Physical properties of liquid scintillators

Abstract

Neutrino oscillation studies at KamLAND in Japan using a liquid scintillator detector concluded the "Solar Neutrino Problem" by observing that the electron anti-neutrinos from the reactors around Japan were oscillating. The parameters of the oscillation matched those deduced from solar neutrinos, so additionally demonstrated that the electron anti-neutrinos are not behaving differently from electron neutrinos in any significant manner. The data fits also eliminate some models competing with the oscillatory hypothesis [I, 2]. The secondary signal observed and presented by the collaboration involves the signal coming from the Earth itself, the "Geo Nu's" or geo-neutrinos [3,4]. The next step in studying the geological neutrinos is to characterize radioactivity within the layers of the earth using anti-electron neutrinos emanating from the Earth's mantle and core. This characterization requires the ability to discriminate radiation coming from the mantle and core separately from the crust. This characterization requires that the detector is far removed from significant background signal contribution due to the crust and man-made nuclear reactors [5, 6]. Modeling the parameters of this type of neutrino experiment demonstrated the need for a 10 kilotonne liquid scintillator detector, ten or more times the size of the KamLAND detector. In order to avoid the neutrino background from regional manmade reactors and escape the contribution from the continental crust, we plan to place a detector over the thin mid-ocean crust. Submerging the detector in the deep ocean (> 3000 m) provides excellent shielding from the cosmic ray muons which otherwise induce background events. A key to the success of this detector is the liquid scintillator and its physical properties. Any design for an underwater neutrino detector at a depth of a few thousand meters adds two new key parameters beyond land-based detectors such as KamLAND [2] and BOREXINO [14]. The first constraint on the scintillator to be employed is temperature; the temperature of seawater is approximately 4°C at depth. The second constraint is pressure; the pressure is approximately -6000 psi (400 atmos.) at 4 km depth. The goal is to find a liquid scintillator candidate that does not exhibit significant deterioration of the scintillators optical properties due to change in pressure and temperature. The concerns are mechanical and optical properties, about which there is little or no available relevant literature. Viable candidates must demonstrate little if any pressure dependence over a temperature between 0°C and 25°C. They must also maintain a stable attenuation length at 0°C comparable to their attenuation length at ambient near surface conditions, between 20°C and 25°C.