Fast timing detectors for high field spectrometers
Introduction
The capability of a time-differential spectrometer to detect high-frequency muon-spin precession (oscillation) signals is defined by the time-resolution of its detector system according to [1]:where is the amplitude of the precession signal of frequency measured with time resolution (standard deviation); is the maximum signal amplitude at . As follows from Eq. (1), at . Accordingly, the detector system of a 10 T spectrometer should have a time resolution better than 150 ps in order to detect muon-spin precession with its Larmor frequency of 1.35 GHz with more than 45% of its low field amplitude. Therefore, the upper limit for the time resolution of an individual (muon or positron) counter is 105 ps.
Realization of such fast-timing detectors in a traditional way, i.e by using photomultiplier tubes (PMTs), is problematic due to the high magnetic field environment. PMTs, whose performance is strongly affected by the field, have to be placed in a low field region rather far from the scintillators, the scintillation light would be delivered to them by long light guides. The attenuation and broadening of the light pulses in the light guides leads to degradation of the time resolution. The only presently existing high field spectrometer is the 7 T HiTime instrument at TRIUMF (Vancouver, Canada), which is equipped with a PMT-based detector system and has a time resolution of [2].
In this work we demonstrate prototypes of high-time resolution scintillation counters which can be used in the presence of high magnetic fields. The insensitivity to the magnetic field is achieved by using novel solid-state photodetectors–multipixel Geiger-mode Avalanche Photodiodes (G-APDs) [3].
Section snippets
Fast-timing G-APD based detectors
Two pairs of identical muon and positron counters have been built for the present tests, their construction is shown in Fig. 1. The linear dimensions () of the counters are characteristic for a 10 T high magnetic field instrument [4], [5] planned at the Swiss Muon source of the Paul Scherrer Institut (PSI, Villigen, Switzerland). The muon counter is intended for use in a “surface” muon beam (momentum ) and thus is based on a thick plastic scintillator.
The measurements
TF measurements
The muon and positron counters described above were used to build a setup for TF measurements (layout shown in Fig. 6). The measurements were performed in magnetic fields of 0.2 and 4.8 T, with transverse muon-spin polarization (the muon spin is rotated by by the beamline separator). The obtained muon-spin asymmetries are and , respectively. Practically the same (within experimental errors) low and high field asymmetry values are consistent with the expected time
Summary
Scintillation counters for fast timing in high magnetic fields have been developed. A time resolution of (sigma) at detection of muons and positrons in 4.8 T field has been achieved.
The obtained results prove the feasibility of a high-time resolution detector system for the 10 T instrument planned at the of the PSI.
Acknowledgements
We thank M. Elender (PSI) for help in building the detectors and in preparing the measurements, and U. Greuter (PSI) for help with the amplifier design. A.S., expresses his gratitude to D. Renker (PSI) and Yu. Musienko (Northeastern University, Boston) for consultations on G-APDs.
This project has been supported by the European Commission under the 6th Framework Programme through the Key Action: Strengthening the European Research Area, Research Infrastructures. Contract no.: RII3-CT-2003-505925.
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