An array of low-background 3He proportional counters for the Sudbury Neutrino Observatory

Center for Experimental Nuclear Physics and Astrophysics, and Department of Physics, University of Washington, Seattle, WA 98195, USA; Los Alamos National Laboratory, Los Alamos, NM 87545, USA; Department of Physics, University of Oxford, Oxford, OX1 3RH, UK; Department of Physics and Astronomy, University of Pennsylvania, Philadelphia, PA 19104, USA; Department of Physics, Queen's University, Kingston, Ont., Canada K7L 3N6; Department of Physics, University of Texas at Austin, Austin, TX 78712, USA; Physics Department, University of Guelph, Guelph, Ont., Canada N1G 2W1; SNOLAB, Lively, Ont., Canada P3Y 1M3; Institute for Nuclear and Particle Astrophysics, and Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA; Department of Physics and Astronomy, University of British Columbia, Vancouver, BC, Canada V6T 1Z1
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 06/2007; DOI: 10.1016/j.nima.2007.05.321
Source: arXiv

ABSTRACT An array of Neutral-Current Detectors (NCDs) has been built in order to make a unique measurement of the total active flux of solar neutrinos in the Sudbury Neutrino Observatory (SNO). Data in the third phase of the SNO experiment were collected between November 2004 and 2006, after the NCD array was added to improve the neutral-current sensitivity of the SNO detector. This array consisted of 36 strings of proportional counters filled with a mixture of 3He and CF4 gas capable of detecting the neutrons liberated by the neutrino-deuteron neutral-current reaction in the D2O, and four strings filled with a mixture of 4He and CF4 gas for background measurements. The proportional counter diameter is 5 cm. The total deployed array length was 398 m. The SNO NCD array is the lowest-radioactivity large array of proportional counters ever produced. This article describes the design, construction, deployment, and characterization of the NCD array, discusses the electronics and data acquisition system, and considers event signatures and backgrounds.

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    ABSTRACT: The Sudbury Neutrino Observatory (SNO) has now completed neutrino detection with 1,000 tonnes of heavy water situated 2,000 meters underground in INCO's Creighton Mine near Sudbury, Ontario. During the first two phases of the experiment, SNO showed that neutrinos change flavour on their way to earth and accurately measured the solar neutrino oscillation parameters. Besides this, SNO was also able to use the data of these first two phases for other analyses. In particular, it recently set a limit on the Solar hep Reaction and the diffuse Supernova background. The third and final phase of operation uses an array of neutron detectors to independently observe the Neutral Current reaction of solar neutrinos on deuterium. Data analysis of this phase is in progress. In this paper, I discuss the latest results of the first two phases of the SNO experiment and report on the operation and calibration of the SNO detector during the it's final phase.
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    ABSTRACT: We present a straightforward method for particle identification and background rejection in $^3$He proportional counters for use in neutron detection. By measuring the risetime and pulse height of the preamplifier signals, one may define a region in the risetime versus pulse height space where the events are predominately from neutron interactions. For six proportional counters surveyed in a low-background environment, we demonstrate the ability to reject alpha-particle events with an efficiency of 99%. By applying the same method, we also show an effective rejection of microdischarge noise events that, when passed through a shaping amplifier, are indistinguishable from physical events in the counters. The primary application of this method is in measurements where the signal-to-background for counting neutrons is very low, such as in underground laboratories.
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