Photon Multiplicity Detectors (PMD) in STAR and ALICE

STAR PMD

The PMD built for STAR consists of a lead converter 15 mm thick, sandwiched between two planes of detectors with high granularity. The detector plane behind the lead detects photons through the electromagnetic showers they produce in the lead; the one in front helps to reject charged-particle hits. The detectors, which are based on the gas proportional counter, are formed from a honeycomb structure of cells with a copper cathode and tungsten wire anode, and a mixture of argon and carbon dioxide as the sensitive medium. The cells are 8 mm deep with a cross-sectional area of about 1 cm2. Copper walls separate the cells in order to prevent cross-talk by confining low-energy \(\delta\)-electrons to a single cell. A special feature of the design is its unusual aspect ratio, with the cell size and depth being of similar dimensions. In addition, the cathode extends on to the printed circuit boards covering the copper honeycomb so that the anode-cathode distance is less than 2 mm even though the physical separation between the copper cathode and anode wire is 5 mm. This extended cathode ensures uniform sensitivity of the detector throughout the cell volume.

The complete detector is constructed from units with a rhombus shape consisting of 24 x 24 cells. A number of these units, varying from four to nine, are housed in gas-tight enclosures called super-modules, and each plane of the PMD has 12 super-modules. The detector is assembled in two halves, which can be separated vertically and moved independently, with a final hexagonal shape.

The signal from the cells is processed using GASSIPLEX chips, developed at CERN, which provide 16-channel analogue multiplexed information. The analogue signals are digitized and read out using the C-RAMS ADC board. The front-end electronics board, consisting of four GASSIPLEX chips, is a 70 mm rhombus and is directly mounted on the unit module, almost covering the entire area of the detector over 8 x 8 cells.

The full PMD in STAR has about 83,000 cells in the two planes and covers an area of about 4 m². It is located near the east wall of the Wide Angle Hall at Brookhaven, 550 cm from the interaction point and behind the forward time-projection chamber.

PMD Fabrication

PMD in STAR @ Brookhaven National Laboratory

Publications

ALICE PMD

Because of the large particle density in the forward region, calorimetric techniques for photon measure-ments are not feasible. The PMD uses the preshower detector, where a three radiation length thick lead con-verter is sandwiched between two planes of highly granular gas proportional counters. The information from one of the gas detector planes, placed in front of the converter is used to veto the charged particles, where as the preshower data from the other detector plane isused for photon identification. The granularity and the converter thickness of the PMD are optimized for high particle density so that the overlap of photon show-ers is minimal. The PMD consists of 221184 honey-comb shaped proportional counters, each of 0.25cm2 area. Each counter has a honeycomb shaped cathode extended towards a 20μm thick gold-plated tungsten wire kept at a ground potential at the centre of each cell. The optimal operating voltage for the detector is −1400 Volts which forms part of the plateau region of the proportional zone. The efficiency is about 96% for charged pions at this voltage. The PMD is assembled in two equal halves. Each half has independent cooling, gas supply and electronics accessories.The front-end electronics consists of MANAS (Multi-plexed ANAlog Signal Processor) chips for anode signal processing and the Cluster Read Out Con-centrator Unit System (CROCUS) for data acquisition.

Testing the prototype models of PMD at CERN

PMD installation in progress at point-2 in ALICE @ CERN June 2009

PMD in final position (2 November 2009)

Publications