The Sudbury Neutrino Observatory (SNO) is a second generation dedicated solar neutrino experiment which has extended the results of our work with the Kamiokande II detector by measuring three reactions of solar neutrinos rather than the single reaction measured by the Kamiokande and SuperKamiokande experiments. The collaborative project includes physicists from Canada, the United Kingdom, and the United States.

The Solar Neutrino Problem

It is possible that the first astrophysics problem ever considered was the question of how the Sun burns, and yet it was only very recently that we have demonstrated that we know the answer. Even as late as the nineteenth century, the belief that gravity was the source of the Sun's power was used as evidence that Darwin could not be right---the contraction that was required could not have lasted the billions of years necessary for the observed diversity of species to have arisen. Sir Arthur Eddington was the first to suggest that it was the energy stored in atomic nuclei which provided the Sun's power, pointing out in 1920 that this source was `well-nigh inexhaustible' and ` maintain [an] output of heat for 15 billion years'. Proving that the Sun is tapping the nucleus for energy is not easy, however---it requires the detection of some specific signature other than the light and heat that we see and feel (most of which is coming directly from the surface of the Sun, not its center). Fortunately, there is a common by-product of nuclear reactions---neutrinos---which can travel through the Sun more easily than sunlight does through glass. Observing solar neutrinos can therefore tell us directly about the processes occuring in the solar core.

It was not until 1967 that Ray Davis and his coworkers constructed the first detector to look for these neutrinos. Davis's experiment was not only expected to be a triumphant confirmation of the nuclear fusion theory, but also to usher in a new era of solar physics in which neutrinos would be used to probe the details of the solar core. Yet, while Davis did prove that solar neutrinos exist, his measurements also produced a surprise: compared to the detailed calculations made by John Bahcall and others (the `Standard Solar Model'), he found that he was seeing far fewer neutrinos than predicted. The five experiments that came after Davis' also saw a deficit, and the mystery of these `missing' neutrinos became known as the Solar Neutrino Problem.

Essentially there are three possible ways in which neutrinos from the Sun might appear to be missing. First, it may simply be that the measurements are incorrect---the neutrinos are there but we have underestimated their number. The different detectors and approaches used by each of the solar neutrino experiments, however, makes a common error between them very unlikely. Second, the Standard Solar Model may be wrong in some way---perhaps the Sun's interior properties are different than we expect, and fewer neutrinos are being created. The model does correctly predict other solar characteristics, however, such as the way in which the solar surface oscillates in response to helioseismic activity, and it is therefore unlikely that it would grossly miscalculate the number of neutrinos. We are left with a third possibility: that what is `wrong' is not our measurement of neutrinos nor our understanding of the Sun, but something about the neutrino itself.

Davis's experiment therefore provided much more than a new way to study the Sun: it raised questions about a part of fundamental particle physics we thought we already understood. Our best theory of the matter in the Universe (called, in a great imaginitive leap, the `Standard Model') holds that there are three separate types (or `flavors') of neutrinos--- electron neutrinos, muon neutrinos, and tau neutrinos. The Sun can only produce electron neutrinos, and therefore the six solar neutrino experiments that followed in the three decades after Davis's primarily looked only for this one flavor. But if the Standard Model is incomplete in some way, and the three flavors of neutrinos are not separate but can change from one type to another, we may have an explanation for the Solar Neutrino Problem: if electron neutrinos born in the center of the Sun may change into one of the other types on their way to the Earth, then the experiments like Davis's will have seen fewer neutrinos than they had expected---the others sail right through the detectors unnoticed.

The changing of neutrinos back and forth from one flavor to another is usually referred to as neutrino `oscillations'. Strong evidence that neutrinos can oscillate from one flavor to another was reported in 1998 by the Super-Kamiokande experiment which looked at muon flavor neutrinos that were produced in the Earth's upper atmosphere by cosmic rays. What Super-Kamiokande observed was that the number of muon neutrinos they measured depended on where the muon neutrinos were produced---above the detector where they needed to travel only 100~km or so before observation or below it all the way on the other side of the Earth where they would need to travel thousands of kilometers. If muon neutrinos can oscillate, they would produce exactly the type of distance-dependent difference in the number of neutrinos seen by Super-Kamiokande.

Unfortunately, for solar neutrinos, the distance from the production point (the Sun) to the detection point (the Earth) does not vary enough to allow the type of measurements made with Super-Kamiokande's `atmospheric' muon neutrinos. Rather than look for distance-dependent effects, the way to prove that the electron neutrinos produced in the Sun are oscillating into muon neutrinos or tau neutrinos is to directly look for muon neutrinos or tau neutrinos. The Sudbury Neutrino Observatory (SNO) was designed to do just that: to determine whether or not solar neutrinos other than electron neutrinos are arriving from the Sun. In the Spring of 2001 and again (and more definitively) in 2002, SNO published its first detection of these non-electron type neutrinos, demonstrating directly that neutrinos can, in fact, change flavor. We therefore have firstly found that our must fundamental theory of matter is incomplete. Just as interesting, however, is that when the number of these muon and tau neutrinos is added to the count of electron neutrions, the total is found to be in excellent agreement with the predictions of the Standard Solar Model---we do, in fact, understand the Sun very well. We can therefore finally go back to Davis's original hope, and begin the new field of solar neutrino astronomy, using neutrinos learn the details of what happens deep inside the solar core.

SNO's Uniqueness

SNO's success in solving the Solar Neutrino Problem was primarily the result of its unique neutrino detection medium: 1000 tons of heavy water (D2O), in which deuterium has replaced hydrogen in each water molecule. The use of ordinary light water (H2O) to detect neutrinos has a long history, most notably with the IMB (Irvine-Michigan-Brookhaven), Kamiokande, and Super-Kamiokande experiments. In these detectors, a large mass of water (up to 40,000 tons in the case of Super-Kamiokande) is used as the `target'--- neutrinos in these detectors scatter electrons as they enter the water and as these electrons travel through the water they do so at velocities above that which light can (the water's index of refraction slows the light down, but not relativistic electrons). When charged particles move through a medium faster than the local speed of light, a shock wave in the medium is created which manifests itself as a cone of blue light (not very different conceptually from the sonic boom created by a jet exceeding the local speed of sound). Although the number of photons created this way is not very large---just 500 for a typical neutrino event---photomultiplier tubes (PMTs) placed at the edges of the water volume are able to detect these tiny light levels. In an average solar neutrino interaction , 50 PMTs will be hit, each registering the arrival of a single Cerenkov photon.

SNO is also a water Cerenkov detector, but neutrinos interact in its heavy water in two additional ways not possible in ordinary H2O. The first of these reactions is the absorption of a neutrino by the additional neutron in each deuteron When the neutrino is absorbed by the neutron, the neutron emits an electron and becomes a proton. As in the electron scattering reaction, the electron moves through the heavy water fast enough that it creates detectable Cerenkov light. This neutrino absorption reaction can only happen if the neutrino is an electron neutrino---a muon neutrino or a tau neutrino interacting in this way must create either a muon or a tau rather than an electron, and solar neutrinos do not have nearly enough energy to do this. The neutrino absorption reaction thus exlusively counts electron neutrinos, the neutrinos which the Sun produces in its central fusion reactions.

The second of SNO's neutrino-deuteron reactions is the deuteron breakup reaction . Here, the neutrino splits the deuteron, liberating the neutron from the proton. In this reaction, no new charged particle is created and it thus occurs with equal probability for all neutrino flavors---a muon or tau neutrino will break the deuteron apart as easily as will a $\nu_e$. The free neutron cannot by itself create Cerenkov light, but after scattering off of the nuclei in the heavy water it is eventually captured by another deuteron, creating a tritium nucleus and releasing a high energy $\gamma$ ray. This $\gamma$ ray then scatters an electron in the heavy water and it is this secondary electron which creates the Cerenkov light detected by the PMTs.

Of course, in addition to these two SNO-specific reactions, the electron scattering reaction seen in light water Cerenkov detectors (which only requires the presence of electrons) also occurs in SNO's heavy water. Although any neutrino flavor can scatter an electron (no new charged particles are created in the process) the probability for a electron neutrino to do so is much higher than either a muon or a tau neutrino (this is just a consequence of the fact that the electron neutrino is a `partner' of the electron; a muon neutrino would scatter a muon more often than an electron neutrino would).

Demonstrating that neutrinos other than the electron type are arriving from the Sun is thus reduced to a simple comparison: if the number of neutrinos counted using either the deuteron breakup or electron scattering reactions is significantly higher than the number counted with the neutrino absorption reaction, then non-electron flavor neutrinos must be present in the solar flux. The Sun can only produce electron neutrinos (its fusion processes lack the energy to produce any of the other flavors) and therefore detecting solar muon or tau neutrinos directly shows that electron neutrinos can transform into one or both of the other flavors, in violation of the basic assumptions of the Standard Model. If we further find that the when the neutrino flux from all the flavors is added that the total flux agrees with the predictions of the Standard Solar Model, then we have solved the 30 year old problem first identified by Ray Davis---the missing neutrinos have been found.

The SNO Detector

The SNO detector is located in a large cavity excavated at the 6800 foot level in the INCO, Ltd., Creighton mine near Sudbury, Ontario. The 1000 tons of heavy water (D2O) are contained in a twelve meter diameter acrylic vessel surrounded by a light water shield. A geodesic sphere of diameter seventeen meters surrounds the D2O and supports over 9500 eight inch photomultiplier tubes (PMTs), each with a light collecting reflector. In order to reduce contamination of the neutrino signal by radioactive backgrounds, the entire underground laboratory is run as a clean room---everything (both people and equipment) must be washed before entering, and only the purest materials were used in the construction of the detector. In addition, both the heavy and light water is periodically purified and then assayed to determine the residual amounts of background that are present.

Penn and SNO

The University of Pennsylvania group's responsibilities to the SNO collaboration cover a broad range of efforts, but have focused primarily on the design and construction of the entire front-end electronics and trigger system, as well as significant contributions to the SNO analysis program. The 9728 channels of electronics built by Penn handle the signals from the PMTs, measuring the time each tube was hit with a precision of about 100 ps (the tubes' intrinsic timing resolution is roughly 1.5 ns), and the charge in each PMT pulse. The trigger system is capable of making a decision on whether or not an event should be kept for later analysis in just over 200 ns, and can store up to a million events in the case a nearby supernova occurs. In addition, each event is time-tagged with an absolute time good to a few hundred nanoseconds.

Penn's contribution to the analysis program include the creation of reconstruction algorithms, measurements of backgrounds, improvement of energy scale calibrations, the extraction of physics signals, and the creation and managament of the data processing chain.

Many people at Penn have contributed to the SNO effort over the past 10 years or more: Chuck Alexander, Gene Beier, Rick Van Berg, Jim Cook, Doug Cowen, Monica Dunford, Ed Frank, Bill Frati, Bill Heintzelman, Paul Keener, Josh Klein, Chris Kyba, Godwin Mayers, Neil McCauley, Doug McDonald, Mitch Newcomer, Mark Neubauer, Scott Oser, Ron Pearce, Vadim Rusu, Richard Van de Water, and Peter Wittich.

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This page written by J.R. Klein Last updated on $Date: 2002/10/03 21:36:37 $ by $Author: jrk $