Radiation can ionize matter caused by the high energy which displaces electrons during interactions with atoms. In the electromagnetic spectrum higher frequency ultraviolet radiation begins to have enough energy to ionize matter.
Examples of ionizing radiation include alpha particles, beta particles, gamma rays, x-rays, neutrons, high-speed electrons, high-speed protons, and other particles capable of producing ions by direct or secondary processes in passage through tissues.
Damage of living tissue results from the transfer of energy to atoms and molecules in the cellular structure. Ionized cells have to repair themselves to remain alive. Generally, healthy cells have a higher capability to repair themselves than cancer cells.

Biological effects of ionizing radiation exposure:
Ionizing radiation are used in a wide range of facilities, including health care, research institutions, nuclear reactors and their support facilities, and other manufacturing settings. These radiation sources can pose a serious hazard to affected people and environment if not properly controlled.

See also Radiation Safety, Controlled Area, Radiotoxicity and As Low As Reasonably Achievable.

Radiation safety concerns the safe use of ionizing radiation. The radiation exposure has to be controlled to protect people and the environment from unnecessary exposure and the damaging effect to the health. Legal regulations require that radiation exposure (individual radiation exposure as well as collective dose) must be kept as low as reasonably achievable.
The electromagnetic spectrum includes x-rays, gamma rays, ultraviolet radiation, visible light, infrared radiation, and radio waves. Additionally, there are several types of particulate radiation e.g., alpha and beta particles. All types of radiation are used in a wide range of medicine, industry, research and communication. Radiation risks can occur due to either long-term low level exposure or short-term high level exposure. A well-functioning dosimetry program is essential for a safe use and for compliance with federal and state regulations.

Three basic rules have to be observed for a safe use of ionizing radiation.

See also Inverse Square Law, Administrative Dose Guidelines and Annual Dose Limit.

Scattering refers to the reflection of radiation, electromagnetic or particulate, when interacting with matter. Scattering results in a loss of energy of the radiation.
Backscattering refers to a reflection of radiation in a direction generally greater than 90 degrees to that of the incident radiation.

Secondary radiation is the result of absorption of other radiation in matter. It could be either electromagnetic or particulate in nature.

X-rays are a part of the electromagnetic spectrum. X-rays and gamma rays are differentiated on the origin of the radiation, not on the wavelength, frequency, or the energy. X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus. X-rays have wavelengths in the range of about 1 nanometer (nm) to 10 picometer (pm), frequencies in the range of 10^-16 to 10^-20 Hertz (Hz) and photon energies between 0.12 and 120 kilo electron Volt (keV). The energy of rays increase with decreased wavelengths. X-rays with energies between 10 keV and a few hundred keV are considered hard X-rays. The cutoff between soft or hard X-rays is around a wavelength of 100 pm.
Because of their short wavelength, X-rays interact little with matter and pass through a wide range of materials. These interactions occur as absorption or scattering;; primary are the photoelectric effect, Compton scattering and, for ultrahigh photon energies of above 1.022 mega electron Volt (MeV), pair production.
X-rays are produced when high energy electrons struck a metal target. The kinetic energy of the electrons is transformed into electromagnetic energy when the electrons are abruptly decelerated (also called bremsstrahlung radiation, or braking radiation) similar to the deceleration of the circulating electron beam in a synchrotron particle accelerator. Another type of rays is produced by the inner, more tightly bound electrons in atoms;; frequently occurring in decay of radionuclides (characteristic radiation, gamma ray, beta ray). The energy of an X-ray is equivalent to the difference in energy of the initial and final atomic state minus the binding energy of the electron.
Wilhelm Conrad Roentgen discovered this type of rays (also called Roentgen-rays) in 1895 and realized that X-rays penetrate soft tissue but are absorbed by bones, which provides the possibility to image anatomic structures; the first type of diagnostic imaging was established. Radiographic images are based on this difference in attenuation for tissue and organs of different density. Today ionizing radiation is widely used in medicine in the field of radiology.

See also Exposure Factors, X-Ray Tube, and X-Ray Spectrum.