High-Resolution Timing for Vertex-Reconstructed Muon-Spin Spectroscopy Using Plastic Scintillators and MuTRiG

Abstract

Vertex-reconstructed muon-spin spectroscopy (vx-μSR) based on silicon pixel detectors has recently demonstrated unprecedented lateral resolution and operation at muon stop rates exceeding 400 kHz. However, the intrinsic timing resolution of current silicon pixel detector technology limits the accessible frequency range and restricts μSR measurements with fast relaxation rates. In this work, we report on the integration of plastic scintillator detectors (PSD) read out with the MuTRiG ASIC into the MuSiP vx-μSR spectrometer. This complements the spatial resolution achieved by using silicon pixel detectors with high-precision timing information for incoming muons and decay positrons. We demonstrate stable operation of MuTRiG in vacuum and achieve sub-300 ps time resolution after time-walk correction. Standard transverse-field μSR measurements on a SiO$_2$ sample confirm that the combined MuTRiG-PSD system resolves precession frequencies beyond 50 MHz, far exceeding the capabilities of silicon pixel detectors alone. These results establish a viable and scalable path towards high-rate, high-resolution μSR with both excellent spatial and temporal performance.

Figures (8)

• Figure 1: A schematic of the detector arrangement of the Si pixels and PSDs relative to the beam direction.
• Figure 2: (a) Photograph of the copper collimator, M counter and B positron detector. (b) View of the rear side of the spectrometer. Note the F positron detector placed after the downstream MuPix11 array.
• Figure 3: ToT distributions from each plastic scintillator counter, shown for hits on a single readout channel without (dashed lines) and with coincidence (solid lines) with the other readout on the same PSD. The asymmetric shape observed for B is attributed to light scattering from the central hole.
• Figure 4: Correlation of the ToT measured on the two readout channels of each PSD: (a) M counter (channels 32-33), (b) B detector (channels 34-35), and (c) F detector (channels 36-37). Only hit pairs with time difference $| \Delta t |<4$ ns are shown. The clear correlations demonstrate consistent light sharing and stable SiPM response.
• Figure 5: Raw time-difference distributions between hit pairs recorded on the two channels of the same PSD for the (a) M counter, (b) B detector, and (c) F detector.