Magnetic shield optimization of PMTs exposed to constant quadripolar fringe fields

The Photon Array for the studies of Radioactive and Ion Stable beams (PARIS) is a multidetector of clusters. Each cluster is composed of 9 units of two-shells phoswiches of LaBr3/NaI scintillators optically coupled to one photomultiplier tube (PMT). PARIS will be used in combination with the VAMOS spectrometer at GANIL. During the experiment, PMTs will be exposed to the constant magnetic fringe fields produced by a quadrupole. Magnetic shielding is essential to efficiently lower the magnetic field inside the PMTs. ANSYS software allowed to simulate fringe fields produced by the quadrupole and to verify the impact of the shield initially foreseen. Obtained results were used to optimize and improve the geometry of the shield and to select the most efficient materials. They are an original illustration of the performance capability of ANSYS software within the fields of nuclear physics, high energy physics and medical imaging instrumentation.