Intrinsic background of EJ-309 liquid scintillator detectors.

In deep-underground experiments, intrinsic detector background can exceed the external one, limiting the detection sensitivity. Intrinsic background in liquid scintillator neutron detectors consists of α particles emitted from radioactive impurities in the detector housing (and possibly in the liquid). We present the results of long background measurements of 12 EJ-309 liquid organic scintillators detectors in the deep-underground Gran Sasso National Laboratory of the Italian Institute of Nuclear Physics. These detectors are a part of the detection array for the SHADES ERC project that aims to measure the very low cross section (down to picobarn) of the astrophysically important reaction 22 Ne (α , n) 25 Mg. With an exposure of ∼ 35 days we identify an actinide concentration in the detector housing of 8. 4 (1. 8) sta (1. 4) sys × 1 0 − 2 ppm and 1. 62 (0. 57) sta (0. 03) sys × 1 0 − 1 ppm of 238 U and 232Th, resulting in 64 α particle counts per hour. The half-life of 214Po was also determined as 163. 7 (11) μ s. [ABSTRACT FROM AUTHOR]