Nuclear Physics: Radiation, Decay Laws and Atomic Forces

Nuclear Physics: Types of Radiation

Alpha rays were identified by Rutherford as helium atoms but with two net positive electric charges, i.e., helium nuclei. Beta rays were identified by Becquerel as cathode rays, and he concluded that they were electrons. Their mass is about 8,000 times lower than that of the alpha rays. Gamma rays were identified as electromagnetic radiation, much like X-rays but of higher energy.

Radioactive Displacement Laws

• When a nucleus emits an alpha particle (α), it becomes another nucleus whose mass number is four units lower and its atomic number is two units less.
• When a nucleus emits a beta particle (β), it becomes another nucleus whose mass number is the same and whose atomic number is one greater.
• When a nucleus emits gamma radiation (γ), it alters its energy content but does not change the number of nucleons; therefore, it does not change the element.

Law of Radioactive Decay

Law of radioactive decay: The number of nuclei in a sample decreases exponentially with time.

Properties of Nuclear Forces

1. They are attractive and about 100 times more intense than the electromagnetic forces.
2. They have a very short range and are practically zero at distances greater than 10^-15 m. For very small distances, they are repulsive.
3. They are saturated; that is, each nucleon is bound only to a certain number of other nucleons, and not all in the nucleus.

Classification of Nucleons

Nucleons are classified as fermions (particles that form matter) and bosons (particles that transmit forces).

The Binding Energy and Stability

It is called binding energy per nucleon the ratio between the binding energy and the number of nucleons. It represents the energy required to remove a nucleon from the nucleus that contains it and is a measure of the stability of a nucleus: the most stable are those with the greatest binding energy per nucleon.

Nuclear Fission and the Liquid Drop Model

Nuclear fission is the splitting of nuclei, usually heavy ones, into two or more lighter nuclei called fission fragments. Nuclear fission can be interpreted by the liquid drop model: a drop, vibrating successively, adopts spherical and ellipsoidal shapes due, firstly, to the restoring force of surface tension, which makes the drop spherical again, and, moreover, to inertia, which makes the transition to ellipsoidal as follows: if the degree of deformation is sufficient, the surface tension is not able to stop the deformation, and the drop splits.

Nuclear Fusion and Atomic Interaction

The union of nuclei to form a larger one is called fusion. It is possible to communicate to small cores enough kinetic energy to overcome the electric repulsion and approach distances where the nuclear forces come into play.