The photoelectric effect describes the following interaction of electromagnetic radiation with a metallic surface: a photon with an energy (frequency) above the binding energy of an electron gets absorbed and the electron is emitted. The positive energy difference is transferred to the electrons kinetic energy. If the photons energy is not high enough for the electron to overcome its binding forces, the photon will be re-emitted. It is not the intensity of a photon beam (amount of photons) which allows the photoelectric effect; it is the energy (frequency) of a single photon which will allow the emission of a single photoelectron.
The discovery and study of the photoelectric effect leads to a new quantized understanding in physics. Albert Einstein was awarded the Noble prize for physics in 1921 'for his services to theoretical physics and especially for his discovery of the law of the photoelectric effect'.
The photoelectric effect is the most important effect in medical radiography. E.g. it is photoelectric absorption that is responsible for most of the absorption in a mammogram which creates the contrast in the image.

See also Photon, Electron.

(p) The pitch (in computed tomography) is the ratio of the patient table increment to the total nominal beam width for the CT scan. The pitch factor relates the volume coverage speed to the thinnest sections that can be reconstructed. In spiral CT, dose is always inversely proportional to pitch.
Among the different manufacturers, there are various definitions of pitch depending on whether a single-detector (single-slice) or multi-detector (multi-slice) CT scanner is used.

The primary radiation is the useful beam emitted from the x-ray source.

A proton is a basic particle located in the nucleus of an atom. The number of protons in the nucleus governs the chemical properties of that element. The positive electrical charge is equal to the negative charge of an electron and the mass is similar to that of a neutron (see also baryon). The proton belongs to the group of hadrons and is composed out of three quarks.
A hydrogen (H1) nucleus is build of one proton.
Proton beam therapy is the preferred treatment for cancers near the eye and the spinal cord.

Radiation shielding is the process of limiting the penetration of radiation into the environment, by blocking with a barrier made of impermeable material. This protective barrier is usually formed of a material with high density, for example lead that absorbs the radiation.
Radiation sources are self-shielded with absorbing material incorporated into the equipment, adjacent to the source to reduce stray radiation to the surrounding area below dose limits.
Rooms with x-ray or other radiation equipment are additionally shielded with lead-lined walls to reduce the radiation exposure to humans within the facility. The amount of shielding required to protect against different kinds of radiation depends on how much energy they have. The shielding calculations are based on the half value layer of the primary radiation beam. Sufficient half value layers of shielding are calculated to reduce the radiation exposure outside the room to reasonable levels.
Personal shielding requirements depending on the type of radiation:
See also Radiation Safety.