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Unformatted text preview: Physics 54 Nuclear Physics It’s very difFcult to make predictions—especially about the future. —Niels Bohr Overview The discovery of radioactivity in 1896 caused a shock among chemists, because it suggested that the atom is not the eternal, immutable object they assumed it to be. This became even clearer within a few years, when radioactive process were studied carefully and it was shown that, in some of them, existing elements in the sample under study disappeared and were replaced by others that had not previously been there. Soon three kinds of “rays”, called α , β and γ , were identiFed among the emissions given off in radioactive “decays”. The α and β rays, which could be de¡ected by strong magnetic Felds, were determined to be charged particles, while γ rays were found to electrically neutral. After much further work it was established that: • α rays have charge +2e and mass 4 times that of hydrogen. • β rays are electrons (mostly — some turn out to be positrons). • γ rays are photons of exceptionally high frequency. Since charge is always conserved, emission of α or β rays must be accompanied by a change in the charge of the nucleus. That is, a transmutation takes place, changing atoms of one element into those of another. (This does not occur in emission of the γ rays, in which the nucleus is undergoing a transition from a higher to a lower energy state.) After helium had been discovered on earth, it was found that α rays quickly become neutral helium atoms. Nearly all of the helium we Fnd in the earth’s crust comes from nuclear decays giving off α rays. It was by using α rays from a strong radium source that Rutherford was able to perform many of his studies, including the ones that revealed the existence of the atomic nucleus. Subsequently he was also able to make a nucleus absorb an α ray and become a different kind of nucleus: the Frst man-made nuclear reaction. PHY 54 1 Modern Physics 2 It was realized early on that the nucleus of hydrogen is an “elementary” particle, and it was named the proton . The other subatomic particle known at that time was the electron, and for about a decade it was assumed that these are the only ones. They are also the only common microscopic particles that are absolutely stable against decay. For all nuclei except hydrogen the mass is a larger multiple of the proton mass than the charge is of the proton charge, so the nucleus cannot consists only of protons. This was explained by assuming that there are also electrons in the nucleus, whose negative charge cancels the charge of some of the protons, but whose mass is negligible. Helium, for example, would have a nucleus consisting of four protons and two electrons....
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This note was uploaded on 10/19/2011 for the course PHYSICS 54L taught by Professor Thomas during the Summer '09 term at Duke.
- Summer '09