The effect of magnetic fields on vertex reconstructed muon-spin spectroscopy

TL;DR

The work addresses the limited stopped muon rate in muSR by proposing a silicon-pixel HV-MAPS spectrometer capable of vertex reconstruction to pair each muon with its decay positron. Using musrSim, the authors simulate two detector geometries under magnetic fields up to 0.75 T to evaluate tracking accuracy, finding robust performance up to about 50 mT and degradation beyond that unless a third layer or a precise field map is used. Matched-event analysis improves tracking quality but reduces the matched fraction as field increases. The results indicate that with a ~40 mm cryostat, lateral resolution near 1 mm is achievable in the low-field regime, enabling roughly a tenfold increase in stopped muon rate and substantially smaller sample requirements.

Abstract

The use of a Si pixel-based particle tracking scheme in muSR will, among others, allow measurements using a ten-fold increased stopped muons rate and samples ten times smaller than currently possible. Here we present simulation results to assess the effects of magnetic fields on two spectrometer configurations using a two-layered tracking scheme for the incoming and outgoing particles. At a low magnetic field of up to ~50 mT, the tracking and reconstruction accuracy is only minimally influenced. Beyond a magnetic field of ~80 mT the tracking capabilities diminish significantly. Operating a two-layered scheme using small magnetic fields hence does not require adaptations. Only at large magnetic fields, a tracking scheme that makes use of an accurate field map or the use of at least three layers must be employed to achieve reliable particle tracking.

Figures (9)

• Figure 1: Depiction of the simulated µ SR spectrometer configurations. (a) The currently used setup mandok2025arxiv, consisting of four layers of Si detectors, each comprised of four Si pixel chips and arranged in an upstream and downstream pairs. (b) A potential future assembly, consisting of four pairs of Si detectors layers and arranged in upstream, downstream, above and below the sample. The outer layers are comprised of four chips, whereas the inner layers are comprised of one chip each. The sample (red disk) sits between the upstream and downstream detector pairs. The gray disk serves as a non interacting (void) reference detector. Typical particle tracks are shown in an applied field of 0.75T, with $\mu^+$ in magenta, $e^+$ in blue, $e^-$ in red and neutrinos in green.
• Figure 2: Schematic depiction of the tracking of incoming muons and corresponding decay positrons. Tracks consist of two hits and are extrapolated to the sample position.
• Figure 3: The STD of $\delta_\mu$ (circles) and $\delta_e$ (squares) in the linear configuration as a function of the separation between the sample and the inner detector layers for different applied magnetic fields. The STD of $\delta_\mu$ changes by less than 2% in the range of 00.75T.
• Figure 4: The STD of $\delta_\mu$ (circles) and $\delta_e$ (squares) in the linear (orange) and the cuboidal (blue) configurations as a function of applied magnetic field for a fixed distance $d=20$ mm between the sample and the inner layers.
• Figure 5: The STD of $\delta_\mu$ and $\delta_e$ in the cuboidal configuration as a function of the separation between the sample and the inner detector layers for different external magnetic field magnitudes in the range from 00.75T. The STD of $\delta_\mu$ for different field magnitudes is represented by one data set, as the sets differ by less than 2%.