Nuclear Physics

I

Introduction

Nuclear Physics, the study of atomic nuclei, and of their interactions with other nuclei and with individual elementary particles (see Atom).

II

Decay of Nuclides

Atomic nuclei consist of positively charged protons and neutral, or uncharged, neutrons. The number of protons in a nucleus is the atomic number, which defines the chemical element. Nuclei with 11 protons, for example, are nuclei of sodium (Na) atoms. An element can have various isotopes, the nuclei of which have differing numbers of neutrons. For example, stable sodium nuclei contain 12 neutrons, whereas those with 13 are radioactive. These isotopes are notated as ®Na and ²Na, where the left-hand subscript indicates the atomic number and the superscript represents the total number of nucleons, or neutrons and protons. Any species of nucleus designated by certain atomic and neutron numbers is called a nuclide.

Radioactive nuclides are unstable: they undergo spontaneous transformation into nuclides of other elements, releasing energy in the process. These transformations include alpha (a) decay (the emission of a helium nucleus, ¸He²⁺), and beta (β) decay or positron (β⁺) decay. In β decay a neutron is transformed into a proton with the simultaneous emission of a high-energy electron. In β⁺ decay a nuclear proton turns into a neutron with the emission of a high-energy positron. For example, ²⁴Na undergoes β decay to form the next higher element, magnesium:

Gamma (g) radiation, like light, is electromagnetic radiation, but by virtue of their much higher frequency, g rays have far more energy. When a or β decay occurs, the resulting nucleus is often left in an excited (higher energy) state. Gamma rays are emitted as the nucleus drops to a lower energy state.

Any characterization of radioactive nuclide decay must include a determination of the half-life of the nuclide, that is, the time it takes for half of a sample to decay. The half-life of ²⁴Na, for example, is 15 hours. The types and energies of radiation emitted by the nuclide are also important in characterizing the decay.

III

Early Experiments

Radioactivity emitted by uranium salts was discovered by the French physicist Henri Becquerel in 1896. In 1898 the French scientists Marie Curie and Pierre Curie discovered the naturally occurring radioactive elements polonium (⁸⁴Po) and radium (⁸⁸Ra). During the 1930s, Irène and Frédéric Joliot-Curie made the first artificial radioactive nuclides by bombarding boron (⁵B) and aluminium (¹³Al) with a particles to form radioactive isotopes of nitrogen (⁷ N) and phosphorus (¹⁵P). Naturally occurring isotopes of these elements are stable.

The German nuclear scientists Otto Hahn and Fritz Strassmann discovered nuclear fission in 1938. When uranium is irradiated with neutrons, some uranium nuclei split into two nuclei, each with about half the atomic number of uranium. Fission releases enormous energy and is used in nuclear fission weapons and reactors (see Nuclear Energy).

IV

Nuclear Reactions

Nuclear physics also involves the study of nuclear reactions: the use of nuclear projectiles to convert one species of nucleus into another. If, for example, sodium is bombarded with neutrons, some of the stable ®Na nuclei capture neutrons to form radioactive ²Na nuclei:

®Na + ¦n → ²Na + g rays

Neutron reactions are studied by placing samples inside nuclear reactors, which produce enormous numbers of neutrons.

Nuclei can also react with each other, but being positively charged, they repel each other with great force. The projectile nucleus must have a high energy to overcome the repulsion and to react with target nuclei. High-energy nuclei are produced in cyclotrons, Van de Graaff generators, or other particle accelerators.

A typical nuclear reaction is the one that was used to produce artificially the next heavier element above uranium (°U), the heaviest element that occurs in nature (see Periodic Law). Neptunium (±Np) was made by bombarding uranium (mostly °U) with deuterons (nuclei of the heavy hydrogen isotope, ªH₁) to knock out two neutrons, forming ±Np:

°U + ªH → ±Np + 2¦n