Contact: Lynn Yarris, [email protected]

Mary Poppins once advised that a spoonful of sugar helps the medicine go down. To that adage she might have added that a spoonful of salt helps make the neutrino detector more sensitive.

By adding common table salt to the heavy water in their detector system, researchers at the Sudbury Neutrino Observatory (SNO) have tripled the sensitivity of what was already the world’s most sensitive detector of neutrinos. Berkeley Lab researchers were key participants in the latest results to be reported from SNO, based on this enhanced sensitivity.

Kevin Lesko, a physicist with Berkeley Lab’s Nuclear Science Division, is the chair of the SNO scientific board and a veteran member of SNO’s international scientific collaboration. He discussed the significance of the latest SNO results, which were reported at the TAUP 2003 (Topics in Astroparticle and Underground Physics) conference held earlier this month in Seattle:

“In the first papers out of SNO, we proved that neutrinos can undergo a transformation in flavor (type) as they journey from the sun to the earth. In this new paper, we presented precise measurements of the parameters that govern the transformation, in particular the mixing of the neutrinos. With these new results, we’ve been able to make predictions for other neutrino experiments regarding the difference in masses.”

Neutrinos are ghostlike subatomic particles with no electric charge and little mass. They’re emitted out of thermonuclear reactions in the core of the sun and other stars. Because they rarely interact with other forms of matter and are unaffected by magnetic fields, neutrinos may prove to be extremely valuable tools for mapping the universe. Their pathway to earth is virtually a straight line from their point of origin. Neutrinos might also hold the key to answering several important scientific questions, including why matter was favored over antimatter in the creation of the universe.

For decades, scientists believed neutrinos were massless. But experimental results at SNO and other neutrino detectors have shown that neutrinos do have mass which, although very tiny, contributes as much to the universe as the all the visible stars and galaxies. It was also believed that neutrinos come in three distinct flavors — the electron neutrino, the muon neutrino, and the tau neutrino. Scientists now believe the picture is more complicated and that neutrino flavors mix and change between flavors.

“What was thought of as an electron neutrino turns out to be a combination of the three different flavors that can change as they travel from the Sun,” says Lesko. “The particles we see being emitted as a part of beta decay, which we label neutrinos, are not fundamental particles but a combination of fundamental mass states. We’re now in the process of measuring the angle that describes the flavor mixing which is taking place. This angle is at its maximum for atmospheric neutrinos, which points to a complete mixing of flavors, while the angle we observe at SNO for solar neutrinos is large but not maximal. This hints that there’s something going on which we can’t yet explain.”

Located about a mile underground in a Canadian nickel mine, SNO is a neutrino “telescope” consisting of a geodesic sphere 17 meters in diameter, suspended in a huge pool filled with purified water. The outer steel surface of the geodesic sphere is studded with ultra-sensitive light sensors called photomultiplier tubes. Inside the sphere is an acrylic vessel filled with 1,000 metric tons of heavy water (deuterium oxide, or D2O).

A neutrino interacts with an atomic nucleus in SNO’s heavy-water-filled acrylic sphere. Surrounding photomultiplier tubes detect the flash of Cherenkov radiation and reconstruct the neutrino’s energy and direction.

When a neutrino passing through the heavy water interacts with a deuterium ion or deuteron, a flash of light called Cherenkov radiation is emitted. The photomultiplier tubes detect these light flashes and convert them into electronic signals that scientists can analyze. SNO is the only detector in the world able to simultaneously measure all three neutrino flavors; it does so by recording all three different ways in which a neutrino and a deuteron can interact.

One of these interactions, called the neutral current reaction, involves the liberation and recapturing of a neutron. The number of these interactions and SNO’s ability to detect them can be boosted with additives to the heavy water, as Alan Poon of Berkeley Lab’s Nuclear Science Division explains:

“In pure heavy water, the free neutron has to be captured by deuterium, which is not very efficient and results in the release of gamma ray that is just barely high enough for us to detect. By adding sodium chloride, for example” — common table salt, NaCl — “the neutron can be captured by the chlorine atom, which has a higher capturing probability than deuterium and gives off a stronger gamma ray signal, which we are more likely to detect.”

To obtain their latest experimental results, the SNO researchers added two tons of high-purity table salt to the detector’s pool of heavy water. Says Poon, who served as deputy analysis coordinator of SNO’s “salt phase” experiments, “After considering a number of alternatives, we found that sodium chloride is the easiest to handle.”

The salt is now being removed from SNO’s heavy water in preparation for another phase of experiments, which will involve the placement of a half-kilometer-long array of ultra-clean detectors to be placed in the heavy water. These detectors are precision instruments that are designed to provide further insights into neutrino properties.

In addition to Lesko and Poon, other Berkeley Lab participants in the SNO salt-phase experiments were the Nuclear Science Division’s Yuen-Dat Chan, Alysia Marino, Eric Norman, Robert Stokstad, and Karsten Heeger, plus the Engineering Division’s Yoichi Kajiyama.

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