4.2.1   Detectors and Environment

There are four systems in CLAS which use photomutiplier tubes (PMTs) to detect low levels of light produced by ionizing particles. These are the gas Cerenkov counters, Time-of-Flight (TOF) scintillation counters, electromagnetic calorimeters (EC) and tagging spectrometer. The last three detector systems use plastic scintillators based on polyvinyltoluene (PVT) purchased from Bicron Corporation (BC-408 and BC-412). The scintillators are attached to the PMTs via acrylic or polystyrene plastic light guides.

There are six Cerenkov detectors, one per sector in CLAS, each with 36 5" Hamamatsu PMTs and approximately 380 ft3 of non-flammable gas (C4F10). The voltage divider is inside the gas-filled volume, with signal and high voltage cables patched through the detector walls to the outside.

4.2.2   High Voltage Supply

The high voltage (HV) for the PMTs of the CLAS detector ¹ is provided by the LeCroy 1450 Modular High Voltage System. The power is provided by the 1458 Mainframe which houses up to sixteen vertical 1461 high voltage cards which supply the actual high voltage to the PMTs. All subsystems require negative voltage (supplied by 1461N cards), except for the Cerenkov detector which requires positive voltage (supplied by 1461P cards). The mainframe can supply a maximum of 1440 W (60 A at 24 V) to all cards. Each card has twelve channels which generate a voltage between 0 and 3 kV and a maximum current of 2.5 mA per channel at 3 kV (7.5 W per channel). Details of the PMTs for the TOF and calorimeter as interfaced to the HV system can be found in CLAS-NOTE 96-010.

Table 1 gives a breakdown of number of HV channels per subsystem. The HV mainframes for the Forward-Angle TOF, calorimeters and Cerenkov counters are on the Forward Carriage (Deck 3). The HV for the tagging hodoscope is located on the floor of Hall B below the spectrometer. The mainframes for the Large-angle calorimeters are located on the south CLAM shell. The mainframes for the Large-Angle TOF are located on the north and south CLAM shells, and one on the space frame.

TABLE 5. Number of High Voltage channels required by each subsystem:

4.2.3   Cables

Each PMT is connected to a LeCroy 1461 HV card with a red RG-59 cable (except for the tagging spectrometer which has black cables) and SHV connectors. Two PMT signals are connected to the electronics, one to the readout and the other to the trigger, with coaxial cables. All cables have a fire rating of CL2X or equivalent.

4.2.4   PMT Voltage Dividers

The voltage divider networks for the TOF, calorimeter and tagger T-counters are based on a University of New Hampshire low-power design, which typically draws 0.5 mA at an operating voltage of 2000 V (1 W), independent of load. Failures which result in the divider network drawing more than 2.5 mA will trip the high voltage channel. In very unusual circumstances, failures could result in drawing up to a maximum of 2.5 mA at 3000 V, or 7.5 W. These conditions, however, should be flagged by the slow control system which will monitor the current draw and operating voltage of each PMT.

4.2.5   HV Shock

If the HV connections to the PMT voltage divider are improperly attached, it is
possible to receive an electrical shock. To avoid shock, the following precautions shall be taken upon initial operation of the system:

• Check grounding of support frame to carriage
• Check grounding of High Voltage supply to carriage

The grounding scheme for the Forward carriage is shown in Figure 2 (similar schemes are implemented for the North and South carriages). All racks and cable trays are grounded with a 4/0 copper ground strap to the grounding grid in Hall B for safety and to minimize sources of noise.

Additional special circumstances for particular detectors include:

4.2.6   Fire

The largest fuel supply in Hall B consists of the inventory of plastics. Most of the plastic is in the cables, although a significant fraction is found in the scintillator detectors. (For more information refer to the Fire Hazard Analysis for Hall B.) Therefore, a concern is any fire which originates in a voltage divider or any electronics connected through cables to the PMT detector. The detector system was designed to minimize the potential for ignition or spread of fires by using low-powered bases (see above), fire retardant cables, and safe connections (SHV). However, the detector material itself, especially of the TOF system is covered by thin sheets of plastic and aluminum foil to achieve the desired physics objectives.

We therefore require that for HV equipment installed in Hall B the following procedures must be followed:

• The LeCroy 1458 Mainframes shall be powered fully loaded with HV cards and burned in for a minimum of 30 hours before installation.
• The detectors will be not be left on without supervision for at least two days prior to routine operation. During this time there will be a visual inspection of energized detectors as they are plugged in and hours after first energized. Look for smoke, distorted plastic and other signs of heat to whatever extent visible.

TABLE 6. Checklist for installation.

FIGURE 2. Grounding connections on the Forward Carriage. Similar grounding schemes are implemented on the north and south CLAM shells.