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 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.
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.
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”.
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