Foundations of Nuclear and Particle Physics

Basic Properties of Finite Nuclei

We begin the discussions of finite nuclei with some general remarks on the systematics of the nuclear ground state. For even-even nuclei (nuclei with an even number of both protons and neutrons) the nucleons making up the nucleus form pairs containing one spin-up and one spin-down nucleon to yield a net spin of zero and even parity. In Chapter 15 we consider elastic electron scattering from such nuclei as the paradigm for what follows. Odd-even and even-odd nuclei often, but not always, have the same spin as the last unpaired valence nucleon; we shall see examples when discussing elastic magnetic electron scattering and magnetic moments also in Chapter 15. Finally, odd-odd nuclei are somewhat unusual. There exist only four stable nuclei having unpaired protons and neutrons, namely, 2H, 6Li, and 14N with spin-parity 1+, and 10B having 3+. The basic characteristics of the known nuclei are the following: they occupy a region in the NZplane whose central valley runs roughly along the N = Z line at values of A below 40 and then bends towards the region having higher values of N than of Z, as shown in Fig. 13.1. Taking cuts across the valley at either constant Z or at constant N, one climbs out of the valley, on the average moving to less bound nuclei until reaching the so-called drip-lines where nuclei are no longer stable to proton or neutron emission. At the bottom of the valley, where the most stable nuclei reside, one finds the binding energy per nucleon to be relatively constant for nuclei beyond A = 40 at a value ̴ 8.5 MeV per nucleon, as shown in Fig. 13.2 and, as the values of N and Z where stable nuclei exist become very large, this valley of stability narrows and then disappears.

One of the key questions in studies of nuclear systematics is: Do “islands of stability” exist at even higher N, Z-values, the so-called superheavy nuclei? An island of stability, first conjectured by Seaborg in the 1960s, is a collection of heavier isotopes of transuranic elements, expected to be more stable than those closer in atomic number to uranium with radioactive decay half-lives of minutes to days.