Irradiation of a 2.5 m long SciFi module with 24 GeV/c protons to the dose profile expected in LHCb

A 2.5 m long and 13 cm wide SciFi module, made of a 6-layer single fibre mat, was irradiated with 24 GeV/c protons in the new CERN PS IRRAD facility. The module is equipped with an aluminised mylar mirror and was previously carefully characterised in a test beam experiment in the CERN H8 zone. For the irradiation the module was tilted by $7 ^o$ w.r.t. the beam axis. A translation table allowed to move the module during the irradiation transverse to the beam in order to generate the dose distribution expected in LHCb after an integrated luminosity of 50 fb$^{-1}$, including the very sharp rise at the mirror end to 35 kGy. The dose distribution was derived from the generation of sodium isotopes in the aluminium strips placed in front of and behind the module. The dosimetry relies mainly on the activity of the long-lived Na-22 isotope, as the short-lived Na-24 gave in our specific case less precise results. The irradiated zone corresponds to an approximately 25 mm wide band running along the fibre mat, at 32 mm from its edge. The other part of the module are expected to have received a negligible amount of radiation and can be considered as reference zone. Within the uncertainty of the dosimetry, the dose distribution comes close to the targeted profile.