Neutrinos from the Sun
Neutrinos, or little neutral ones, are very close to being nothing at all. They move at or very near the velocity of light, have no electric charge, and have so little mass that until very recently scientists were not sure if neutrinos have any mass at all. Lacking any bonds to matter, neutrinos are extraordinarily antisocial. They just don’t like to interact with anything in the material world.
To neutrinos, the Sun is transparent, and large amounts of them move out into all directions of space at nearly the speed of light. About 3 million billion solar neutrinos enter every square meter of the Earth’s surface facing the Sun every second, and pass out through the opposite surface unimpeded. Each second there are about 100 billion ghostly solar neutrinos passing through the tip of your finger, and every other square centimeter of your body, whether you are indoors or outdoors, or whether it is day or night, and without your body noticing them, or them noticing your body. At night they go through the entire Earth before reaching you.
The only way to detect solar neutrinos is through their exceptionally rare collisions with ordinary matter. Although the vast majority of neutrinos pass right through matter more easily than light through a window pane, there is a finite chance that a neutrino will interact with a sub-atomic particle. When this slight chance is multiplied by the enormous quantities of neutrinos flowing from the Sun, we conclude that once in a great while a solar neutrino will score a direct hit, and the resulting blast of nuclear debris can signal the existence of the otherwise invisible neutrino.
The Sun produces neutrinos with a range of energies, and both the amount and energy of solar neutrinos depend on the particular reactions that produced them. Their expected flux at the Earth is calculated using large computers that produce theoretical models, culminating in the Standard Solar Model that best describes the Sun’s luminous output, size and mass at its present age. The results of these calculations (Fig. 3.1) indicate that the great majority of solar neutrinos have the lowest energy, and that they are generated during the nuclear fusion of two protons, the reaction that initiates the proton-proton chain. Smaller amounts of high-energy neutrinos are produced from the decay of boron 8 during a rare termination of the proton-proton chain. Different neutrino detectors are sensitive to different energy ranges (Fig. 3.1), and so tell us about different nuclear reactions in the Sun.
Fortunately for science, a miniscule proportion of the Sun’s neutrinos do collide with more palpable sub-atomic particles, and when such a collision occurs inside a massive detector its effects reveal the neutrino’s presence. The neutrino detectors contain large amounts of material, literally tons of it, to measure even a few of the solar neutrinos. The massive tanks must also be placed deep underground, beneath a mountain or inside a mine, so that only neutrinos can reach them (Fig. 3.2). The thick layers of intervening rock are transparent to neutrinos, but they filter out other energetic particles, generated by cosmic rays, and shield the detectors from their confusing signals. So, all of the solar neutrino hunters work deep underground where the Sun never shines.
Massive, subterranean detectors have been snagging just a handful of elusive neutrinos for more than a quarter century. The first such experiment, constructed by Raymond Davis Jr. in 1967, is a 615-ton tank containing 100 thousand gallons of cleaning fluid, technically called perchloroethylene or “perc”.
This pioneering chlorine detector measured solar neutrinos for more than a quarter century, always detecting about one third of the expected amount (Fig. 3.3). It measures an average neutrino flux at the Earth of 2.55 ± 0.25 SNU. In contrast, the Standard Solar Model predicts that it should observe a flux of about 8.0 ± 1.0 SNU. This discrepancy between the number of detected neutrinos and the number predicted is known as the solar neutrino problem.
A second experiment, that began operating in 1987, was located 1,000 meters down in the Kamioka zinc mine under Mt. Ikena in the Japanese Alps. It was also limited to the rare, high-energy solar neutrinos, but it could uniquely tell where the neutrinos come from. The detector consisted of a 3,000-ton tank of pure water.
The Kamiokande experiment was updated in 1996 to a high-tech, $100 million Super Kamiokande status, also located deep underground in the Kamioka zinc mine. The new detector is a stainless steel cylinder, roughly 40 meters in diameter and height, that contains 50,000 metric tons, or 12.5 gallons, of ultra-pure water. About 13,000 light sensors are uniformly arrayed on all the inner walls of the cistern (Fig. 3.4). These photo-multiplier tubes are so sensitive that they can detect a single photon of light – a light level approximately the same as light visible on Earth from a candle on the Moon. Since there are practically no impurities in the very clean water, light can travel for almost 100 meters without being noticeably attenuated; for ordinary water its less than 3 meters. This means that the light sensors can monitor the entire water volume for the bluish Cherenkov light generated by an electron recoiling from a direct hit by a neutrino.
The Sudbury Neutrino Observatory, or SNO pronounced “snow”, is located 2,000 meters underground in a working nickel mine near Sudbury Ontario. Like the previous water detectors, it will see only boron-8 solar neutrinos with energies above 5 MeV. But unlike Super Kamiokande, the heart of the SNO detector contains heavy water. One thousand tons of heavy water, with a value of $300 million, has been placed in a central spherical cistern with transparent acrylic walls (Fig. 3.5, Fig. 3.6). Since the scientists cannot afford its cost, the heavy water has been borrowed from Atomic Energy of Canada Limited, who have stockpiled it for use in its nuclear power reactors – the heavy water moderates neutrons created by uranium fission in the reactors.
A geodesic array of 9,500 photo-multiplier tubes surrounds the vessel to detect the flash of light given off by heavy water when it is hit by a neutrino. Both the light sensors and the central tank are enveloped by a 7,800-ton jacket of ordinary water (Fig. 3.6), to shield the heavy water from weak natural radiation, gamma rays and neutrons, in the underground rocks. As with the other neutrino detectors, the overlying rock blocks energetic particles generated by cosmic rays. If the detector was put on the surface of the Earth, the high-energy, cosmic-ray particles would make the detector glow like a giant neon sign.
Solving the Solar Neutrino Problem
In one attractive solution to the solar neutrino problem, the neutrinos are produced at the Sun's center in the quantity predicted by the Standard Solar Model, but the neutrinos change form and switch identity as they propagate out from the Sun. The neutrinos have an identity crisis on their way to us from the center of the Sun, transforming themselves into a form that we have not yet detected from the Sun and a flavor that we have not yet tasted.
The resolution of the solar neutrino problem rests with the refinement of our detection techniques. Experiments currently under way should reveal new secrets of the Sun’s core, and settle the question of whether solar neutrinos switch identities while travelling to Earth. They will also tell us about neutrinos themselves, providing stringent limits to, or measurements of, the neutrino mass. This may yield an improved estimate of the mass of the Universe.