Magnetic shielding studies of the LHCb rich photon detectors

The LHCb experiment will use two RICH detectors to provide particle identification. The Hybrid Photon Detectors (HPDs) adopted for these detectors are required to operate in the fringe field of the 4 Tm LHCb dipole magnet. In fields in excess of 15 G, photoelectrons are lost from the active area of an HPD. Shielding the HPDs from the fringe field is therefore essential for the efficient operation of the RICH detectors. This paper describes the technique used to calculate the residual field inside the relevant magnetic shields. The size of the problem, together with the disparity in the geometric scales involved, makes the calculations intractable using a finite element model of the entire magnetic environment. As a result, a sub-modelling method has been used. The model indicates that the photon detectors will operate efficiently.