What forms the major portion of an ionization chamber?

The major portion of an ionization chamber consists of a gas-filled region that is contained within an electric field. This gas serves as the medium where ionization occurs when radiation interacts with the gas molecules. When ionizing radiation passes through this gas, it ionizes some of the gas atoms, creating positive ions and electrons. The electric field generated in the chamber then accelerates these charged particles toward their respective electrodes, which generates a measurable electric current or signal.

This characteristic is crucial for the function of the ionization chamber as a radiation detection device, as the amount of current generated is proportional to the amount of radiation present, allowing for accurate measurements. The design of the ionization chamber, including the chamber shape and the nature of the gas used, influences its sensitivity and response to different types of radiation.

The other choices do not form the major part of an ionization chamber: an ionization chamber itself describes the entire device rather than its internal components; a vacuum tube does not contain gas for ionization; and an electric transformer is used to change voltage levels, which does not relate directly to the ionization process within the chamber.