SNO - Neutrinos, Dark Matter and the Sun
We
went to a lecture the other day (March 21, 2016) at the University of
Alberta, by Dr. Art McDonald of Queen’s University. Dr. MacDonald
has been a major player in the SNO (Sudbury Neutrino Observatory) and
SNOLAB observatories, which are neutrino detectors (and more) located
2 miles underground, near Sudbury Ontario. He and various colleagues
recently shared the Nobel Prize in Physics, for their work, which
showed that neutrinos can change their “flavour”, and thus have
mass. Dr. McDonald was born and raised in Cape Breton, Nova Scotia,
and his career has spanned many decades at various highly regarded
universities, Princeton for one. He is now a professor emeritus at
Queen’s in Kingston, Ontario.
The
University of Alberta was an appropriate venue for such a lecture, as
it has a long history of collaboration in these projects. Among
other things, the U of A fabricated many key parts of the equipment.
It is also currently involved in a new venture in the same
underground lab, the PICO dark matter search.
Neutrinos
Neutrinos
are sub-atomic particles, conventionally thought to be massless,
though now they are thought to have a small mass. They are very
abundant – billions pass through one’s fingernail (about a square
centimeter) every second. But they have very little interaction with
regular matter. It is said that a neutrino could pass through a lead
wall one light year thick, with only a 50% chance of interaction with
another particle. Basically, they have to hit an atomic nucleus or
electron “head-on”, to interact with regular matter. The
probability of this happening is vanishingly small, since most matter
consists almost entirely of empty space.
The
Big Bang theory says that they were one of the first particles
created, shortly after the event that started things off. They are
also created in various nuclear reactions, including the reactions
that power the sun, via nuclear fusion. Some other radioactive
processes, such as beta decay result in neutrinos, and they are
released in huge quantities during supernovas. Depending on the
process that created them, they can come in three “flavours”, the
electron neutrino, the muon neutrino and the tau neutrino. This
becomes central to the solution of the “solar neutrino problem”,
which is a key reason why Dr. McDonald and his team won the Nobel.
It
should be noted that neutrino research could have practical benefits,
in terms of developing working fusion reactors, which could provide
power in the future.
Neutrinos, SNO and the Sun
A
noted above, the nuclear reactions which create the energy that we
receive from the sun produces neutrinos. These particles are needed
to balance the nuclear equations, and carry away some of the energy
from the proton-proton chain reactions (a somewhat complex series of
nuclear reactions). Early detection apparatus (e.g. Homestake mine
observatory) showed that there were not as many neutrinos coming out
of the sun as expected – only about a third to a half.
Some
explanations of the deficit involved major changes to our models of
the sun’s structure and the associated solar reactions, for example
different pressures and temperatures . But various other
measurements (e.g. helioseismology) supported the original models.
Another
possibility was that solar neutrinos changed their flavour, during
the journey from sun to Earth. This theory of neutrino oscillation
was originally proposed in the 1950’s. It stated that neutrinos
could change their flavour (oscillate), due to certain quantum
mechanical effects. This also implied that neutrinos had mass, which
the standard model did not predict, in its original form.
Originally, neutrinos were thought to be stable and unchangeable.
The theory of neutron oscillation was further elaborated over the
next couple of decades.
The
SNO observatory was designed to detect all three flavours of
neutrino, and it did detect the expected number of neutrinos in 2001,
about equally distributed between the three flavours. The first
observations at Homestake could only detect electron neutrinos, and
that explained the discrepancy. The conclusion therefore, was that
oscillation did occur and neutrinos have mass (though extremely small
mass, perhaps 1/500000 of an electron). That’s the key work that
earned Dr. McDonald the Nobel, along with Takaaki Kajita of Japan,
who made a concurrent discovery with the Super-Kamiokande Observatory
in Japan.
SNO itself
First
off, let’s get this out of the way – “there’s no business
like SNO business”. Well, strictly speaking there are a few
underground observatories around the world, but it is a small number.
The
reason for going two miles underground in a working mine, it to get
away from radiation that would confuse the results of the experiment.
Mainly, that means shielding from cosmic rays and their by-products.
A very low level of radioactivity is required, such a low level that
the type of paint used for some of the equipment became a problem.
This concerned a little yellow submarine that allowed access inside
the detector, which had to be changed from yellow to grey, as the
yellow paint was too radioactive.
Basically,
the detector was a huge (6 meter radius) acrylic sphere, containing
1000 tons of heavy water, which was surrounded by an array of
photomultiplier tubes, that registered the nuclear reactions that
were the signature of neutrinos. Heavy water was a key to the
design, as the interaction with deuterium was involved. Heavy water
contains more than the normal amount of deuterium (hydrogen with both
proton and neutron). The detector used $300 million dollars of the
stuff, lent to it by Atomic Energy Canada. The mining corporation
INCO provided the space in the mine.
The
detector could discriminate the three flavours of neutrino, due to
the differing details of the resulting energy levels, directions, and
locations of detection. Note that these interactions didn’t occur
very often, maybe about once per day. The detectors sensitivity has
been compared to “seeing a candle on the moon”.
The
working area had to be kept immaculately clean, rather like the rooms
in which computer chips are made. Obviously that posed some
problems, two miles underground, in a working mine. Dr. McDonald
noted that his mother once visited the facility, and was impressed
with how clean they managed to keep it.
SNOLAB
Since
the original experiment, SNO has morphed into a larger facility,
SNOLAB. This has a number of experiments running, including some
that are involved in the search for dark matter.
Dark
matter is what it says it is – matter that we know exists but can’t
yet detect. We know it exists from a number of arguments:
- Galaxy rotation is such that galaxies shouldn’t be stable over long periods of time, unless some unseen matter was providing the necessary gravitational attraction to hold the galaxy together.
- Similarly for galaxy clusters. They ought not to remain bound as long as it appears that they are, so there must be unseen mass involved there as well.
- Cosmological “big bang” theories imply that there is a lot more mass in the universe than we can see.
A
leading candidate for dark matter are the class of theorized
particles known as WIMPS (weakly interacting massive particles). The
Atlas experiment at SNOLAB is looking for these particles via recoil
reactions produced by the rare interactions, which could be
differentiated from “regular matter” interactions. There have
also been attempts to produce these particles directly at CERN,
though there have been no conclusive positive results.
The
PICO Bubble Chamber experiment is another one at SNOLAB, which is
looking for certain recoil reactions, that would be consistent with
WIMPS. The U of Alberta is involved in this project as well.
The Nobel Prize Ceremony
Dr.
McDonald closed off the talk with some photos of the 2015 Nobel
ceremony and some anecdotes on this event. Many of the experiment
participants were able to attend. The Nobel ceremonies are very
elaborate and go on for quite a few days. It sounded fun but
exhausting. The Swedish royal family were “just folks”,
according to the professor.
By
the way, SNO and SNOLAB have had many illustrious visitors, including
Stephen Hawking, who curiously enough, has not won the Nobel. He is
pictured here with Dr. McDonald, who noted that Stephen Hawking was
extremely patient, brilliant and has a wicked sense of humour.
Also
of note, is that the observatory has been featured in Robert Sawyer's
SF novels (Hominids and companion books), and the solar neutrino
problem was a key plot element in Arthue C. Clarke's “Songs of
Distant Earth”.
A
note on the photos: Most are from the SNOLAB site, via Google
Images.
And
it is only fair that I mention one of our Dodecahedron Books SF
novels, since I can't be expected to blog entirely without
self-interest :). Book one of the Witches' Stones series (Rescue
from the Planet of the Amartos) includes
some
references to
dark matter and a neutron star plays a pivotal role in one action
scene. Neutron
stars, of course, are the result of supernovas, which produce an
incredible
numbers of neutrinos.
Plus, there's a nice neutron star on the cover, as
well as a rather fetching heroine. You
should buy it and read it, if only for the neutrinos. :)
Amazon
Canada: https://www.amazon.ca/dp/B008PNIRP4
