Proton-Arm Spectrometer

Responsible: GSI

Work is performed within the Plastic Scintillators Working Group.

Contact person: Michael Heil.

General Information     Detector Concept   Simulations    Detector Performence    Literature

The Proton-Arm Spectrometer (PAS) is one of the main R³B detector systems dedicated to the detection of protons and other light-charged particles with beam-like velocities. The detector will be positioned behind the GLAD magnet in the High-Energy Cave (HEC) at FAIR.

The large acceptance of GLAD, including its adjacent vacuum chamber, allows for kinematically complete measurements of reactions involving charged-particle decays of excited fragments. The bending-angle acceptance of up to 50 deg, compared to the beam and residual-fragment deflection angle of 18 deg covers a wide range in magnetic rigidity for charged decay products, which will be fully covered by the PAS tracking system.

PAS is mandatory for experiments involving continuum excitations of proton-rich nuclei that require precise detection of protons in coincidence with the beam-like residual fragment. It will also play a critical role for lighter and medium-mass nuclei, where charged-particle decays can occur when systems are excited to high-lying continuum states.

PAS is based on scintillating fiber technology, enabling the construction of large, modular, and highly granular detectors. Developed in close collaboration with the Detector Laboratory at GSI, the design profits from established infrastructure and expertise in producing scintillating-fiber detectors for heavy-fragment tracking within the R³B setup. The primary engineering challenge was extending the scintillating-fiber concept to reliably track minimum-ionizing particles.

The PAS setup comprises two fiber detectors located downstream of GLAD:

• First Detector (x1): Features an aperture of 195x100 cm² and consists of 15 modules..
• Second Detector (x2): Features an aperture of 247x100 cm² and contains 19 modules.

In both detectors fibers are orientated vertically providing x-position information to determine deflection angles in the magnetic field.

The active area of both detectors is built from Kuraray square-cross-section scintillating fibers with a width of 0.5 mm. The fibers are arranged in ribbons containing three staggered layers (with a ribbon width of 13 cm and a total length of 132 cm). This structural offset prevents particles from passing undetected through the non-scintillating claddings. Each ribbon is laminated with a 10um-thick black Pokalon foil, protecting the active area from ambient light while minimizing material in the beam path.

Ribbons are wound using the specialized winding machine at GSI (for details, see P. Garcia Gil et al., DOI: 10.3390/jmmp10100399).

Scintillation light is read out at both ends using Hamamatsu S13552 Multi-Pixel Photon Counter (MPPC) arrays. Analog signals from the SiPMs are processed by SciFi_652 readout modules developed by the GSI Electronics Department, featuring:

• 128 channels per module with discrete, low-power preamplifiers (G=25-30).
• Individual threshold discrimination.
• Leading and trailing edge recording via a 170 ps RMS FPGA TDC.
• Data transmission via a 5 Gbps optical serial interface.
• Local SiPM bias generation (>52 V) via Hamamatsu C14156 boost converters powered by a 5V input.

Geometry and operational parameters are optimized to detect minimum-ionizing particles with an efficiency greater than 97 %, a spatial resolution (σ) better than 0.25 mm, and an angular resolution (σ) of 0.25 mrad, achieving a momentum resolution of dp/p ≈ 10⁻³.
The PAS tracking system is complemented by a time-of-flight wall (PASyToF) built from horizontal scintillation bars, which serves as a trigger detector and measures the y-position of light-charged particles. A rough energy measurement allows separation between protons and heavier fragments. PASyToF has an aperture of 250100cm2 and is currently in the design phase.

The primary goal of PAS is to detect protons with high efficiency and determine their momentum to an accuracy of 0.1 %. Because the energy loss of protons passing through matter is low, a specific material thickness is required to ensure high detection efficiency. However, increasing thickness raises multiple Coulomb scattering, which degrades momentum resolution. Similarly, finer granularity and smaller fiber widths improve spatial resolution and momentum accuracy, but significantly increase the number of readout channels and overall system cost.

To optimize fiber thickness and granularity, simulations were performed using the R3BRoot framework. Tests with 1 GeV protons across various fiber geometries confirmed that quadratic fibers ranging from 0.1 mm to 2.0 mm all yielded momentum accuracies better than the target 0.1 %. Balancing simulation results against available commercial components and photosensors, a detector thickness of 1.5 mm was chosen implemented by stacking three 0.5 mm fiber ribbons.

The first test of a single PAS module took place in July 2023 at the COSY facility (Jülich) using two trigger scintillators and a small 0.2 x 0.2 mm² double-plane fiber detector which served as position reference. Measurements demonstrated:

• Proton detection efficiency of 98.8(5) % for 1 GeV protons and 99.0(3) % for 430 MeV protons. 
• Spatial resolution better than 0.25 mm (further improvements are anticipated with double-sided readout).

Two PAS modules were further characterized in 2025 at the Heidelberg Ion-Beam Therapy Center (HIT) using 220 MeV protons. These measurements yielded:

• Spatial resolution: better than 0.2 mm
• Time resolution: 342.7(24.6) ps
• Detection efficiency: 97.8 %

Results from both test campaigns confirm that the PAS detector fully satisfies its design requirements and is well-suited for high-energy proton tracking.

• "Addendum to the Technical Report for the Design, Construction and Commissioning of the Tracking Detectors for R3B - PAS: Proton Arm Spectrometer", T. Aumann et al, https://edms.cern.ch/ui/file/3129262/1/TDR_R3B_TrackingDetectors_PAS_public.pdf