First commissioning measurements with a reconfigurable cosmic ray mini-array

This paper describes the first operational tests of a low-cost, reconfigurable mini-array for cosmic rays. Such facility is intended both as an educational tool for master and PhD students in a cosmic ray laboratory and to carry out quantitative investigations in cosmic ray physics. Each detection module is based on a 400 cm(2) scintillator tile, optically coupled to a Wavelength Shifter (WLS) bar and a Silicon photomultiplier (SiPM) for light collection and readout. Individual signals from SiPMs are amplified, discriminated and shaped, to be sent to an Arduino MEGA board for trigger, data acquisition and storage. Time stamping of the events is also provided, by means of a GPS module connected to Arduino. The tests carried out with a set of 30 detection modules, spread over an area of about 50 m(2), are here reported. About 300k shower events were collected during a period of 13 days, with a trigger involving at least a three-fold coincidence between the modules. Individual rates of the modules, together with environmental data (pressure and temperature) were also measured along the acquisition period. Multiplicity distributions in stand-alone mode and in coincidence with an additional detector located some distance apart were extracted from the data. The dependence of the shower rate on the atmospheric pressure was investigated for different multiplicity bins. The array may be further expanded and employed in various geometrical configurations, depending on the number of modules and on their relative distances.