A multiplexed optical-fiber system for the PMT calibration of the Borexino experiment

A multiplexed system of optical fibers has been designed for the photomultiplier calibration of the Borexino experiment at Gran Sasso. Both time and energy calibration are of paramount importance in Borexino for the measurement of the solar 7Be neutrino flux. Equalization of photomultipliers within 1 ns is required to define a Fiducial Volume to isolate a very pure internal region of observation, and accurate energy determination and resolution are crucial for the spectral shape recognition of the neutrino signal. The size of the detector, its tightness and radiopurity constraints require special care in the material selection and mechanical handling of the system. The solution of multiplexed fiber chains has been realized for the first time in a large underground detector. In this paper we illustrate the PMT calibration system design and the results of various feasibility tests performed.