Radio-active Counters

This slide discusses on Radio activiy counters, its types, applications and limitations.

1. Radio-activity Counters Presented By NIZAM ASHRAF OST-2018-22-15

2. Radioactivity Radioactivity is a phenomenon that occurs naturally in a number of substances. Atoms of the substance spontaneously emit invisible but energetic radiations, which can penetrate materials that are opaque to visible light. The effects of these radiations can be harmful to living cells but, when used in the right way, they have a wide range of beneficial applications, particularly in medicine. Radioactivity has been present in natural materials on the earth since its formation. However, because its radiations cannot be detected by any of the body’s five senses, the phenomenon was only discovered 100 years ago when radiation detectors were developed.

3. Radio-active detectors A wide range of radioactivity detectors are in use today. The main detector is the Geiger–Mueller counter or G-M counter. A Geiger counter is an instrument made of glass or metal tube used for detecting and measuring ionizing radiation. It has a thin window, usually made of mica at one end to enclose the gas. It detects ionizing radiation such as α -particles, β - particles, and gamma rays using the ionization effect produced in a Geiger–Müller tube. In wide and prominent use as a hand-held radiation survey instrument, it is perhaps one of the world's best-known radiation detection instruments. 1. Geiger–Mueller counter

4. Principle of operation A Geiger counter consists of a Geiger–Müller tube (the sensing element which detects the radiation) and the processing electronics, which displays the result. The Geiger–Müller tube is filled with an inert gas such as helium, neon, or argon at low pressure, to which a high voltage is applied. The tube briefly conducts electrical charge when a particle or photon of incident radiation makes the gas conductive by ionization. The ionization is amplified within the tube by the Townsend discharge effect to produce an easily measured detection pulse, which is fed to the processing and display electronics.

5. The electronics also generate a high voltage, typically 400–900 volts, that has to be applied to the Geiger–Müller tube to enable its operation. To stop the discharge in the Geiger–Müller tube a little halogen gas or organic material (alcohol) is added to the gas mixture.

6. The detected radiation can be readout as two: counts or radiation dose. The counts display is the simplest and is the number of ionizing events detected displayed either as a count rate, such as "counts per minute" or "counts per second", or as a total number of counts over a set time period. The counts readout is normally used when alpha or beta particles are being detected. While the radiation dose rate is displayed in a unit such as the sievert which is normally used for measuring gamma or X-ray dose rates. A Geiger–Müller tube can detect the presence of radiation, but not its energy, which influences the radiation's ionizing effect.

7. Limitations There are two main limitations of the Geiger counter. Because the output pulse from a Geiger–Müller tube is always of the same magnitude, the tube cannot differentiate between radiation types. Secondly, the inability to measure high radiation rates due to the "dead time" of the tube. This is an insensitive period after each ionization of the gas during which any further incident radiation will not result in a count, and the indicated rate is, therefore, lower than actual.

8. Types and Application The GM counters can be generally categorized as "end-window", windowless "thin-walled", "thick-walled“ and hybrids of this types. GM counters are mainly employed in the Particle Detection. Gamma and X-Ray Detection. Neutron Detection – (Boron trifluoride or Helium-3) Physical Design.

9. 2. Ionization Chamber The ionization chamber is the simplest of all gas-filled radiation detectors. Widely used for the detection and measurement of certain types of ionizing radiation such as X-rays, gamma rays, and beta particles. Ion chambers have a good uniform response to radiation over a wide range of energies and are the preferred means of measuring high levels of gamma radiation. They are widely used in the nuclear power industry, research labs, radiography, radiobiology, and environmental monitoring.

10. Principle An ionization chamber measures the charge from the number of ion pairs created within a gas caused by incident radiation. It consists of a gas-filled chamber with two electrodes; known as anode and cathode. A voltage potential is applied between the electrodes to create an electric field. When gas between the electrodes is ionized by incident ionizing radiation, ion-pairs are created and the resultant positive ions and dissociated electrons move to the electrodes of the opposite polarity under the influence of the electric field. This generates an ionization current which is measured by an electrometer circuit.

11. Types and Applications The following chamber types are commonly used: Free-air Chamber Vented Chamber Sealed low pressure Chamber High pressure Chamber Parallel-plate Chamber The following are the Applications of Ionization Chamber In Nuclear industry Smoke detectors Medical radiation measurement

12. Counter 3. Scintillation Counter A scintillation counter is an instrument for detecting and measuring ionizing radiation by using the excitation effect of incident radiation on a scintillating material, and detecting the resultant light pulses. It consists of a scintillator which generates photons in response to incident radiation, a sensitive photodetector (usually a photomultiplier tube (PMT), a charge-coupled device (CCD) camera, or a photodiode), which converts the light to an electrical signal and electronics to process this signal.

13. Scintillation counters are widely used in radiation protection, assay of radioactive materials as they can measure both the intensity and the energy of incident radiation. When high energy atomic radiations are incident on a surface coated with some fluorescent material, then flashes of light (scintillations) are produced. The scintillations are detected with the help of a photomultiplier tube, that gives rise to an equivalent electric pulse. Principle

14. Scintillator The Scintillator is made from a single crystal that should have following characteristics: Available in proper form High efficiency Transparent to light Suitable value of refractive index High resolution power Stable under experimental conditions Popular types of crystals used as Scintillators are: Cesium Iodide, Zn Sulphide, Xenon, Organic Phosphors for detection of Gamma rays.

15. Photomultiplier Tube Around 10 dynodes are specifically designed and properly positioned, for automatic focusing of electrons. Each dynode have a particular function: Collection of photoelectrons from previous dynode Emission of low energy electrons

16. Working The radiations are allowed to enter the scintillators through a window of pyrex glass. When high energy radiations strike the crystal, short duration scintillations are emitted. The photoelectrons emitted from cathode are directed towards 1 st dynode that give rise to secondary emission of electrons. The secondary electrons, emitted from the surface of 1 st dynode, get accelerated towards 2 nd dynode. The process repeats and electron get much more multiplied in number. A high energy pulse is delivered to the counting device through the anode. The electric pulse is then delivered to the electronic counting device, through a discriminator.

17. Applications Used in hand held radiation survey meters, personnel and environmental monitoring for radioactive contamination Medical imaging Radiometric assay The ability to accommodate samples of any type, including liquids, solids and gels. The ability to count separately different isotopes in the same sample, which means dual labelling experiments can be carried out. Scintillation counters are highly automated

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