Deployment and background characterization of the Sudbury Neutrino Observatory Neutral Current Detectors
The Sudbury Neutrino Observatory (SNO) is a heavy water Cherenkov detector designed to observe 8B neutrinos from the sun via elastic-scattering interactions with electrons and charged-current and neutral-current interactions with deuterium. SNO is currently taking data in its third phase, in which the detector's capabilities have been enhanced by the addition of the Neutral Current Detectors (NCDs), an array of 3He proportional counters that detect neutrons liberated in neutral-current interactions. The NCDs were designed and constructed to minimize radioactive impurities that can produce backgrounds. A variety of innovative materials and methods were used in the construction and deployment of the NCD array in order to meet the stringent purity requirements.
It is important to quantify alpha backgrounds in the NCDs and to verify that the impurity levels are within design specifications. Analysis techniques relying on pulse shape characteristics, time coincidences, and energy spectra were used to identify and measure bulk 238U and 232 Th chain decays and surface 210Po decays in the nickel NCD bodies. The measured bulk impurities in the NCD array are equivalent to [special characters omitted] picograms 232Th per gram of nickel and [special characters omitted] picograms 238U per gram of nickel. In addition to the [special characters omitted] alphas per day in the NCD array from these sources, 210Po surface contamination produces 127 ± 2 alphas per day, and another [special characters omitted] alphas per day are produced by activity in the upper portions of the disequilibrium 238U and 232Th chains. The alpha background studies presented here indicate that the alpha rates in the NCD array are in line with expectations, and will not pose a significant problem to the successful extraction of the neutral-current signal from the NCDs using pulse shape analysis techniques.
0798: Particle physics