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 `sufficient...to 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.
Back to the Penn/SNO home page
This page written by
J.R. Klein
Last updated on $Date: 2002/10/03 21:36:37 $ by $Author: jrk $