Rudolf Mößbauer (1988) - The solar neutrino problem

Ladies and gentlemen, Our good, old Sun – and a lot of physics is contained in these two adjectives – our good, old Sun usually stands in the sky, and doesn't attract much attention. Sometimes this changes. In the last few days we've read in the newspapers that there were large solar protuberances that also affect communications on Earth, but only from time to time. Today, too, the Sun will also affect you a bit, because you'll come half an hour later for lunch because of the Sun. Today I'd like to tell you something about the Sun, but not about the Sun’s surface. We know that it shines, we know that it emits a lot of light, which means that we know how much energy it emits. The Sun's luminosity is well known. But the light that the Sun emits is very old light. It needs hundreds of thousands of years to strenuously make its way from the place where it was generated in the Sun's interior to the Sun's outer surface. Today I don't want to talk to you about this outer surface, but instead about the Sun's interior. And we still know relatively little about the Sun's interior. We have very precise theoretical ideas, and we're also very convinced that these ideas are right. The Standard Solar Model was mentioned briefly. However, we actually still have relatively few technical measurement data. What do we know about the Sun's interior? Well, we have two experimental possibilities; one is more recent, and the other is a bit older. The more recent one is this helioseismography, where, in the final analysis, the Doppler effect is utilised that has already been mentioned today, and where we measure local wavelength shifts in the light emitted from the Sun. So, you have the Doppler effect. The biologists and chemists had it explained earlier, they now know how the Doppler effect works when a bear approaches you, and then goes away again – this was demonstrated. I'm going up to somewhat higher frequencies, we can take an example – just for the chemists and biologists again – where we say – if you drive through a red light in your car and the police catch you, then you can say to the policeman that you saw a green light due to the speed conditions in the case of this car. You can only do this in Germany where we don't have speed limits, but in America you generally pay 1 dollar or something for every mile above the speed limit. And if you approach the speed of light, then it gets very expensive. Then it's better to just pay for the red light instead. So, I don't wish to say anything about this seismographic effect, not least because we continue to have major problems with interpreting these data, although there's a lot of such data. I'd like to say something about the reactions occurring in the Sun's interior, and specifically about the nuclear fusion processes that occur there. The Sun is of course the only star, the only star out of an incredible number of stars that is so near to us in our Universe that we can really conduct experiments, and really perform measurements on the Sun relating to its interior. So, it's the prototype of a star, and everything we imagine about stars, such as with nuclear reactions, with energy production and so on occurring in the Sun's interior, at the moment we can only test this in experiments on our Sun. And that's what I want to talk about now. Now, the medium we test with are those particles, those strange particles that come from the Sun's interior to us here on Earth. And only around a quarter of the Sun's radius is burning, creating nuclear fusion, those strange particles that come down from the Sun's interior to us here on Earth, and whose existence we can prove, and which we call neutrinos. These neutrinos were introduced into physics as a hypothesis around 60 years ago by Wolfgang Pauli. It was then around 1956 or so when Cowan and Reines were able for the first time to show in an experiment that they really exist, but today we know relatively little about these particles, and actually, solar experiments are not only interesting in order to find out something about the Sun's interior, but also something about neutrinos. Now, if I speak about neutrinos, then these are interesting particles to the extent that they are the only particles that are exposed only to weak interaction. I'm not talking about gravitation, that's even weaker, so, I'm talking only about electromagnetic strong and weak interactions, and there it's the weakest type of particle. It has no charge, it's not exposed to other interactions, and so you can study weak interaction in pure culture with neutrinos, and this has been done at the big accelerators, meaning CERN, Fermilab, and Stanford and so on. I'm speaking here about relatively low-energy neutrinos, let's say MeV or typical nuclear-physics energies that are much more difficult to measure. Here, too, I can also now mention, for example, that there are particles that are even more difficult to measure, namely that neutrino background radiation that must have remained from the Big Bang in the same way as the electromagnetic background radiation – if the entire Big Bang theory is real, and not just a hypothesis. In other words, we have Penzias and Wilson, which was already mentioned here at the conference. Electromagnetic background radiation has been found. Neutrino radiation has not yet been seen because of its extraordinarily low energy. Measuring neutrinos at 3 degrees or so is very, very difficult, and particularly for the younger among you I can say that there are a lot of Nobel Prizes just lying around waiting to be won, if a good idea occurs to you, then just go with it. Now, the neutrinos I'm talking about here are neutrinos from the Sun's interior. They reach us in the range of some MeV energies, and they tell us what's going on in the Sun's interior. I've already said that these are particles that are subject only to weak interaction. And to make this clear to you, I'd like to say that these neutrinos, for example, enter the Earth at the front, and exit it at the back, and don't even notice that the Earth is there at all. And instead of this small, measly planet I could also take larger formations such as the Sun. They enter the Sun at the front if they come from the cosmos, and exit the Sun at the back – practically unaffected – these neutrinos lose almost nothing at all. Now, how can we then measure such particles at all with terrestrial experiments, perhaps with a detector, let's say, typically with 1 cubic metre of volume. In other words, incredibly tiny compared with this huge formation I've just mentioned. You can only measure it by having enormous numbers of these particles available, by letting them run through your detector, and perhaps every once in a while one gets caught. And you hang on to this one particle, and write lots of papers about it, and you then speak of this rare event. So, if you want to conduct neutrino experiments you need huge volumes of such particles. And there are, in practice, two sources to get such numbers of particles, such large numbers of particles, and these two sources that we have are, firstly, nuclear reactors and, secondly, solar fusion. In nuclear reactors we have the situation that we have nuclear fission, in other words, uranium-235 or plutonium-239 or one of these unpleasant things. So, they are split in the nuclear reactors, and as the heavy nuclei are very rich in neutrons and as they decay into medium-sized nuclei during this fission, we have too many neutrons, and the nuclei need to get rid of them in their excited states, and this occurs through this beta decay, the neutrons decay into protons, electrons and electron-antineutrinos. An analogous beta decay process occurs in the Sun. Here it's the other way round; you take protons and make neutrons, positrons and electron-neutrinos out of them because fusion processes occur in the Sun. In other words, you make one helium-4 nucleus out of 4 protons in each case. In the final analysis, this is what happens in the Sun, I've written this down here. It says, if I want to use 4 protons to make one helium-4 nucleus, which consists of 2 protons and 2 neutrons then, of course, I need to convert 2 protons into 2 neutrons in each case, that is to let this process run. So, the nuclear reactors produce electron-antineutrinos in large quantities because we have many such reactions per second, and the Sun makes exactly as many neutrinos because we have many of such processes per second in the Sun, in line with its size. That these reactions here exist is not something I want to talk about today. We've been trying for many years to measure with these things. We're now at the point of doing experiments with these here. That there are many of such processes is something we know from the theories of Bethe, Weizsäcker etc., but we'd like to verify this through experiments, of course. So, these neutrinos are what matter, and you might now first ask – "Why is the process so complicated?" That is, why is the situation such that we can't just forget the neutrinos at this point? That beta decay, for example, works with these two particles alone – this could also be possible. It's troublesome that nature had to add this third particle, troublesome also where tests are concerned that we have to take such things into account. It would be much simpler if we had just two of such particles. Now, I can say straight away that it's very, very important that these particles also come out here. And, in fact, Wolfgang Pauli introduced them for the following reason: first it was thought that there were actually only two decay processes during this transition, or during this beta decay. If this were so, then the well-defined energy of the starting condition that primarily enters there in kinetic energy would need to manifest itself by this particle also delivering a well-defined energy, in other words, a monochromatic line. Exactly as here. But this wasn't so. We've known since the 1920s that the beta spectrum is a continuous spectrum. Which led Wolfgang Pauli to say, in order to rescue the principle (of the conservation) of energy in physics, that something else needs to come out. And this "something else" that was supposed to come out was first a proton, which was obvious. So, he said, this is a three-body decay and not a two-body decay, this beta process, and there's also a third particle that comes out. And so now, experimental physicists among you, have a go at trying to find this photon … Many people, including Liese Meitner, spent many years trying to find this photon and couldn't find it, and Wolfgang Pauli, who, as a theoretician, was a particularly sarcastic man, naturally initially ascribed this to the experimenters' ineptitude. As it became clear over the course of the years that this point of view was untenable, Wolfgang Pauli simply invented a new particle, and this was a major achievement at the time, namely, a particle that was such that experimental physicists couldn't see it, and which has weak interaction so that it escapes their observations. And so weak interaction entered physics, and this particle is what we refer to today as the neutrino and the neutron had not yet been discovered and so on, all of which is unimportant for now. Now, why are neutrinos so important for us, why do they only make the tests more difficult? They are important for us because these processes are weak interaction processes. If we did not have this, then it could be attributed to an electromagnetic or strong interaction process, but this is now really a weak interaction process, in other words, it proceeds extremely slowly. I explained before what weak interaction really means, and extremely slowly means that in this manner we have found a way to exist, because our life has a certain timespan, of course, it doesn't happen in just seconds. Although it's ridiculously short compared with the age of the cosmos, it's still quite a lot compared with the time unit of the second. I'd like to remind you that we have processes in the cosmos that actually occur very much faster. Let me just remind you that we had a supernova explosion last year, and such a thing explodes in a matter of seconds. Such a star explodes in a matter of seconds, and if the stars that we have in the cosmos were to turn to dust within seconds, then our existence would be extremely imperilled. The fact that our Sun has already been living for around 4.5 billion years, and will live for as long again, means that we had time to develop ourselves, and we can thank weak interaction for this, we can thank these neutrinos for our existence. So, it's quite crucial that this weak interaction is not just a caprice of nature, but that it has made life possible. Now, people have been looking for these neutrinos. I'd now like to briefly show you an image of the Sun, and I'm going to show you this proton-proton fusion again. I'd like to remind you again, we know, of course, that we not only know about the existence of this proton-proton fusion from the calculations of Bethe and Weizsäcker and so on, but we also know that there is such a thing as a hydrogen bomb. It's not a very pleasant example, but the only thing that I can provide here that we have directly available. Here, the same processes are underway, the same that are responsible for energy release in the Sun. And for decades attempts have been made here on this Earth to get this controlled nuclear fusion going, this fusion of protons to helium. This would be the energy source that would make it possible for us to then get rid of nuclear energy, but it doesn't exist yet. Although we're getting ever closer to it, we've not yet got far enough to ignite. And even if we could ignite, which might perhaps be the case within the next ten years, then from there to the actual exploitation of this energy source, this practically inexhaustible energy source, could take an incredibly long time indeed. Now, in the Sun it was the case that it reached a state around 4.5 billion years ago that corresponds roughly to the state we have today as far as temperature and radius are concerned. So, it has been burning pretty much constantly for 4.5 billion years, and will continue to do so for about as long again. What happens, of course, is that the hydrogen – indicated here in red – gets consumed over the course of time, so that the helium portion grows. The burning core is indicated here, this is just the actual interior of the Sun, the interior of the Sun becomes ever richer in helium at the expense of the Sun's hydrogen. You see, a lot of the hydrogen has been burnt off – here's where we are today – but there's still a lot left, and we don't need to worry about the near future, about the Sun running out of fuel. Now in the next image I want to show you a little more precisely the neutrino spectrum that comes to us from the Sun. And here on the lower scale I'm first showing you the energies that are emitted there. Here I'm showing you the typical range of solar neutrinos – let's say from 0.1 MeV up to 10 MeV – and written up here in some units is the flux, the number of neutrinos that arrive here on this Earth per second and square centimetre. I'd just like to draw your attention to two ranges of this neutrino spectrum, namely this green branch and this blue branch. The blue branch is the actually important proton-proton fusion process. You see the logarithmic units here; this is absolutely dominant, in other words, almost everything, 98 percent of the solar energy, is connected with the blue branch. But the green branch is also important. It's a quite small, uninteresting ancillary process of the Sun, but it's currently important for experimental physicists because it's the only branch that we've been able to measure. It has not been possible to measure the blue branch to date. In other words, we had to make do with this green range here that arrives with paltry intensities, but high energies. And the high energy is important, firstly because the reactions that are used there have thresholds, and unfortunately they are so high that we only get the upper part as far as energy is concerned. And what helps us in this context is that the effective cross sections are quadratically effective in relation to energy, meaning our entire apparatuses prefer the high energies. It's easier, or only possible at all, to measure them. The reaction that was used here is chlorine 37+ neutrino, gives argon 37 + electron. This is the famous experiment conducted by Ray Davies and his colleagues, which was performed in South Dakota in a goldmine. You need to go so deep below the Earth because the count rates are ridiculously low due to the weak interaction. So you get a magnitude of around, let's say, 10^10 neutrinos per second and square centimetre, and the count rate that Ray Davies got in a massive detector of 620 tonnes, 620 tonnes of a chlorine substance, these count rates were around 1 neutrino every two days that manifested in this detector. Now, I've given this reaction here according to its threshold value. The threshold lies here for this reaction, and we can only measure what's above it, and you see this is essentially this green branch that was measured because of the high threshold. The shocking thing about this experiment, I've got a few details here, perhaps I should show you this first before I report about the shock that this generated in physics … So, this is Ray Davies and his colleagues in the Homestake Mine in South Dakota. Here's the reaction again. I also said that the reactor is very big. Perhaps it's not uninteresting to say that this liquid that you see here, this C2Cl4 is the same liquid that chemical cleaning companies use to remove fat stains from your jackets. It's a very cheap liquid, which made it affordable to create these enormous detectors in this goldmine. And the shocking thing was that, of course, you can now, in principle, calculate according to the Standard Solar Model how many neutrinos should arrive, and only about a third of them actually arrive. Now, you might say that this is not so shocking, but the shocking thing is the fact that the Standard Solar Model is a very tricky model. It includes relatively few parameters, and if you fiddle around a little with the parameters then the entire Sun goes up in smoke, then it doesn't have the radius, temperature and energy radiation and so on that it has. Which means, the Standard Solar Model is a model that many, many people believe in. There are many others, mainly theoreticians, who derive a living, of course, from the fact that they diverge from the Standard Solar Model. People don't quite believe in these theories, and they're unwilling to give up the Standard Solar Model. But this factor 3 that is there is quite disturbing. Deviations of 10 percent or so can be explained, but a factor of 3 is not so easy to accept unless you want to throw overboard a lot of physics in which we believe. Now, the difficulty with explaining this factor of 3 is that there is one explanation, it could be that the Standard Solar Model isn't right. And the problem is that this green branch, this small side branch, we're not quite sure that we know about it. It depends to an incredible extent on the temperature in the Sun's interior. It works at the power of 17 or so. And we still have a few excuses, but we don't have these excuses any longer if we could measure the blue branch, because the blue branch is directly connected with the Sun's luminosity, which is very well known. In other words, if we measure the blue branch, and we don't find the spectrum's flux again, if we could measure it, we wouldn't find it again, then actually nothing can be wrong with the Sun any longer. Then something needs to happen with the neutrinos on their way from the Sun, from the place where they are generated in the Sun's interior to the Earth, they must be doing something strange. This means that we would have a chance if we could find such deviations, to learn something about the neutrinos, especially about the mass of neutrinos, because this is something that we haven't known at all to date. Today, we all assume that neutrinos have mass; it's just that we have no idea how big it is. For this reason, we assume that they have a mass because, if they had no mass, there would need to be a symmetry principle in nature that disallows this mass, exactly as is the case with photons. There we have the gauge invariance that says that the photon rest mass is zero. We need to have precisely such a symmetry principle for neutrinos, but nobody has seen it anywhere to date, and it's a bit unlikely for this reason that it would have escaped us up to now that they have a zero mass. For this reason, they should have a mass, but we don't know how big it is. And theoreticians give us some indications that currently lie at the upper limit, let's say in the case of electron neutrinos at 100 electron volts, and a lower limit perhaps at 10^-6 electron volts, so that whenever we measure one, the theoreticians can say, "Look, that's what we predicted", but it's also the case that these limits can be readily extended upwards or downwards if need be. In other words, as an experimental physicist you have no indications of what these masses should actually be, except that they are not incredibly large if the particles are stable. So, if we could measure the blue branch, we have a chance to verify whether the Standard Solar Model is right, or whether something's wrong with it. This can't be good. If we find the full flux, then it's right, and if we didn't find the full flux, then something must be amiss with the neutrinos. Now, what is the problem, why don't we measure the blue branch? Well, it can be measured. And down here I've written a reaction with which it would be possible, namely the reaction gallium 71 + solar neutrinos = germanium 71 + electron, and this reaction has a very low threshold. It's here, and you see that, if I go up here, then I get most of this spectrum, if I were to use this reaction instead of that. The difference why this has not been done long ago is due to the fact, as I said previously, that this substance is very cheap – whereas this substance is incredibly expensive, so it's purely a question of price. And actually there was around - it is almost ten years ago – an American-German collaboration, the group at the Max Planck Institute for Nuclear Physics in Heidelberg on the German side, and Brookhaven National Laboratory on the US side, came together to conduct this experiment, which is one of the important experiments of this decade. But the German side was able to raise one-third of the money for the gallium. They needed around 50 tonnes of gallium at the time, which was equivalent to around 20 million dollars back then. The Americans failed to raise their two-thirds. This was probably due to the fact that it wasn't easy to raise 20 million dollars; it's relatively easy to find a million, and also relatively easy to find 500 million, but in between there's a range that's difficult to negotiate within committees. It's too high for low energy, and too low for high energy. And after years of frustration, this collaboration was dissolved, and the decision was then taken to try to raise the money in Europe, and this is precisely what has now happened. There's now a European cooperation, the European Gallex Project, Gallex stands for gallium experiment, and this collaboration consists of the following members. First there's a big group, the Max Planck Institute for Nuclear Physics in Heidelberg. This group is responsible for making the counters. Then there's a lot of chemistry in this experiment, and the responsibility for this lies with the Karlsruhe Nuclear Research Centre, the groups in Rehovot at the Weizman Institute in Israel, and here at the end is the Brookhaven National Laboratory again. They still haven't paid their entry subscription that has now shrunk to 1.5 million dollars, as I said, from 20 million dollars. They still haven't paid it yet, and we very much hope that they will do so in the not too distant future. So, the chemistry is being performed by these three groups. Then there's our group in Munich, we're responsible for the data processing. Then there are the Italian groups, because the experiment – as I will come on to explain – is being conducted in Italy, in Milan, and particularly in Rome. And then there are the French groups here in Grenoble, in Paris and in Nice, who are responsible for the so-called source experiment, which I'd also like to talk about if I have time. Now, what does this Gallex Experiment consist of? I've already shown you the reaction, it's able to register the low energy portion of the neutrinos. It's an experiment, the reaction is up here again, that now uses just 30 tonnes of gallium. These 30 tonnes of gallium are used in the form of gallium chloride, a highly acidic solution, the whole thing amounts to around 83 tonnes. And the idea is now, in this tank of 83 tonnes of gallium that we're going to use, to capture the neutrinos from the Sun. In this tank of this substance, we have around 10^29 gallium nuclei, and of these 10^29 gallium nuclei one will be converted into a germanium 71 nucleus every few days, if all goes well. And the problem is first to fish out a nucleus from these 10^29 nuclei, and to then hound it through a certain amount of chemistry, and to then get it into a proportional counter, and to then measure in this counter the transition back from germanium 71 back to gallium 71. And then this provides an indication that this process actually occurred previously from left to right, and that an absorption of a neutrino actually happened. The half-life of this germanium 71 here is of the order of 11 days. In other words, this fishing of nuclei will be conducted every 14 days, so you have to get them out relatively quickly. This takes around a day. And so we'll fish out not even as many as 14 nuclei, perhaps just 10 nuclei, which then need to be treated chemically and put into a proportional counter. Now, how is it at all possible to fish out an individual nucleus from 10^29 nuclei? It's not as difficult as it seems. You can do it with efficiencies of around 98 percent, and in the following manner: Gallium chloride has the pleasant property that it converts into germanium 71 chloride in the neutrino absorption, although this is 3 and this is 4 valence, and the 4 valence doesn't feel very well in the environment of the 3 valence, which gives it a tendency to move out relatively easily. A bit of neutral germanium carrier is then put into this substance, and blows the entire thing out with air or helium gas. When you have got this mixture of this gas with a few little germanium chloride nuclei in it, you then need to clean it, and this is where all the chemistry comes in, and you need to take care that you don't lose it somewhere along the way. So we have a quite large series of chemical processes, which I only understand in part so far and about which I would be better advised to say nothing here in this lecture, and finally resulting in a conversion of germanium chloride into germane. This substance here is called germane, and we think that it's very important that, particularly with this experiment, a French group, gallium, is involved at the start, and a German group, germane, is involved at the end. This is already a very good omen for this experiment. Now, this germane is then mixed together with xenon in a counter, the actual counter gas, and the counter – I must say this – has an active volume of around half a cubic centimetre, and the Heidelberg-based Max Planck Institute has been developing such counters for ten years. This is also an interesting story in fact. Here, if you have a nucleus that is activated per day, of course you need to take care that you work in an extremely background-free environment, that you work very cleanly, in other words, you don't have any students with dirty fingers around. You need to work extremely cleanly, but even this is not yet enough, it also needs to be backed up by refined electronics that make it possible to distinguish between correct and false impulses. I should perhaps in this connection also point out that there is a competitor company to this Gallex Experiment, namely a Russian group is currently conducting this experiment in the Caucasus, in the so-called Baksan Valley. I've been there a few times, and I'm very familiar with the situation there. The Russians have the huge advantage over us that they already have the gallium. In fact, they have 60 tonnes of gallium available there in large tanks. We're not going to have our full amount of gallium until the end of next year; actually, we still need approximately the equivalent of one year's global production of gallium for this experiment. But this will soon change because gallium production is now operated at many sites. In particular, it's also underway in Japan now, because gallium arsenide seems to be an important substance in the semiconductor industry, which is why people are interested in it. It's perhaps also quite interesting from the financial side. A lot of money was raised for these 30 tons of gallium, partly by the German Federal Ministry of Education and Research, and partly by the Max Planck Society, around 22 million deutschmarks, which is quite a lot of money, but the gallium isn't used up by this experiment, of course. So, every few days we convert a nucleus, but even the nucleus returns, as you'll have noted, so that, when the experiment is finally concluded – it will start in around two years' time, so at the end of next year. And all in all it will then last for around four years according to what we extrapolate – so that after around six years we can return this gallium to the market, and it may well be that we then get a lot more money for it than was previously invested. It was perhaps a good investment quite apart from the experiment. In any case, the Russians already have their gallium, so they're two years ahead of us, but we have the detector, and the Russians don't have their detector yet. And it's not yet quite clear what will happen. It's also interesting that with us, Brookhaven National Laboratory, with its wealth of experience in chemistry but that Los Alamos National Laboratory is involved with the Russian group, and that now in the age of Perestroika we have the interesting situation that, for the first time, two American national laboratories face each other across the Iron Curtain, and what will happen is something that we will see over the course of some years. It's certainly also very good that with this experiment, which is never done in the same way, that the Russians have another chemical extraction. Namely, they're not working with gallium chloride, but instead with gallium, with gallium metal. It's a very good thing that such an important experiment is being conducted at two different locations, as this will of course then boost the results' credibility massively. Because we want to find the full flux in this experiment, then everything will be okay. Then the Standard Solar Model is correct, then nobody will give two hoots about it, if you will If we then find less flux, then we must ascribe this to neutrinos, neutrino properties, neutrino masses, neutrino mixing angles, and, of course, nobody will believe us if this isn't confirmed by independent experiments, although we have another possibility which I will speak about shortly, and which we can use with this experiment to make it more credible. I'll now briefly show you an image of how an artist imagines we conduct the experiment. So, we're going to have a large tank here in the middle where we put the substance, the gallium chloride. As it's a highly acidic liquid, it's not all that trivial. We then have the major chemistry going on here, we then also have of course, in an emergency the possibility, if something happens, that we don't lose the gallium down here, after all it's a lot of money that we can then catch. The entire thing happens underground, where we have the huge advantage that we don't need to go into a goldmine, because the operating conditions are very difficult there, it's very hot, and the access routes are very narrow. You need to spend a huge amount of time talking with students to persuade them to go and spend a long time down there, so this type of doctoral work lasts five years, of course, and it's not much fun. So, we have a much more elegant method, we do it in Italy, in the Gran Sasso National Laboratory that's being built there. And first I'll show you – for my American colleagues I'm showing you the whole of Italy here – and for the Europeans I'm showing the central section, and so this is central Italy enlarged, here is Rome, and around 150 kilometres east of Rome is where the Abruzzo massif where the Gran Sasso is located. And through this Gran Sasso the Italians have built a motorway, and in the centre of this motorway, where the massif mountains above it are at their highest, large underground caves were built into the sides of the mountains, the largest caves that have ever been created artificially, and this is the Italian neutrino laboratory. Assergi is the nearest town, and L’Aquila is the nearest larger city, and this is where our experiment, among others, is currently being set up. A lot of other experiments are also planned at this site, and if Mr. Rubbia were here, he'd be able to report on another such experiment. This underground laboratory is extraordinarily large, I'm just giving you a small picture of it here. So, here are the caves that have been excavated, here's one side of the motorway, the other isn't shown. Cars travel on the other one, this side is still closed to traffic so that these laboratories can be constructed undisturbed. It would be impossible to do such a thing in Germany, where once a motorway is finally completed it naturally goes straight into operation. It has already been finished here for two years in principle, but until now we've had the opportunity to get on with building it undisturbed. Our laboratory is here, this small cavity that you see here, I'll show it to you next as it was when it was still a building shell. A lot of large cavities are planned for other experiments, including for laser experiments and so on and so forth. I'm showing you just an old picture of how it looked in its rough state, and just this small cave here in which we will work, it looks something like you see it here. Here you see one of our physicists who we have included here to give you an idea of the dimension of this cave. This cave has the advantage compared with gold mines that we can of course enter it with extraordinarily large vehicles. We can put all of our gallium in a tank, and this is extremely important because the relationship between outer surface and volume is thereby very favourable. The outer surface affects us because, of course, radioactivity intrudes everywhere, from the walls, from below, radon and so on, and naturally from the tank itself, and so this experiment offers this fantastic opportunity to circumvent this problem. Now, I briefly mentioned that we are going to do something to boost this experiment's credibility, and what we will do is that, into the middle of our detector, I'm showing you it again, here's the tank, twice during the experiment we'll place into the middle of our detector an artificial neutrino source, which must compete with the Sun, in order to calibrate the experiment. Although we know to some extent and in many cases a great deal about the many, many individual steps that are required, in order to ensure that a gallium atom that is converted here into germanium, and is then pulled out, pushed through the chemistry and finally lands in the counter, in order to ensure that and with what probability it will occur. We know all, all, all of these steps very precisely, with the exception of the effective cross section that we know precisely to some extent, perhaps to around exact 10 percent, but nobody will really believe you if it hasn't been verified. And for this reason, we'll introduce an artificial neutrino source here where we can then calculate precisely how much production rate, how many neutrinos we will measure in our counters. And if this number then agrees with the prediction, then naturally the credibility of this experiment is then immensely boosted. So, this source is produced, and our French and American colleagues are mainly involved in this; the French colleagues to the extent that they will introduce the reactor radiation, of the order of around 1 megacurie The Americans will carry out the isotope enrichment for the source. We need around 120 kilogrammes of chromium, which we will use as neutral chromium as an output material. We will then enrich it, and we will then work with around 40 kilogrammes – and these are huge volumes – and this 1.5 million dollar admission price for the Americans, that's the price to manufacture this enriched isotope that we need for this source experiment. And now, at the very end of my talk, I'd just like to show you what we can learn about neutrinos now if things go well, if we measure less flux than the Standard Solar Model gives us. I'm showing you here a drawing with two simple parameters, a quite simple model, the simplest I can use here. And the neutrino mixing angle is drawn down here. In the case of the quarks, this would be called the Cabibbo angle, but that's now the mixing angle for potential lepton mixing, theta, in logarithmic units, and drawn up here is the squared mass difference, the size that we can measure there, in other words, this would be, for example, M1^2 minus M2^2, where M1 and M2 are the masses of two potential neutrino mass eigenstates. What can namely happen on the way from the Sun to the Earth is that the neutrinos that are produced as electron neutrinos convert into muon neutrinos, revert into electron neutrinos, but also perhaps into tauon neutrinos or perhaps into others that we don't about yet, so that we lose electron neutrinos, and this would mean that our counter measures less, because it's sensitive to only the one type. This conversion – and that's a long story – will not occur so much in the vacuum or quasi-vacuum between the Sun and the Earth, but in the Sun's interior. The fact that electron neutrinos have another interaction than all other neutrinos with the electrons in the Sun, results in this, or can result in this. And this means that, depending on how sensitive they are to this conversion that is determined by the mixing angle, which says how well electron neutrinos convert into others, and once the neutrino masses enter and not necessarily eigenstates of the weak interaction. Now, I've indicated here in blue the sensitivity range of the Gallex Experiment to these two parameters. So you see, as far as the mixing angle is concerned, in the mass range of 10^-4, we're extraordinarily sensitive to as far down as 10^-4, further, in the case of even small masses, we're less sensitive. I've also indicated for comparison in red here another experiment that we conducted a few years ago, and about whose beginnings I've already spoken here, the Gösgen reactor experiment, where we measured the electron antineutrinos, and looked for neutrino oscillations. Where we found nothing, but could nevertheless state limits due to our measuring errors, where these parameters here are no longer possible, and this red area is excluded by these earlier experiments. Back then it was much more dramatic. I've cleverly not shown you a logarithmic scale here, but instead a linear scale, and then this naturally overlaps with most of this image here, and you see that neutrinos can now only exist in small ranges relating to these parameters. But now, when we replace the distance between the detector and the reactor that we had then, of up to 65 metres by the distance between the Earth and the Sun, we get a great deal more sensitivity, which is why I now make use of a logarithmic scale here and there, and you see that this is the range that is accessible to our Gallex Experiment. So, if mass differences arise here in this range, then we will see them, but we might be unlucky with the whole thing; it could be that it is precisely the neutrino masses that are in them. And it's not accessible to either this experiment, because it has a large distance, or to this experiment, because it has a small distance, we would then need to make a particular effort. And there are actually a lot of groups that currently want to get into this range. I'm somewhat against this because I think we should first wait for this experiment, which is in any case being conducted for quite other reasons. If it then supplies a positive result, in other words, if we are in the blue range, then these experiments are useless. If it's not the case, then they can still be done a few years later. The reason why I'm against experiments in this range at the moment is that you can relatively easily bring it down by a power of ten; but secondly it will then become incredibly expensive. So, the reason is essentially of a financial nature. The money would be better given currently to solid state physicists and biologists before this experiment is done, and if we then see that we must get into this range, then we could still do it in a few years' time. But this is all still a long way off, as I said, in around two years we'll begin, and by around four months after the start we'll have the first data that tell us whether the flux comes from the Sun or not. And we will then measure for a total of around four years, and perhaps during this time I'll have the opportunity to then report on the current status of the experiment. So, in around six years from now – if all goes well – the experiment will be concluded, and we will know whether we lie in the blue range with our neutrino characteristics or not. Since I don't wish to look that far into the future now, I'd like to conclude at this point.

Rudolf Mößbauer (1988)

The solar neutrino problem

Rudolf Mößbauer (1988)

The solar neutrino problem

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In his first three neutrino lectures at the Lindau Meetings, Rudolf Mößbauer described his experimental project to detect a possible non-zero neutrino mass through the observation of neutrino oscillations. As a source for neutrinos, he used nuclear fission reactors in France and Switzerland. At the time of the present lecture, these experiments were finsihed and Mößbauers interest had moved to using a much larger source of neutrinos, the sun. In the first part of the lecture, which gives the background to his involvement in a new experimental collaboration, Mößbauer describes what is called the standard solar model and how the neutrinos are created in nuclear fusion reactions in the interior of the sun. He also gives a very nice short history of the concept of neutrino particles, which derives from Wolfgang Pauli’s attempts to understand the continous energy spectrum of electrons emitted in radioactive decay and in which, among others, Lise Meitner was involved. Mößbauer also puts forward the hypothesis that the universe is filled with very low energy neutrinos, much like the cosmic background radiation discovered by Arno Penziaz and Robert Wilson. Since the detection of very low energy neutrinos is extremely difficult, he tells his audience of students and young researchers that finding a method to detect a cosmic neutrino background surely would lead to a Nobel Prize. As far as I know, this background radiation has so far (2012) not been detected, so there is still lots to do! Mößbauer then describes the so-called solar neutrino puzzle, by which was meant the fact that only one third of the expected neutrinos from the sun were ever detected during many years of experiments in the Homestake mine in the US. Finally, he describes a new project which would complement the Homestake measurements by looking for neutrinos of a lower energy. This new project, planned to start taking data in the beginning of the 1990’s, had been given the name GALLEX and was to be constructed in the Gran Sasso tunnel north of Rome. Mößbauer gives an inspired account of the problems in starting this new project, acquiring funding of about 20 million US dollars (too much for the low-energy committees, too little for the high-energy committees!), and buying about 100 tons of gallium chloride. In this amount of gallium chloride, about one gallium atom per day would be hit by a solar neutrino and thus transmuted into a radioactive germanium atom. Since the radioactive germanium atom would have a half-life of about 10 days, the 100 ton detector fluid would have to be washed trough every fortnight in order to find the radioactivity! Mößbauer shows pictures of the laboratory and the planned experiment and also explains what information could be gained from the measurements. Having listened to several lectures by Mößbauer, both recorded and in real life, I have seldom heard him be so enthusiastic! Anders Bárány

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