X-ray

X-Rays # Discovered in 1895 by Roentgen # “X” Rays because he didn’t know what they were! # An ionising radiation at a higher level on EM spectrum # Higher frequency or shorter wavelength

Nature of X-Rays X-rays are electromagnetic radiation ranging in wavelength from about 100 A o to 0.01 A o. The shorter the wavelength of X ray, the greater is its energy and its penetrating power. Types of X-rays 1- Soft X rays 2- Hard X rays 3- White X rays 4- Monochromatic X rays X-ray

X-ray production High energy electrons hit a (metallic) target where part of their energy is converted into radiation X-ray 5 target electrons X-rays Low to medium energy (10-400keV) High > 1MeV energy

X-ray Production There are two types of x-rays produced in the target of the x-ray tube. The majority are called Bremmstrahlung radiation and the others are called Characteristic radiation.

Bremmstrahlung x-rays are produced when high-speed electrons from the filament are slowed down as they pass close to, or strike, the nuclei of the target atoms. The closer the electrons are to the nucleus, the more they will be slowed down. The higher the speed of the electrons crossing the target, the higher the average energy of the x-rays produced. The electrons may interact with several target atoms before losing all of their energy. Bremsstrahlung Radiation (Also known as braking radiation or general radiation)

Bremsstrahlung X-ray Production + High-speed electron from filament enters tungsten atom Electron slowed down by positive charge of nucelus; energy released in form of x-ray Electron continues on in different direction to interact with other atoms until all of its energy is lost

Bremsstrahlung X-ray Production Maximum energy High-speed electron from filament enters tungsten atom and strikes target, losing all its energy and disappearing The x-ray produced has energy equal to the energy of the high- speed electron; this is the maximum energy possible +

Characteristic Radiation Characteristic x-rays are produced when a high- speed electron from the filament collides with an electron in one of the orbits of a target atom; the electron is knocked out of its orbit, creating a void (open space). This space is immediately filled by an electron from an outer orbit. When the electron drops into the open space, energy is released in the form of a characteristic x-ray. The energy of the high-speed electron must be higher than the binding energy of the target electron with which it interacts in order to eject the target electron. Both electrons leave the atom.

Characteristic x-rays have energies “characteristic” of the target material. The energy will equal the difference between the binding energies of the target electrons involved. For example, if a K-shell electron is ejected and an L-shell electron drops into the space, the energy of the x-ray will be equal to the difference in binding energies between the K- and L-shells. The binding energies are different for each type of material; it is dependent on the number of protons in the nucleus (the atomic number). Characteristic Radiation (continued) K-shell M-shell L-shell

Characteristic X-ray Production L K M High-speed electron with at least 70 keV of energy (must be more than the binding energy of k-shell Tungsten atom) strikes electron in the K shell, knocking it out of its orbit Ejected electron leaves atom Recoil electron (with very little energy) exits atom vacancy X-ray with 59 keV of energy produced. 70 (binding energy of K-shell electron) minus 11 (binding energy of L-shell electron) = 59. Electron in L-shell drops down to fill vacancy in K-shell