Excellent Particle IDentification (PID) is crucial to the success of the PANDA physics program. Two fast and compact Cherenkov counters, using leading edge Detection of Internally Reflected Cherenkov light (DIRC) technology, will provide efficient and clean PID in the PANDA Target Spectrometer.
Cherenkov radiation was first observed more than 100 years ago by Marie and Pierre Curie, studied experimentally by Pavel Cherenkov in 1934, and explained theoretically by Ilya Frank and Igor Tamm in 1937, leading to a Nobel Prize in 1958. Cherenkov light is emitted when charged particles pass through a dielectric medium at a speed larger than the phase velocity of light in that medium. The emission angle of the Cherenkov photons with respect to the particle trajectory, as well as the number of photons emitted, correlate with the speed of the particle. This information, in combination with the momentum measurement from tracking detectors, provides high-quality PID for hadrons and leptons over a large range of solid angles and momenta.
In contrast to conventional Ring Imaging Cherenkov (RICH) counters, DIRC detectors use high-precision quartz bars or plates as radiator material to produce the Cherenkov light. The Cherenkov photons are then captured by total internal reflection in the fused silica radiator and transported to photon detectors outside the path of particles to be measured there. This principle allows DIRC detectors to be much more compact than conventional Cherenkov detectors. Two DIRC detectors will be installed in PANDA: the Barrel DIRC and the Endcap Disc DIRC.
Both DIRC detectors will utilize radiator bars or plates made of synthetic fused silica. This material provides excellent transparency, even after large radiation doses, and can be machined and polished with high precision. These properties ensure that the photon transport inside the radiator is very efficient and that the photon angles are conserved during hundreds of internal reflections. Focusing optics image the Cherenkov photons on arrays of MicroChannel Plate PhotoMultiplier Tubes (MCP-PMTs), which can be operated in a magnetic field of about 1 T and measure the location and arrival time of the photons with an excellent time resolution of 40ps or better.
The PANDA Barrel DIRC
The Barrel DIRC will cover the polar angle range of 22-140 degrees and cleanly separate charged pions from kaons for momenta between 0.5 GeV/c and 3.5 GeV/c with a separation power of at least 3 standard deviations. The design is based on the successful BABAR DIRC and the SuperB FDIRC R&D with several important improvements to optimize the performance for PANDA, such as wider and shorter radiator bars, a spherical focusing lens system, fast timing, a compact fused silica prism as expansion region, and lifetime-enhanced MCP-PMTs for photon detection.
The Barrel DIRC consists of sixteen optically isolated sectors, each comprising a bar box and a synthetic fused silica ("quartz'') prism, surrounding the beam line in a 16-sided polygonal barrel with a radius of 476 mm. Each bar box contains three synthetic fused silica bars of 17 mm thickness, 53 mm width, and 2400 mm length (produced by gluing two 1200 mm-long pieces end-to-end), placed side-by-side, separated by a small air gap. A flat mirror is attached to the forward end of each bar to reflect photons towards the read-out end, where they are focused by a 3-layer spherical lens on the back of a 300 mm-deep solid prism, made of synthetic fused silica, serving as expansion volume (EV). An array of 12 MCP-PMTs per sector, each with 8x8 pixels of about 6.5 mmx6.5 mm size, is placed at the back surface of the prisms. The MCP-PMTs and the FPGA-based readout electronics detect the photons and measure their arrival time on a total of about 11,300 pixels with a precision of 100 ps or better.
The reconstruction of the Cherenkov light image is performed using a maximum likelihood test, based on the photon arrival time in each MCP-PMT pixel. Detailed Monte Carlo simulations and measurements with complex prototypes in particle beams at GSI and the CERN PS predict that the PANDA Barrel DIRC will be able to separate charged pions from kaons with a separation power of 4-18 standard deviations for the kaon final state phase space in the Barrel DIRC.
The Endcap Disc DIRC
The Endcap Disc DIRC (EDD) consists of four independent quadrants which form a dodecagon and will cover polar angles between 5° and 22°. It will provide particle identification of charged pions and kaons up to 4 GeV/s with a separation power of at least 3 standard deviations. The EDD is a novel type of DIRC detector, providing a very compact read-out region, which is achieved by using photon sensors with a high spatial granularity.
The thickness of the radiator is 2 cm and the largest diagonal distance of the radiator surface amounts to 1.5 m. In the readout region so-called Focusing ELements (FELs) convert the angle information of the Cherenkov photons into a position information, which is read-out by the MCP-PMTs. The resolution of the FELs matches the pitch of the MCP-PMT pixels, which is 0.5 mm or better. Dispersion effects, which are a main error source in case of the EDD, are reduced by an optical filter. This allows to directly reconstruct the Cherenkov angle by measuring single photons down to a precision of few mrad, which translates into total resolution below 2 mrad if multiple Cherenkov photons are registered.
In total 96 MCP-PMTs will be operated, which adds up to more than 28,000 channels in total. An ASIC-based readout will be responsible for processing this datastream towards the DAQ. On average between 18-25 photons will be registered per particle track, which allows to measure the Cherenkov angle with a resolution of 2 mrad or better. Recent testbeams using hadronic and electron beams at CERN and DESY have verified he anticipated detector performance with a small-scale prototype.