Natural radioactivity effects on the scaler operation mode of the ARGO-YBJ detector

ARGO-YBJ is an extensive air shower detector located at the Yangbajing Cosmic Ray Laboratory (4300 m a.s.l., 606 g/cm2 atmospheric depth). It is made by a single layer of Resistive Plate Chambers (RPCs, total surface » 6700 m2) grouped into 153 units called ”cluster”. The lower energy threshold of the experiment is obtained using the ”scaler operation mode”, i.e. counting all the particles hitting the detector without measurement of the energy and arrival direction of the primary cosmic rays. For each cluster the signals generated by these particles are put in coincidence in a narrow time window (150 ns) and read by four independent scaler channels, giving the counting rates of ¸ 1, ¸ 2, ¸ 3 and ¸ 4 hits. The study of these counting rates has given unexpected results: while the MC simulations can account fairly well for the coincident counting rates, the expectation for channel ¸ 1 is about half of the measured value. Moreover as discuss in [2], the regression coefficient with the atmospheric pressure for channel ¸ 1 is also about half of the value measured for the coincident counting rates: seemingly half of these counts did not cross the atmosphere. A measurement of the radioactivity of the ground below the detector and a MC simulation to estimate the contribution from this local effect on our counting rates is presented and discussed