tailieunhanh - Ebook Introductory nuclear physics (2nd edition): Part 2

(BQ) Part 2 book "Introductory nuclear physics" has contents: Nuclear collective motion, microscopic models of nuclear structure, nuclear reactions, nuclear astrophysics, nuclear physics: present and future. | Chapter 6 Nuclear Collective Mot ion The experimental observations outlined in the previous two chapters on energy level positions, static moments, transition rates, and reaction cross sections provide us with the basis for nuclear structure studies. Many of the observed properties of a nucleus involve the motion of many nucleons “collectively.” For these phenomena, it is more appropriate to describe them using a Hamiltonian expressed in terms of the bulk or macroscopic coordinates of the system, such a mass, radius, and volume. s 6-1 Vibrational Model We have seen earlier in the discussion of nuclear binding energies in $1-3 and $4-9 that, in many ways, the nucleus may be looked upon as a drop of fluid. A large number of the observed properties can be understood from the interplay between the surface tension and the volume energy of the drop. In this section, we shall take the same approach to examine nuclear excitation due to vibrational motion. For simplicity we shall take that, at equilibrium, the shape of a nucleus is spherical, ., the potential energy is minimum when the nucleus assumes a spherical shape. This is purely an assumption of convenience for our discussion here. It is made, in part, for the reason that spherical nuclei do not have rotational degrees of freedom, and it9 a result, vibrational motion stands out clearly, without complications due to rotation. In practice, the most stable shape for many nuclei is deformed, as we shall see later in $6-3, and vibrational motions built upon deformed shapes are also commonly observed. Breathing mode. When a nucleus acquires an excess of energy, for example, from Coulomb excitation due to a charged particle passing nearby, it can be set into vibration around its equilibrium shape. We can envisage several different types of vibration. For example, the nucleus may change its size without changing its shape, as shown in Fig. 6 - l ( a ) . Since the volume is now changing while the total amount of nuclear