Probing the Deep Interior of the Sun.
A Point-Form Summary
This section of the course notes, and the associated PowerPoint presentations, make the following critical points:
the reactions which fuse hydrogen into helium in the sun take place in stages, with a slow build-up through a series of steps where pieces of intermediate size are synthesized
not all the steps in the process happen equally quickly: some reactions are more likely than others
as a result, varied amounts of the intermediate products are produced depending on whether or not there is a 'bottle-neck' in that stage of the proceedings
you do not need to know the details. The important point is that astrophysicists must know and understand these things because the rate of fuel consumption, and thus the potential lifetime of the sun, will depend on these slow changes in the composition and structure
moreover, we have to be aware of other possible ways of producing energy. It turns out that there are a few other ways of fusing hydrogen into helium
one involves the momentary creation of other light elements, including Lithium, Boron, and Beryllium, which participate in subsequent reactions -- but always with the net effect of converting hydrogen to helium. These are described as alternative branches of the pp cycle
there is yet another way to convert hydrogen to helium, if you have a star which contains a small amount of Carbon. In the so-called CNO cycle, the elements Carbon, Nitrogen and Oxygen act rather as a catalyst. That is, they get involved in the reaction process, but the net effect is still to turn Hydrogen to Helium, with no net change to the overall abundance of C,N, or O
incidentally, this is not really a catalytic process because a true catalyst does not participate in the reactions it encourages -- instead, it merely acts as a favourable site for them to take place. Here, the C,N, and O do actively participate, but at the end of the cycle they are unchanged
in any event, the CNO cycle is really only an important source of energy in stars considerably hotter than the sun. The reason is that protons have to be moving really fast if they are to be slammed into and made to fuse with C, N, or O nuclei. This is because those nuclei contain 6,7, or 8 protons, and their cumulative positive charge strongly repels any approaching single protons
putting it all together, astrophysicists now believe that they have a complete understanding of the reactions happening deep in the core of the sun. Ideally, we would like to have a probe of that: that is, to be able to measure the actual rate of energy release deep within
unfortunately, the present release of radiant energy from the surface of the sun really only tells us about the reactions that were going on almost a million years ago. This is because the energy released there makes its way out through various interactions and collisions with other particles, in a phenomenon which can be modelled as a drunkard's walk
we'd prefer to have some way of gauging the rate of reactions going on right now in the core of the sun. Ideally this means that we should study something that comes directly out, as though the sun were transparent. There are such particles: the neutrinos.
neutrinos scarcely interact at all with ordinary matter, and they pass essentially unaffected through the sun's outer parts. The problem is that they also pass through any detectors we build on Earth (and indeed through the Earth itself) so they are hard to detect and count
it is actually quite straightforward to predict the number of neutrinos passing by as a result of the Sun's nuclear reactions. In fact, there are reckoned to be about 100 billion of them passing through every square centimeter of the surface of the Earth -- that means through you, too! -- every second. The problem is to detect them so that we can test this
neutrinos sound a little mystical, and in fact they were predicted long before they were even discovered. The prediction was intended to save the principle of the conservation of linear momentum. (Look back at the Physics 015 notes, or read pages 136-137 of your text, to remind yourself what this means.) In the 1930's, physicists discovered that neutrons were decaying into two pieces in a way which did not conserve linear momentum, and the easiest 'out' was to suppose that there was a new particle being created which had not yet been observed. It would have to move in a way which accounted for the 'missing' momentum
neutrinos can be produced in large and predictable numbers by nuclear reactors, which we can turn on and off. Although they are elusive, there are also ways of detecting a tiny fraction of those produced, and in this way we were able first to detect them in 1956. So neutrinos do exist!
in fact there are three kinds (or flavours) of neutrinos. They can be produced in different ways by different particles in various interactions and decays. The pp-cycle in the sun produces just one kind of neutrino: the so-called electron neutrinos
we can now search for those coming from the sun. The first such experiment used a tank of cleaning fluid deep in a mine (shielded from cosmic rays). This experiment was not ideal, in several respects. There was no directional sensitivity (so there was no guarantee that the neturinos were even coming from the sun); there was no time discrimination (so there was no way of telling exactly when the neturinos arrived); the use of chlorine-rich cleaning fluid was not the most sensitive technique available; and the experiment was sensitive to only one kind of neutrino. Still, it was an important first attempt
this experiment detected fewer than half the number of neutrinos expected. Why? There were three obvious possibilities: 1 maybe the experiment was faulty in some way
2 maybe the sun was not producing as many neutrinos as theory predicted
3 maybe the neutrinos left the sun in one form, but some of them turned into a different kind of neutrino en route, thanks to a mechanism called neutrino mixing. If so, the transformed neutrinos would not be detected
repeated experiments confirmed the deficit, using quite independent methods, confirming that there was indeed a problem. Either we were wrong about the nuclear reactions in the sun, or we were wrong about the nature of neutrinos -- but which was it?
the Sudbury Neutrino Observatory, which has its headquarters here at Queen's, is an experiment deep in a mine in Sudbury (2 km underground). It has directionality, time discrimination, better sensitivity, and the ability to detect all three kinds of neutrinos -- so it is better in all respects than previous experiments. (It still sees only a minuscule fraction of all the neutrinos that pass by, of course).
the neutrino observatory consists of a huge underground vessel filled with a thousand tons of 'heavy water' -- water in which ordinary hydrogen has been replaced by deuterium. This makes it much more likely that the neutrinos will interact as they pass through
the neutrinos can interact with the heavy water in a few ways, but the most likely result is that an electron will be accelerated to a speed greater than the speed of light in water. Charged particles moving at these 'superluminal' speeds emit light in the direction of motion, rather like turning on the headlights of a speeding car; and the flash of light is picked up by inward-looking phototubes (of which the SNO experiment has nearly ten thousand). So we record the time of arrival of the neutrino, and the direction from which it came. The most important point, however, is the sensitivity to all three kinds of neutrinos.
the SNO experiment has shown that there are as many neutrinos flowing from the sun as predicted -- but that some of them have converted into other flavours on the way. We were simply unable to detect them in our earlier experiments. So we are quite right about the Sun and the rate of its nuclear reactions
the profound revelation is that neutrino mixing does occur: the neutrinos are changing flavour (from one kind to another) as they travel. This has enormous implications for our basic understanding and represents a considerable overthrow of the 'Standard Model' of particle physics.
Associated Readings from the TextPlease look at: Chapter 15, pages 496-517. and visit the website of the Sudbury Neutrino Observatory at http://www.sno.phy.queensu.caDelayed at the Airport.Last day, I described the series of reactions which constitute the proton-proton chain by which hydrogen is converted to helium. As you saw, there are various steps in the reaction. It is interesting that not all the steps are equally speedy, a piece of physics which controls the rate at which everything happens. To understand what I mean by this, imagine yourself flying back into Canada after an overseas trip. You arrive at Pearson Airport and go through the usual procedures: you wait while the aircraft taxies to the gate, and then while the `bridge' is extended out to the aircraft so that the doors can be opened; you have an interview with an immigration officer; you wait at the baggage carousel for your luggage to arrive; you wait in a queue for customs inspection. What you notice in this familiar process is that the various stages are not all equally speedy. It may take five minutes to get through the immigration process if you are a returning citizen, longer if you are a visitor; the baggage can be delayed for half an hour; the customs can be long or slow depending on whether you have anything to declare (and on whether the inspectors believe you!). Similar considerations pertain to our discussion of the interior of the sun. The best way to understand this is to consider what would happen if you were able to paint one proton blue so that you could recognize it. What would happen to it as you watch it move around in the core of the sun? Well, your expectation is that it would eventually run into another proton and fuse with it (the first step in the P-P chain). This is correct, but the average proton does not suffer this fate until after about 14 billion years of randomly moving around. Since the sun has been around for only about 5 billion years, this means that less than half of the protons in the core have suffered this fate yet. (Don't forget that protons get used up in other ways as well, in other steps in the p-p chain!) Thus most protons can `enjoy' quite a long life in the core, just as most people can look forward to a long life, although a few tragically will not have that happy experience. By contrast, once a pair of protons do fuse to form a deuteron, it is very easy prey. For reasons to do with the detailed nuclear structure, a deuteron is very susceptible to merging with almost the first proton it encounters! In fact, the average deuteron lasts only six seconds before being converted to `light helium' via the second step in the p-p cycle. (These various delays and reaction times are tabulated for the P-P cycle in the figure below, but I would emphasise that I do not want or expect you to memorize them!) After that, the light helium lasts typically a million years or so before being converted to `regular' helium by fusing with another nucleus like itself. I used a traffic analogy in class. If you were to randomly pick any single car travelling along Canada's highways, chances are it would not suffer an accident for many tens of thousands of miles of travelling. In this respect, it is analogous to a proton in the sun. But if it suffers a glancing collision, say, while travelling along Highway 401, it is likely to spin out of control in the midst of a stream of fast-moving cars and almost instantly be hit by yet another vehicle. This is the fate of the dueteron: once the proton suffers one collision, and becomes a deuteron, it is almost instantly involved in another serious smash - and becomes an isotope of light helium. There is one obvious implication. At any given time, there are almost no deuterons in the sun, because as soon as one is formed, it is converted to something else (`light' helium). The analogy to the airport works again: if the immigration officers merely wave everyone through, there will be no line-up there (no deuterons), but there may be a big backlog at the customs inspection, where people get processed more slowly. The net reaction rate (the speed at which people get completely passed through the formalities at the airport) is most determined by the single process which is the slowest in the whole system. In nuclear physics, we can determine these reaction rates experimentally or theoretically, but obviously we have to know them if we are to understand the sun at all.Other PP Branches.I have oversimplified the nuclear reactions taking place in the sun in two ways, the first of which was that I ignored alternate branches in the p-p cycle. Let us return to my airport analogy. Not every visitor has to follow exactly the path I described through the terminal. For instance: someone in a wheelchair might be granted the favour of bypassing some of the line-ups, or might be allowed to go through the doors usually reserved for officials and air crew. if you have lately returned from an area where malaria is common, you might need to see the health inspectors. pilots do not usually go through immigration control, and are often given only cursory inspection at customs or bypass it altogether. These are what you might call small variations on the main direction of processing. The P-P cycle is rather like this. For instance, there are other (but less common) ways in which the deuteron can react with something else, so the total chain is rather complex. A couple of these alternative branches, called P-P II and P-P III, are shown in equation form below. You will see that the light elements of Beryllium, Boron, and Lithium play a role here. A realistic solar model has to take all these possibilities, and their relative probabilities, into account. The probabilities are called branching ratios. For example, ninety-nine percent of the light helium nuclei formed may follow the simple step in the main P-P cycle, but one in a hundred may follow a different path. Once again, if we are to understand what is going on in the sun, we need a deep understanding of all of the relevant numbers. Which reaction is more probable, and by how much, and why?The CNO Cycle.My second oversimplification is more serious. There is a second reaction chain, the so-called CNO Cycle, which is fundamentally different from the P-P Cycle. It is rather like what a chemist might call a catalysed process. Let me first remind you of the phenomenon called catalysis. There are some chemical reactions which typically proceed at a modest rate in a test tube, but which are enormously sped up if something is inserted into the tube - a piece of platinum, for instance. The remarkable thing is that the platinum itself does not participate in the reaction: it remains unchanged. But its presence somehow encourages the reaction, perhaps because its surface acts as a particularly favourable location for the chemical species to combine. The substance used in this way is called a catalyst. Within the sun, there is not only hydrogen and helium; there are also small amounts of other elements, including carbon, nitrogen and oxygen (C,N, and O). As we have noted, the energy source in the sun is simply the conversion of hydrogen to helium, with the P-P Chain being the dominant process. But there is a chain, the CNO Cycle, in which the same effect is achieved (hydrogen is converted to helium) through the catalytic role played by C, N, and O. To be precisely correct, I really must qualify this remark: the C,N, and O actually participate in the reactions, so are not catalysts in the precise sense that the platinum is in the example quoted above. But at the end of the cycle, there is no net change, as we will see. What happens is summarized in the set of equations below; you will see it in more diagrammatic form on page 560 of your text. (Please do not bother to commit this cycle to memory!) First, a nucleus of carbon 12 (i.e. one containing 6 protons, which makes it carbon, and 6 neutrons) collides and fuses with a proton , turning into an isotope of nitrogen-13. That isotope, being unstable, decays into an isotope of carbon (one with 7 neutrons, so a total atomic mass of 13) by emitting a positron and a neutrino The carbon-13 meets a second proton and is turned into nitrogen-14. The nitrogen-14 meets a third proton and is converted to oxygen-15. The oxygen-15 decays to yet another isotope of nitrogen, N-15, by emitting a positron and a neutrino. Finally, the nitrogen-15 meets a fourth proton , with the production of carbon-12 plus a helium nucleus. Notice, therefore, that at the end of step six we wind up with a nucleus of carbon-12, exactly the same thing we started with. The chain of reactions has had the net effect of combining, in stages, four protons into one helium atom (just as the P-P chain did). While the C, N, and O participated in the individual reactions, the net effect leaves the situation unchanged. The set of equations also shows the timescale for each of the steps. As you can see, for instance, the isotope of nitrogen-13 is quite unstable: once it is produced, it typically decays away in about 7 minutes. When you look at the numbers, you will see that all the times are rather short (remember by contrast that the first step in the P-P chain had a characteristic timescale of about 14 billion years for a given proton). In other words, a nucleus of carbon-12 is unlikely to survive very long before merging with a proton. Eventually the carbon-12 nucleus reappears, and then the whole process starts again.The Importance of Temperature, Revisited.Look again at the first step of the reaction chain in the CNO cycle. You can see that it requires a positively-charged proton to fuse with a carbon nucleus carrying six positive charges. The repulsive force between them is very strong, so only the fastest moving protons can get in there at all. This suggests that at moderate temperatures, such as in the middle of the sun, the PP reactions will be proceeding fairly quickly (since it is relatively easy to mash two protons together) but the CNO reactions will be less important (since it is harder to push a proton into a carbon nucleus). That is why, in the sun, the PP cycle is the dominant source of energy. The CNO-cycle contributes only a little. But now consider a more massive star, one which is hotter in the core. In such a star, everything is moving around faster, so fusing any two particles should be easier, and the reactions should be going faster. That is true. But let us think about this quantitatively. If I were to raise the temperature by ten percent, would the reaction rates increase by ten percent as well? Or more? Or less? Well, it turns out that the PP cycle and the CNO cycle have different temperature dependences - not surprisingly, considering that they involve completely different chains of reactions. But there are two important features: both sets of reactions (PP and CNO) are very sensitive to temperature, so that a somewhat higher temperature leads to much more vigorous nuclear reactions; and the CNO cycle is more sensitive to temperature than the PP cycle is. For instance, certain reactions in the CNO cycle depend on the temperature-to-the-sixteenth power, which means that a mere five percent increase in the temperature can lead to the reaction rates more than doubling. This tells us that in a star which is hotter than the sun, the PP cycle will be going faster, but the CNO cycle will be going much faster, and in the more massive (hotter) stars this will be the dominant reaction cycle. Do not lose sight, however, of the fact, however, that the net result is the same in these stars. (In both cases, hydrogen is being fused into helium.) The important point here is not so much the details of the CNO cycle as the fact that all of these things have to be known before we can even hope to understand the nature and evolution of the stars.Probing the Solar Interior: The Drunkard's Walk.Much of the energy released at the sun's center is in the form of photons (gamma rays and other high-energy photons). But it does not come shooting straight out of the sun. Because the material is so dense there, the photon can only travel a small distance (perhaps a few millimeters) before it collides with a particle, and bounces off in some other direction, or is absorbed and then promptly re-emitted in some random direction. The net result of all this is that the light comes out very slowly. (See the figure on page 508 of your text.) The energy released about one million years ago is only now escaping the surface of the sun and (eight minutes later) reaching us here on Earth. The light we see, therefore, does not tell us very much about the `state of the furnaces' in the sun's core at the time the energy was produced. The way the photons come out through the sun's outer layers is a manifestation of what is known as the `drunkard's walk' problem. If you imagine a person so drunk that he or she can only stagger, completely at random, in one direction or another, then it is clear that they are unlikely to make much progress in any particular direction. If they start from the pub door and take one hundred staggering steps, they may later find themselves only a few paces away from the door because a step in one direction may be followed by one in exactly the opposite way. In fact, it can be shown statistically that after N steps in total, the drunkard is most likely to be about square-root-of-N steps away from the starting point, in some random direction. Thus, after 100 random staggering steps, the drunkard will probably be about 10 steps from the door. This is only a statistical estimate of course, so it is easiest to think of repeated experiments, night after night, or many drunkards leaving all the taverns around town. After 100 steps taken, the average distance actually covered will be 10 steps, if the drunkards really do have a completely random 50:50 chance of staggering one way or the other. (Generally they are not quite so incapable as that.) It is this sort of analysis that describes the way photons gradually percolate out through the sun, and why it takes so long for the energy to escape. Here are some numbers for those interested. The sun has a radius of about 700 bilion milli meters. Since light travels at a speed of about 300 billion millimeters per second, it could leave the center and get right out of the sun in a little over two seconds, if the sun were transparent. But in fact each `step' a photon takes before running into another atom is only about 1 mm in length, so covering the N steps (the 700 billion steps) from the center to the outer edge of the sun requires each photon to take a total of NxN (= 700 billion x 700 billion) steps, like the drunkard. At the speed of light, this takes (700 billion x 700 billion)/(300 billion) seconds which is just about 16 trillion seconds, or about half a million years. (The actual calculation is a bit more complicated than this because there are three dimensions in which the photons can move, unlike our drunkard staggering left or right along the sidewalk, and the steps get a bit longer in the outer parts of the sun, where the material is less dense. But the number is roughly correct.)Old News.Imagine a very crowded cocktail party where you, the host, are anxious to keep the supply of snacks on the table replenished. Suppose a friend plunges in to get a fresh drink and some snacks, then battles his way back out through the thick crowd. You ask him how the snacks are holding up, and he replies that the tray is still half full of goodies. He may, of course, be quite wrong! In the few minutes it took him to fight his way out through the crowd, the tray may have been emptied. The information you have is out of date , and perhaps should not be trusted to reflect the present state of things. In similar fashion, the brightness we see now for the sun mostly demonstrates just how prodigious the energy production was many hundreds of thousands of years ago near the centre. How do we know that there are any reactions at all going on in the core? One simple reason is that it is difficult to imagine how the reactions could vary significantly over time: the stability of the sun, as discussed earlier, suggests that it should be pretty much unchanging over the aeons. But a better approach would be to try and identify some information which comes straight out from the core, more or less immediately. The special kind of subatomic particle called a neutrino provides just this sort of information. As I noted earlier, the neutrinos which are released in the nuclear reactions within the sun's core come shooting straight out of the center of the sun. To them, the overlying layers of the sun are almost completely transparent. The number of neutrinos flowing out is therefore an immediate indication of how vigorous the reactions are in the sun's core, so all we have to do is measure how many are flowing past the Earth. But this is not easy! The neutrinos scarcely interact with matter at all! They are governed by what is called the weak interaction in physics, and a given neutrino could pass through literally trillions of miles of lead before interacting with one of the atoms there. This is what allows them to come streaming out of the sun, but of course it also means that they will go shooting straight through us, our telescopes, and the Earth! So how can we detect their presence and their total number? Before answering that, let me give you some idea of the numbers. If we are right about the nuclear reactions in the sun, then every square centimeter of the Earth's surface, and of your body, has about one hundred billion neutrinos passing through it every second. But we want to test this hypothesis, to see if it agrees with our predictions. We must do this in two steps. First, we rely on the saving fact fact that not absolutely every neutrino will get through. Occasionally - very occasionally! - one of them does interact with an atom of matter. We need to find a way of recognizing that such an interaction has taken place: a detection method. Secondly, we need to know how efficient our detection method is. If we succeed in detecting only one in every trillion neutrinos passing through, say, then a detection of six neutrinos in our experiment means that there were in fact six trillion actually flowing by. We need a complete understanding of the experiment. Before I describe the attempts to carry this out, let me first give you some interesting history.Neutrinos Predicted: A Victory for Classical Physics.In the 1930's, various atomic experiments were centered on the phenomenon of what was known as `beta decay', a process wherein a single neutron (an uncharged particle) sitting in empty space decayed into a positively-charged proton and a negatively-charged electron moving away from each other. Now, remember that physicists since Newton had observed that the universe seems to obey various conservation laws. Here is how the thinking went in this context: The neutral proton splits into a positively-charged proton and a negatively-charged electron, so the total charge is still zero. Charge is conserved! The mass of the neutron is very closely the same as the mass of the proton plus the mass of the electron. Mass is conserved! The neutron, originally sitting still in space (with zero momentum), decays into two particles moving apart, each of which has a measureable amount of linear momentum. But the measured momenta do not in fact exactly cancel out. Apparently the Law of the Conservation of Momentum is violated! One of the cornerstones of Newtonian physics has just been weakened. Do we abandon this long-trusted conservation law? No. Rather than give it up, a physicist named Wolfgang Pauli suggested that there must be another particle produced by the beta decay. You can see that it must carry no charge be of very low mass interact very little with ordinary matter (or else it would already have been detected) carry the right amount of momentum to make up the shortfall. For these reasons, Pauli named it `neutrino' (Italian for `little neutral one.') His insight and his reluctance to give up one of the classical laws of physics were vindicated when the abundant neutrinos produced in a nuclear reactor were first detected in 1956, fully twenty-five years after his first prediction.Neutrino Observatories: The First Attempts.In your text, on pages 506-508, you will find a discussion of the first experiment to detect solar neutrinos, a famous experiment run by Ray Davis. As you will see in the text, his `telescope' consists of a huge tank filled with cleaning fluid deep in a mine. The chamber in which the tank sits is flooded with water before each observing run. As unlikely as it sounds, this works! Here is why: The cleaning fluid contains many chlorine atoms (it is similar to carbon-tetrachloride, a well-known solvent used in dry cleaning), and these atoms can be converted to radioactive argon, which is a gas, when struck by neutrinos. The tank is deep in a mine to shield it from cosmic rays, energetic particles which arrive from outer space and come showering down onto the Earth. They can induce unwanted reactions in the fluid in the tank. The chamber is flooded so that the radioactive emissions from the surrounding rocks are prevented from reaching the cleaning fluid, since they too can induce unwanted reactions. The experiment itself works as follows: helium gas is bubbled through the cleaning fluid to sweep it clean of any argon. Then the tank is allowed to sit for several weeks or months. Another sweep with helium gas brings out any argon atoms which have been produced, and because they are radioactive their total number - typically several hundred atoms at most - can be determined by measuring the radioactive decay rates. Knowing the efficiency of the whole enterprise, we can convert this measured number into an estimate of the total number of neutrinos which came through the tank, and compare that to theory.Limitations on the Experiment.The Davis experiment has several important limitations. It has no directionality. Even if we can correctly deduce that several neutrinos have arrived during our observing period, we still have no absolute assurance that they came from the sun. Theory suggests that the sun should be the dominant source of neutrinos on Earth, but it would be nice to test this important point. How can we be sure where they come from? It has no time discrimination. When we collect an argon atom from the tank, we have no idea if it was created by an incoming neutrino a month ago, a week ago, or five minutes ago. This may seem unimportant since we are interested in measuring the steady, long-term emission of neutrinos from the sun, but it would be nice to recognize if they come out in bursts, for instance, or to identify a burst from some other source (like an exploding supernova, about which we will talk more later). How can we record each neutrino as it arrives? The experiment is not very senstive and is rather cumbersome. There may be better, more sensitive ways to do this than originally thought up by Ray Davis (although his efforts, now forty years old, were heroic and the best available at the time).The Results.If the Ray Davis experiment yielded data which was in good agreement with our theoretical expectations, all would be well. As it happens, however, there was a problem. The Davis experiment, which ran for several decades, found significantly fewer neutrinos than predicted, results which have been confirmed by other more recent experiments. Some of the possible reasons for this are described in your text, but can be summarised succinctly as follows: Perhaps the experiment is flawed in some way - maybe the extraction of the argon is not as efficient as we believe, for example. Tests seem to disprove this, however, and independent experiments find a similar shortfall using different detection methods. Perhaps the neutrinos change in some way as they travel from the sun to the Earth, so that they can not be detected any more. There are in fact three kinds of neutrinos (particle physicists call these 'neutrino flavours'), and some theoretical models suggest the possibility of a phenomenon known as neutrino mixing, wherein neutrinos change in flavour as they travel. The nuclear reactions in the sun produce only one flavour -- the so-called electron neutrinos -- and the Davis experiment and most others are able to detect only these. Any transformed neutrinos would be missed in the count. One way of visualising this is to imagine a friend throwing you a baseball which turns into a football en route. You would, of course, still see the football -- and maybe even still catch it, despite your surprise! -- but the neutrino experiments would simply be 'blind' to the other neutrino flavours and would never know that this had happened. It is an important issue, because if neutrino mixing does happen, it has very important consequences for our understanding of the fundamental particles of nature -- indeed, it would be a tremendously profound discovery. Perhaps the sun is not undergoing as many nuclear reactions at the core as the `standard solar model' tells us it must be. In that case, we are misunderstanding something very basic about the deep interior of the sun, a situation which we want to resolve. Please remember that we believe that the nuclear reactions keep the interior of the sun hot, which supplies the sustaining pressure to hold it up. If the reactions are not as vigorous as we think, what holds the sun up? A strong magnetic field? Or what?The Sudbury Neutrino Observatory: How It Works.The Sudbury Neutrino Observatory, described fully at its website http://www.sno.phy.queensu.ca, is a multi-national neutrino-detection experiment with headquarters right here at Queen's University. Quite a few of our Physics professors (but not me!) are involved in the project. The experiment itself is housed deep (2 km underground) in a mine near Sudbury. It is continuing to collect data right now, in one of several phases of complex observations, but the remarkable thing is that it has already completely resolved the solar neutrino problem, after the first few years of data collection. Before I tell you about that, let me describe how the SNO project differs from the Davis experiment. In the SNO, there is an enormous spherical vessel filled with heavy water (remember that this means that each hydrogen atom in each molecule of the water is in fact the hydrogen isotope deuterium rather than everyday hydrogen). When a neutrino passes through this vessel, which is housed in a chamber as tall as a ten-story building but a mile underground, there is a chance that it will interact with one of the deuterons. Without going into the details of the interaction, I can tell you that this can lead to an electron being formed and ejected from the nucleus. The electron winds up moving in approximately the same direction as the neutrino itself was originally travelling. More importantly, the emitted electron will in general be moving faster than the speed of light in water. This will surprise you if you think that `nothing can travel faster than light.' This is not correct! It is true that nothing can travel faster than light itself travels through a vacuum, like the emptiness of space. But in water, light is slowed down considerably and a fast-moving particle can outrun it. For reasons I will not go into, such a fast-moving particle gives off a cone of light in the direction it is travelling (this is called Cerenkov radiation ), rather as though the electron were to turn on a small flashlight to see where it will end up. The inside of the SNO tank is covered with about ten thousand inward-looking ultra-sensitive photomultipliers which detect this flash of light and, by their positions and responses, indicate the direction from which the neutrinos came. (On page 507 of your textbook, you will see a figure showing a similar setup in the Super-Kamiokande experiment in Japan. The fundamental technique in that experiment is similar to what happens in SNO, but differs in some respects -- the most important of which is the fact that it does not use heavy water. The Super-K experiment is thus able to detect only one flavour of neutrinos, which is why it was not able to resolve the problem.) I think now that you can see why we will realize some enormous gains when we use the SNO detector. First, we have directionality -- the flashlight-like cone of light points roughly in the direction the neutrino was travelling. Second, we have time discrimination --the photomultipliers record the precise moment at which the flash of light is seen, so we know how steady the neutrino rate is. Third, although it is not apparent from my description of the SNO, the whole experiment is much more sensitive than the Ray Davis experiment and other experiments of various kinds. Fourth, and most important, SNO can detect neturinos of all flavours as they flow outward from the sun, so we can get a good count on the total numbers and see if indeed the neutrinos change from one form to another as they travel from the sun. And that is indeed the answer! SNO has shown that there are just as many solar neutrinos passing through the earth as the astrophysicists have always maintained: the nuclear reactions in the sun are behaving as we thought. But the particle physicists have a lot of fresh thinking to do: the existence of 'neutrino mixing' requires nothing less than a radical overhaul of the 'standard model' of particle physics. Really profound changes are required, and the SNO experiment has understandably attracted an enormous amount of international attention. Previous chapter:Next chapter0: Physics 016: The Course Notes, spring 2005. 1: The Properties of the Sun: 2: What Is The Sun Doing? 3: An Introduction to Thermonuclear Fusion. 4: Probing the Deep Interior of the Sun. 5: The Sun in More Detail. 6: An Introduction to the Stars. 7: Stars and Their Distances: 8: The HR Diagram: 9: Questions Arising from the HR Diagram: 10: The Importance of Binary Stars: 11: Implications from Stellar Masses: 12: Late in the Life of the Sun: 13: The Importance of Star Clusters in Understanding Stellar Evolution: 14: The Chandrasekhar Limit: 15: Supernovae: The Deaths of Massive Stars, 16: Pulsars: 17: Novae: 18: An Introduction to Black Holes: 19: Gravity as Geometry: 20: Finishing Off Black Holes: 21: Star Formation: 22: Dust in the Interstellar Medium: 23: Gas in the ISM: 24: The Size and Shape of Our Galaxy: 25: The Discovery of External Galaxies: 26: Galaxies of All Kinds: 27: The Expanding Universe: 28: Quasars and Active Galaxies: 29: The Hot Big Bang: 30: The Geometry of the Universe: 31: Closing Thoughts: Part 1:Part 2:Part 3: |
(Monday, 20 April, 2026.)
