More details on the ALICE PhotoMultiplicity Detector (PMD)

The physics topics to be addressed by the PMD are closely interwoven with those of other ALICE detectors. The reaction plane determined from the PMD can be used to study flow parameters of various particle species in the TPC. While the azimuthal anisotropy can be determined independently using the PMD alone, a correlation between the anisotropy determined from the PMD and the FMD will be very useful to establish collective flow. One can study quarkonium suppression as a function of reaction plane in order to distinguish between rescattering effects and deconfinement as possible causes of suppression.

PMD uses gas as the sensitive medium, as the other options were either too expensive or not compatible with ALICE baseline detectors. The choice of detector technology for use in a preshower detector is dictated by the considerations that a) charged paticles should be confined preferably to one cell, b) low-energy δ-electrons should be prevented from travelling to nearby cells and from causing crosstalk among adjacent channels, c) the technology should be amenable to modular design with minimum dead space at the boundaries and finally the detector should be placed in contact with the converter without large air gaps. 

After surveying various technologies in the field of gas detectors, we have selected a novel design having honeycomb structure and wire readoud. The honeycomb cells are physically isolated from each other by thin metallic walls to contain δ-rays. The metallic wall of the honeycomb forms the common cathode and is kept at a large negative potential. The individual anode wires in the cells are kept at ground potential and connected to the readout electronics. Honeycomb geometry is selected because of its closeness to a circular approximation, providing almost circular equipotentials within a cell. This geometry also facilitates close packing of large arrays. The same design can be used for both the preshower part and the charged-particle veto. 

The detector has the form of unit modules with dimensions that are convenient to handle during the assembly. The unit module consists of 24 x 24 hexagonal cells in a rhombus array. The rhombus shape of a honeycomb array provides minimum dead space at the boundaries. A set of 3 x 3 unit modules are contained within a supermodule. Each supermodule is a rhombus of side 80 cm and provides a gas-tight enclosure. 

 

  • University of Panjab, Chandigarh, India
  • University of Rajasthan, Jaipur, Rajasthan, India 
  • VECC, Calcutta, India 
  • University of Jammu, Jammu, India 
  • Institue of Physics,, Bhubaneswar, India

The Charged-Particle Veto (CPV) detector is designed to suppress detection of charged particles hitting the front surface of the PHOS. The CPV consists of four separate modules, each of them placed on top of one PHOS module. The physical requirements on the CPV are the following:

-large-area modules of 230 200 cm2 

-high detection eciency for charged particles, close to 100%,

-two dimensional readout with localization accuracy of 3 mm,

-minimum amount of material, less than 5% of X0, electronics included

-minimum dead-zone, 

-low neutron sensitivity.

The detector based on the MWPC with pad readout and Ar/CO2 gas mixture ful lls all the above requirements. In particular due to the absence of hydrogen in the operating gas the CPV will have low sensitivity to neutron uxes. The CPV modules are positioned at approximately 5 mm upstream of the PHOS modules. In offline charged particle hits in the veto planes are projected onto the PHOS modules, and for each hit the corresponding region in the PHOS is marked as a ected by charged particle. The showers in the PHOS with coordinates in these regions will be then rejected as produced by charged particles.

The prototype of the CPV detector was designed and produced during summer | autumn of 1998. Low-mass construction materials were used for the production to minimize radiation length, material budget and detector weight. It was made as a plane structure with dimensions 18 18 cm2 lled with eight proportional tubes equipped with cathode pad readout based on the GASSIPLEX chip as front-end electronics. The key element of the CPV construction is lithographically produced pad cathode. 0.8 mm thick cooper-clad G10 foils was used to form the cathodes of proportional tubes, the outer and inner cooper cladding play role of the detector electromagnetic shield and cathode sheet with a fine pad structure. Transversal (relative to the anode wires of prototype) cross section of each tube is rhombus shaped with small deviation from a square, in order to improve spatial resolution in the direction transversal to the anode wires. The wire pitch is 2:2 cm. The cathode pad sizes, 2:2 2:2 cm2 , are the same as the transversal dimensions of the scintillating crystals in the PHOS. The other main parameters of CPV prototype are as follows:

- anode wires have diameter 50 m and made of gold-plated tungsten with 3% renium,

- anode-cathode distance is D = 1:1 cm,

- work gas mixture as 80% Ar + 20% CO2

 

Read here more details on the CPV detector.