The calibration system for the Borexino experiment

Borexino is a large liquid scintillator detector designed to study solar neutrinos in real-time. Since the beginning of data-taking in 2007, Borexino has been able to perform a complete spectroscopy of neutrinos from the Sun, thanks to its unprecedented radiopurity. Calibrations have been crucial for the success of the experiment. In this paper, we describe the Borexino calibration system, emphasizing its most critical aspects, in particular to those related to radiopurity. We also discuss some of the results of the calibration campaigns performed in Borexino Phase-1.