Hans-Georg Zaunick

We present the design concept and first experimental results of RUBIK, a three-dimensional resolving plastic scintillation detector developed as a scientific payload for the Research and Observation in Medium Earth Orbit satellite mission of the Institute of Space Systems of the University of Stuttgart. The name RUBIK is inspired by the Rubik's Cube, a familiar example of a three-dimensional segmented structure, reflecting the 5×5×5 modular arrangement of the sensitive volume. The sensitive volume consists of 125 PVT-based plastic scintillator unit cubes, each penetrated by two perpendicularly aligned wavelength-shifting fibres, which guide scintillation photons to silicon photomultipliers (SiPMs), providing 50 independent readout channels. Custom readout electronics, comprising two discriminator boards with per-channel programmable thresholds, an FPGA-based data-acquisition board, and a dedicated power and bias distribution board, have been developed exclusively from commercial off-the-shelf components. A single-layer prototype with ten readout channels was used for concept validation through SiPM bias voltage and temperature characterisation, and a time-over-threshold based amplitude reconstruction method was validated against radioactive source and cosmic ray measurements. First hit-pattern measurements with 220 MeV protons at a clinical proton therapy facility confirmed that spatially resolved hit clusters consistent with the expected beam geometry were observable in the single-layer readout, demonstrating the capability to resolve particle-induced signal patterns as a first step towards 3D track reconstruction in the full instrument.

Accurate measurements of proton depth-dose distributions in low-dose environments are an underutilized region in proton therapy. These measurements are additionally useful for detector development, Monte Carlo validation, and emerging proton imaging applications. Conventional dosimetry systems are optimized for high-dose clinical environments, making them unsuited for applications where the collectible charge is small. We present a compact, segmented depth-dose detector based on PbWO ₄ scintillators that are individually read out by SiPMs through digitizers, allowing single particle energy deposition tracking. The high density of PbWO ₄ enables full stopping of clinical proton energies within a short detector length while the relatively low light yield avoids signal saturation in the Bragg-peak region. Single-particle sensitivity is achieved through a fast custom designed SiPM readout using coincidence methods. The detector response is modeled using a dedicated response function that accounts for scintillation statistics and SiPM microcell saturation, allowing energy spectra to be unfolded on a per-layer basis, increasing dose accuracy. The detector depth is converted to water-equivalent thickness. Measurements were performed at the Marburg Ion-Beam Therapy Center with a 221.6 MeV proton beam at kilo count per second particle flux, both with and without homogeneous polymethylmethacrylat (PMMA) phantoms of varying thickness. The reconstructed depth-dose distributions show clear Bragg peaks and allow determination of proton range and phantom water-equivalent thickness with sub-millimeter accuracy. Reconstructed PMMA thicknesses agree with nominal values within 0.5 %, corresponding to absolute deviations below 0.3 mm. These results demonstrate that the presented detector concept enables precise depth-dose measurements under low-dose conditions.

The PANDA (antiProton ANnihilation at DArmstadt) experiment will study the strong interaction in annihilation reactions between an antiproton beam and a stationary cluster jet target. The PANDA detector will be composed of several sub-detectors designed for tracking, particle identification and calorimetry. The Micro-Vertex Detector (MVD) is the innermost part of the tracking system surrounding the interaction region, which is designed for precise vertex and tracking detection. It consists of silicon pixel and double-sided microstrip detectors. For the readout of the microstrip sensors an ASIC called ToASt (Torino Asic for Strip readout) is being developed in 0.11 μm CMOS technology at INFN Turin. The ASIC takes advantage of both Time-over-Threshold and Time-of-Arrival methods to accurately measure the event's energy and timestamp. To sustain the acquisition of the microstrip sensors a MDC (Module Data Concentrator) ASIC is under development at KIT. Up to eight ToASt front-ends' data streams are multiplexed, read out and processed by one MDC. The data of several MDCs are collected and processed by the off-detector readout card MMB (MVD Multiplexer Board), also under development at KIT. The processed data is then transferred via 100 GbE optical links to the computing nodes. The ToASt chips have been integrated with the FPGA implementation of the MDC to form the first fully functional detector module. Beam tests have been performed at the COSY facility in Jülich. This paper focuses on the design of MDC ASIC and MMB board, the integration with the ToASt and presents preliminary beam test results.

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.

Plastic scintillation materials play a crucial role in the construction of large area detectors in high energy physics experiments. Future detector concepts for HEP experiments, particularly at collider facilities, will require an unique combination of the material features and of affordable price. Mandatory is a minimal level of radiation damage due to the electromagnetic part of ionizing radiation and energetic hadrons as well: tolerable deterioration of the optical transmission, negligible impact on the scintillation mechanism and small contribution of radio-luminescence due to secondary reaction products in the detector material. A systematic study of the radiation hardness of inorganic optical and scintillation materials has been performed by us since several years. It resulted in the understanding of the damage effects in particular in self activated, Ce ³⁺ -doped and cross-luminescent crystalline materials. Here we report on first results for the industrially produced plastic scintillation material EJ260 (ELJEN). This plastic is a bright, fast and green light emitting scintillator with properties similar to other materials such as BC428 and NE103, respectively. Samples of a thickness of 1.25 cm were tested before and after irradiation with γ-quanta (1.2 MeV, Giessen) and protons of 190 MeV (KVI, Groningen) and 24 GeV (CERN, PS), respectively. No damage of the material properties was observed after irradiation with γ-quanta up to an absorbed dose of 1 kGy consistent with similar studies. However, the exposure to 190 MeV protons with a fluence of 5×10 ¹³ p/cm ² showed a large reduction of the optical transmission and a slowing down of the scintillation kinetics. In this study we also focused on the impact of high energetic protons. We obtained evidence that light fragments of the (p, ¹² C) reactions play a significant role in the damage of the matrix and the luminescent organic dye centers.