Determination of CsI(Tl) scintillation time constants and intensities by direct fitting of digitized waveforms in the CHIMERA 4π multidetector

The design of modern multi-detector arrays for Intermediate Energy Nuclear Physics experiments has to face the problem of the improvement of the Charge and Mass Identification of Heavy and Light Charged Products. Most of the detector arrays developed for the identification of mass and atomic number of the emitted fragments, use detection units composed by thin silicon detectors and by thick scintillator crystals read-out by photodiodes. The light output of the CsI(TI) crystal can be modeled - at least in a given energy range - with a combination of two exponential functions with different time constants. The possibility of storing the digitized waveforms allows performing further off-line analyses. In the present work we determine - by direct least-square fitting of the base-line subtracted output waveforms - the four-vector defining the light output (τ_f, τ_x, h_f, h_s). In this way it is possible to provide an experimental determination of the scintillator parameters for a wide energy range and for different particle types. The proposed method has been applied to the waveforms acquired with the CHIMERA 4π detector array during runs at the INFN Laboratori Nazionali del Sud Superconducting Cyclotron in Catania, Italy with different beam/target pairs. In this paper we present the results obtained with a 21.5 MeV/u ²⁰Ne beam bombarding a ¹²C target.