The Crystal Ball spectrometer

Figure 1: Schematic view of a part of the Crystal Ball. Two NaI crystals are replaced by Ge detectors (yellow).

Figure 2:The Crystal Ball at Cave B

The detection properties of the Crystal Ball, which are still the same as measured during the commisioning of the ball in 1983, are summarized in table 1.

Currently the electronics of the CB consists of NIM and CAMAC moduls; the 162 energy and timing signals are read out by a Fastbus system. To increase the functionality, improve the reliability and enhance the maximum trigger rate, a completely new VXI based electronics system [CHE] is currently being built in Heidelberg. The whole system will be housed in two VXI crates, each containing a VXIbus Resource manager, a Readout controller and 10 NaI modules providing 8 NaI channels each. The system can run in stand alone mode and will have its own data storage. The maximum event rate of the system is expected to exceed 10kHz. For experiments where the set-ups include ancillary detectors, the VXI system will be interconnected by a differential VSB interface to the common data acquisition system. For each channel the new NaI boards process the energy and two timing (CFT, LE) informations. Set-up, inspection and monitoring of the moduls is computer controlled, enabling an enhanced diagnosis of analog and digital parts of the VXI boards. A DSP on each VXI board constructs a compact event data format and allows histogramming of the data.

To enhance the reliability and stability and to reduce the size and power consumption of the HV supply, new photomultiplier bases are developed with active HV regulators enabling low base currents. A distributed system of control units will support 8 bases each. Besides set-up and inspection these units provide pulser signals to check the whole data acquisition path starting from the preamplifiers.

The CB is mounted on a compact platform of 2 times 4 meter size, which is movable on air cushions or wheels. The construction is adjustable to different beam heights and allows to open the CB perpendicular to the beam direction. The complete electronics and data acquisition system can be placed on the platform. Therefore external connections are reduced to data bus connectors, a power cable and cooling water connections.

[CB] V. Metag, D. Habs, and D. Schwalm, Nucl. Part. Phys. 16, 213 (1986).

[CHE] VXI Electronics for Euroball, Ch. Ender, Universität Heidelberg, Heidelberg 1993.

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