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Apart from making films of the test material, autoradiography can directly be measured by various counters like Geiger-Muller counter and Scintillation Counter. While counting the back ground count should also be considered.
Counter # 1. 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 eg. isopropanol or isobutene or cooking gas (which is a mixture of hydrocarbons).
It has a thin window, usually made of mica at one end to enclose the gas. Depending upon the thickness of the window and the energy of radiation of the radio-isotopes a fraction of the particles emitted enter the counting tube and ionize the gas mixture inside.
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The small current generated is magnified at high voltage, and measured with a scaler 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, etc.
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,
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(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 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 300 µ sec. after each ionizing event. It is used for X-rays, Ƴ-rays, electrons and β-particles detection (Fig. 35.10).
Counter # 2. Scintillation Counter:
Certain organic and inorganic materials emit light flashes or scintillation when charged. Flashes or particles, X-rays, Ƴ-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 proportional to the intensity of ionizing radiations (Fig. 35.11).
