The Long-Term Stability of Liquid Organic Scintillators Used for Gamma–Neutron Separation

Since the beginning of using liquid scintillators for gamma–neutron separation, there have been many articles dealing with long-term degradation, especially due to oxygen dissolved in the cocktail or presented during scintillator encapsulation. Oxygen is a well-known quenching agent causing a significant reduction of light yield. The effect of aging of liquid organic scintillators, namely EJ301, EJ309 (both Eljen Technology), and new custom-made cocktails based on 1-Phenyl-3-(2,4,6-trimethylphenyl)-2-pyrazoline and 2,5-Bis (5-tert-butyl-benzoxazol-2-yl)thiophene luminophores, were investigated for more than half a year. The research was focused on the Compton edge shifting of gamma particles since the position is proportional to the light yield of the selected scintillator. It was proven that all investigated cocktails suffered from different deterioration of the light yield in the investigated period, especially those containing a low concentration of luminophore. On the other hand, the gamma–neutron separation was improving up to the 100th day of measurement. Furthermore, the gamma–neutron separation was observed and quantified using figure of merit (FoM) for samples prepared and filled under normal and nitrogen atmosphere during the mentioned period. All stated parameters of liquid scintillator NE213 manufactured by Nuclear Enterprises Ltd. opened more than three decades ago were measured and used for comparison. It was observed that even after 30 years, this cocktail was still able to discriminate gammas from neutrons.