Gaug, M

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29th International Cosmic Ray Conference Pune (2005) 5, 375378Calibration of the MAGIC TelescopeM. Gaug , H. Bartko, J. Cortina and J. Rico (a) Institut de F·sica d’Altes Energies (IFAE), 08193 Bellaterra, Barcelona, Spain(b) Max-Planck Institute f¤ur Physics, 80805 Munich, GermanyPresenter: J. Rico (jrico@ifae.es), spa-rico-J-abs2-og27-posterThe MAGIC Telescope has a 577 pixel photo-multiplier tube (PMT) camera which requires precise and regularcalibration over a large dynamic range. A system for the optical calibration consisting of a number of ultra-fast and powerful LED pulsers is used. We calibrate each pixel using the F-Factor Method with signals inthree different wavelengths. The light intensity is variable in the range of 4 to 700 photo-electrons per PMT.We achieve an absolute calibration by comparing the signal of the pixels with the one obtained from a 1 cmPIN diode. This device is calibrated with the emission lines of two different gamma-emitters (Am andBa) which produce a precise reference signal in the active region of the PIN diode. The time resolutionof the entire MAGIC read-out system has been measured to about 700 ps at intensities of 10 photo-electronsreaching 200 ps at 100 photo-electrons. With an external calibration trigger, it is possible to take calibrationevents interlaced with normal data at a rate of 50 Hz. The entire system has been used on-site for one year.1. IntroductionThe MAGIC Telescope [1] houses a camera of 397 inner pixels (0.1 	 ) and 180 outer pixels (0.2 	 ), each readout with 300 MSample/s flash-ADCs [2] and an optical link of 260 MHz bandwidth to transfer the electronicsignal over 160 m to the counting house. The quantum efficiency (QE) of the MAGIC PMTs strongly depend onthe incident wavelength. Moreover, differences in the exact shape of QE(  ) between PMTs had been observed.It is therefore desirable to calibrate the PMT response at different wavelengths.We use a system of very fast (3–4 ns FWHM) and powerful (10  –10photons/sr) light emitting diodes [3](NISHIA, single quantum well) in three different wavelengths (370 nm, 460 nm and 520 nm) and differentintensities (up to 700 photo-electrons per inner pixel and pulse) so that we are able to check the linearity of thewhole readout chain.2. Excess noise factor methodIn the last year, the camera was calibrated using the F-Factor. Assuming that the number of photons impingingon the photo-cathode has a Poisson variance, that the photon detection efficieny is independent from the placewhere and under which angle the photo-electron is released and that the excess noise introduced by the readoutchain does not depend on the signal amplitude, one can derive [4]: ﬀﬁﬂﬃ! #"$&%"$('Here," describes the signal extractor resolution (mostly due to noise of the night sky background)," themeasured standard deviation of the signal peak andﬁis mean reconstructed (and pedestal-subtracted) signal. stands for the excess noise factor, previously measured in the lab.The method yields one value of ) per pixel the average of which is used to extract an averaged photo-electron fluence per light pulse and inner pixel *+)$, . All reconstructed signals from data are then multi-plied with a conversion factor -.)*/01,2435.676ﬃﬁ. where 8 stands for the pixel index and 39.676 for theratio of covered areas. 39.676 is 1 for all inner pixels and 4 for all outer pixels.Figure 1 left shows the evolution of the mean number of photo-electrons over one night. One can see that,376 M. Gaug et al.despite some long-term dependency, *:  , remains stable to about 1% on minute scales. More preciseinvestigations revealed a dependency of the light output with ambient temperature of about 2 ;ﬃ4<.Figure 1 right shows *=>ﬀ, , calculated at different intensities and with different light colours.Time [min.]0 50 100 150 200 250 300 350> [1]phe<N3031323334353637383940> [FADC cnts]µMean Charge <102 103> [1]phe<N10102Green PulsesBlue PulsesUV PulsesFigure 1. Left: Evolution of the mean number of photo-electrons over one night. Right: Mean reconstructed number ofphoto-electrons vs. mean reconstructed charge at various intensities.3. PIN-Diode methodWe measured the absolute light flux with a PIN diode, situated at 105 cm distance from the calibration lightpulser and read out with a charge sensitive pre-amplifier (shaping time 1ﬁs) [3]. The PIN diode was calibratedwith an?Am source emitting 59.95 keV photons and aBa source having an emission line at 81.0 keV [5].The assumed average energy to create an electron-hole pair by ionization is 3.62 eV at 20 	 C producing apeak in the recorded PIN Diode spectrum due to photo-absorption. Figure 2 shows the obtained spectra andthe signals from two typical calibration light pulses. The quantum efficiency of the diode was obtained bycomparison with a calibrated PIN diode. An average QE is obtained by folding the LED spectrum with the QEfor each wavelength.Table 1 shows the result of two dedicated calibration runs where both calibration methods (F-Factor and PINDiode) have been applied to the same data. One can see a good agreement between the two results, althoughthere is still need to reduce the systematic uncertainties (see caption).4. Time CalibrationThe photo-multipliers introduce a time delay, the “transit time (TT)”, in the amplified photo-electrons signal,depending on the applied high-voltage (HV). Together with smaller relative delays due to different lengths ofthe optical fibers, these delays have to be calibrated relatively to each other in order to obtain a correct timinginformation for the analysis.Calibration of the MAGIC Telescope 377Table 1. Average number of photo-electrons for one pixel,calculated with both calibration methods for two different lightintensities. For the PIN Diode Method, an additional systematic error of @BADCFEHG and for the F-Factor Method, I 5% pluspossible degradations of the overall PMT quantum efciency and transmission coefcients have to be added.Number of LEDs fired *J>K, *=0K,F-Factor Method PIN Diode Methode10 Leds Blue 291 L 3 294 L 320 Leds Blue 605 L 6 613 L 6channel0 50 100 150 200 250counts10102103104 20 Leds Blue10 Leds BlueAm (60 keV)241Ba (81 keV)133Ba (Comp. edge 207 keV)133Figure 2. Spectra taken with the PIN Diode and various sources: bottom: MONP Am with a 60 keV photon emission line,center: P#QRQ Ba with a photon emission line at 81 keV and a Compton egde at 207 keV, top: the spectrum obtained from lightpulses with two combinations of LEDs.Using the light pulser at different intensities, we measured the time offsets and time spreads of the readout anddetection chain. Event by event, the reconstructed arrival time difference of every channel with respect to areference channel was measured and its mean and RMS calculated. The former yields the measured relativetime offset while the latter is the convolution of the arrival time resolution of the measured and the referencechannel.Figure 3 (left) shows the time offset versus the applied HV for each PMT. Like expected, one can see a clearanti-correlation. The smaller the applied HV, the longer the signal takes to travel from the first dynode to theanode.Figure 3 (right) shows the time resolution (RMS of the arrival time differences histogram, divided by the squareroot of 2), measured at different intensities. The measurements have been fitted by the following function:S)TUT*=V:,ﬃ4WYXYZ)[T*JV:,ﬃWYX1Z[T]\ (1)whereT parameterizes the combination of intrinsic arrival time spread of the photons from the light pulserand the transit time spread of the PMT,T parameterizes the reconstruction error due to background noise andlimited extractor resolution andT is a constant offset which might be due to the residual FADC clock jitter.378 M. Gaug et al.HV [V]950 1000 1050 1100 1150 1200 1250 1300 1350 1400 T [ns] (arb. ref.)∆-4-3-2-101234mean signal [ph.el.]10210 310Time Resolution [ns]-1101 / ndf 2χ 49.92 / 10 [ns]: 1T  0.16± 1.69  [ns]: 2T  0.78± 5.68  [ns]: 0T  0.02± 0.21 Green Pulses Inner PixelsBlue Pulses Inner PixelsUV Pulses Inner PixelsFigure 3. Left: Calibrated arrival time offsets ^&_ vs. applied HV. Right: Calibrated time resolution for various intensities.5. ConclusionsThe LEDs pulser calibration system has been used to calibrate the MAGIC camera for about one year. If runin standard mode, it fires UV-pulses of 370 nm in dedicated calibration runs and additionally as interlacedcalibration events with a frequency of 50 Hz. This frequency allows to accumulate enough statistics beforereaching the typical time scales of residual short-term fluctuations of the optical transmission gains. Thecamera is such continuously monitored and re-calibrated [6] using the F-Factor method.In dedicated calibration runs with different colours and intensities, the response of the system and signal ex-traction methods [7] have been tested.Using a calibrated PIN Diode, the light flux of the pulser has been measured with an independent methodyielding consistent results with the F-Factor method. The systematic error of both methods is still above 5%(8% for the PIN Diode method) and will be reduced in the future.Using the light pulses to calibrate the arrival times extracted from each channel, we find an upper limit to thetime resolution of the MAGIC telescope for cosmics pulses of:S>Ta`Rbcedgf `Rc]hU ikj1l*mVn,ﬃ4WYXYZ[ o(pj1l*=Vn,ﬃWYX1Z[p\pikjql\ (2)References[1] R. Mirzoyan for the MAGIC coll., these proceedings[2] F. Goebel et al., Proc. of the 28 r  ICRC, Tokyo (2003)[3] T. Schweizer et al., IEEE Transactions on Nuclear Science, San Diego (2001)[4] R. Mirzoyan and E. Lorenz, Proc. of the 25 r  ICRC, Durban (1997)[5] L. Holl, E. Lorenz, G. Mageras, IEEE Transactions on Nuclear Science, Vol.35, Nr. 1 (1988)[6] J. Cortina, these proceedings[7] H. Bartko, M. Gaug, A. Moralejo, N. Sidro, astro-ph/0506459

Calibration of the MAGIC Telescope

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