A photon is a discrete packet of electromagnetic energy. The amount of energy depends on the frequency (wavelength) of the photon. Highest frequency, most energetic photon radiations are gamma rays, up to 300 EHz - 1.24 MeV. In addition to energy, photons are also carrying momentum.
Photons have no electrical charge or rest mass and exhibit both particle and wave behavior.
Photons are traveling in vacuum (without interactions with matter) with the constant velocity of 2.9979 x 10⁸ m/s (c, speed of light).
Photons get absorbed or scattered away from their original direction of travel when interacting with matter.
High energy photons as for example x-rays cause damages to exposed tissue and cells. Radiation exposure is measured in roentgen, radiation absorption in Roentgen//min.
Photon radiation in the frequency ranges of x-rays and gamma rays are used for medical diagnostic and treatment.

See also Photon Energy and Gamma Ray.

Radiotoxicity refers to radioactive materials that are toxic to living cells or tissues. Radiotoxicity results from the type of radiation, the radioactive half-life of the used radionuclide, the biological half-life in the tissue and the radioactivity absorbed in the organ. Radiotoxic substances can be collected following ingestion, inhalation and absorption.

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

Slow neutrons have a speed of around 2.2 km/s (0.025 eV). They could be generated in a reactor and could lead, because of their much larger effective neutron absorption cross-section than quick neutrons, to neutron activation.

See also Neutron Activation, Neutron Activation Analysis, Neutron Capture and Neutron Radiation.

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.