Forces Between Nucleons

By comparing these two answers we can see that the repulsive force between two protons in the nucleus is about ten billion times as great as the repulsive force between two protons bound together in a hydrogen molecule. In order to overcome these enormous intranuclear coulomb repulsions and hold the nucleus together there must exist some very strong attractive forces between the nucleons. The nature of these forces is not understood and remains a very important problem in physics. 

Atomic nuclei are extraordinary particles. They contain all the protons in the atom crammed together in a tiny volume, despite their positive charges (Fig. 17.1). Yet most nuclei survive indefinitely despite the immense repulsive forces between their protons. In some nuclei, though, the repulsions that protons exert on one another overcome the force that holds the nucleons together. Fragments of the nucleus are then ejected, and the nucleus is said to decay. ... 

The internal energy is sometimes called chemical energy because it is the consequence of all the motions of particles and forces between particles molecules, atoms, nucleons, and electrons. 

The simplest bound nuclear system, the deuteron, consists of a neutron and a proton. The deuteron is known to have a quadmpole moment, 0.00286 barns, which tells us that the deuteron is not perfectly spherical and that the force between two nucleons is not spherically symmetric. Formally, we say the force between two nucleons has two components, a spherically symmetric central force and an asymmetric tensor force that depends on the angles between the spin axis of each nucleon and the line connecting them. 

The observation of the masses of mirror nuclei suggests the strong or nuclear force between a neutron and a proton is the same. This equivalence leads naturally to considering the neutron and the proton as corresponding to two states of the same particle, the nucleon. (A similar simation holds for the it meson, where the it0, tt+,... 

Nuclear forces between any two nucleons are very much alike, whether they are neutrons or protons, and arrangements contg equal numbers of each tend to be most stable in light nuclei. As the size and weight of the nucleus increase, however, Coulomb repulsion gains in relative importance because it falls off only with the square of the distance, much more slowly than the nuclear force. Thus, with increasing atomic weight the number of neutrons increases more and more, relative to the number of protons... 

Atoms are first stripped of their electrons at very high temperatures this creates a plasma (ionized gas) of positive ions. Then the positive ions must be brought into close enough proximity, so that the strong attractive force between nucleons can overwhelm the Coulomb repulsion between them. Magnetic fields can confine hot plasmas of ions, provided that collective instabilities of these plasmas can be controlled. For a successful nuclear fusion reactor, three requirements must be met (1) The density of the plasma must exceed some critical value p. (2) The plasma confinement time must exceed some critical value t. (3) The temperature of the plasma must exceed some critical value 9... 

Theoreticians thought that stable heavier elements might be in prospect. The stability of a nucleus (based on a model of nuclear stability analogous to that of the Rutherford-Bohr model of electronic structure) is determined by the inter-nucleon forces (nucleons are protons and neutrons), an attractive force between all nucleons and a Coulombic repulsion force between protons, the latter becoming proportionately more important as the number of protons increases. Extra stability is associated with filled shells of nucleons, magic numbers for neutrons they are 2,8,20,28,50,82,126,184, and 196 and for protons they are 2, 8, 20, 28, 50, 82, 114, and 164. 

In 1935, the Japanese physicist Hideki Yukawa proposed that a force between protons that is stronger than the electrostatic repulsion can exist between protons. Later research showed a similar attraction between two neutrons and between a proton and a neutron. This force is called the strong force and is exerted by nucleons only when they are very close to each other. All the protons and neutrons of a stable nucleus are held together by this strong force. 

Figure 24.6 The electrostatic force, represented by the purple arrows, acts between two charged particles. It is repulsive between two protons. The strong nuclear force, represented by the green arrows, acts between any two or more nucleons and Is always attractive. Infer What is the effect of the electrostatic force between two neutrons Between a proton and an electron ... 

The interaction between nucleons. Detailed calculations of nuclear structure and in particular the estimation of the energies of excited states, must be based on knowledge of the force between nucleons in a nucleus. Since no experiment gives directly the properties of this force, it is customary to make certain definite assumptions and to see how far experimental results can be so explained The first assumptions, against which there is no evidence, are that the... 

One of the most recent theoretical advances has been to correlate almost all n-p and p-p data below 300 Mev in terms of boundary conditions on S, P and D states with only one energy dependent parameter (see Breit and Feshbach ). However, there is still no answer to whether it is possible to write an analytical expression as a function of energy for the law of force between two nucleons. It may be that the need of a Lorentz invariant expression for the interaction is the fundamental reason why potentials (even of the repulsive core type (Jastrow )] specifically have not been able to fit all of the data. 

P) Many-body forces. One of the old questions of nuclear physics is whether there are many-body forces between three or more nucleons. The combination of a lack of knowledge about the two-body forces and the difficulties of theoretically treating many-body problems has left unanswered even the qualitative question of whether there are many-body forces. 

---