Submitted by mu.schmidt on Thu, 27/09/2018 - 22:21.
The PANDA detector at the future FAIR facility at GSI is planned as a fixed- target experiment for proton-antiproton collisions at momenta between 1.5 and 15 GeV/c. It will be used to address open questions in hadronic physics. In order to achieve a sufficient particle identification, two different DIRC detector concepts have been developed. This talk will cover the Endcap Disc DIRC detector which is placed at the forward endcap of the PANDA target spectrometer and will provide a 3σ separation of pions and kaons up to a momentum of 4 GeV/c for polar angles from 5° to 22°. The most important component of the DIRC detector is a 2 cm thin fused silica radiator plate that is divided into 4 identical quadrants. The surfaces are polished with high precision in order to guarantee little photon losses by total reflection and conserve the Cherenkov angle during propagation through the optical system. Intrinsic chromatic errors will be minimized by the implementation of an optical filter. The readout system consists of 96 readout elements with focusing optics and attached MCP-PMTs to focus the photons that are produced by the Cherenkov cone of the traversing particle and acquire their position and timing information. This new detector concept requires the development of dedicated reconstruction and particle identification algorithms which permit an efficient analysis of the measured time-correlated photon patterns. Time and event based simulations with dedicated Monte-Carlo simulation frameworks have been used to validate the PID requirements of the DIRC counter. Additionally, the Monte-Carlo simulations have been used to estimate the radiation dose in different detector volumes in order to compare these values with actual measurement results.
The PANDA experiment at FAIR will use DIRC detectors for the separation of hadrons. The compactness of the PANDA detector requires the image planes of these detectors to be placed inside the magnetic field of the solenoid. Due to this and other boundary conditions MCP-PMTs were identified as the only suitable photon sensors. Until recently the major obstacle for an application of MCP-PMTs in high rate experiments like PANDA were serious aging problems which led to damage at the photo-cathode and a fast declining quantum efficiency as the integrated anode charge (IAC) increased. With new countermeasures against the aging, in particular due to the application of an atomic layer deposition (ALD) technique to coat the MCP pores, the lifetime of MCP-PMTs has meanwhile increased by a factor >50 which is fully sufficient for PANDA. The recent results of our long-term lifetime measurements are discussed. New 2-inch MCP-PMT prototypes from Hamamatsu show an encouraging behavior. However, the currently best performing MCP-PMT is a 2-inch PHOTONIS tube with two ALD-layers which reaches an IAC of >16 C/cm2 without any visible sign of aging. In the second part of these proceedings a new data acquisition system of the PADIWA/TRB type is presented which allows a quasi-parallel measurement of many MCP-PMT performance parameters. Especially unwanted effects like dark-count rate, crosstalk, ion afterpulsing, and recoil electrons can be studied in more detail than ever before. Exemplary results for these parameters are shown. The discussed DAQ system will be used for the comprehensive data quality checks of the MCP-PMTs being built into the DIRCs.
The PANDA experiment is a fixed target experiment where antiprotons collide with stationary hydrogen atoms. The main physics program of the experiment is to study open questions in hadron physics by performing charmonium spectroscopy by precise measurements of width, mass and decay branches and investigating possible exotic states like glueballs and hybrids. The Barrel Time-of-Flight detector (Barrel TOF), which is built in the PANDA target spectrometer, located between the DIRC detector and the EMC, has been designed to precisely measure the time at which a charged particle transits the detector with a resolution superior to the other sub-detectors of PANDA. A time resolution below 100 ps (sigma) is mandatory for this sub-detector to fulfill the requirements of good event separation and particle identification below the Cherenkov threshold. The implementation of the Barrel TOF is based on very fast organic scintillator tiles with a size of 87x29.5x5 mm3 coupled to Silicon Photomultipliers. The total of 1920 tiles are read out each by 8 SiPMs and cover almost the full azimuthal angular range and polar angles from 22.5 deg to 140 deg and an area of about 5 m2. The current prototypes achieve ~60 ps, well below the design goal. The detector R&D is now in a matured stage.
The favored photon sensors for the DIRC (detection of internally reflected Cherenkov light) detectors at the PANDA (Anti-proton Annihilation at Darmstadt) experiment at FAIR (Facility for anti-proton and ion research) are microchannel-plate photomultipliers (MCP-PMTs). The main problem until a few years ago was the limited lifetime of the MCP-PMTs caused by a rapid decrease in quantum efficiency (QE) of the photo cathode (PC) with increasing integrated anode charge (IAC). These limitations are overcome by applying an atomic layer deposition (ALD) coating on the MCPs, as recently done by PHOTONIS and Hamamatsu. During the last years tests of lifetime enhanced MCP-PMTs were performed and their results were compared. For this the QE was measured in dependence of the IAC as function of the wavelength and position dependent across the PC. The best performing tubes show a lifetime increase compared to not enhanced devices by a factor > 50 with an IAC > 13 C/cm2. Additionally, performance results of new 2 inch Hamamatsu tubes and a new high QE PHOTONIS tube are presented.
Microchannel-plate (MCP) PMTs are the favored photon sensors for the DIRC detectors of the PANDA experiment at FAIR. Until recently the main drawback of MCP-PMTs were serious aging eects which led to a limited lifetime due to a rapidly decreasing quantum effciency (QE) of the photo cathode (PC) as the integrated anode charge (IAC) increased. In the latest models of PHOTONIS and Hamamatsu an innovative atomic layer deposition (ALD) technique is applied to overcome these limitations. During the last five years comprehensive aging tests with ALD coated MCP-PMTs were performed and the results were compared to tubes treated with other techniques. The QE in dependence of the IAC was measured as a function of the wavelength and the position across the PC. For the best performing tubes the lifetime improvement in comparison to the older MCP-PMTs is a factor of >50 based on an IAC of meanwhile >10 C/cm2. In addition, the performance results of a new 2-inch ALD coated MCP-PMT prototype from Hamamatsu with a very high position resolution (128x6 anode pixels) is presented and the first conclusions from investigations concerning the PC aging mechanism will be discussed.