Results on the LHCb hadron calorimeter prototype

The hadron calorimeter HCAL of the LHCb experiment at CERN is the key device which provides data for the level-0 high pt trigger and can perform fast trigger signal construction with reasonable background suppression when B decays are reconstructed. During 1997-1999 an HCAL prototype stack of 6 modules of 0.96 x 0.96 m2 front surface and with different internal structure has been constructed and exposed to the X7 test beams of the SPS at CERN. HCAL technology consists of a scintillator/iron sampling structure with scintillating tiles parallel to the beam axis and a passive radiator made of steel plates. The absorber plates were supplied by Romanian, CERN and IPT Kharkiv groups. Scintillating tiles have been produced in Russia using a modern casting technique. In the prototype we use Pol.Hi.Tech.(S250) single clad wave-length shifting fibers of 1 mm in diameter with a polished and aluminized mirror at one end. Parts of two HCAL modules have been recently re-assembled with new faster fiber of type Bicron BCF-92 and irradiated components. Two types of Russian photomultipliers have been used in the CAL Prototype beam-tests: FEU-84-3 and FEU-115M. A calibration system with radioactive source and pulsing LED has been used and checked. Beam tests with the HCAL Prototype have been done at the X7 beam-line of the West Area experimental hall at CERN in a wide beam momentum range from 5 to 100 GeV. A rotating table on the platform allows to turn the detector on up to 180 angle around he vertical axis and a lateral and an up-down displacement of 2 m. A computer controlled High Voltage distribution scheme, developed by IFIN-HH group, allows individual setting of the HV on each PMT for the gain adjustment with a minimal amount of cables. The system consists of three parts: 1) the control box which includes the low voltage power supply, the RS232 interface to a PC and three modules of the high voltage power supply: two with Vmax = 2000 V, Imax = 10 mA and one with Vmax = 2500 V, Imax = 20 mA; 2) two types of multichannel HV distributors with an individual voltage setting within 512 V range; 3) a software package to control all settings and refresh them periodically. A new design for a HV power supply system which satisfies the LHC requirements is in progress. The main goal of the beam tests was to measure in beam intrinsic properties of the calorimeter like resolution, response uniformity, signal time properties, shower lateral dimensions and to decide upon the cell size and depth of the hadron calorimeter. The measured energy, angular and coordinate dependences of the HCAL response and resolution were found to correspond to the LHCb design requirements. The resolution as function of the beam energy for two prototype depths of 7.3 λ (5 modules) and 5.6 λ ia shown. The angular and X-Y coordinate uniformity have been checked and compared with stand-alone Monte-Carlo simulation program predictions. A good uniformity within the total angle range has been observed. The average hadronic shower profile on both X- and Y-directions obtained from the scan data are presented. (authors)