Stefan Diehl

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.

Characterization and Applications of New High Quality LuAG:Ce and LYSO:Ce fibers

Inorganic scintillating fibers have a large potential as highly granular detector components in hadron and particle physics as well as in medical applications. With the micropulling-down-method a fast and cost efficient technique was developed over the last decades to grow such fibers from the melt. This paper will present the recent development of the quality of LuAG:Ce and LYSO:Ce fibers in terms of light output and light attenuation inside the fiber. For this purpose different steps in optimizing the fiber growth will be compared with the achieved fiber performance. In addition the response of the fibers to low energetic gamma rays will be studied and different readout concepts for the fibers will be discussed and compared.

Impact of Non-Uniformity in Light Collection on the Energy Resolution of the PANDA Electromagnetic Calorimeter at Photon Energies Below 1 GeV

The electromagnetic calorimeter (EMC) of the PANDA detector at the future FAIR facility comprises more than 15,000 lead tungstate (PWO) crystals. The barrel part will consist of 11 crystal geometries with different degree of tapering, which causes a non-uniformity in light collection as an interplay between the focusing and the internal absorption of the light. For the most tapered crystals the detected light is enhanced by 40%, if the scintillation process is created in the front part of the crystal. Due to the shower development and its fluctuations the non-uniformity leads to a reduction of the energy resolution. To reduce this effect, one lateral crystal side face has been de-polished to a roughness of 0.3 μm. Measurements confirm an increase of the light yield in the rear part of the crystal. In contrast, only a slight decrease can be observed in the front part. The overall non-uniformity is significantly reduced below 5%. This paper will discuss the experimental studies based on GEANT4 and optical simulations to understand the impact of a de-polished side face on the light collection. For consequences on the future performance, a 3x3 sub-array of de-polished crystals was directly studied using a tagged photon beam in the energy range from 50 MeV up to 800 MeV, respectively, performed at the tagged photon facility at MAMI, Mainz. The comparison to an array composed of polished crystals confirms a significant improvement of the constant term of the energy resolution from above 2 % down to 0.5 % and only a small increase of the statistical term. The results can be reproduced in GEANT4 simulations.

The light collection non-uniformity of strongly tapered PWO crystals and its impact on the energy resolution of the PANDA electromagnetic calorimeter in the energy region below 1 GeV

– The barrel part of the target EMC of the PANDA detector at the future FAIR facility will consist of 11 crystal geometries with a varying degree of tapering. The tapered shape introduces a focussing effect to the light collection, which in combination with the absorption of the scintillation light within the crystal causes a non-uniformity in light collection. For the most tapered crystals, light generated in the front part of the crystal is enhanced by approximately 40% compared to light generated in the rear part of the crystal. Due to the distribution of the electromagnetic shower within the crystal, this non-uniformity leads to a smearing of the energy response, resulting in a deterioration of the energy resolution. To avoid this effect, the light collection has been made uniform by de-polishing one lateral crystal side face to an average roughness Ra of 0.3 μm. Applying this method, the non-uniformity of the most tapered crystals has been decreased to a level of 5 % with a slight decrease of the light yield from the front part and a significant increase in the rear part of the crystal. This paper will discuss the observed effects of the light collection and compare the response of a 3×3 array of crystals with one de-polished side face with an identical array of completely polished crystals in the energy region below 1 GeV. In the energy region above 200 MeV a significant improvement of the energy resolution has been achieved.

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.

PANDA Forward Spectrometer Calorimeter

The PANDA experiment is one of the challenging projects being constructed on new FAIR facility near Darmstadt. It will use the antiproton beam from the High Energy Storage Ring colliding with an internal proton target and a general purpose spectrometer to carry out a rich and diversified hadrons physics program. One of the main PANDA detectors is a calorimeter complex consisting of a central PWO crystal based calorimeter and a forward shashlyk-type sampling calorimeter. The design of the PANDA Forward Spectrometer Calorimeter (FSC) is completed and validated by several test-beam measurements. FSC Technical Design Report recently was approved by FAIR experts committee and construction of the detector is expected to be started in the next year. The detector production-ready status including mass-production techniques developed at IHEP,Protvino is presented in the report.

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.

Research activity with different types of scintillation materials

Nowadays there is a growing interest and demand in the development of new types of scintillation materials for experimental high energy physics. Future detector developments will focus on cheap, fast, and radiation hard materials, especially for application in collider experiments. The most recent results obtained by the Giessen group in close cooperation with colleagues from different institutes will be presented. The new start of the mass production of high quality lead tungstate crystals (PbWO4, PWO) for electromagnetic calorimetry was started by the company CRYTUR (Turnov, Czech Republic). We will present a detailed progress report on the research program of lead tungstate performed in the last two years. The latest results in the development of LuAG:Ce, YAG:Ce and LYSO:Ce inorganic fibers, grown by the micro pulling down method and cut with the heated wire technique as well as new glass ceramics material BaO*2SiO2 (DSB) doped by Ce and Gd will be presented. In addition, different samples of the organic plastic scintillator EJ-260 produced by the company Eljen Technology (Sweetwater, USA) have been characterized. The study has focused on the change of performance after irradiation with 150 MeV protons up to an integral fluence of 5-1013 protons/cm2 as well as with a strong 60Co gamma-source accumulating an integral dose of 100 Gy.

Readout concepts for the suppression of the slow component of BaF2 for the upgrade of the TAPS spectrometer at ELSA

For the measurement at extremely high interaction rates with fast scintillators, pile-up of consecutive events is a limiting factor. With a decay time of 600 ps of the fast crossluminescence component, Barium Fluoride (BaF2) is one of the fastest inorganic scintillators known today. However, the dominating slow component with a 3 orders of magnitude longer decay time of 630 ns limits the rate capability. To circumvent this limit, different approaches have been made in the past. The slow component can be suppressed for example by doping the crystals with rare earth ions like La3+. The paper will give an overview over the various concepts investigated in the past and present the suppression via optical band pass filters. This method has been chosen for the upgrade of the BaF2 crystals in the most forward region of the TAPS-spectrometer at ELSA in Bonn. It allows to reuse the existing crystals and to achieve a high degree of suppression of the slow component. The focus of the paper will be on the selection of the filters, the achievable rate capability and the energy resolution of the fast component.

Response of the shashlyk forward spectrometer of PANDA to photons below 800 MeV energy

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 m2 surface and comprising 54×28 cells of a shashlyk type design. Each cell has a cross section of 5.5×5.5cm2 (~ 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 X0. 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.