A Calorimeter for the Precise Determination of the Activity of the Ce-144-Pr-144 Anti-Neutrino Source in the SOX Experiment

We describe the design and the performance of a high precision thermal calorimeter, whose purpose was the measurement of the total activity of the Ce-144-Pr-144 anti-neutrino source of the SOX (Short distance neutrino Oscillation with BoreXino) experiment. SOX aimed at the search for eV-scale sterile neutrinos by means of the Borexino detector at the Laboratori Nazionali del Gran Sasso in Italy and of a very powerful artificial anti-neutrino source located at 8.51 m from the detector center. In order to obtain the required sensitivity, the activity of the source (approximately 150 kCi) had to be known at 1% precision. In this work we report the design of the experimental apparatus and the results of calibration measurements performed with an electrical mockup of the source. We show that the calorimeter sensitivity exceeds design goals, reaching a precision of a few parts per thousand.