T. Kuske

The PANDA Electromagnetic Calorimeter—A High-Resolution Detector Based on PWO-II

The electromagnetic calorimeter of the PANDA detector at the future Facility for Antiproton and Ion Research is one of the central components to measure high-energy photons over a wide dynamic range down to a few tens of mega-electron volts. The calorimeter comprises almost 16 000 PWO-II crystals of 200 mm length with high quality standards in optical performance, light output, and radiation hardness. The running mass production has passed more than 7000 crystals. The observation of stimulated recovery even of cooled crystals by illumination with external light sources integrated into the detector modules has opened up a new design concept for PANDA and future calorimeters.

Stimulation of Radiation Damage Recovery of Lead Tungstate Scintillation Crystals Operating in a High Dose-Rate Radiation Environment

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 degradation of the optical transmission of these crystals occurs due to creation of color centers. In addition to the recharge by γ-radiation of the point structure defects, which exist a priori in the crystals, additional damage occurs to the crystal matrix due to the interaction of hadrons. Thus radiation induced optical absorption can limit the energy resolution of the calorimeter. To reduce the recharge by γ-radiation we have both minimized the concentration of point structure defects during manufacture, and used light from visible to infrared to stimulate the recovery of the color centers. In this paper we show that method of stimulated recovery is also applicable to recover from degradation of the crystals optical transmission caused by hadron interactions. The mechanisms of the damage under γ- and hadron-irradiation are discussed.

Comparison of Radiation Damage Effects in PWO Crystals Under 150 MeV and 24 GeV High Fluence Proton Irradiation

Radiation damage effects induced by the hadronic part of ionizing radiation in experiments at high-energy and high-luminosity accelerators will play a significant role as limiting factor of the long term stability when operating an experimental setup. Measurements of the deterioration of the optical transmission of lead tungstate (PbWO 4, PWO) scintillation crystals were performed after irradiation with high and low energy protons. One sample with CMS specification was irradiated in 2010 with a 24 GeV/c proton beam at the CERN Proton Synchrotron (PS) with a flux of about 109 p/(cm2s) up to an accumulated fluence of 3.0 103 p/cm2. Eight more crystals of PWO-II quality with dimensions of 2×2×5 cm3 were selected from the set of crystals produced for the PANDA electromagnetic calorimeter at the future FAIR facility at Darmstadt. Four crystals were produced by the Bogoroditsk Technical Chemical Plant (Russia) and the others by the Shanghai Institute of Ceramics (China). These samples were irradiated in 2012 with a 150 MeV proton beam at the AGOR accelerator at KVI (Groningen, The Netherlands) with a flux of 3 109 up to an integral fluence of about 1.0 1012 p/cm2 and 1.8 1013 p/cm2, respectively. Due to the proton induced activation of long-lived radioactive secondaries the optical inspection of all samples had to be performed several months after irradiation for safety reasons. However, the samples were kept continuously cold to minimize thermal recovery. Both irradiations produced a similar set of induced absorption bands. Moreover, a shift of the fundamental absorption edge at short wavelength appears even after irradiation with low energy protons. The contribution will show in detail the set of experimental data and discuss a possible mechanism for an understanding and interpretation of the observed effects.

High-quality PWO crystals for the PANDA-EMC

The paper provides a status report on the crystal production and quality control of a major part of the PbWO 4 crystals for the PANDA-EMC. The results confirm the excellent performance of the new g ...

Influence of the Light Collection Non-Uniformity in Strongly Tapered Lead Tungstate Crystals on the Energy Resolution of the PANDA Electromagnetic Calorimeter at Energies Below 1 GeV

The electromagnetic calorimeter of the PANDA detector at the future FAIR facility, will be one of the central components to achieve the physical goals in studying the interaction of cooled antiprotons with a fixed target. The barrel part of the target electromagnetic calorimeter will consist of 11 crystal geometries with a different degree of tapering. Due to tapering the crystals show a non-uniformity in light collection, which is resulting from an interplay between the focusing and the intrinsic absorption of light in the crystal. For the most tapered crystals the light detected by the photo sensor is enhanced by a factor of > 1.4, if the scintillation light is created in the front part of the crystal. Due to the spread of the electromagnetic shower within the crystal and due to its fluctuations, this effect leads to a smearing of the response, resulting in a reduction of the energy resolution. Therefore, one lateral crystal surface has been de-polished for 9 crystals to a roughness of 0.3 μm, which decreases the non-uniformity from up to 40% to less than 5%, with a tolerable decrease of the light yield. This paper will compare the response of a 3×3 array of crystals with one de-polished side face with an identical matrix of completely polished crystals using high energy photons from 56 MeV up to 767 MeV, respectively. The results are compared to GEANT4 simulations and show a significant improvement of the energy resolution at energies above ~ 200 MeV with no deterioration down to 50 MeV.

Trigger-less readout of the PANDA electromagnetic calorimeter

The PANDA collaboration at the future FAIR facility at Darmstadt, Germany, will employ antiproton annihilations to investigate resonances in the charmonium mass region. In order to gain high flexibility for physics event selection, a readout system without hardware trigger, i.e. a trigger-less data-acquisition system will be employed and is currently being developed. The event-selection is based on high-level reconstructed information, e.g. invariant mass, secondary vertices, and time correlations, which may be contributed from all sub-detector systems. Prototypes of the PANDA Electromagnetic Calorimeter have been exploited to verify the desired event-selection approach. The prototype readout system includes a dedicated Sampling ADC (SADC), data concentrator (DCON) and compute-node (CN) modules. An optical link connects several SADC digitizers to a DCON module from where the data are streamed to the CN network. Data, collected by the SADCs are processed online in FPGAs by a feature-extraction algorithm. This processing technique guarantees a dead-time free operation and allows including an on-line pile-up recovery process for single detector counting rates up to 1 MHz. The prototype of the readout-chain has been evaluated using pulse generators and high-energy photon beams; the proper event correlation could be verified. The applied FPGA has been irradiated with protons to demonstrate its applicability in the radiation environment of the PANDA experiment.

Construction and assembly of one barrel slice for the electromagnetic calorimeter of the PANDA detector

The electromagnetic calorimeter (EMC) of the PANDA detector at the future FAIR facility is composed of two endcaps and a barrel covering the major part of the solid angle consisting of more than 11.300 tapered PbWO4 crystals. The individual scintillator modules are readout via two large area avalanche photo diodes. The signal processing is performed with a custom made ASIC-preamplifier providing a large dynamic range, low noise and reduced power consumption since the calorimeter will be operated at a temperature of −25°C. The first major assembly stage outlined in this paper is going to be conducted beginning in mid 2016 by assembling one single barrel slice segment. The construction of this segment comprises a full length slice beam holding a total of 18 module blocks, each one being a matrix of 4×10 crystals, in place. The assembly procedure of single detector modules, 40-crystal module blocks and the overall slice segment, respectively will be discussed and important findings during the procedure mentioned. In order to lay out and optimize the assembly procedure, the results and experiences gained with an earlier 80-crystal fully functional prototype detector were accounted for, which are reviewed in this contribution. Test results of single components and fully assembled detector modules will be discussed and compared with earlier prototype in-beam and lab tests as well as with the envisaged PANDA requirements.

Performance of Prototypes for the Barrel Part of the ANDA Electromagnetic Calorimeter

The performance of the most recent prototypes of the ANDA barrel electromagnetic calorimeter (EMC) will be compared. The first large scale prototype PROTO60 was designed to test the performance of the improved tapered lead tungstate crystals (PWO-II). The PROTO60 which consists of 6 × 10 crystals was tested at various accelerator facilities over the complete envisaged energy range fulfilling the requirements of the TDR of the ANDA EMC in terms of energy, position and time resolution. To realize the final barrel geometry and to test the final front end electronics, a second prototype PROTO120 has been constructed. It represents a larger section of a barrel slice, containing the most tapered crystals and the close to final components for the ANDA EMC. The performance of both prototypes will be compared with a focus on the analysis procedure including the signal extraction, noise rejection, calibration and the energy resolution. In addition, the influence of the non-uniformity of the crystal on the energy resolution will be discussed.

Radiation damage and recovery mechanisms in cooled PbWO 4 crystals

The Electromagnetic Calorimeter (EMC) of the future PANDA detector relies on a stabile performance of the PbWO4 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 6.75×1011 protons/cm2 protons will impinge and be completely stopped in the test samples.

Limitations of the performance of PWO-II crystals due to radiation damage

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 PbWO4 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).