Isotopes and analogs of hydrogen--from fundamental investigations to practical applications.

3.3 New particles There was never really a time when the world seemed to consist only of the particles of "ordinary matter": protons, neutrons, and electrons. Although it was widely believed that the nucleus was comprised of protons together with other particles of about the same mass and zero charge, it was easiest to assume that these were some kind of bound state of the two known particles, the proton and the electron, coexisting in the form of a "neutral doublet"; because of this notion the neutron, though "conceived" around 1920, was not "born" until 1932, when James Chadwick discovered it experimentally as a distinct particle (113). (That the nucleus does not contain electrons is clear from [sup.14]N. In the "neutral doublet" theory it should contain 14 protons and seven electrons; given that each of these particles has a spin of 1/2, the total spin of the nucleus should also take a half-integer value, but experimentally the [sup.14]N nuclear spin is known to be (114).)Almost immediately after the neutron was discovered the list of known particles had to be expanded to include the positron: this particle, with charge +e and mass [m.sub.e], was discovered in cosmic rays by Carl Anderson in August 1932 (115,116). Track photographs from his Wilson cloud chamber experiments showed particles exhibiting curved paths in a magnetic field of several kilogauss roughly equal in degree of curvature but opposite in direction compared to electrons; the direction of travel was confirmed by noting the change in curvature as the particle lost momentum on passing through a lead plate. Strangely enough, there was already a place in theory for the positron. Paul Dirac's theory of the electron contained an aspect that proved very difficult to explain; a set of (infinitely many) negative energy solutions describing states that had to be occupied before stable positive energy states for the electron were possible (117). While at first this appeared only to be a blemish on an otherwise beautiful theory, over a period of two years or so as the Dirac theory was used and discussed by scientists such as Oppenheimer, Heisenberg, Blackett, and Dirac himself, the interpretation of these states evolved slightly; instead of negative energy states of the negatively charged electron, the states began to be viewed as positive energy solutions corresponding to a hitherto unseen particle with the same mass as the electron but a positive charge. If there was already a place in theory for the positron at the time of its discovery, the same could not be said for the next particle to be discovered, the muon. In the mid-1930s, now that the contents of the nucleus were known, the puzzle was to discover the nature of the force holding it together. Whatever it was must be much stronger than the Coulomb repulsion of the protons in the nucleus for one another, but only operate over short distances. In the theory of quantum electrodynamics that was emerging at the time, the electromagnetic interaction was beginning to be seen as a process whereby charged particles interacted by exchanging "virtual" photons as permitted by the uncertainty principle. With the photon's zero rest mass and infinite lifetime, the range of the interaction is infinite, with inter-particle forces obeying an inverse square law and the Coulomb potential taking the form V(r)[varies] - 1/r. (35) Hideki Yukawa surmised that a similar law should govern the interactions within the nucleus, but with the variation that the force carrier particle must have nonzero rest mass; if this were the case, the uncertainty principle would limit its lifetime according to [DELTA]E[DELTA]t[less than or equal to]h/2 (118). The potential would be modified by an exponential decay dependent on the lifetime of the exchange particle according to V(r)[varies]- [e.sup.-[lambda]r]/r. (36) If the nucleus has a diameter on the order of [10.sup.-15] m, a particle traveling at c can traverse it in 3. …