Development of a Thin-Wall Superconducting Magnet for the Positron Spectrometer in the MEG Experiment

Abstract

Abstract-A thin-wall superconducting magnet was developed for the positron spectrometer in the MEG experiment. The magnet is specially designed to provide a gradient magnetic field to achieve good features of the spectrometer such as constant projected bending radius for monochromatic positrons and much quicker sweep of positrons than in the conventional uniform solenoidal field, which allows a stable operation of the spectrometer in a high rate muon beam. A high-strength aluminum-stabilized conductor was developed so as to minimize the thickness of the coil between the target and photon detector. A pair of compensation coils is implemented in the magnet to cancel stray field around the photon detector to be placed closely to the magnet. Design of the magnet and results from the excitation tests to measure performance of the magnet will be presented here.

Search for the lepton flavour violating decay $$\mu ^+ \rightarrow \mathrm {e}^+ \gamma $$ μ + → e + γ with the full dataset of the MEG experiment

“…29a-e, where all the fitted spectra are in good agreement with the data spectra. The agreement is also confirmed by the relative signal likelihood R sig defined as R sig = log 10 S(…”

“…The COBRA (constant bending radius) magnet [10] is a thinwalled, superconducting magnet with an axially graded magnetic field, ranging from 1.27 T at the centre to 0.49 T at either end of the magnet cryostat. The graded field has the advantage with respect to a uniform solenoidal field that particles produced with small longitudinal momentum have a much shorter latency time in the spectrometer, allowing stable operation in a high-rate environment.…”

Muon polarization in the MEG experiment: predictions and measurements

“…The muon decay products are detected by a spectrometer with a gradient magnetic field and by an electromagnetic calorimeter. The magnetic field is generated by a multi-coil superconducting magnet (COBRA) [7,8], with conventional compensation coils; the maximum intensity of the field is 1.26 T at the target position. The positron momenta are measured by sixteen drift chambers (DCH) [9], radially aligned, and their arrival times by means of a Timing Counter (TC) [10][11][12], consisting of two scintillator arrays, placed at opposite sides relative to the muon target.…”

New Limit on the Lepton-Flavor-Violating Decayμ+→e+γ

“…The positron spectrometer consists of a set of drift chambers (DC) [7] and scintillation timing counters (TC) [8] located inside a superconducting solenoid with a gradient field [9] along the beam axis, ranging from 1.27 Tesla at the centre to 0.49 Tesla at either end. The photon detector [10], located outside of the solenoid, is a homogeneous volume (900 ℓ) of liquid xenon (LXe) viewed by 846 UV-sensitive photomultiplier tubes (PMTs) submerged in the liquid.…”