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After reading this article you will learn about:- 1. Meaning of Radioactivity 2. Derivation of Radioactivity 3. Units 4. Measurement.
Meaning of Radioactivity:
Radioactivity is a phenomenon in which nuclei of certain elements undergo spontaneous disintegration with emission of anα or β particles and the formation or synthesis of the nucleus of a new element.
Half-Life Period:
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The time required for a given amount of a radio element to decay to one-half its initial value is called its half-life. Hence, the half-life period T for which n/n0= 1/2 is given by,
T = 2.303/K log 2 = 0.693/K
As for an example, the half-life of radium is 1620 years, that means, one-half of a given amount of radium remains after 1620 years, 1/4 remains after another 1620 years and so on. The shorter the half-life, the greater is the number of atoms that disintegrated.
Derivation of Radioactivity:
The rate of disintegration of radioactive element is directly proportional to the amount present at time, t. So,
Units of Radioactivity:
The curie is the unit for specifying amounts of radioactivity. Rutherford (rd) is also used as a unit of radioactivity.
One rd = 1 × 106 disintegrations per second (d.p.s.)
Therefore, 1 curie = 3.7 × 10 rd.
It is a measure of the emission rate. The biological effect of radiation is measured in rems.
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1 Becquerel = 1 d.p.s.
Measurement of Radioactivity:
1. Ionisation Principle:
Instruments and equipment’s based upon ionisation principle may be classified into two groups as follows:
(i) Pulse counting type where pulses are counted separately (Non-integrative type).
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(ii) The integrating type in which the pulses are not separated but recording device will indicate the average current flow to the chamber.
Subbiah (1993) outlines the basic principles of radiation detection as follows:
Ionisation Chambers:
All counters involving gas ionisation including proportional and Geiger Muller counters are ionisation chambers. In this process there are no secondary ions produced due to acceleration of primary ions under the influence of higher potentials.
The ionisation chamber is essentially a closed one which contains two plates (P), maintained at a potential difference of about 500 volts by means of a battery (B).
The chamber contains gas (e.g. air)—when a particle enters the chamber through a very thin mica window (W), it ionizes the air between the plates. Then a mild current is produced through the movement of ions to the plates and it is used to operate a counting device (C) in the circuit.
However, in this method, a-particles are detected even at as short intervals at 10-4 seconds. But the detection of β-particles may not be possible in this method because of feeble ionising effect as well production of inadequate electric pulse. Therefore, P-particles are generally detected with the help of Geiger Muller Counter.
2. Detection by Geiger Muller Counter:
Geiger Muller Counter is a glass tube containing a hollow metal cylinder (C) of about 8 cm length and 1.5 cm diameter, having a thin wire of steel, aluminium or tungsten (W) along its axis.
The wire and the cylinder are electrically insulated from one another and are part of a circuit containing a battery (B), a resistance of high capacity (D) and an electrical counting device (E). When anα and β particle enters into the tube through a window, it ionises the gas.
The ions formed are so much accelerated in the strong electrical field that they in turn produce further ionisation. The feeble current caused by original ionisation is thus amplified. Then the discharge of the ions between the walls and the central wire causes a flow of current through the circuit which operates the counting device in the Geiger Muller counter.
However, from this method, an electric pulse caused by a single α or β particle may be readily detected by such a device and the number of particles given off by a radio-active source may be easily counted.
3. Solid Scintillation Counters:
The production of light flashes with certain substances termed as fluors (phosphors) is the basis in operating all scintillation detectors. The fluorescent substances consisting of anyone of the number of substances which is appropriately suited to the detection of specific types of radiation.
For α, β, and γ-paiticles, zinc sulphide activated by silver crystals of anthracene or naphthalene containing small amount of anthracene; and crystals of sodium iodide containing a trace of thalium iodide as an activator, Nal respectively are used for their detection in the solid scintillation counters.
4. Liquid Scintillation Counting:
Liquid scintillation counting, discovered by Kallman and Reynolds, 1950, is usually based on the property of some fluorescent substances in aromatic solvents to emit, light flashes due to interaction with radioactive emanations. The radioactive sample is intimately mixed in solution with an organic scintillating matial in an organic solvent.
Thus there is no self-absorption of β or a particles because each radioactive atom or molecule is surrounded by molecules of the scintillator and hence liquid scintillation counting method is the best choice to count β particles of low energy. The liquid scintillation counter can be successfully used for certain high energy β emitters (Emax> 1 MeV) by employing Cerenkov counting techniques.
5. Autoradiography:
Through this process, the phenomenon of radioactivity was discovered by Becquerel (1896). In this process, silver halide is affected by ionising radiations in photographic emulsions. When radioactive material is placed on a photographic plate or film, blackening will be produced on the development of the emulsion.
The blackened area is termed as “a self-portrait” of the activity in the material. The magnitude of blackening at a given place will be a function of the time of exposure and the amount of activity in the sample at that place or position.
This method is particularly suitable for the distribution of radioactive compound in a biological material to be studied. However, care should be taken so that there is no chemical or pressure effects on the emulsion since it may also produce blackening.
Autoradiography may be of macro and micro-techniques with specific advantages and disadvantages for each technique. However, micro auto-radiography is being used to solve problems involving site initiation, duration and rate of synthesis of physiologically active substances namely DNA, RNA, proteins, polysaccharides, lipids etc.