Nuclear Reaction

Nuclear Reactions Nuclear reactions are completely independent of chemical reactions, in the sense that nuclear energies are several orders of magnitude larger. Thus an atom which has undergone a nuclear reaction reacts the same chemically as an identical atom which has not. There are four major types of nuclear reaction: 1. "fission", the splitting of a nucleus into two "daughter" nuclei: n + 235U -> 141Ba + 92Kr + 3n; 2. "fusion" of two "parent" nuclei into one daughter nucleus: p + p -> 2 H + e+ + ν + .42 MeV, where ν stands for a "neutrino" (we will see in Section E below that the proton "turned into" a neutron, a positive electron (!) and the neutrino); 3. "neutron capture", used to create radioactive isotopes, in which ○ the nuclear charge (Z, the atomic number) is unchanged, ○ the nuclear mass (A = number of protons + neutrons, the atomic mass) increases by one, and ○ the number of neutrons (N) increases by one (note that N always = A - Z); and 4. various "decay modes", in which nuclei "spontaneously" eject one or more particles and

lose energy to become nuclei of lighter atoms. We will meet them in the next section. Note that fission and neutron capture start out the same; the difference in fission is that the excited nucleus (with the extra neutron) is too unstable to stay together. In neutron capture, the new nucleus is simply "radioactive", that is, it participates in the decay modes of the next section. The most common use of fission is in the nuclear reactors which provide power and pollution, ostensibly an improvement over their previously most common use as atomic weapons. Fusion is much more interesting as the source of life on this planet in the form of the sun. The sun has "burned" for about 5 billion years; the burning is actually nuclear fusion processes (which occur at the core) called the "PP" (proton - proton) chain: Reaction p + p -> 2 H + e+ + ν e+ + e- -> γ 2 H + p -> 3He + γ 3 He + 3He -> 4He + p + p total

Energy yield .42 MeV 1.02 MeV 5.49 MeV 12.86 MeV 26.72 MeV

(Note that the first three steps occur twice for each occurrence of the fourth step.) This process takes place in all "main sequence" stars. For our sun, it will continue for about another 5 billion years, when the Hydrogen (p) supply will be used up. The sun will then swell to encompass the orbits of the first four planets as a "Red Giant". After that point, Helium is the main fuel:

. Reaction He + He -> 7Be + γ 7 Be + p -> 8B + γ 8 B -> 8Be + e+ + ν 8 Be -> 4He + 4He total 3

4

Energy yield 1.59 MeV 13 MeV 10.78 MeV .095 MeV 12.595 MeV

After the Helium is exhausted, progressively more massive nuclei fuse until iron is produced. Iron fusion is endothermic (requires more energy than it produces), so the star collapses under its own weight and either forms •

a white dwarf (the remnant of a small star); or it will supernova (explode) and the core will become

•

a neutron star (whose density is that of nuclei) or

•

a black hole (from which nothing can escape - even light!),

depending on its initial mass. Our sun will probably end up as a dwarf, since its mass is below the "Chandrasekhar" mass required to become a black hole. It is interesting to note that everything in our solar system (except "primordial" hydrogen and helium, which was made during the creation of the universe), including all of us, is made from matter which was created by fusion processes in a star and dispersed into space in a supernova explosion. For this reason, our sun is a "second (or perhaps third, etc.) generation" star. The next section is about nuclear decay and radioactive series.