Ultra-low background measurements in a large volume underground detector

A large volume (4.8 m3) liquid scintillator detector has been running in Hall C of the Gran Sasso Underground Laboratory since February 1995. This detector is called the “Counting Test Facility” (CTF). The main goal of the detector facility is the measurement of ultralow background levels in scintillators and the development of processes able to purify them at this level. The detector has been designed to have exceptional sensitivity using a variety of methods to identify backgrounds. With the CTF, records were achieved in the domain of low background large volume detectors. Limits of 3.5 ± 1.3 × 10−16 g/g and 4.4−1.2+1.5 × 10−16 g/g for the 238U and 232Th daughters, respectively, and 1.85 ± 0.13 ± 0.01 × 10−18 for the isotopic abundance of 14C relative to 12C were obtained. These results are very encouraging and point towards the feasibility of low energy, real time scintillation detectors for solar neutrinos, such as Borexino.