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Detecting Radio Pulses from Air Showers with LOPES: Prototype of a LOFAR Station

LOPES (LOFAR Prototype Station) aims to measure radio emission from air showers, determine radiation mechanisms, and calibrate data with existing air shower arrays. This text provides an overview of LOPES hardware, setup, and data processing steps.

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Detecting Radio Pulses from Air Showers with LOPES: Prototype of a LOFAR Station

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  1. Detecting Radio Pulses from Air Showers with LOPES Andreas Horneffer for the LOPES Collaboration

  2. LOPES(LOFAR PrototypeStation) • prototype of a LOFAR station • frequency range of 40 – 80 MHz • set up at the KASCADE-Grande site • 10 antennas in the first phase, 30 antennas in the second phase • Goals: • develop techniques to measure the radio emission from air showers • determine the radiation mechanism of air showers • calibrate the radio data with theoretical and experimental values from an existing air shower array

  3. Hardware of LOPES

  4. Hardware of LOPES LOPES-Antenna • short dipole with “inverted vee shape” • beamwidth 80°-120° (parallel/ perpendicular to dipole)

  5. Hardware of LOPES Receiver Module • direct sampling of the radio signal with minimal analog parts: amplifier, filter, AD-converter • sampling with 80 MSPS in the 2nd Nyquist domain of the AD-converter

  6. Hardware of LOPES Memory Buffer aka. TIM-Module (Twin Input Module) • uses PC133-type memory • memory for up to 6.1 seconds per channel • pre- and post-trigger capability

  7. Hardware of LOPES10 Clock & Trigger distribution board • 1 master & 3 slave boards • master board generates clock and accepts trigger • slave boards distribute clocks and trigger

  8. LOPES: Setup & Status • 30 antennas running at KASCADE (10 antennas in first phase: LOPES10) • triggered by large event (KASCADE) trigger (10 out of 16 array clusters) • offline or online correlation of KASCADE & LOPES events • KASCADE provides starting points for LOPES air shower reconstruction • core position of the air shower • direction of the air shower • size of the air shower • Averages 2500 – 3000 events per full day • ca. 10 GByte uncompressed data per day •  ca. 3.6 TByte per year

  9. Data Processing • steps of the data processing: • instrumental delay correction from TV-phases • frequency dependent gain correction • filtering of narrow band interference • flagging of antennas • correction of trigger & instrumental delay • beam forming in the direction of the air shower • optimizing radius of curvature • quantification of peak parameters

  10. Delay correction • TV-transmitter with picture- and two sound carriers • relative phases between antennas lets us correct for delay errors delay corrections residual delays

  11. Gain calibration • measured the gain of the different parts “in the lab” • combined all to a frequency dependent gain curve • biggest uncertainty: match of antenna to LNA LNA (dark blue), cable (green), receiver Module (red), total (light blue)

  12. Digital Filtering raw data: power spectrum: filtered data: blocksize: 128 samples blocksize: 64k samples

  13. Beamforming Electric field and power before time shifting Electric field and power after time shifting

  14. Event Discrimination • criteria for “good” events: • existence of a coherent pulse • position in time of pulse • uniform pulse height in all antennas • selection currently done manually Bad Event Good Event

  15. Example events 1 Antenna Data Pulse undetected by program Formed Beam Bad Event Good Event

  16. LOPES10 Data • LOPES10 ran from January to September 2004 • 630 thousand events total • used selection for further study: • KASCADE array processor didn’t fail • distance of the core to the array center < 91m • shower size (number of electrons) > 5e6 or truncated muon number > 2e5 • → 412 events

  17. Detected Events • 228 out of 412 events considered good • Fraction of “good” to “bad” events increases with increasing Muon number and increasing geomagnetic angle • → fraction also increases with zenith angle

  18. Dependencies: Geomagnetic Angle • Divided pulse height by muon number • Fit results to the cosine of the geomagnetic angle • Fit exponential decrease to distance

  19. Dependencies: Size, Nµtrunc and Energy • Divided pulse height by the results from previous fits. • Added undetected events with height 2σ • Only little dependency on electron number • Power law is a good fit for muon number or energy

  20. Summary • LOPES measured radio pulses from air showers for the first time since 30 years • with digital filtering and beam forming these radio pulses can be measured even in a radio loud environment • measured radio pulse height depends on the angle to the geomagnetic field • radio pulse height correlates well with Nµ and energy and not so well with Ne • radio can give useful complementary information for air shower analysis • additional value for energy and mass determination • independent direction measurement • position of the shower center

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