Calibration of secondary emission monitors of absolute proton beam intensity in the CERN SPS north area

Secondary Emission Monitors (SEMs) are used at the SPS for measurements of the absolute proton beam intensity. As an accurate measurement was recently required by the NA56 (SPY) experiment, the responses of all the SEMs used in the SPS North Area, including those used for NA56, were calibrated. The independent measurement of the absolute proton intensity needed for this calibration was obtained by an activation method, where the radioactivity induced in a thin aluminium foil is measured with a gamma counter immediately after a short exposure of the foil to the proton beam. The foil activity was in turn calibrated using the absolute proton intensity measurements provided by the Beam Current Transformers (BCTs) available in the West Area Neutrino Facility. A description of the experimental method and results of the calibration are given. SEM calibration factors were determined with an error that, even in the worst case, does not exceed 3%. The response per unit proton intensity of the new titanium SEMs installed in 1995 is about 9% smaller than that of the traditional aluminium SEMs. The measured calibration factor of the titanium monitor used by NA56 has been shown to be stable within ±1% over a period of almost one month. By combining this additional uncertainty to the error on the determination of the calibration factor from the activation measurement, the absolute proton intensity delivered to NA56 could be measured with an uncertainty of 1.7%. (orig.)