Description
Vertex-reconstructed muSR (vx-muSR) combines thin silicon pixel detectors with custom tracking
algorithms to extend the capabilities of muon spin relaxation measurements. Conventional muSR
spectrometers at continuous muon sources are typically limited to stopped muon rates of approxi-
mately 40 kHz and require sample dimensions of several millimeters. By exploiting recent develop-
ments in monolithic active pixel sensors, we have implemented a prototype silicon-pixel-based muSR
spectrometer at PSI that enables tracking of both incoming muons and decay positrons, providing
spatially resolved muSR measurements with substantially increased rate capability.
The spectrometer is based on the MuPix11 detector, operated in a four-layer telescope configu-
ration with two tracking planes upstream and two downstream of the sample. The detector’s high
spatial resolution (≈23 µm) and low material budget (≤100 µm sensor thickness) allow accurate
reconstruction of particle trajectories from individual pixel hits. We present a dedicated reconstruc-
tion pipeline that transforms raw detector data into muSR observables through (i) hit clustering,
(ii) track reconstruction from clusters, and (iii) matching of muon stopping points with positron
emission trajectories and vertex determination. Spatial vertex matching provides intrinsically low
uncorrelated background while enabling operation at stopped muon rates exceeding 400 kHz.
We demonstrate the performance of the system by mapping the magnetic field distribution pro-
duced by two NdFeB magnets mounted on a silver plate. By reconstructing muSR spectra from
regions as small as 1.5 mm × 1.5 mm, we obtain a two-dimensional magnetic field map directly
from the tracked events. Beyond high-rate operation, the vx-muSR approach enables measurements
of multiple samples simultaneously, three-dimensional surface tomography, and the investigation of
millimeter scale magnetic structures that are inaccessible to conventional scintillator-based muSR
instruments.