The use of radioactive radiations to obtain the photographic film of the test material, incorporated with the radioactive tracers, is called autoradiography and the film obtained is called autoradiograph. After development the irradiated areas appear on the film as dark areas corresponding to the distribution of the tracer.
Autoradiography can be detected either directly with a scintillation counter or indirectly via their effect on photographic film. At the light microscopic level the autoradiography is based on the principle that if a photographic emulsion is brought in to contact with radioactive material, the ionic radiations will convert the emulsion as dark spots of silver at certain points.
Radioactive substances are introduced into the test material either in a given chemical form or tagged with certain metabolic precursors. Nucleic acid can be made radioactive by incorporation of radioactive phosphate during nucleic acid synthesis.
The following three types of radiations are used in autoradiography:
Alpha rays - The alpha rays particles which consist of 2 neutrons and 2 protons and infact charged helium atoms. Radium 226 is their source.
Beta rays - The beta rays are electrons ejected or emitted by nuclei. The energy levels of these electrons may vary. When highly charged they are called hard beta as of p32 but soft beta is emitted from C14.
Gamma rays -The gamma rays are electromagnetic rays and resemble X-rays. CO60 is gamma emitter.
Autoradiography can directly be measured by various counters like Geiger Muller counter, Scintillation counter, Propotional counter, etc. While counting the back ground count should also be considered.
a) Geiger- Muller Counter - It is made of a glass or metal tube containing a mixture of gases, an inert gas like helium or argon and an organic vapour or a gas e.g. isopropanol or isobutene or cooking gas (which is a mixture of hydrocarbons). It has thin window, usually made of mica at one end to enclose the gas. Depending up on the thickness of the window and the energy of radiation of the radioisotopes a fraction of the particles emitted enter the counting tube and ionize the gas mixture inside. The small current generated is magnified at high voltage, and measured with a clear calibrated to record disintegrations per unit time. The counts recorded include counts due to ionization of the gas mixture by cosmic rays passing through the tube, any radioactivity present in the surrounding area. This is known as the background. These background counts have to be substracted from the counts given by the sample. Corrections also have to be applied for a. half life of decay, b. geometry,c. coincidence loss, and d. self absorption. If measurements are done at the same shelf and same position, no correction for geometry is measurements are done at the same shelf and same position, no correction for geometry is necessary. For long lived isotopes half life of decay is unimportant. The results of Geiger- Muller counter are expressed in counts/min. The main limitation of gas counter is its dead time. After the initial ionization the cloud of slow moving positive ions formed potential on the cathode. Thus the counter is dead for about 300µ sec. after each ionizing event. It is used for X- rays, Y-rays, electrons and ß- particles detection.
b) Scintillation Counter- Certain organic and inorganic materials emit light flashes or scintillation when charged. Flashes or particles, X- rays, Y-rays pass through them. There are three stages in the operation of this counter: absorption, scintillation process, and conversion of light into electronic impulsions i.e. the light flashes from the scintillators fall on a photomultiplier tube and then the signal is amplified. The amplified signal is made to be propotional to the intensity of ionizing radiations.
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