1. 1 Max-Planck-Institut fuer Physik, Muenchen, Germany, 2 Humboldt-Universituet Berlin, Germany, 3 Univ. Complutense, Madrid, Spain, 4 ETH, Zurich, Switzerland, 5 Universitaet Wuerzburg, Germany T.Y. Saito 1, M. Shayduk 1,2, M.V. Fonseca 3, M. Hayashida 1, E. Lorenz 1,4, K. Mannheim 5, R.Mirzoyan 1, T. Schweizer 1, M. Teshima 1 on behalf of the MAGIC Collaboration MAGIC Webpage @ http://wwwmagic.mppmu.mpg.de/ Abstract Recently Developed HPDs from Hamamatsu with a GaAsP photocathode, namely the R9792U-40, provide a peak QE of more than 50% and a pulse width of ~2 nsec. The afterpulsing rate is very low compared to that of conventional PMTs, i.e. the value is 300 times lower. Photocathode aging measurement showed the lifetime to be more than 10 years under standard operating conditions of Cherenkov Telescopes. Temperature dependence of avalanche gain can be suppressed to the same level as that of a PMT gain by a simple compensation circuit. MAGIC Telescope AfterPulse Temperature Compensation Lifetime Quantum Efficiency Ph.e. Resolution The MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescope, with a reflector diameter of 17 m, is the world’s largest Imaging Atmospheric Cherenkov telescope (IACT). Since fall 2003 it has been in operation on the Canary Islands of La Palma (28.75 o N, 17.90 o W and 2200m a.s.l). Low afterpulsing probability is important to avoid explosion of the event rate for low energy threshold settings. Afterpulsing probability has been measured as a function of the threshold level of the afterpulse by using two different intensity light pulses. (See figure on the right). Compared to the PMT used in the current telescope, the probability is 300 times lower above 2 ph.e. level. If the threshold of the afterpulse is set at a 1 ph.e. level, the probability increases by two orders of magnitude. Due to large amplification (~1500) by electron bombardment Excellent resolution up to 6 ph.e. Useful for the light intensity calibration Useful for the gain calibration The avalanche gain has strong temperature dependence (2%/ o C). Simple temperature compensation circuit has been developed by using a thermister (See figure to the right). Temperature dependence of the avalanche gain was suppressed to the level of ~0.3%/ o C from 25% to 35%, which is the same level as that of PMTs. We tuned the system for a mean temperature of 30 degrees but can easily shift it by changing resistors of the circuit. Lifetimes of 5 HPDs have been measured by accelerated tests. All 5 HPDs showed more than 10 years of lifetime In order to lower the current energy threshold further and increase the sensitivity, a second 17-m diameter telescope, located at 85-m distance from the first telescope, is being constructed. We call this stereoscopic observation by two telescopes the MAGIC-II project. In the MAGIC-II project, in addition to the gain of stereoscopic observation, we are planning to use high quantum efficiency Hybrid PhotoDetectors (HPDs) with GaAsP photocathodes as alternative photo sensors to PMTs, which are used in IACTs. HPD R9792U-40 Photocathode 18 mm  GaAsP Bombardment Gain ~1500 @ 8kV Avalanche Gain ~50 @ 450V Dynamic Range Up to 5000 ph.e. Dark Count Several 10 kHz Leakage Current of AD ~5 nA @ Av. Gain. Of 50 Sensor Shape Hexagonal 28 mm Pulse Width ~ 2 nsec Due to a GaAsP Photocathode More than 50% at around 500 nm WFS coating enhances Q.E. in UV range Two times more photoelectron compared to PMTs used in the current telescope. An HPD R9792U-40 consists of a GaAsP photocathode and of an Avalanche Diode (AD) serving as an anode. When applying a ~8kV high tension to the photocathode, photoelectrons are accelerated in the high electric field and bombarded to the AD. This electron bombardment produces ~1500 electron-hole pairs per one photoelectron. These electrons subsequently induce avalanches in the active volume of AD and provide an additional gain of 30-50 with a bias voltage of a few hundred volts. Defining that  Lifetime : Period when Q.E. degrades by relatively 20%.  1 year : 1000 hour operation under 300MHz background photons (standard conditions for IACTs) AD bias voltage IN (0-2V) OUT (0- 400V) Thermister Ishizuka 103AT-2 Arrival time distributions of afterpulses are shown below. Above 2 ph.e. (left figure), several peaks can be seen, (45, 60,90,135,180 nsec). Assuming a uniform electric field with a potential difference of 8kV and a distance of 2.8 cm, the travel time of molecules from the AD to the photocathode can be estimated as ~45 (M/M p /Z) 1/2 nsec. Afterpulses above 2 ph.e. level may be due to ion-feedback from the surface of the AD. On the other hand, above 1ph.e. (right figure), an exponential decay structure can be seen. This may be due to photon-feedback from AD. Afterpulsing probability (P AP ) : P AP = N AP : number of afterpulses N MP : number of main pulses M MP : number of ph.e. in a main pulse N AP N MP *M MP 1500 4 were measured in Hamamatsu Photonics and 1 was measured in Max-Planck-Institut fuer Physik. Possible candidates responsible for feedback ions are written over the peaks. Arrival time Distribution of AP (>2 ph.e.) Arrival time Distribution of AP (>1 ph.e.)  = ~140 ns  = ~800 ns Sum of two exponentials fits to the arrival time distribution of afterpulses above 1 ph.e..