The lack of agreement is what became known as the solar neutrino problem. Also, researchers extensively examined the nuclear cross-section measurements relevant to the pp chain, but no explanation for the discrepancy was apparent. One of the more reasonable effects was that the interior of the Sun was essentially devoid of heavy elements. Between 19, many effects that might reduce the central temperature of the Sun and hence the 8B solar neutrino flux had been considered. By 1978, solar neutrinos had been definitely detected ( 1), but the flux was still about 2.5 times lower than the predicted flux ( 9). Over the next decade, both the experiment and the theoretical calculation were refined. The calculation of these fluxes requires a detailed solar model, and at the time the prediction of the 8B flux was made (1968), Bahcall felt that the theoretical uncertainty was sufficient to contradict the idea that the observed deficit meant “that something fundamental was really wrong” ( 8).
A rate of approximately 2 events per day was expected, and the first result published in 1968 ( 7) reported a limit ∼2.5 times smaller than predicted.Īlthough the total number of solar neutrinos is strongly constrained by the Sun's luminosity, the relative neutrino fluxes in the pp chain are not in particular, the competition between 3He + 3He → 4He + 2 p and 3He + 4He → 7Be + γ and between e − + 7Be → 7 Li + ν e and p + 7Be → 8B + γ determines the energy spectrum of the neutrinos shown in Figure 1. The Q-value of the reaction is such that the experiment was sensitive to 7Be, CNO, pep, and, predominantly, the weak 8B branch in the pp chain.
#SUDBURY NEUTRINO OBSERVATORY SERIES#
Solar neutrinos allow the direct study of the nuclear fusion reactions in the Sun that convert hydrogen to helium with the release of energy via a series of reactions, summarized by It is the story of a massive experimental adventure, involving more than 500 people over 24 years, that followed from an inspired idea of Herb Chen, whose life was tragically ended by leukemia in 1987 before construction started in 1990. This article describes the extensive work that went into the experiment, ranging from the initial design, to a complex construction period, to the commissioning, operation, and removal of the heavy water, and summarizes the analysis of the data. This article is the story of this experiment and of how it showed that electron neutrinos from the core of the Sun change to other types of neutrinos, requiring extensions to the Standard Model of elementary particles and confirming that solar models are accurate. To make these measurements, the SNO collaboration built an ultraclean detector containing 1000 tonnes 1 of heavy water (valued at ∼$300 million), 2 km underground in an active nickel mine, and spent a number of years making observations. By comparing the fluxes observed by these two reactions, it is possible to determine ( a) whether electron neutrinos produced by one of the nuclear reactions powering the Sun change to other active types in transit to the Earth and ( b) the total flux of these neutrinos to compare with solar model calculations. Chen and George Ewan acted as cospokesmen for the SNO collaboration after it was founded in 1984.ĭeuterium provides the opportunity to observe two reactions for solar neutrinos: the charged-current (CC) reaction, which is specific to electron neutrinos at solar neutrino energies, and the neutral-current (NC) reaction, which is equally sensitive to all active neutrino types. The advantages of using heavy water in this measurement were described in a seminal paper by Herbert Chen ( 4). This discrepancy had come to be known as the solar neutrino problem.
This measurement was motivated by the measurements taken by Davis and coworkers ( 1), which indicated that fluxes of electron neutrinos from the Sun were lower than had been calculated by the comprehensive solar models ( 2) developed by Bahcall and others ( 3). The Sudbury Neutrino Observatory (SNO) was designed to make a definitive measurement of solar neutrinos, clearly determining whether they change from one type to another, by using heavy water as a target.
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