Purification of Liquid Scintillators for Nuclear Astrophysics Applications

Abstract

To study the occurrence of rare events such as those associated with solar and atmospheric neutrino detection, experiments require high sensitivity which are ultimately limited by the detector volume and the detector background (Alimonti et al. 2009). Borexino is an international experiment located in the underground Gran Sasso National Laboratory in Italy designed to detect the low energy neutrinos from the decay of 7Be in the sun by neutrino-electron scattering (Alimonti et al. 2009). The entire detector is located 1400 m underground with the outer tank filled with ultra-pure water (2600 tons) and an inner sphere made of stainless steel which supports the phototubes and contains two pseudocumene buffer regions (1300 tons), with the center containing a pseudocumene/PPO scintillator mixture (Ianni 2012). 210Bi and 210Po are two of the most problematic radioactive impurities within contaminating the water and the scintillator. They originate from the decay of 210Pb in the Rn chain. The motivation for this thesis is to determine an efficient method for the removal of bismuth from deionized water. Bismuth trioxide is used as the source of bismuth and two main removal methods are explored, auto and electro-deposition. Experiments have been performed to replicate the results of experiments seen in literature which claim complete deposition of bismuth using either of the two methods. First, optimal pH and temperature condition for dissolving bismuth trioxide in solution are determined. After, auto-deposition experiments are performed using silver and copper planchets in nitric and hydrochloric acid solutions, and electro-deposition experiments are performed using silver as the cathode and either silver or graphite as the anode in hydrochloric acid solutions. Experimental results show that complete deposition is not possible in most instances for either method, even when following the procedures suggested by literature. Not only is complete deposition not achievable, but the rapid corrosion of the silver and copper planchets at short timescales prove to be a significant problem which literature does not address.