Article

A Monte Carlo simulation of the Sudbury Neutrino Observatory proportional counters

04/2011;
Source: arXiv

ABSTRACT The third phase of the Sudbury Neutrino Observatory (SNO) experiment added an
array of 3He proportional counters to the detector. The purpose of this Neutral
Current Detection (NCD) array was to observe neutrons resulting from
neutral-current solar neutrino-deuteron interactions. We have developed a
detailed simulation of the current pulses from the NCD array proportional
counters, from the primary neutron capture on 3He through the NCD array
signal-processing electronics. This NCD array Monte Carlo simulation was used
to model the alpha-decay background in SNO's third-phase 8B solar-neutrino
measurement.

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    ABSTRACT: The Sudbury Neutrino Observatory is a heavy-water Cherenkov detector designed to detect 8B neutrinos from the sun. It is sensitive to neutrino elastic-scattering interactions with electrons, and neutral-current (NC) and charged-current (CC) interactions with deuterium. SNO uses its extensive array of photomultiplier tubes (PMTs) to measure both the electron neutrino flux and the total neutrino flux. In doing so SNO conclusively demonstrated the existence of solar neutrino flavor change. For its third phase of operation the detector was enhanced with an array of 3He proportional counters called the Neutral Current Detection (NCD) Array. The counters detect neutrons from the NC interactions and measure the total solar neutrino flux independent of the PMT measurements. A unique and highly detailed Monte Carlo (MC) simulation of the NCD array was developed to assist in the understanding of the detector. It has been and continues to be an essential part of the analysis of NCD Array data. The NCD MC is used to classify pulses from the NCD Array as either signal or background events. The data pulses are fit with libraries of simulated pulses composed of signal neutron-captures and background alphas. The fit results identify a subset of neutron pulses that are unique from the alphas. This subset is used to determine the total number of neutrons detected with a simple cut-based analysis. The result of this pulse-shape-based analysis is a measurement of the NC solar neutrino flux based on one third of the NCD data: fNCDNC=5.9 3+/-0.43stat. +0.27-0.26sy st.ncm- 2s-1 . This is in excellent agreement with the BPS08(GS) Standard Solar Model (SSM) prediction. This NC neutrino flux measurement is placed in the context of the global set of solar-neutrino experiments. The experimental results combined with the solar luminosity constrain the values of the neutrino mixing angles and the solar neutrino fluxes. The experiments can also neutrino-based measurement of the solar luminosity. The effects of this NCD-based NC measurement are examined with this global solar-neutrino analysis. The results show excellent agreement with the SSM expectations, and the new measurement of fNCDNC reduces the uncertainties on the 8B solar-neutrino spectrum and the neutrino mixing angles theta12 and theta 13.
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