v1.0.1 / chapter 6 of 20 / 01 sep 07 / greg goebel / public domain
* At first, the discovery of new subatomic particles did not cause much confusion among physicists, since new particles like the neutron seemed to fit so well into what was already known, and so the discovery of the positron, or "antimatter electron", was not too big of a shock. When a particle was discovered that seemed to fit the description of Hideki Yukawa's strong-force exchange particle, that seemed to be further vindication of existing theory.
After World War II, particle physicists found that this particle, which would become known as the "muon", was a different beast from the real strong-force exchange particle, which had been discovered as the "pion". There was also some consternation over Enrico Fermi's prediction that some types of radioactive decay produced a particle, the "neutrino", that was so hard to detect that many simply judged it a fiction. Whatever the case, the "particle zoo" was beginning to become increasingly complicated.
* While the theorists were coming up with proposals for new particles, the experimentalists had been hunting for new particles on their own. They looked to the sky for them. In 1910, a Jesuit priest named Theodor Wulf (1868:1946) had gone up the Eiffel Tower in Paris with an electroscope and found that the radiation levels were surprisingly high, higher than would be expected if the radiation were coming from the Earth, and that this mysterious radiation might be coming from the upper atmosphere or space. He suggested that balloon flights might be conducted to confirm this notion, and in 1911:1912 an Austrian physicist named Victor Hess (1883:1964) made a series of balloon flights to investigate.
Hess did discover that radiation increased with altitude. There was considerable skepticism over his claims, but a German physicist named Werner Kollhoerster performed a series of flights that confirmed Hess's results. The war interrupted studies of this mysterious radiation, but after the conflict, in the 1920s, Robert Millikan conducted extensive studies of his own on the matter. His work would end up being a comedy of errors: while Millikan could be hidebound, he was still a superb experimentalist, and unfortunately it would turn out that everything he touched on this particular subject would be afflicted by confounding effects that made him come to consistently wrong conclusions.
At first, Millikan didn't think cosmic rays existed, resulting in a running quarrel with Hess and Kollhoerster. Once he had been convinced, Millikan ended up giving the radiation the name of "cosmic rays", but he then went down the wrong track again by insisting they were gamma rays. However, in 1929 Kollhoerster and Walter Bothe created what they called, with some overstatement in hindsight, a "cosmic ray telescope" that consisted of a pair of the newfangled Geiger-Mueller proportional counters, with each tube connected to an electroscope as an output device. The idea was that a gamma ray would be absorbed by a gas atom in one of the tubes and cause the atom to eject an electron, which would be indicated by the electroscope attached to the tube. A stepping camera automatically kept track of events.
A gamma ray would only set off one tube at a time, but in reality, most of the events set off both tubes. The two researchers even tried placing a gold bar between the tubes to block the cosmic ray, only to find out it still got through about three-quarters of the time. Obviously, cosmic rays weren't gamma rays; the only other option was that they were charged particles, since neutral particles wouldn't set off the Geiger-Mueller tubes.
Millikan continued to insist that they were gamma rays, but Bothe and Kollhoerster pointed out that if cosmic rays were charged particles, they would be influenced by the Earth's magnetic field. That meant their flux would vary with latitude, being strongest at the poles and weakest at the equator. After another comedy of errors, Arthur Compton confirmed this was the case, with Millikan then abruptly changing his position and even trying to deny he had been on the wrong side of the argument. Despite the years of effort Millikan spent on cosmic rays, he barely mentioned them in his memoirs, clearly finding the subject something he didn't want to remember. Hess got the Nobel prize for his work in 1936; possibly Millikan thought he was welcome to it.
The Italian experimental physicist Bruno Rossi (1905:1993) came up with an improved detector scheme, using vacuum-tube electronic circuitry to automatically indicate coincident events between Geiger-Mueller tubes. He set up systems of three tubes arranged in a triangle and linked to his "coincidence circuit", with lead shielding to protect them from confounding radiation from the Earth. The three tubes were often all set off at once; since there was no way a single particle could do that, obviously cosmic rays were generated as "showers" of particles. It was eventually realized that a cosmic ray hitting the top of the atmosphere would collide with the molecules in the air and generate a set of particles; these particles would then produce more particles by decay or collision, which would be detected on the ground.
* Further progress depended on taking a better look at cosmic rays. The tool needed for the job has actually been around for some time. The "cloud chamber" had been invented in 1911 by the British physicist Charles Thomas Rees Wilson (1869:1959), one of J.J. Thompson's disciples at Cambridge Cavendish Laboratory; Wilson had actually come up with the basic idea in the 1890s, but he'd been sidetracked by other things and had to put it aside for over a decade.
The cloud chamber was a cylinder with a glass lid and a piston that could be raised or lowered. The cylinder was filled with moist air; if a charged particle zipped through the chamber, it would leave an ionization trail behind it. If the piston was then lowered to increase the volume of the cylinder, the water vapor would cool and condense around the ionized air molecules, revealing the "contrail (condensation trail)" or "track" of the charged particle that had just passed through. The piston could then be raised again to "reset" the device for another observation.
The heavier the particle, the denser the track. A magnetic field placed top to bottom across the chamber would cause particles to curve one way or another, depending on their charge, allowing the particle charge to be determined. Wilson got the Nobel prize for the cloud chamber in 1927.
Another British researcher, Patrick Blackett (1897:1974), who had given up a career in the Royal Navy to work in physics, got his hands on the cloud chamber and automated it. Manually activating the cloud chamber and then watching for particle tracks through it was of course tiresome and error prone; Blackett came up with a scheme in which the cloud chamber was activated in a cycle, about once every ten or fifteen seconds, with a movie camera taking a picture on a single frame of movie film once per cycle. Before the cloud chamber, researchers could detect the presence of particles using scintillation screens and the like, but observing their precise trajectories was impossible. Blackett's automated cloud chamber changed all that; he took tens of thousands of pictures of cloud chamber tracks in the early 1920s, observing particles and radioactive interactions in an unprecedented level of detail.
In 1930, Millikan set one of his students, Carl David Anderson (1905:1991), to observing cosmic rays using a cloud chamber with a powerful water-cooled magnet, sited on top of a mountain. Initial results showed a roughly equal mix of positively and negatively charged particles. That puzzled Millikan, since he still believed that cosmic rays were gamma rays, the charged particles being nothing more than electrons the gamma rays had knocked loose. Clearly, at least half of them weren't electrons.
Millikan thought the positively-charged particles were protons, since nobody knew of any other positively-charged particle at the time. Anderson wasn't so sure: the curvature of the cloud-chamber tracks was about the same for both positively and negatively charged particles, which meant that the heavy protons would have to be moving much more slowly than the light electrons. Anderson wondered if what appeared to be positively-charged particles were actually electrons moving up from the bottom of the chamber instead of down from the top.
He ran some tests to be sure, and found out that both Millikan and himself were wrong. Anderson put a lead plate across the cloud chamber, believing that any charged particle that struck it would lose energy and would demonstrate a wider curve in the magnetic field. The positively-charged particles were not coming from the bottom of the chamber, but from the top, and they were clearly very similar to electrons except for different charge. Anderson was still unsure of what to make of matters, but he was confident enough to defy Millikan's objections and publish a note in the physics press in September 1932 that he had discovered positively charged particles that were smaller than the proton. He wasn't more specific than that, his objective being to establish priority of discovery, and went back to his investigations.
A further improved version of the cloud chamber was available by this time to validate Anderson's discovery. The problem with Blackett's original automated cloud chamber was that it took pictures on an interval, and finding a particle passing through the chamber was basically a matter of getting lucky. Working at Cavendish, Blackett and a clever Italian researcher named Giuseppe "Beppo" Occhialini (1907:1993) had come up with an improved scheme, with a Geiger-Mueller tube above and below the chamber. The two tubes were linked to a coincidence circuit; if a charged particle zipped through both tubes at one time, the cloud chamber lid was automatically raised and a picture taken, revealing the ionization trail of the particle that had just passed through the chamber.
The improved cloud chamber trapped events much more predictably than the old scheme, and in 1932 Blackett and Occhialini took thousands of pictures of cosmic ray tracks. They hadn't correctly interpreted the tracks of small positively-charged particles, but once they heard of Anderson's discovery, they found their pictures were full of such tracks. In fact, they found vee-shaped tracks caused by an electron and a identical particle with positive charge. It was clearly Dirac's antielectron, with pair production being observed in practice. Occhialini was so excited at the discovery that he ran to Rutherford's house and, in a gesture somewhat more out of place in England than it would have been in Italy, kissed the maid when she answered the door.
Anderson published a formal paper on the "positive electron" or "positron" in early 1933. He won the 1936 Nobel prize in physics, sharing it with Victor Hess. Incidentally, Millikan's memoirs also said little about Anderson.
Ernest Rutherford was a little startled that the theorists, in this case Dirac, had beaten the experimentalists to the discovery. Rutherford had long been of the viewpoint that the experimentalists did the real work and then tossed the results over to the theorists to play with as they chose. He had to comment: "It seems to be to a certain degree regrettable that we had a theory of the positive electron before the experiments ... I would be more pleased if the theory had appeared after the establishment of the experimental facts." Rutherford clearly sensed that the day of his style of practical experiments was ending, and that in the future the theoreticians would be increasingly in the driver's seat and not just passengers in the back.
* Dirac had been vindicated. Vindicating Yukawa would take more work. While Anderson was hunting positrons, he noticed tracks of a more penetrating particle in the photographs, and in 1936 he and his colleague Seth Neddermeyer (1907:1988) announced that they had discovered a new particle that was 207 times the mass of the electron, or about a seventh the mass of the proton. It was named the "mesotron", meaning "middle particle". It came in both positively charged and negatively charged forms and was unstable in isolation, breaking down in 2 millionths of a second on the average. The muon seemed like it was the right size to be Yukawa's mysterious strong-force exchange particle at first, with a cosmic-ray impact on atoms in the upper atmosphere smashing it out of the nucleus and allowing it to be detected during its short lifetime.
Then things started to get more complicated. In 1938, a British physicist named Nicholas Kemmer pointed out that since the strong force existed between protons and protons, protons and neutrons, and neutrons and neutrons, then there needed to be three strong force exchange particles -- one with a positive electric charge; one with a negative electric charge; and one neutral, with no electric charge.
There was no reason there couldn't be three mesotrons, though only positive and negative forms were known; everyone knew that it was hard to track down neutral particles, and so the neutral mesotron was likely to be found in time. However, during World War II, two young Italian scientists, Marcello Conversi and Oreste Picconi, were conducting research on the decay of the mesotron in a basement laboratory in Rome, where they were hiding out from German occupation forces so they wouldn't be hauled off to a forced labor camp. Their work suggested that the mesotron wasn't likely to be Yukawa's exchange particle; after the war ended, the two researchers teamed up with another Italian physicist, Ettore Pancini, to perform a more methodical study under more comfortable circumstance.
In summary, they set up a simple system of Geiger counters, materials to slow down cosmic rays, and large iron bar magnets to focus charged particles. In their initial trials, they used iron to slow down the cosmic rays; they found that positive mesotrons decayed quickly but that negative mesotrons didn't. That seemed consistent with the idea that the mesotron was the strong force carrier, with the negative mesotrons being so quickly "sucked up" by atomic nuclei that they didn't have time to decay.
Then they used carbon instead of iron to slow down the cosmic rays; the carbon of course was not as big an obstacle to the cosmic rays as the heavier iron had been. To their surprise, the negative mesotrons usually passed through the carbon and broke down. If the mesotron had actually the strong force exchange particle, the carbon should have sucked it up just as readily as iron. They published their results in 1947, showing that it took a nucleus 10^-6 seconds to soak up a mesotron. That might not sound like much time, but Yukawa's exchange particle should have been soaked up in 10^-23 seconds. The ratio of time in these two cases was, scaled up, equivalent to several billion years to a single second. The mesotron was clearly not the strong force exchange particle.
* Yukawa had already had his suspicions about this, forming up a study group during the war to examine the issue. Now his suspicions had been confirmed. Once it was realized that the mesotron wasn't likely to be the strong force exchange particle, it was suggested that the real exchange particle wasn't being seen because it was so easily absorbed by nuclei. Any such particles resulting from cosmic rays would disappear at high altitude in the atmosphere.
In the 1930s, the British physicist Cecil Frank Powell (1903:1969), of Bristol University, had developed a new technique for obtaining the tracks of charged particles, using stacks of photographic plates, eventually obtaining special fine-granularity photographic emulsions for the task. In 1947, he set up stacks of such emulsions in the Pic du Midi de Biggore astronomical observatory, 2,850 meters high in the French Pyrenees. The emulsions clearly showed an intermediate-mass unstable particle that at first sight appeared to be much like the muon. It was a bit heavier, with a mass 273 times that of the electron, but it strongly interacted with protons and neutrons: it was obviously Yukawa's exchange particle. It was named the "pi meson" or "pion". As had been predicted, three particles were found, with the negatively-charged "pion-" and "pion+" discovered initially and the neutral "pion0" discovered in 1950. Yukawa won the Nobel prize in physics in 1949; Powell got it in 1950.
As far as the mesotron went, it was something of a puzzle, and in fact remains something of a puzzle even today. It was a half-spin particle that differed from the electron only in being bigger; like the positron and electron, there were positively and negatively charged versions, but not a neutral one. It was renamed the "mu meson" for a time, but the particles known as mesons would prove to be something very different, and so it was finally renamed the "muon", with any implication of being a meson being firmly denied. Once the muon's true identity was known, the Polish-born American physicist Isidore Isaac ("I.I.") Rabi (1898:1988) famously asked: "Who ordered that?"
* Incidentally, an extensive series of postwar balloon flights carrying stacks of photographic plates finally pinned down the nature of the "primary" cosmic rays that hit the upper atmosphere and produced the showers of particles detected at lower altitudes. They generally turned out to be charged particles -- usually protons, sometimes alpha particles, and on rare occasions heavier nuclei. Nobody was sure then, and nobody's sure now, where they came from, since space magnetic fields scrambled their trajectories, and so tracing them back to a source is difficult. Observations of cosmic rays would lead to the discovery of more new particles, a matter discussed later.
* An understanding of the strong force led to an understanding of why nuclei became more unstable as they grew more massive, and why the ratio of neutrons to protons increased as well. The protons in a nucleus drove each other apart with the long-range electromagnetic force, while the neutrons held them together with the short-range strong force. As the number of protons increased, so did the "electromagnetic force couplings" AKA "Coulomb force couplings" between each proton and all the others. The strong force couplings only worked between neighboring particles, and so more neutrons were needed to keep the nucleus together. The balance between the long-range electromagnetic force and the short-range strong force became ever more fragile until, above a certain atomic number, it was impossible to form a stable nucleus. It was no coincidence that the strong force was about a hundred times stronger than the EM force, and that the heaviest stable nuclei had an atomic number of a little under a hundred.
This is an intuitive way of looking at the issue, and in a domain as counterintuitive and bewildering as quantum physics, relying strictly on intuition is a good way to step on a mine. In the post-World War II period, Maria Goeppert-Mayer (1906:1972), a German physicist who came to the US and became an American citizen, came up with the idea that the particles in the nucleus occupied energy levels and energy shells, very much along the lines of those that are occupied by electrons in atoms. She and her colleague Hans Jensen (1907:1973) published a text on the concept in 1955, and the two shared the Nobel prize in 1963.
The theory of nuclear energy levels is of course complicated, but a
simplified model can illustrate how it relates to nuclear stability. Since
both protons and neutrons are fermions, they both obey the Pauli exclusion
principle, meaning that each level can accommodate only two particles, one
with spin UP and the other with spin DOWN. Both protons and neutrons have
their own energy levels; in this simplified model, the neutrons have
equally-spaced energy levels, but due to the electromagnetic repulsion
between protons the spacing between protons is greater:
proton neutron
< > < >
< >
< >
< >
< U >
< U D >
< U D > < U D > base energy level
This means that as the number of nucleons in a nucleus increases, the neutron
energy levels tend to fill up more quickly than the proton energy levels. If
the number of neutrons gets too far ahead of the number of protons, however,
the nucleus is not at its lowest energy level and neutrons will tend to
decay.
* The Ukrainian-American physicist George Gamow (1904:1968) came with an alternative theory in 1928, which became known as the "liquid drop" model of the nucleus. The basic concept is that the nucleons are like the molecules in a droplet of water, with the strong force acting as something like "surface tension" to keep the droplet together. Of course, nobody knew about the neutron in 1928, and so Gamow was not able to flesh out the idea properly; Niels Bohr hung on to it and developed it in later years, and Bohr is often credited incorrectly with coming up with the concept.
* While one faction of physicists discovered the neutron and the strong nuclear force, another faction was discovering a second nuclear force.
The story is also convoluted. The discovery of the neutron and the strong force did much to explain alpha decay, but it complicated finding an explanation of beta decay. In the days when the nucleus had been thought to be composed of protons and neutrons, beta decay seemed like merely like an electron escaping from the nucleus. The discovery of the neutron emphasized that there were no electrons in the nucleus, and so beta radiation was the result of the decay of the neutron into a proton and an electron.
To no surprise, this led to more puzzles. The first was that in beta decay, the kinetic energy of the proton and the electron produced in the breakdown of the neutron didn't seem to add up -- there was a deficit. This deficit had actually been noticed in beta decay before the discovery of the neutron. In 1930, Wolfgang Pauli proposed that the missing energy was being carried off by some mysterious neutral particle that to that time hadn't been detected.
Following the discovery of the neutron there were suggestions that it was the mystery particle, but this notion didn't match the details of beta decay at all, and in fact analysis of the energy deficit involved in beta decay suggested that the mystery particle would have little or no mass. The photon is a massless particle, or more precisely it is a particle with no rest mass. According to Einstein's theory of relativity, objects get more massive as they approach the speed of light; their mass would be infinite if they reached the speed of light, which is one reason why objects with mass can't do that. Through a strange balance, the fact that the photon by definition travels at the speed of light gives it a certain amount of mass even though it otherwise wouldn't have any at all. The mystery particle involved in beta decay might be a similar massless particle.
In 1934, Enrico Fermi published a detailed analysis of the expected properties of this mystery particle. When somebody wondered if it was actually the neutron, Fermi, who had a impish sense of humor, replied: "No, it's just a neutrino." -- implying something like a "baby neutron". The flippant name stuck. Of course it was neutral and massless, or nearly so, but Fermi also pointed out that it would be hard to detect even if someone was looking for it; the neutrino was able to zip through planets with very little likelihood of being stopped. The neutrino seemed a little like invoking the tooth fairy to many physicists and there was widespread suspicion of the idea -- Fermi had trouble getting his paper published, being initially rejected by the British journal NATURE because it was "too speculative" -- but as discussed later, the neutrino would actually be discovered in the 1950s.
* There remained the issue of why beta decay occurred. Dirac's analysis of the interactions of electrons with photons encouraged Fermi to use a similar approach to model the emission of an electron and a neutrino by a neutron in beta decay. In 1933, Fermi managed to come up with a scheme that worked, indicating the existence of a very short-range force that only operated on the distance scale of a single neutron. It was a very subtle force that didn't glue one particle to another so much as it glued a particle to itself, controlling beta decay. For this reason that physicists often prefer to semantically waffle and call it an "interaction" and not a "force".
Though it was originally called the "Fermi force", it became known as the "weak nuclear force" or just "weak force", because it was so much weaker than the strong force. Fermi had originally envisioned the weak force as being mediated by an exchange particle or particles that effectively had zero range and infinite mass, but in 1938 the Swedish physicist Oscar Klein (1894:1977) suggested a particle with finite mass instead. However, the fact that the weak force also had a much shorter range than the strong force still implied that it was mediated by a much heavier particle or particles. Nobody had any clear idea of the details, and in fact no real progress would be made on that front for decades.
It has to be noted that the existence of antiparticles suggested that beta decay was a bit more complicated than it had seemed. First, it was determined that the neutrino involved in beta decay was actually an antineutrino. Since the neutrino has no electric charge, obviously an antineutrino has the same charge as a neutrino, but they do differ in having spins with opposed orientations to their direction of propagation, and they will annihilate each other on contact. That meant that the familiar beta decay process was as follows:
neutron -> proton + electron + antineutrino
This was not surprising from a consideration of energetics: the mass of the neutron was greater than the sum of the mass of the proton and electron, and so neutrons tended to break down, just as stones tend to roll downhill, with the half-life of the decay of a free neutron being about 12.8 minutes.
Second, with the discovery of the positron, physicists realized that there was a "mirror" beta decay process. The decay of the neutron had to be referred to as the "electron" beta decay or "beta-" decay, because it then became obvious that there was a "positron" beta decay as well, which went as follows:
proton --> neutron + positron + neutrino
Although positron beta decay was much less common than electron beta decay -- the mass of the products is greater than the proton, meaning their creation requires a net input of energy -- "positron" beta decay or "beta+" decay was quickly observed. In this document, all following references to "beta decay" will actually mean beta- decay; if beta+ decay is being discussed, it will be specified as such. Incidentally, both these decay processes can be run in reverse:
proton + electron + antineutrino --> neutron
neutron + positron + neutrino --> proton
The reverse processes are, however, very rare; since a free neutron undergoes beta decay with a half-life of 12.8 minutes, emitting an antineutrino and an electron, an antineutrino must on the average be in contact with neutrons and electrons for 12.8 minutes for inverse beta decay to occur. Even with a dense material like lead, the cross-sectional area of the nucleus where neutrons reside is vastly smaller than the cross-sectional area of the atom itself, and since neutrinos and antineutrinos move at the speed of light, or at least were presumed to at the time, it takes thousands of light-years of lead to have a high probability of stopping any one neutrino.
* By the mid-1930s, the mysterious processes of radioactive decay were understood, at least in general. Alpha decay was the result of a tug-of-war between the electromagnetic and strong forces. Although a raw consideration of relative balance of the forces indicated that the strong force should win far more often than it seemed to, George Gamow showed that adding in the concept of quantum-mechanical tunneling tipped the balance back towards the electromagnetic force. Beta decay involved considerations of the weak force.
Both were entirely random processes: there was no "clock" inside the atom that said it would decay at some time or other, there was just a certain probability of it happening that resulted, in the scale of large numbers of radioactive atoms, in a highly specific half-life. In alpha decay. the half-life was directly related to the probability of tunneling: over the long run, if there was a chance of the alpha particle escaping, it would do so eventually, with the average time required proportional to the tunneling probability.
