In order to reach a nearly 4π detection efficiency for electromagnetic calorimetry in PANDA at the FAIR facility, the target calorimeter is complemented by a calorimeter in the Forward Spectrometer ...designed as a sampling calorimeter covering 4.6 m 2 surface and comprising 54×28 cells of a shashlyk type design. Each cell has a cross section of 5.5×5.5cm 2 (~ 1 Moliere radius) composed of 380 layers of lead absorbers (0.275mm thick) and plastic scintillator tiles (1.5mm thick) with a total length corresponding to 19.6 X 0 . The scintillation light is collected and accumulated via 18 wavelength shifting fibers, which are inserted through holes and form loops on the front side of the module. Photomultipliers (18 mmØ) are used as photo sensors since the calorimeter is placed at sufficient distance to the tracking magnets. The design is optimized for electromagnetic probes at very low detection threshold to cover a photon energy range down to about 10 MeV.
The Electromagnetic Calorimeter (EMC) of the future PANDA detector relies on a stabile performance of the PbWO 4 crystals of enhanced PWO-II quality. In spite of the expected moderate radiation dose ...imposed by hadrons and electromagnetic probes during operation, the interplay between damaging and statistical recovery processes is strongly influenced by the low operating temperature of T=-25° C. Previous investigations have shown that relaxation mechanisms become extremely slow and the limit on the asymptotically reached light output depends on the intrinsic concentration of defect centers in the crystal. The process of stimulated recovery leads in case of damage due to γ-rays at room temperature to an almost complete recovery using even infrared light. At low temperature, the mechanism leads to a substantial recovery but on significantly longer time scales. The paper will discuss a series of detailed measurements in order to understand recovery mechanisms at low temperature and the sensitivity to different external light sources. Part of the program is EPR-based measurements to identify the population or extinction of well-known color centers. A second experimental set-up allows measuring the change of the optical transmission spectrum of a permanently cooled crystal in situ using a photo spectrometer. The transmittance over the relevant region of wavelength is controlled before and after irradiation as well as during the recovery with different external light sources. The studies of the damage due to a high flux of γ-rays are compared complementary to the impact of 90 MeV protons performed at the cyclotron at KVI (Groningen Netherlands). Up to a fluence of 6.75×10 11 protons/cm 2 protons will impinge and be completely stopped in the test samples.
We studied the light yield of a pure polystyrene slide coated with wavelength-shifter molecules, coupled to a photomultiplier, using beta particles from a 90-Sr source, as a possible easy-to-build, ...low-cost plastic scintillator detector. Comparison measurements were performed with an uncoated polystyrene slide as well as with uncoated and coated PMMA slides, the latter which can only produce Cherenkov light when being traversed by charged particles. The results with the single (double) coated polystyrene slides show about 4.9 (6.3) times higher detected photon yield compared to the uncoated slide. For comparison, the light yield of a polystyrene-based extruded plastic scintillator material doped with PTP and POPOP was measured as well. The absolute detected light yield motivates future studies for developing easy-to-build, low-cost polystyrene-based plastic scintillator detectors.
More than 7,300 PbWO 4 crystals of PWO-II quality have been delivered by BTCP (Bogoroditsk Technical Chemical Plant, Russia) and tested, which corresponds to almost 50% of the electromagnetic ...calorimeter of the future PANDA detector at FAIR. The existing crystals comprise both end caps completely and a small part out of the 2×11 crystal geometries forming the barrel. Beside the verification of the geometrical shape and surface quality the very stringent controlling process focuses on the longitudinal and transversal optical transmittance, radiation hardness and the relevant scintillation parameters such as light output and decay kinetics. All crystals are exposed at room temperature to an integral dose of 30 Gy ( 60 Co) to determine the radiation hardness, which is a crucial parameter due to the operation of the calorimeter at T= -25°C, when recovery mechanisms are slowed down. Significantly less than 10% had to be rejected. The overall performance will be compared to crystal samples of identical geometry provided by SICCAS (Shanghai, China).
Scintillation crystals of the lead tungstate family - PWO, PWO-II - became widely used in electromagnetic calorimeters in high energy physics experiments at high luminosity accelerator facilities. ...During the operation of electromagnetic calorimeters a damage of the optical transmission of the crystals occurs due to creation of color centers. In addition to the recharge of a priori in the crystals existing defects by γ-radiation additional damage of the crystal matrix occurs due to hadrons. Therefore, radiation induced optical absorption can limit the energy resolution of the calorimeter. We have minimized radiation damage effects by technological efforts to obtain perfect crystals with a minimal concentration of the defects creating metastable color centers as well as the recoven' of color centers by stimulation with infrared light. In this paper we discuss the method of stimulated recovery of radiation induced absorption which can be even applied in situ. The mechanisms of the damage under γ- and hadron irradiation are described.
For the first time, full size lead tungstate crystals of different suppliers, quality and dopant concentration have been irradiated with gamma-rays at low temperatures down to -25degC at IHEP ...Protvino. In contrast to the behavior at room temperature, increased damage and extremely slow recovery processes have been observed. These first results are discussed in the light of several very different interpretations. Further more sensitive measurements are under preparation. The outcome will have a strong impact on the presently assembled ALICE-PHOS detector and the design of the EM calorimeter of PANDA at the future FAIR facility.
The presented paper documents the overall quality of the PWO-II crystals for the PANDA electromagnetic calorimeter focusing in particular on the radiation hardness and the consequences on the ...operation at T=-25°C. The imposed radiation damage due to ¿-rays can be significantly cured applying the process of stimulated recovery, which was observed for the first time for PbWO 4 scintillators. The fast recovery can be achieved even using photons in the infrared region and can be applied to operating detectors parallel to damaging irradiation in case the photo sensor is not sensitive to the external light. The technological application is part of a submitted patent (TM-382-DE).
Detailed studies of the radiation hardness of cooled high-quality PWO-II crystals for the electromagnetic calorimeter of the target spectrometer of PANDA at the future FAIR facility have been ...performed. Operating at low temperatures, such as T=−25°C, slows down significantly any recovery process in competition with the radiation damage. The investigations show, that one can overcome this problem by extremely radiation hard crystals selected at room temperature. In spite of light losses, the effective gain of the light yield at T=−25°C still remains at a factor close to three.
The influence of the irradiation of PWO crystals on the characteristic timing parameters of changes in their optical transparency and subsequent recovery during the working cycle of the LHC ...accelerator (CERN) was studied. PWO crystals were irradiated with a working dose corresponding to the radiation load in the electromagnetic calorimeter. A method for evaluating the influence of the change in the optical transparency of scintillation crystals on the statistical term in the energy resolution of the electromagnetic calorimeter in intense radiation fields is proposed. This method is applied to the development of the technology of mass production of radiation-hard scintillation crystals used in electromagnetic calorimetry and to its certification.
