Dzmitry Kazlou

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.