3. PROPERTIES OF X RAYS.pptx

PROPERTIES OF X RAYS • X-rays are defined as weightless packages of pure energy (photons) that are without electrical charge and that travel in waves along a straight line with a specific frequency and speed. • The properties of X-rays may be classified into four broad categories: A. Physical • B....

PROPERTIES OF X RAYS • X-rays are defined as weightless packages of pure energy (photons) that are without electrical charge and that travel in waves along a straight line with a specific frequency and speed. • The properties of X-rays may be classified into four broad categories: A. Physical • B. Chemical • C. Biological • D. Physiochemical. PHYSICAL PROPERTIES 1. X-rays belong to a family of electromagnetic radiations having a wavelength between 10 Å and 0.01 Å. 2. They travel through space in a wave motion. 3. In free space they travel in a straight line. 4. They travel with the same speed as that of visible light (i.e. 1,86,000 miles per second). 5. As they travel through space, they can produce an electrical field at right angles to their path of propagation and a magnetic field at right angles to the electric field. 6. They are invisible to the eye and cannot be seen, heard or smelt (they remain undetected by the human senses). 7. They cannot be focused by a lens. 8. They cannot be reflected, refracted or deflected by a magnet or electric field as they do not possess any charge. 9. They show the properties of interference, diffraction and polarization, similar to that of visible light. 10. They do not require a medium for propagation. 11. X-rays are pure energy, no mass and they transfer energy from place to place in the form of quanta (photons). (E = h). 12. In free space they obey the inverse square law, which states that for a point source of radiation the intensity (I) at any given place varies inversely as the square of the distance (d) from the source to the place at which the intensity is being considered. I  1/d2 or I = k/d2 , where k is a constant. Intensity is determined by the number or quantity of X-ray photons in a beam. • 13. X-rays are produced by the collision of electrons with tungsten atoms. The collisions which occur are of two types, thus giving rise to two types of spectra (Fig. 3.1): i. Continuous Spectra (General Radiation, Bremsstrahlung Radiation or Braking Radiation) (Fig. 3.2). The incoming electron penetrates the outer electron shell and passes close to the nucleus of the tungsten atom. The incoming electron is dramatically slowed down and deflected by the nucleus with a large loss of energy, which is emitted in the form of X-rays. The amount of deceleration and the degree of deflection determines the amount of energy lost by the bombarding electron and hence the energy of the resultant emitted photon has a wide range or spectrum of energies and therefore called Continuous Spectrum • ii. Characteristic Spectrum or Line Spectrum (Fig. 3.3): The incoming electron collides with an inner shell tungsten electron, displacing it to an outer shell (excitation), or displacing it from the atom (ionization), with a large loss of energy and subsequently the orbiting tungsten electrons rearrange themselves to return the atom to neutral or ground state. This involves electron 'jumps' which results in the emission of X-ray photons with a specific energy called Characteristic Spectrum. • 14. X-rays can penetrate various objects and the degree of penetration depends upon the quality of the X-ray beam, and also on the intensity and wavelength of the X-ray beam. Quality (penetrating power of X-ray beam) is defined as the energy carried by the X-ray beam. The quality of the Xray beam is determined by the kilo voltage, milli amperage, distance between the target and the object, time or length of exposure, filteration and target material. • 14. X-rays can penetrate various objects and the degree of penetration depends upon the quality of the X-ray beam, and also on the intensity and wavelength of the X-ray beam. Quality (penetrating power of X-ray beam) is defined as the energy carried by the X-ray beam. The quality of the X-ray beam is determined by the kilo voltage, milli amperage, distance between the target and the object, time or length of exposure, filtration and target material. • 15. Property of attenuation, absorption and scatter; when passing through matter the intensity of radiation is reduced (attenuation) both because radiation energy is taken up by the material (absorption) and some is deflected from the original path, to travel in a new direction (scattering). • Effect of Interaction of X-rays with Matter In the case of the diagnostic X-ray beam there are three mechanisms by which these processes take place: 1. Coherent scattering. 2. Photoelectric effect. 3. Compton scattering. 1. Coherent or Elastic Scattering is a process by which radiation is deflected without losing energy. • X-rays when passing close to an atom causes the "bound electrons" to vibrate momentarily at a frequency equal to that of the incident photon. • The incident photon then ceases to exist. • The vibration causes the electron to radiate energy in the form of another X-ray photon of the same frequency and energy as that in the incident beam. • Usually the secondary photon emitted is at an angle to the path of the incidental X-ray. • In short, the direction of the incident X-ray has been altered, usually low energy photons undergo coherent scattering. • At energy levels employed in diagnostic radiology, the effect of coherent scattering is negligible in production of fog. This property is used to investigate internal molecular structure of materials, by the method of X-ray diffraction, called X-ray crystallography. 2. Photoelectric Effect is a process of interaction of the incident photon and the bound electron leading to emission of characteristic radiation. • It occurs when an incident photon collides with a bound electron in the atom of the absorbing medium. • The incident photon ceases to exist and its energy helps to eject a bound electron from its shell to become a recoil electron or a photo electron. • The kinetic energy imparted to the recoil electron is equal to the energy of the incident photon minus that required to overcome the electron binding energy. • The ejection of the electron will ionize the atom. • The orbital vacancy caused by the electron reshuffle and the neutrality is obtained by attracting an electron from outside. • During this rearrangement characteristic radiation is emitted. • About 30 percent of photons absorbed from a dental Xray beam are absorbed by the photoelectric process. 3. Compton Scattering or Inelastic Scattering is an interaction of photons with free or loosely bound outer shell electrons. • The photon gives some of its energy to the electron and it, itself continues in a new direction (scattered or modified compton attenuation), but with reduced energy and hence with increased wavelength. • The ejected outer shell electron is called compton or recoil electron. • The angle through which the photon is scattered depends upon the energy lost by the photon. • If scattered through a small angle, very small amount of energy is lost to the outer electron. • In case of a head on collision, the photon is turned back along its track and maximum energy is transferred to the recoil electron (unmodified compton attenuation), and all the energy is lost. • The recoil electrons undergo further ionizing interactions within the tissues, and gradually lose energy along their tracts by causing secondary radiations and consequent biological damage. • The scattered photon may undergo further compton interactions and/or photoelectric effects and/or escape from the tissue. It is the latter which forms scattered radiation of concern in the clinical environment. • 16. Due to their energy X-rays can release photoelectrons from the metals, when allowed to fall on them. • 17. Heating effect: The production of heat is one of the initial results of the slowing down of the primary electrons, it also arises as an end product of the chemical reactions induced by radiations. This change in temperature is very small and can only be detected by very sensitive instruments. • 18. Fluorescence: When X-rays fall upon certain materials, visible light is emitted called fluorescence, and it was this very property which led to the discovery 19. Ionization: This is a process of converting atoms into ions. a. X-rays can ionize gases and thus increase the electroconductivity of a gas through which it passes. Ionized air is accepted as the basis for X-ray measurement and for the definition of unit of X-ray quantity (Roentgen). Various devices that work on the principle of ionization are: i. Ionization chamber. ii. Thimble chamber. iii. Condenser chamber. iv. Geiger Muller counter. v. Scintillation counter. b. Ionization also takes place whenever an X-ray photon strikes matter producing secondary radiation, and the effects of Xrays is largely due to this process. c. X-ray radiation by virtue of its intrinsic ionizing potential can activate and dissociate silver ions in silver halide. This property is used in diagnostic radiology. CHEMICAL PROPERTIES • The outer electrons of the atoms play an important role in chemical combinations and therefore any disturbance in the outer electron configuration of an atom brings about a chemical change. 1. X-rays induce color changes of several substances or their solutions. i. Methylene blue gets bleached. ii. Sodium platinocyanide which is apple green turns to darker shades, then light brown and finally dark brown. These color changes are used to detect cases of forgery and dose measurement. 2. X-rays bring about chemical changes in solutions which are otherwise completely stable. This is because X-rays produce the highly active radical 'OH' in water, which reacts with the solutes. i. This brings about molecular changes in biological molecules • ii. Organic compounds get oxidized to carbon dioxide with release of hydrogen when exposed to radiation, because water in organic substances undergoes oxidation and reduction reactions when irradiated. • iii. X-rays can cause oxidation of ferrous sulphate to ferric sulphate and this is used as a method of measuring X-ray dosage (Frickle Dosimeter). 3. X-rays can cause destruction of the fermenting power of enzymes, which are vital substances for the metabolism of cells of all living materials. BIOLOGICAL PROPERTIES • When X-rays are incident on an atom, one of the reaction it produces is 'excitation.' This state of 'excitation' in biological materials enable it to take part in a chemical process into which in the normal state it would not enter. This is an important cause of biological damage produced by radiation. i. This property of excitation is used in the treatment of malignant lesions. ii. X-rays also have a germicidal or bactericidal effect and are used for sterilization and preservation of food. • The biological effects of X-rays may be classified into two types: i. Somatic effect: This ranges from a simple sun burn to severe dermatitis, to changes in the blood supply and/or malignancy. The effect is cumulative and depends upon the type of tissues and intensity of the radiation. ii. Genetic effect: This effect is due to radiation induced mutation of genes and chromosomes. These effects are usually seen in the off-springs of the irradiated parents. The fetus is more sensitive to radiation in the early stage of development. PHYSIOCHEMICAL PROPERTIES • The photographic effect: photographic paper or film when exposed to X-ray radiation and then developed will be found blackened. The irradiation effects the silver salts in the emulsion, so that after the chemical process called developing, the radiograph metallic silver is released and the film or paper appears blackened. This blackening is known as 'film density' and the degree of blackening depends upon: • i. Amount of radiation. • ii. Quality of radiation. • iii. Characteristic of a film. • iv. Concentration and age of developing solution. • v. Length of developing time. • vi. Use of intensifying screens. The difference between the degree of density is known as 'film contrast.' • Thank you…