P. Drexler

High-Energy Photon Detection With LYSO Crystals

For the first time, the response function to high-energy photons of a 3 times 3 matrix comprising large volume LYSO crystals was measured using energy marked photons provided by the tagged photon facility of MAMI. The crystal quality was determined based on the optical transparency, the intrinsic radioactivity and the luminescence yield. Energy and time resolutions for photons up to 490 MeV photon energy have been deduced from the reconstruction of the electromagnetic shower deposited into the crystal array and the data delivers very promising results.

Performance of PWO-II Prototype Arrays for the EMC of PANDA

The response function of a 3 times 3 matrix of 200-mm-long PWO-II crystals has been measured with energy-marked photons up to 675 MeV energy. For the first time the crystals were readout individually with one large area avalanche photo diode of the second generation operating at a temperature of 0degC. The reconstructed electromagnetic shower distributions show excellent energy resolutions down to 40 MeV. Reducing the temperature and increasing the light collection by an additional large-area avalanche photo diode can achieve further improvement.

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.

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.

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.

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.

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.

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.