James Cronin (2012) - Spontaneous Ionization to Subatomic Physics: Some Vignettes from Cosmic Ray History

While they're setting this up let me just make a few remarks. When you do physics for 50 years in your life, you realise that it's not so easy to do. When I was a kid, a student and undergraduate, I was asked to read the biography of Rutherford and write a report on it. And my professor was very disappointed because it didn't seem to me at the time that Rutherford did all that much stuff that was so difficult. But after 50 years you realise that a career in physics involves lots of things, successes, happiness, sadness and one then turns at the end of one's career to looking back on the history of the particular, you know physics that you did or were close to. And the last 20 years of my life I have been working in cosmic rays. And I became fascinated with the history of cosmic rays. You will see there are certain features, particularly those talked about by Professor Osheroff. That in doing science there are certain rules and these will lead to surprising discoveries. And certainly in the history of cosmic rays that is the fact. The question at the end of the 19th century is, does air ionise spontaneously? Crookes in 1865 showed that if you charge up an object it gradually loses its charge. If you charge it up and place it in a vacuum it does not lose its charge. So the conclusion was that somehow there are free ions in the air, is that air ionising spontaneously. So this was an important question in physics and you can do these experiments, you probably did it, you've used maybe a gold leaf electrometer. And if you charge it up with rod and cat's fur or something like that, the gold leaf will extend out. But then gradually collapse. So anyway you can imagine that. So this is a really important question in physics. And the solution came no doubt with the discovery of radioactivity by Becquerel in 1896. So now one has the source of ionisation coming from the earth, everything is a little bit radioactive. And so this was presumed to be the explanation of why these electroscopes lost their charge. And a very careful study of that was done by C.T.R. Wilson, the inventor of the cloud chamber, an extraordinary physicist in general. And he studied leakage of electricity through dust free air and did some very careful experiments. The results of which, or some of them is the rate of the leak of ionisation is approximately proportional pressure, when you lower the pressure the leakage gets lower. And then he quantified this with the charge in the electron which was a little bit higher than eventually measured to be about 20 ions per CC per second of either sign. And then he did a very interesting experiment using his electroscope. And exposing it to radium and giving the relative rates of discharge depending on the gas, air, hydrogen and so forth. And then he exposed it to polonium and got the same thing more or less. And then the spontaneous ionisation when you took all the radioactive sources away, more or less looked like the same thing. So the presumption would be that the ionisation produced in the ground and so forth was really just a matter of local radioactivity. And this was a very interesting study which was followed by, as I showed, C.T.R. Wilson, Elster and Geital, Rutherford, Eve, even Erwin Schrödinger worked on this. And he calculated because there was radon emanation that that would go up in the atmosphere. And people calculated if the radiation came from the earth, how high it would go. But in particular the last person here, Pacini was a meteorologist, he did very careful studies over sea water, over land and so forth. And he wrote this article which essentially said that, if I translate it right, shows that a pressure part of the radiation penetrating in the air and particularly that which is subject to variations, has an origin independent of the action, direct action of substances active in the layers, upper layers of the crust of the earth. And actually his estimate of what was the unexplained ionisation was 2 ion pairs per CC per second. And that's precisely at sea level what the muon radiation, the cosmic rays give. So he had this just right. And there have been recently a lot of discussions that maybe he was the ultimate or the real discoverer of cosmic rays. I don't think so because the radiation that in the end amounted to cosmic rays, nobody had any idea what it was. And then a new series of experiments began in a particularly nice electroscope, this is just schematic, was developed by a Jesuit priest, Wulf, and you would charge this up and then the two wires were suspended under tension. And they would split apart by repulsion if there were charge. And this was the kind of instrument that was used, first by Wulf himself who reasoned as would be natural that if I get the ionisation detector away from the earth, it will show a decrease in ionisation. So he lived in Holland, he travelled to Paris and at the bottom of the Eiffel tower made certain readings. Then a certain set of readings up and then back on the ground again and back home. And you can see maybe there's some slight tendency for these readings to be a bit lower at the top of the Eiffel tower. However there are no errors here and I don't think this paper would be accepted by the physical review letters at all because of its incomplete analysis of the earth. But nevertheless what I think is striking is that going up all this distance, really the ionisation rate hardly changed at all. This was followed by a balloon flight of a Swiss named Gockel. And then a very careful set of balloon flights by Victor Hess who was at the radium institute in Vienna. And he began with two flights at the rather low altitude, first 300 metres and then up to about 1,000 metres. And again he measured the ionisation with one of these Wulf electrometers. And found no change in ionisation with altitude. So already that was a bit intriguing. And then these balloons used by Hess were Austrian military balloons. And ultimately he in his 7th flight, he had a balloon filled with 1,600 cubic metres, hydrogen filled balloon to get to some height. And he was able to achieve a height of some 4,000, almost 5,000 metres. And he had three different detectors, two of them were sealed and then one was open to the air. And one corrected, gave I think the most convincing results that the ionisation by the time he was up at 4,500 metres or so, had increased by a factor of 2. But there is no data here at all and the experts on Hess say that he became ill and had to descend very quickly to relieve his illness. But this represents what was ultimately accepted as the evidence for radiation coming from above the earth, from outer space if you like. Now Hess was followed by a German physicist from Berlin, Werner Kolhörster who did many, many important things in cosmic ray physics. But the one that was the most sensational was that in a balloon he achieved a height of 9,300 metres. In order to do this he had to breathe oxygen. And it's really quite a daring thing to do when you think about it, to go up in a balloon in a little basket and make these measurements. But his measurements were quite sensational. In flights in 1913 and 1914 showed that maybe there's a slight decrease in the radiation but that it climbs, climbs, climbs by almost a factor of 10. And this is really a dramatic demonstration that as you climb in height, the radiation significantly increases. The World War I. came in 1914, all research stopped. Then it was picked up again and principally among many of the people who picked it up was Robert Millikan who we know from the photoelectric effect and the charge on the electron. And he was a very confident, self-confident physicist and he began to make measurements. And his early measurements had flaws which I don't have time to go into but he with a student, Otis, concluded in some of his early papers in '24: He is arguing that there is no such cosmic radiation. And others, Hoffmann, a German, made this conclusion, measurements at high altitudes of cosmic rays. He says, how they got there is another question". But the whole idea of cosmic radiation in the '20s, early '20s, was called into question. And of course you can imagine that Hess didn't care for this. But then Millikan himself had a complete reversal of his conclusions studying penetration into deep lakes. And he in a lecture at Leeds University wrote, that at the high altitude rays do not originate in earth's atmosphere, very certainly not in the lower 9/10 of it. And it justifies the designation of cosmic rays". You may note that Hess and Kolhörster called these rays 'Höhenstrahlung', which was a very natural thing to do, we still live with bremsstrahlung. So there's no reason why 'Höhenstrahlung' might not have remained. But Millikan re-coined the term. And for all of this Hess was not very happy. And so he wrote in one paper, that he tells a story of the discovery of 'Höhenstrahlung' that could easily be misunderstood, recent determination by Millikan and his colleagues of the high penetrating power of 'Höhenstrahlung' has been an occasion for American scientific journals such as Science, Science Monthly to introduce the term "Millikan rays". Millikan's work is only a confirmation and extension of the results obtained by Gockel and myself and by Kolhörster from 1910 to 1930 using balloon born measurements of the rays. To refuse to acknowledge our work is an error and unjustified". So you can see quite a bit of controversy. And then Millikan trying to grab the credit for the discovery of cosmic rays. And Millikan in particular, all his life believed that the cosmic radiation consisted of gamma rays. In fact everybody would assume that they were gamma rays in 1910 because that's the only kind of penetrating radiation one was aware of. But this phenomenon of cosmic rays involved a whole new source of radiation and particles and so forth that nobody had any idea about. And it took about 40 years to unravel the thing. And this is the point. But new experiments and new detection techniques showed that for the most part, the cosmic radiation consisted of charged particles, not neutral particles. And one of the most important early experiments was by Bothe and Kolhörster who had gone high in the balloons, using the new techniques of Geiger counters and they built this apparatus and had two Geiger counters and a gold, solid gold absorber that they could take in and out. And I can't go through the details but they showed that for the most part the radiation at least on the surface of the earth was corpuscular, charged particles. And I guess naturally you think they were electrons but there's some problem with penetration of electrons through such a thick gold absorber. So it became clear that maybe one is dealing with something else. And Kolhörster and Bothe's experiment got Bruno Rossi involved in the study of cosmic radiation. And he writes that, but I felt that the cosmic rays certainly were neutral but it's an experimental point to establish". And he began his work after the conference at the University of Rome. Here he is discussing things with Fermi. Fermi never did direct experiments in cosmic rays but followed it very closely and was very helpful to Rossi. Now Rossi did many, many things and I don't have time to go through it but one that was very dramatic was using his new coincidence circuit which was far superior to what Bothe had built and received the Nobel Prize for, coincidence technique. He could make tripe coincidences, quadruple coincidences. So he made this stack of lead which was 1 metre high and to define a cosmic ray corpuscle going through 1, 2, 3 counters. And the result of this experiment was at least 50% of the corpuscular radiation on the ground would penetrate 1 metre of lead. Now that's pretty sensational and you're certainly not going to explain this with primary photons. And then a Dutch man named Clay suggested that if the primary radiation is charged, then the earth's magnetic field should have an effect. And at high magnetic latitudes you would have more cosmic rays because they were not deflected by magnetic field compared to the equator. And then Clay's ideas influenced Arthur Compton and Compton influenced in turn by Rossi organised a worldwide survey of the dependence of cosmic ray intensity on geomagnetic latitude. Now this is to test then whether the primaries were charged. You could imagine that on the ground if there were secondary interactions, they could be charged. But maybe the primaries were not charged. And the sensitive way to do that is see if they were affected by the earth's magnetic field. So this is Compton, very dapper man, I always wonder how long it takes for him to lace up his boots. And he has a high pressure ion chamber calibrated by standardised radioactive sources. And distributed to colleagues all over the world, in these places where there are black dots. And so the result of this, done quite quickly was the fact that at sea level, there was a depression in the cosmic ray rate near the galactic equator. And near the pole it went up and the effect became stronger the higher the altitude you were. So this showed quite definitely that even for the most part, the primary radiation was charged particles entering the atmosphere. And there was a famous debate between Millikan and Compton at an American Association for Science meeting in Atlantic City December 31st 1932. And you can read "debate of rival theories brings drama to session of nation's scientists". Their data, that is Compton and Millikan at variance. And here is that William Lawrence wrote. the two protagonists protested their views with vehemence and fervour of those theoretical debates of bygone days when learned men clashed over the number of angels that danced on the point of a needle. Doctor Millikan particularly sprinkled his talk with remarks directly aimed at his antagonist scientific acumen. There is obvious coolness between the two men when they met after the debate was over". And Millikan never got over the fact or always felt that the cosmic rays were neutral. Now new instruments came into play and there's a certain serendipity here. This is a cloud chamber of Skobelzyn who was working in Leningrad and he had a cloud chamber to study Compton scattering. But very occasionally, just at random a very high energy particle, maybe not that high by LAC standards of 7 MeV/C would come into his chamber. And this was totally unexplained by any phenomena that would be local. And then Blackett and Occhialini, Occhialini had been working with Rossi, expert on electronics and counters. So Blackett invited Occhialini to come to England and then make a cloud chamber that would expand on the passage of a cosmic ray. So this would enrich the number of pictures enormously because if you just do it by random, maybe 1 in 50 pictures will have a cosmic ray. And among many things he saw these events which were like showers of particles. And you can see very clearly there are ones which deflect one way and ones deflect the other way. The ionisation looks pretty much the same. So just looking at this you might conclude that there is a positive component in the shower. It looks just like electron. And Blackett and Occhialini wrote this beautiful article covering everything. And this is a summary of their conclusions. leads to a confirmation of the view put forward by Anderson", this is Carl Anderson, a colleague of Millikan. but with a mass comparable with that of an electron rather than a proton". So they acknowledge that Anderson played an important role here. And in fact he wrote, published a little bit, submitted a little bit later the article on the positive electron. Now this was a totally experimental thing, he was not influenced by Dirac, didn't even mention Dirac in the whole theory. And so this is what has been attributed to be the discovery of the positron. But Blackett and Occhialini certainly were very close. And so in 1936 the Nobel foundation, Nobel Prize was awarded to Carl Anderson for the discovery of the positive electron and Hess for the discovery of cosmic radiation. Now when you look into some of the material of the Nobel committee, the committee really felt they could never give Anderson the prize which used the cosmic rays which were discovered by Hess. So that was really the reasoning why they both received this prize in 1936. And there are some other interesting things in the Nobel literature. Now in 1938 a new thing was done and that was the discovery of Pierre Auger of massive extensive air showers. And he did a very simple experiment, first in his laboratory of two counters to define a particle and a third counter to ask, is there a second particle in coincidence, also maybe studying the absorption of that second particle. So this is in Paris at sea level. And when the distance was 2 metres he got 1.7 counts per hour, 5 metres 1.4 and at 20 metres 0.9. So there were coincidences between a common source of cosmic rays which extended over quite a distance. And I should add that Roland Maze did a number of things to increase very much the resolving time of a Geiger counter system so that one wasn't just measuring chance coincidences here. And then Auger and his collaborators did the same thing at high altitudes and they measured a rate of coincidences which reached out to 300 metres. And by that time one had electron shower theory. Bhabha-Heitler and coincidences at 300 metres require primary cosmic ray particles with energies up to 10^15 eV. That's a huge energy compared to what could be artificially accelerated or what you had in radioactivity. And one year later there was a conference at the University of Chicago, July 1939, you remember the invasion of Poland was September 1, 1939. And this just shows the interest in this subject of cosmic rays, because you have Hans Bethe here, you have Bothe, you have Heisenberg, you have Pierre Auger. I know these are one pixels for you but they are there. There's Rossi, there's Edward Teller and with the big shock of hair that's Robert Oppenheimer. And there are other very distinguished people in this meeting. And of course it represented the last significant scientific meeting before the beginning of the Second World War. And after the war Fermi came to Chicago and he was always thinking about cosmic rays. And just before Christmas he wrote down in his note book the theory of cosmic ray acceleration which imagine collisions of charged cosmic rays with moving magnetic fields. And this is what has become the second order Fermi acceleration. And this is one month later, he sent in a paper to the Physical Review. And it was published on April 15th, just a few months. And this is his abstract, but one thing that's fascinating to me is he writes, the heavy nuclei observed in the primary radius". Even in his abstract he's pointing out the faults of his work. And that's because there's too much radiation loss which overcomes the acceleration. And so by 1952 a large number of new particles had been discovered in the cosmic radiation. Beyond just the positron, there's the muon. And in a conference in 1952 an outline of the particles that were existing with the exception of the photon and the proton and neutron. All of these particles, pi mesons, mu mesons, hyperons, k mesons were all discovered in cosmic radiation. And there was a conference then organised at Bagnères-de-Bigorre in the Pyrenees organised by Leprince-Ringuet and Patrick Blackett. Here is Blackett, here is Leprince-Ringuet and Rossi standing behind them, dedicated all 6 days of the conference only to the particles discovered in the cosmic rays. And some of the conclusions, Rossi was asked to summarise the whole conclusion and among other things he wrote, which is already in the course of the conference. It is the very close similarity between the masses of two of the best established particles. I mean the charged tau particle with a mass of 970, that decayed in 3 pi. That's not your tau lepton that you know about now you youngsters. And the theta particle was a decay, neutral decay to pi+ and pi-, mass 970. This looks hardly like an accident. And on the other hand it is very difficult to see how the theta zero particle could be the neutral counterpoint of the tau particle". This was then the beginning of what we call, years ago, famous tau theta puzzle which led to the proposal of parity violation which was then confirmed. What I am trying to say here is that in cosmic rays by 1953 one had identified a whole new set of particles. And even, and this was just about the time accelerators began, the Cosmotron in '53, Bevatron, higher energy accelerators. And the theories of the new particles, kays and hyperons and so forth were well written out by Pais, Gell-Mann and others before any accelerator experiment was done. So the point I'm trying to make is cosmic rays did a good job for particle physics up until the high energy accelerators came to take over the work. And here is the conclusion of the, written by Leprince-Ringuet, remembering things in '82, back to this '53 conference. He writes, "the congress at Bagnères-de-Bigorre, I would say, sounded the death knell for cosmic rays". And it was Powell himself in his closing discourse who said And Leprince in his, had a beautiful group of great students, Peyrou, Gregory, Lagarrigue, Armenteros, Muller, Astier and he moved this whole group from Pic du Midi in southern France to CERN which was just being built. And his progeny have done huge amount of beautiful work. The other thing he feels responsible for and he says naming the hyperon, hyperons. And he says, "I have to say that this was my principle contribution to physics. I announce the word hyperon. And now in Bagnères-de-Bigorre you will find a street Hyperon Avenue". Now after Bagnères-de-Bigorre as I said sub-atomic physics which was so well established in the cosmic rays moved to accelerators and cosmic rays principally moved to space, but not on the surface for the highest energies because the flux was so low. And one used the showers that were first put into evidence by Auger to study the cosmic rays on the ground. So I'm done, sub-atomic physics today, we heard about it, we heard about the discovery of the Higgs and that was from a magnificent machine that is just, only you can admire the technology and the care in which this machine was built to produce those great results we heard. And in cosmic rays at the highest energies one has built very large rays to study their energy. Thank you.

James Cronin (2012)

Spontaneous Ionization to Subatomic Physics: Some Vignettes from Cosmic Ray History

James Cronin (2012)

Spontaneous Ionization to Subatomic Physics: Some Vignettes from Cosmic Ray History

Abstract

Spontaneous Ionization to Subatomic Physics:
Some Vignettes from Cosmic Ray History

In the 1879 Crookes discovered that air seemed to ionize spontaneously.
With the discovery in 1896 of radioactivity by Henri Becqueral it appeared
that the mystery was solved. However a number of physicists sought a
quantitative agreement between the "spontaneous ionization" and the
radioactivity in the earth. The persistance of these physicists led to the
discovery of another source of radiation which appeared to come from the
heavens. The nature of this "cosmic radiation" involved phenomena that were
completely unknown. Coming to an understanding of the nature of this cosmic
radiation took about 40 years between 1912 and 1953. This history involves
extraordinary scientists and the invention of dramatic new detection
techniques. The story finishes with a remarkable conference organized by
Patrick Blackett and Louis Leprince-Ringuet 1953 in the Pyrenees town of
Bagneres de Bigorre.

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