with thelimitation of measurements only in dark, clear, and, moonless nights. There is additionally a third method toinvestigate high-energy EAS: The detection of radio emission generated by electron positron pairs deflected inthe Earth’s magnetic field. These radio emission of the EAS particles superimposes to a short coherent radiopulse easily detectable on the ground. The LOPES experiment [1] is measuring this radio emission from EAS.2. EAS and radio emissionHigh-energy particles, cosmic rays, arriving at the Earth atmosphere undergo a first interaction with the ni-trogen or oxygen nuclei in the upper layers. From this first and the following interactions a cascade ofsecondary particles arises. The air shower consists of different types of particles, for example hadrons andleptons. The electromagnetic component interacts also with the nuclei of the air by photo pair production andbremsstrahlung. Within these processes relativistic photons, electrons, and positrons are produced were thecharged particles are deflected in the Earth’s magnetic field. They will emit synchrotron radiation stronglybeamed in the forward direction with an opening angle  , with the Lorentz factor ﬁﬀ ﬃﬂ "! .Theoretical studies of this so called geosynchroton mechanism were published by Huege & Falcke [3]. Fromall emitting electrons and positrons a beamed coherent short radio pulse is generated. This pulse containsinformation about the electromagnetic component of the EAS integrated over the whole shower development.Some properties of radio emission from EAS were obtained by earlier experiments and summarized in a reviewby Allan (1971) [5]. For a given energy #ﬃ$ of the primary particle, an angle % to the geomagnetic field, zenithangle & , and distance ' to the shower center an approximate formula for the received voltage (*) per unitbandwidth can be written as:(),+.-0/#$*-21435687:9<;>=@?%BADCE92?&2AD5*FHG:/ﬂI''KJ?MLON&PAH7RQTS6U8VXWZY\[(1)with a parameter ']J around 110 m at a frequencyLof 55 MHz. Compared with recent theoretical studies [4]it could be shown that this formula is in the right order of magnitude and describes the observed dependenciesrather well. Now experiments like LOPES have to verify the radio emission in EAS by detecting the amplitudeof the radio pulse in dependence of the shower parameters.3. LOPES30 hardwareThe hardware design of LOPES is based on the concept of the ”Low frequency array” - LOFAR which will bean astrophysical research project in the frequency range 10 - 200 MHz. For this radio telescope, a large arrayof 100 stations with 100 omnidirectional dipole antennas, each is planned to be installed in the Netherlandsand partially in Germany. In LOFAR and LOPES the entire data stream can be stored for a certain period oftime and therefore transient phenomena like EAS will be detectable.Using the antenna design from LOFAR, LOPES was built at the site of KASCADE-Grande at the researchcenter Karlsruhe, Germany. It is measuring in the frequency range 40 - 80 MHz, avoiding the short-wave andthe strong FM-band. Within this range there are only a few radio transmitters with small bandwidth allowinga relatively good RF suppression. In the first stage of LOPES, ten antennas were running over nearly one year.Since early 2005 LOPES is extended to 30 antennas, fully in data acquisition since March. Analysis resultsbased on LOPES10 data are subject of further contributions at this conference [2, 6].The inverted V-shaped dipole antenna with an opening angle of 82.5 ^ is connected to a low noise amplifiermounted on top of a pyramidical antenna frame (see Fig.1, left). All antennas are equipped with dipoles inLOPES30: A digital antenna... 47DAfrontendPCRAMmodule2 GByteactive antennaPCDAQGB>100dig.data opticaltransmit.80 MSPS40−80 MhzRF1 Gbit/samp+filter80 MHz sample clock40 MHz digital clocksync signal digital dataon optical fiberdig.data PCIbusopticalreceiverMemory Buffer (TIM−Module)2 x 1 Gbit/s2nd input40 MHz80 MHzClock Card5 MHz input1 Hz inputtrigger inputvetotime−stampMaster ClockModule& distributionclock generationsync signaldistributiondistributionsync signaldistributionclockSlave Clock Modulesync signalfrom KASCADEsync signal2 m, 100 m,or 150 mRML (Receiver Module LOPES)ethernet100 m or 180 mcoax cableFigure 1. Antenna and scheme of LOPES30. Incoming radio pulses from EAS are transmitted over 100 m to 180 m coaxcable to the Receiver Module (RML). Over an optical ber the digitized signals are sent to a memory buffer. Receivingtriggers from KASCADE-Grande a master clock module distributes the synchronization signal to slave clock modules.east-west direction, measuring the east-west polarization of the radio emission. The received voltage is againamplified after transmitting the signals from the field antenna to an electronic cluster dedicated to ten antennas.The radio signal is filtered in a sophisticated bandpass filter and afterwards digitized with 12-bit ADCs. Thenecessary dynamic range to detect weak pulses while not saturating the ADC with radio interference is achievedwith these 12-bit ADCs. They are working with 80 MHz, allowing 2nd Nyquist sampling of the signal andresulting in a 12.5 ns time resolution. The 30 antennas are divided into three clusters having the same samplingclock for the ADCs. The sample clock is generated by a master clock module and is then distributed over slaveclock modules to all A/D-boards (see Fig.1, right). This allows to combine data from all antennas as a phasedarray and thus enhances the sensitivity. After the digitization the signal is converted into an optical signaltransferred via fiber optics to a memory module and stored in a front-end PC. From the KASCADE-Grandeexperiment LOPES30 receives a trigger if a majority condition 10 out of 16 clusters of the KASCADE arraydetecting an EAS is fulfilled, which requires _ 10 PeV primary energy. Under this trigger condition 800Ssecof data from the memory buffer of all antennas are saved on a central DAQ-PC.4. Status of LOPES30With LOPES10 the “proof of principal” in detecting radio signals was achieved by comparing relative fieldstrengths in the antenna array [7]. The analysis was done without a precise absolute calibration. For a de-tailed comparison with theoretical predictions of the expected electric field strengths and polarization of radioemission in EAS an absolute calibration of the LOPES30 antenna system is essential. Therefore a conversionfactor for received electric field strengths from EAS and measured voltage amplitudes of the antenna system isdetermined. In a first step the electronic part is calibrated, including receiver module (RML), optical transmis-sion, and TIM-module (see Fig.1) resulting in a frequency dependent amplification factor. In a second step thewhole system will be calibrated, which will be done with a biconical shaped reference radio source mountedon a wooden frame or a stationary balloon. The analysis of the absolute calibration is in progress and willgive us a calibrated antenna system to compare theoretical predictions with received voltages of the radio fieldstrength in EAS.The LOPES30 system is currently triggered by the KASCADE array giving an upper limit in energy of 80 PeVfor detecting EAS. By including the KASCADE-Grande trigger this limit is extended to energies of _ 1 EeV.48 S. Nehls et al.Figure 2. Left: LOPES30 antenna positions (numbered squares) at the KASCADE-Grande experiment. The latter forms aquadratic grid with a central detector and a muon detector. 5 Grande stations are represented by rectangulars. Center andright: Voltage amplitude received in antenna 20 (mid.) and antenna 27 (right) during one event trigger.The antenna layout of LOPES30 is depicted in Fig.2, left. During a shower detection there are RF interferencesgenerated in the detector huts surrounding an antenna. The RF interference correlates with deposited energyin the huts making it more difficult to distinguish the radio signal from induced noise. Four antennas (27 to30) are placed outside of the KASCADE array. They were installed to investigate the noise during radio wavedetection. For these four antennas on the field the RF interference from the huts is negligible, see an example inFig. 2. The center and right picture shows the signal of a triggered single event in a time window of 300Ssecfor antenna 20 (inside the KASCADE array) and antenna 27 (outside the array). A further noise reductionwill decrease the detection limit of LOPES30 allowing to investigate threshold effects of the detection and thelateral distribution of the radio emission. Having a maximum baseline of  270 m a lateral distribution of theradio signal on single event basis can be seen and comparisons with theoretical predictions will be done. Theextension to 30 antennas significantly improves the pointing accuracy compared to LOPES10. This can beused as additional information for the KASCADE-Grande reconstruction of the EAS.5. OutlookThe LOPES antenna system as a LOFAR prototype showed the possibility to measure radio emission fromEAS. With a calibrated system we will be able to verify theoretical predictions about the field strength of radioemission in EAS. Measuring both polarizations of the radio emission will directly verify the geosychrotroneffect as the dominant emission process in EAS.References[1] Haungs, A. et al. - LOPES coll., Proc. 22 `Ha Texas Symp., 2004, eConf C041213, 2413[2] Horneffer, A. et al. - LOPES coll., Proc. 29 bdc ICRC Pune, 2005, these proceedings[3] Huege, T. and Falcke, H., A&A, 2003, 412, 19[4] Huege, T. and Falcke, H., A&A, 2005, 430, 779[5] Allan, H. R., Prog. in Elem. part. and Cos. Ray Phys., 1971, 10, 171[6] Badea, A. F. et al. - LOPES coll., Proc. 29 bdc ICRC Pune, 2005, these proceedings[7] Falcke, H. et al. - LOPES coll., Nature, 2005, 435, 313

LOPES30: A digital antenna array for measuring high-energy cosmic ray air showers

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