Results of Hybrid Photodiode irradiation by 200 MeV protons Hybrid Photodiodes (HPD, [1]) will be used as the photodetector for the Compact Muon Solenoid (CMS) Hadron Calorimeter (HCAL) readout [2] . The HPDs are required to operate in a high radiation environment, where the HCAL detector will receive a total ionizing dose of about 330 rads and a fluence of 4x 10 11 n/cm 2 over a 10 year running period [3] . Effects of HPD irradiation by low energy neutrons were studied and reported previously

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... l]. In these studies, high energy protons are used to study possible effects of single event burnout [ 4], since high energy protons are more likely to induce large energy transfer within the HPD silicon. The HPDs were irradiated by 200 Me V protons at the Indiana University Cyclotron Facility [IUCF, 5] . The results of the study are presented below. Setup The principles of HPD operation have been described in an earlier note [6] . The HPD schematic view is shown in Fig. 1 . The input fiber-optic glass window thickness is 5 mm. A photocathode (PC) is deposited on the internal surface of the window in a sealed vacuum area. The distance between the PC and the silicon (Si) is 3.35 mm+/-0.02 mm for the HPD. The HPD silicon thickness is 200 µm and the sensitive area diameter is 25 mm. Fig. 2 shows the positioning of the 73 pixels in the HPD. The flat-to-flat distance of the hexagon shaped pixel is 2.8 mm. The HPD was positioned on a moveable table to simplify its alignment in the beam. The proton beam incidence was normal to the optical window and to the silicon of the HPD. The setup is shown in Fig. 3 . For these studies, the beam flux was 5x10 7 protons/cm 2 /s. The non-uniformity of the beam intensity across the HPD sensitive area was less than 20%. The signals were taken from the central pixel, number 37, only. The remaining pixels were grounded. The signal from pixel 37 was amplified by factors of 30 and 300 using two amplifiers. A QVT module was used to analyze the HPD signal spectra. A Tektronix TDS 3054, 500 MHz digital oscilloscope was used to observe and register the HPD signals. The HPD bias current was measured by a TENMA digital multimeter, model number -72-6202. A schematic view of the apparatus is shown in Fig. 4 .