Rudolf Mößbauer (1982) - The world is full of neutrinos (German presentation)

Dear students, ladies and gentlemen. I would like today … I would like today to talk a little about neutrinos, a topic which has been very much at the focus of interest in physics in recent years. And would like, as someone who comes from Germany, to start with a few historical remarks. I won’t begin with the ancient Greeks, but somewhere around the year 1930 with Wolfgang Pauli, who is really the inventor of the neutrino. Inventor in the best sense of the word, at the time it was just a stopgap which later turned out to be correct. Let me quickly outline the problems that faced us in the 1930s. Radioactive beta decay had been observed, in which a nucleus of atomic number Z emits an electron and is transmuted into a new nucleus of atomic number Z+1. Since the original and the final nucleus are both characterised by a well-defined energy, one should assume that the electron, apart from its own mass, carries this energy difference, in other words that the emitted electron would have a sharply defined energy. But that was not the case, it was known at that time that the electron spectrum did not have a sharply defined energy, but that it was a whole spectrum ranging up to a maximal energy which corresponded with the expected sharply defined energy. That was a puzzle, understanding this spectrum, because it really looked as though the conservation of energy no longer applied, and in fact no less than Niels Bohr showed himself prepared to sacrifice the law of the conservation of energy in this special case. Wolfgang Pauli did not think much of this at all and he racked his brains a lot, and I would like perhaps to quote a letter which he wrote at the time, also to show the students something, that scientific progress mostly consists of false starts, retreats and a zigzag course, and that we only occasionally have the luck to find, or, I would almost say, to guess the right one. Wolfgang Pauli initially had assumed that, besides this electron here, something else was also emitted, that is a normal gamma quantum, and he believed that it had been overlooked in the experiment and he expressed himself on this in the following way, and I now quote from a letter that he wrote to Klein in February 1929. So, I quote: that gamma rays must be the cause of the continuous spectrum of the beta rays, and that Niels Bohr is completely on the wrong track with his remarks in this connection about a violation of the conservation of energy. I also believe that the experimenters who measured the energy made some kind of mistake and that they have so far missed the gamma rays simply as a result of their lack of skill. But I understand experimental physics too little to be able to prove this view and so Bohr is in the pleasant position, for him, to exploit my general helplessness in the discussion of experiments; he can invoke Cambridge authorities, incidentally without bibliographical references, and is able to fool himself and us however he likes.“ So much for the quote from Wolfgang Pauli. Now, in the following year Wolfgang Pauli had gained a little more faith in the assertions of the experimenters, who said they simply did not see any gamma rays, and then he invented the neutrino. Invented in the sense that he said that, besides the electron, another particle is emitted, a particle which he initially named the neutron. Later the neutron was used for something else and it was then called the neutrino, from the Italian for “little neutron”. So this neutrino was, so to speak, invented to save energy conservation, and this invention later turned out in practice to be right. But it still took until the year 1956 before Reines and Cowan provided direct evidence that this neutrino exists, in a direct experiment, in a direct experimental reaction that I have written down for you here and that I will discuss still further. Reines simply fired electron-neutrinos – this slash means antineutrinos, that is not so important now – at protons and then produced a neutron and a positron and showed, using the evidence of these two particles, there really is such a thing, such a thing as a neutrino exists. Today we believe we know three sorts of neutrinos, the electron-neutrinos, the muon-neutrinos and probably also the tau-neutrinos. The tau has already been seen, the associated neutrino not really yet, but we do believe that it exists. Neutrinos have, as I said earlier, a great significance at the moment in physics and I have briefly assembled the principal reasons for this here. First of all, they are very interesting, since they are solely subject to what is known as the weak interaction. Almost everything that we measure in physics today is subject either to the strong interaction, the interaction which is responsible for the stability of atomic nuclei, or at least the somewhat weaker electromagnetic interaction, whenever charges are involved then there is an electromagnetic interaction. These are very strong interactions compared with what is known as the weak interaction which is responsible for this radioactive decay up here and which one normally cannot see because the strong interactions I mentioned overshadow it completely. Only with the neutrinos is it the case that these other interactions are not there and that we can then study the weak interaction in isolation. A second important reason that we are so interested in neutrinos in these years is that the neutrinos are responsible for the reactions which take place in the Sun, that they are essential for these reactions. In the Sun, protons, hydrogen nuclei, are somehow fused together and you finally end up with helium. This kind of fusion of protons, which includes a transformation of protons into neutrons, only works if neutrinos exist. This means that our existence is ultimately dependent on the existence of neutrinos. Further reasons are that after the great successes that Glashow, Weinberg and Salam had a few years ago, efforts are being made today to unify the weak, excuse me, the electromagnetic interaction with this weak interaction. Efforts are now being made to extend this schema and place the strong interaction under a common roof with these other two interactions. Here, too, the neutrinos could play an important role, insofar as a mass scale plays a role with these unifying principles, a role about which we know very little for certain even today. That focuses attention on the problem of what mass the neutrinos have, if they have one at all, and if so, which. And finally, really briefly to conclude, the astrophysicists and the cosmologists, in particular the astrophysicists are very interested in the neutrinos because they could help them to solve and to understand a whole range of problems which are causing them great difficulties at the moment. There is not only the problem whether our universe is an open or closed one, we of course assume, as we just heard in Herr Alfvèn’s talk, that our universe is expanding, but we do not know whether that is so forever, or if it expands at a decreasing rate, if it finally slows down so much that it turns around and then, so to speak, contracts again. Something like that would require sufficient mass to be present in this universe. We can make a whole range of statements about the masses, we can deduce them, the so-called visible masses which we see today are not sufficient to make the universe into a closed one. But it could still be that there are very many neutrinos present, which I will come back to later. And that these neutrinos have sufficient mass and the universe could be a closed one. Perhaps a more aesthetically satisfying aspect, but otherwise perhaps not so enormously important. A much more important aspect is that there is a whole range of indications in astrophysics that a large amount of hidden mass is present in the universe. Mass that we do not see. There is a whole range of phenomena, I just want to mention, for example, that the clusters of galaxies, this accumulation of galaxies, is hard to understand, what holds the whole thing together. How gravitation can hold it together, there is not enough mass present there in what we can see. One supposes that very much more mass must be present, which we don’t see. And this mass in turn could, that would be the simplest possibility, be present e.g. in neutrinos, and neutrinos would then be of decisive significance for the understanding of this phenomenon. There is a whole range of further astrophysical clues about such hidden masses, and there are many people who would like to attribute this hidden mass to the neutrinos. Now let me say a few words about the neutrino sources we have available today, we want after all to perform experiments, in the end we have to prove what we may be supporting here as a hypothesis, prove experimentally and for this we need neutrino sources for our measurements. One of our most important neutrino sources, one of our most interesting neutrino sources, as I have already mentioned, is the Sun. In the Sun, fusion processes take place, like the fusion of protons, the transmutation of protons into neutrons, we need neutrinos for that. And since we assume that all the Sun energy which we receive here essentially relies on such fusion processes, these neutrinos must participate in this, and these neutrinos, of course, come to us here on Earth. They have such a weak interaction that they don’t see the Earth at all, they go in at the front and out again at the back. Almost nothing happens there, the Earth hardly exists for this weak interaction of the neutrinos, but one can still, if one has a sufficiently larger number, and the numbers really are large, 6 times 10 to the power 10 per cm^2 and second, just from this actual fusion process extraordinarily high numbers of neutrinos. One can, in principle, measure them with very sensitive detectors. I have written down for you here two groups of neutrinos. One corresponding to the main fusion reaction and another group which comes from a small side reaction, but which has the advantage that it delivers neutrinos of very high energy, 14 MeV, with very much lower fluxes, nearly 10,000 times lower that this main chain. This little side chain is still very interesting at the moment since it is the chain which can be measured with terrestrial methods. These are the famous experiments of Davis and colleagues with headquarters in Brookhaven, and which were performed in a deep mine in the USA, and where the attempt was made to measure this solar neutrino flux which actually supplies us with the only direct expression of what goes on in the internals of the Sun, we only see the external Sun after all when we observe it optically. So the attempt was made to measure this solar neutrino flux, in order to see whether our ideas are correct about what is happening in the interior of the Sun and it is one of the really great puzzles at the moment that this flux was not found. The flux that is found is a factor of at least 3 lower than what was expected, and recently it almost looks, and of course that all has to be experimentally much more exactly checked, as though everything that is measured may be explicable with underground measurements. That is really a great puzzle and it would really be a catastrophe if this flux would not be there, or if we don't understand why it is not there. There are then all sorts of other possible excuses. I will have something to say about one of these excuses. It would be a catastrophe if this flux were not there, since that would then mean that we don't understand how the Sun produces its energy. We believe that we understand this very, very well. But as I said, we have to be sceptical, and one of the great puzzles is this missing solar neutrino flux, at the moment. There are new efforts, in part German laboratories are also involved here, to address this low energy area here, these are very expensive experiments, the BMFT is already groaning about it. Experiments, which are planned to be performed in a few years’ time with the help of gallium. There the thresholds for the reactions are very much lower and one can hope to study these reactions. Very expensive experiments, but extremely important experiments, for this missing solar neutrino flux is one of the really great puzzles that we have to live with at present. It is highly probable that a further large source of neutrinos is those remaining from the Big Bang which many people believe in. Our world came into being in a Big Bang, originally these neutrinos, which were in equilibrium at this Big Bang with the other particles at very high temperatures, were present in large numbers, and then, as the temperature moderated, fell, because the whole thing had expanded, they were no longer in equilibrium, but they should still be here today, with very low energies, but in very large numbers. We assume, we have not seen them so far, we assume that there are around 500 photons per cm³ here on the Earth, in the whole universe on average. That has been measured. One assumes that about the same number of neutrinos is present per cm³, a little bit less for reasons of statistics. So you can see that the world is full of neutrinos, around 400 such neutrinos per cm³ from this Big Bang with very low energies, but from the Sun too come gigantic numbers of neutrinos which pass through us continually from all sides. The environmental lobby has not yet noticed this and so they have not yet done anything about it. Now something about the artificial sources of neutrinos. Here is the most important source of neutrinos, the nuclear reactor. The fusion – not the fusion reactor, the fission reactor - which sends us so-called electrons antineutrinos, whether they are anti- or neutrinos, that is a matter of definition. In this case, as it is defined, here they are antineutrinos, with energies of a few MeV and quite remarkable fluxes. The flux strengths which I have quoted here are realistic insofar as I provide them at the location of the experiment. One obtains many powers of 10 higher fluxes if one goes to the centre of a nuclear reactor, but one cannot survive there, neither can our apparatus, in the places where we can station detectors there are fluxes of this order of magnitude, and you can see that is much more than what comes from the other sources. Then there are also accelerators, meson factories etc. which supply such neutrinos with higher energies, but considerably lower fluxes. First of all I will talk about the neutrinos produced in nuclear fission reactors and which, as I said, are completely harmless for our human health, as far as we know – the Earth is absolutely transparent and we of course, since we are puny in comparison with the diameter of the Earth, are all the more transparent for these neutrinos. It is extraordinarily difficult to prove their existence. The only hope is to detect them in the laboratory because their number is so extraordinarily high. Now I have already indicated several times the important aspect of the mass, the rest mass of the neutrinos, and would like to say a few words on what we know about that. Physics has lived quite happily for around 40 years with the opinion that the rest mass of the neutrino is zero. There were, in other words, no experiments that contradicted this, and the theoreticians appreciated it very much to include the zero rest mass in their theorems, since the theories were thus extraordinarily simplified. Now, critical, as we must be as physicists, in recent years, also for the reasons that I mentioned earlier, the idea arose that there is actually no good reason for the rest mass to be zero. At least there would have to be a new principle there which we do not understand, and one should therefore measure what the rest mass of this neutrino is. Now, nobody had been able to date really to measure this neutrino mass. I can only give you limits here. We know that that the mass of an electron-neutrino is something less than 35 electron-volts, how small it really is, no one knows. The muon neutrinos are lower than 510 KeV, and the tau neutrinos are lower than 250 MeV. You can see that these are enormous energies we are talking about here, and about which one can in principle say nothing at all. Again, because this weak interaction is so incredibly weak, because these neutrinos manifest themselves so enormously badly. The reason why it is so difficult to measure these masses is that the kinetic energies with which we normally work, especially with the electron neutrinos, are extraordinarily high compared with the rest mass, if there is one, and that this small fraction here is very difficult to measure next to this large fraction. There is a Russian measurement by Tretyakov and colleagues who believe they have seen the neutrino mass, they give values for the electron neutrino mass of between 14 and 46 electron-volts. But this experiment is a single experiment, it should definitely be verified by other laboratories or also by the same group. It is very difficult to make definite statements here, whether solid state effects might not be influencing matters and playing a dirty trick here, before these masses have been measured on a range of solid bodies. With the use of various solid bodies it is really too early to say that this is a valid measurement. Finally I should mention that the cosmologists provide limits on the mass of neutrinos. They say the sum of all neutrino masses of the different types should be less than about 50 electron-volts. Then there is also the possibility that very heavy neutrinos exist, but they would probably not be very long-lived. All that is still very much open, in any case that is a limit which the cosmologists believe in, and some of us more or less, that it is correct. So one has to look below this range if one wishes to find neutrino masses. Now there is an interesting possibility, first pointed out by Pontecorvo and a Japanese group, which is that the neutrinos we produce in the laboratory with beta decay, that these neutrinos are not single states of the weak interaction, in the context of which they were produced, but that these neutrinos may have more fundamental neutrinos behind them, in other words that the neutrinos produced in the context of the weak interaction are not stable, but can transform themselves into each other. That would lead to the possibility of so-called neutrino oscillations, and we have in fact conducted experiments in this direction, started a search for such oscillations in recent years. That would mean that neutrinos, say electron neutrinos, which are produced, in the course of time, as they fly, with practically the speed of light, transform into muon neutrinos and then e.g., that is a particularly simple two-neutrino model here, change back into electron neutrinos, back into muon neutrinos, that such neutrino oscillations take place. In concrete terms, that means that if I have a fission reactor here in the centre of my circle producing electron neutrinos or antineutrinos for me, and they shoot out in some direction or other, in all directions of course, e.g. in this one, then, after a particular time of flight or distance of flight, say at this red position, they would transform into muon neutrinos. Somewhat later they would again be electron neutrinos, a bit later again they would be muon neutrinos and so on and so forth. Thus you have such oscillations here, along this region here. And if you set up a detector here, that e.g. only reacts to the green sort, then you would find such neutrinos here, here you see nothing, here again you find these neutrinos, so you would observe an oscillation in intensity in this detector and therefore be able to establish the existence of these neutrinos directly. Now, what can we learn from such oscillations? What we in principle can learn from that is whether these oscillations appear at all, that means whether these neutrinos of the various types can transform into each other at all, whether they are mixed and I can express this mixing with what is known as a mixing angle, that is one quantity, and we also learn something about this neutrino mass, since the length of these oscillations depends, as can be shown quite simply, I don't have the time for this here, on the mass, more precisely on the difference in mass of the neutrinos involved. So if I assume I have, say, an oscillation between electron and muon neutrinos, then this involves the difference of mass between electron and muon neutrinos, or the fundamental neutrinos behind them. So I can learn something about the neutrino masses and I can learn something about the mixing of these neutrinos. Now, we have been performing such experiments for several years, firstly in an experiment, which was an American-French-German cooperation at the research reactor of the Institut Laue-Langevin in Grenoble. We took measurements at a fixed distance of 8.76 m from the reactor and essentially studied the energy dependence of our neutrinos, so we absorbed neutrinos and had a look to see whether this absorption behaved the same at all energies or whether it is energy-dependent. If it is energy-dependent, that would suggest neutrino oscillations. We now have, in the meantime, I told you three years ago that we planned this experiment, this experiment has now been carried out, a second experiment in turn has also been completed, an American-Swiss-German joint project at the power reactor of the nuclear power station in Gösgen in Switzerland where we carried out a new oscillation experiment at a distance of about 38 m, and I would like to tell you a few details of this experiment just so as to give you a feel for how such experiments run in detail. We use this reaction that I have already mentioned, that we fire electron antineutrinos at protons. So we have a detector which contains very many hydrogen nuclei. In our case it is a liquid which contains a lot of hydrogen and which serves us both as a detector and also as evidence for the neutrinos received, so the protons undergo these reactions, they are transformed here into neutrons and positrons and we measure these two particles together in coincidence, that is in both temporal as well as spatial coincidence. In temporal coincidence, in the sense that they have to appear simultaneously if this reaction occurs at all, that is when we observe neutrinos. And in spatial coincidence, that helps us to solve our substantial underground problems. Because although we have very high neutrino rates, it is only very occasionally that one is caught, so we get very low count rates and you can imagine that it is very difficult with such low count rates to fight against all the other processes which of course occur. You always have natural radioactivity from the environment, not from the nuclear reactor, that provides us with nothing at all, but from our detector itself; radiation comes from the glass in the multipliers, radiation comes out of the concrete walls which we use for shielding, and of course radiation pours down on us from space, cosmic radiation, one has to fight against all that, and that is served by this location-sensitive evidence which I am outlining here. Now, I don’t want to bother you with all the details, I just want to mention briefly once more that the oscillation which I have written up formally here, that is a simple function here, this cosine term is significant, a trigonometric function. It depends firstly on the energy of the neutrinos, as I said, if we had no neutrino oscillation it should not be dependent on the energy. Then it depends on the distance, then it depends on this mass difference which I mentioned and which I have written down exactly here, Delta^2. This quantity which occurs here in the argument of the trigonometric function, depends on the masses M1 and M2 of the two neutrinos which I refer to in the square expression as I have written it here, and finally then the mixing angle is involved. You can see immediately that when the mixing angle is zero then there is no neutrino oscillation, that the whole expression here is zero and then 1 comes out quite simply. That means that nothing at all happens, neither as a function of the distance nor as a function of the energy does anything at all appear. But when a mixing angle is present, that is when the phenomenon of oscillations exists, then an oscillation term appears and from the length of this oscillation, from the argument of the trigonometric function we obtain information about this quantity Delta^2, about this square of the mass difference of the neutrinos, which we are interested in. Now, how does that look in practice? Such a detector looks very roughly like this sketch of mine here. We have here 30 such white boxes, they are the proton counters, the whole thing is roughly the inner counter, say 1m by 1m by 1m. So we have 30 such proton counters in which the neutrinos which come from the reactor, which come from somewhere outside here and arrive here, occasionally experience a transformation, react with a proton and produce a neutron and a positron. And we have to now detect this neutron and this positron. We detect the positron directly in these counters, there photomultipliers just sit at the end, the flashes of light which the positron makes in this scintillation counter, in the scintillation liquid which he have in here, are detected and they give us direct evidence of the positron. And the neutrons which are thereby produced, they come over here in these big helium-3 chambers marked in yellow, in which they are detected by neutron capture. This means that we detect the positron here, we detect the neutron here, and if the two occur simultaneously or practically simultaneously then we know that a real neutrino absorption event has occurred with high probability. The rest of this apparatus, and that is the greater part, that consists of what are known as anti-coincidence counters, which tell us that something wrong is coming from outside, some particle from cosmic radiation, we can then exclude that, then we don’t count it. Then there also a lot of other things here which I don't want to go into. The decisive thing is that we have many metres of concrete around the whole thing. The whole detector weighs around 1000 t, so they are large items of apparatus. In fact the Swiss Army helped us to drive this 1000 t around the area because we are not in a position ourselves to do this without help and we are not keen on spending our limited research funding on the transport of concrete. Now perhaps I will show you this inner counter once again more exactly, how it looks in detail. Here you see once more an enlarged view of the inside, you can see these 30 counters here, here the covers are removed in some cases, those are the photomultipliers which sit in front and behind these counters, and allow us to detect the light impulses, here in between, yellow, are these big helium-3 chambers, we put 400 l of the very rare isotope helium-3 in our detector. Now I would like to show you the results very briefly. What you see here is up above the count rate, this upper curve, recording the count rate per hour. You see, we have typically about 2 per hour here as a function of the neutrino energy or the positron energy which is directly linked with the neutrino energy. You see this upper curve here for the case that the reactor was in operation. We spent about half a year measuring this curve. You see that one needs time with this low count rate, and here below you see the curve we get when the reactor is switched off. That means they are so to say undesired side-events which we have to subtract from the desired events here above. I may be allowed to mention on the side that it was very difficult for this curve here below, we could only make measurements for about 4 weeks on this lower curve, these power reactors in the generating stations have the unpleasant characteristic that they are always in operation and are very rarely switched off. As a physicist, one would rather have reactors which run half the time so that one can measure this curve, and are switched off half the time, so that one can measure this curve. But one must of course subordinate oneself to the conditions which really exist. Now, the difference of these two curves here is that what you see recorded here below, that is the real spectrum and a curve is also drawn here which is not something like a fit to these experimental data, but is the curve that we would have had to expect if we had no neutrino oscillations. So all deviations between this continuous curve and our experimental data, and you can see a little bit here and maybe there, with a lot of imagination, all this would indicate oscillations. What we can state in this case is not masses, we saw no neutrino oscillations in the framework of the statistics that we applied, I will justify that in a moment, but we can state limits for these masses, and that is shown in this figure here, they are the latest, not yet published data where I can state the most accurate limits for the neutrino masses and the mixing angle. What you see here, and that is the green curve, in fact the continuous curve drawn on the right hand edge of the green curve, that is the so-called 90% confidence level for the exclusion of neutrino oscillations. And what is excluded is the whole right-hand area, to the right of this curve, and permitted, on the basis of 90% confidence, is what lies to the left of this curve. Displayed here is up above, this Delta^2, this squared mass difference of the neutrinos, and to the right the mixing angle. So here mixing angle zero and here complete mixing on this side. You see that a further region here is excluded, but everything here that remains to the left of this curve, in the left area, that is the region where neutrino oscillations and thus neutrino masses and neutrino mixing angles are still possible as before. We are now involved in extending these measurements to still greater distances. Here our goal is firstly to examine in more detail this region here, which I will have something more to say about soon, and in particular to go still further downwards here below. That means, if neutrino masses are there, with relatively high values, that is in the region of eV as the cosmologists and astrophysicists would have it, then such masses could only appear with extremely small mixing angles, that is with mixing angles which are clearly smaller than the angle that we like to use today in high energy physics, the Cabibbo angle and the Weinberg angle, which are both a little greater than 0.2. So we are already below this angle in this region here. Of course there is no reason why this mixing angle should agree with this other angle here, but still one thinks a bit in this direction, maybe it is a gift that they are of this order of magnitude. But that is not the case; here they are already, as you can see, below this region. So we can already extrapolate up to arbitrary masses. We can do that because here in these high masses, in this high mass region our oscillation term is averaged out, and that is e.g. one of the hopes which one has to explain the absent solar neutrino flux. It could in fact be that the neutrinos which come to us from the Sun undergo oscillations, that they transform into other kinds of neutrinos, that the electron neutrinos from the Sun may become muon neutrinos and tau neutrinos and the whole thing is mixed up, so that then, when it is roughly equally mixed - and one would assume that, if the oscillation lengths are small compared with the distance from the Sun to the Earth – that we can then assume, if we have three types of neutrinos, that each type accounts for about 1/3 of the intensity, and that would be about what we are measuring at the moment. But that is still very strong wishful thinking, I would say, that must still be verified much more precisely. In any case, in this left region here oscillations are still possible, in this right-hand area they are excluded, here below with really small neutrino masses, and there could of course be some, it could be the case that the neutrinos have this small mass, there they are still fully compatible with all mixing angles. Now I would like to show you, perhaps without going into numerical detail, another picture which caused a bit of excitement about a year ago. And here I show you the same curve again which you just saw in green, in lilac here, and simultaneously I show you measurements here which were made by Reines and his colleagues, very famous measurements, in which neutral and charged flux reactions were carried out with the help of neutrinos on the example of the deuteron, and in which these measurements were interpreted under the assumption that neutrino oscillations exist. Here it is the case that the permitted region is to the left with us, here the permissible regions is to the right. Here you can 90% confidence curve from Reines, to the right you can see the 90% confidence curve of our experiment, here there is no overlap of these curves, which means that the two experiments, at least on the 90% basis, and that can be extended further, in clear contradiction to each other. So we and our experiments do not agree with the assertions of this American group. Now, finally, a last summary of those data which we obtained at the nuclear reactor in Gösgen. It shows you very nicely the difference of distance and also of reactors. I show you again here in blue what you have already seen in other colours, that is our measurements which we carried out in Switzerland, at the Swiss power reactor, and at the same time I show you here in red, once more…

Rudolf Mößbauer (1982)

The world is full of neutrinos (German presentation)

Rudolf Mößbauer (1982)

The world is full of neutrinos (German presentation)

Comment

Rudolf Mößbauer received his Nobel Prize in 1961, only three years after he reported on what was later known as recoilless nuclear resonance fluorescence – or the Mößbauer effect in short. With an age of 32 at the time of the award, he belongs to the ten youngest Nobel Laureates ever. This early success probably facilitated his decision of eventually changing his research focus entirely after accepting a position at the Technical University of Munich in 1965. Around 1970 he became interested in the neutrino, an elusive elementary particle, which had been detected for the first time in 1957. In his 2001 Lindau lecture, Mößbauer would state: “I fooled around with the Mößbauer effect for 15 years and then I had it. […] When the neutrinos came up, I immediately caught fire.” And indeed, the neutrino and its peculiar properties should stay at the focus of his research interest until his retirement in 1997 and beyond. Of the 12 lectures he gave in Lindau during his life, 8 had the neutrino and its properties as their topic (1979, 1982, 1985, 1988, 1994, 1997, 2000, 2001).
In the present lecture, Mößbauer gives a very clear and concise overview of the history of the neutrino, its properties, sources and the scientific questions connected to it at the time. He differentiates three problems in particular: the solar neutrino problem, the question of neutrino mass (traditionally, neutrinos were assumed to have zero mass) and neutrino oscillations. Interestingly, from a today’s view, the three problems are actually but one: the solar neutrino problem, which boils down to the fact that for a long time only one third of the expected solar neutrinos were detectable on earth, could be largely resolved around the turn of the century. At that time experiments showed neutrino oscillations, which also implied a non-zero neutrino mass. However, Mößbauer was not actively involved in these discoveries. Major contributors were the teams running two large underground detectors, the Sudbury Neutrino Observatory in Canada and the Super-Kamiokande in Japan.
In any case, in 1982 the field was still open. In the second part of his talk, Mößbauer describes how his team is using a neutrino detector attached to the nuclear fission reactor in Gösgen, Switzerland in order to search for neutrino oscillations. Since the interaction of neutrinos with matter is so weak, neutrino detectors have to be huge: Mößbauer mentions how the Swiss Army helped the scientists to move their 1 000 ton apparatus into place. And still, despite the massive detector size, measurement periods of half a year were needed for a single curve, as Mößbauer explains. As a comparison, the Super-Kamiokande experiment mentioned above was even bigger, employing a 50 000 ton detector.
Despite the fact that Mößbauer’s work in Gösgen did not result in groundbreaking contributions to the field of neutrino physics, his Lindau lectures on neutrinos are particularly valuable. That is partly due to his well-structured and clear way of presenting. But first of all, his Lindau lectures cover the development of a highly topical research field over a significant period of time (1979-2001) and thus allow for a unique insight into the - sometimes apparently rather slow - workings of science.

David Siegel

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