Measurements of liquid scintillator properties for the Borexino detector

The optical properties of various scintillators (both single components and their mixtures) have been measured in order to choose the scintillator composition offering the best compromise among the requirements of the large volume detector of the Borexino experiment on solar neutrinos. A pre-selection of the most interesting binary or ternary mixtures has been carried out by using a relatively simple and fast optical method. This is based on the UV excitation of the solvent and the observation of the emission properties in the spectral region of interest. The results obtained allowed the light output, response time and attenuation lengths to be evaluated for a broad range of mixtures. The mixtures offering the best properties for Borexino have been investigated in greater depth by using ionizing radiation as excitation source. The results were found to be in good agreement with those obtained by the optical method. In addition, the α quenching and the time response of the scintillators to α and β particles have been tested to evaluate the αβ discrimination capability. mparison of the results obtained in four laboratories using different techniques and methodologies, offered an exhaustive picture of the scintillator properties and provided the criteria for a good choice of the mixture for Borexino.