This article throws light upon the seven methods used for detection and measurement of radioactivity. The methods are: 1. Photographic Emulsions 2. Ionization Chamber 3. Proportional Chamber 4. Scintillation Counters 5. Semiconductor Detectors 6. Scaler 7. Pulse Height Discriminators.
Method # 1. Photographic Emulsions:
According to this method an ionizing particle or ray can cause activation and thus the path becomes visible by the subsequent darkening of photographic plate. The darkening of the plat is a measure of the total activity. This method is used to locate the exact distribution of radioactive material in a thin slice of the sample.
Method # 2. Ionization Chamber:
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In the ionization chamber, an electric field is applied between two electrodes separated by a gas. Charged particles passing into the chamber will produce ion-pairs that, in absence of an electric field, will recombine and will not be easily detected.
As the voltage between the electrodes is increased, the electrons will be collected at the anode and positive ions at cathode at low voltage as shown in Fig. 2. in the region up-to Vλ.
At voltages greater than V-1 there is a region of constant pulse height, indicating that all of the ions produced by the radiation are collected before they have a chance to recombine. This is called as the saturated region. The pulse indicates the disintegration. The currents produced in ionization chambers are of the order 10-15 amp.
Method # 3. Proportional Chamber:
As the voltage in ionization chamber is increased beyond V-2 the current increase because the electrons moving through the higher electric field acquire sufficient energy to cause secondary ionization. Thus the original ionization is multiplied and a larger pulse is obtained. The α particles lie in the voltage plateau region, which produce a large number of primary ions shown in fig (3).
In the same region the β particles can also be seen which are 100 times less ionizing. At higher voltage where both α and β -particles will be counted there the multiplication factor is greatest. The recovery time that is dead time between the pulses is very short, i.e., 1µ sec. and the counter is useful for rates up-to 200,000 counts/sec.
Method # 4. Scintillation Counters:
The various types of rays can be counted and measured easily. For counting Beta particles crystal of anthracene release pulses of photons that are detected by an adjacent photo multiplier tube. Liquid solutions of stibene in Xylene are good scintillators for Beta particles.
For the measurement of Gamma rays, large crystals of sodium iodide are preferred shown in Fig. 4. The crystal is activated with 1% of Thallium iodide. Scintillation counters have very high counting efficiencies and a very short dead times (0.26µ sec for NaCl crystal).
Method # 5. Semiconductor Detectors:
An thin layer of lithium vapour is deposited on a crystal of highly purified p-type Germanium. The temperature is increased and some of the Lithium atoms diffuse into the central portion of the crystal. Such a crystal operates as ionization tube.
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A gamma ray passing through the crystal creates pairs that can be collected as a current pulse. A high sensitivity is achieved. These detectors are now widely used for gamma rays and X rays spectrometry. The schematic diagram of Semiconductor detector is shown in Fig. (5).
Method # 6. Scaler:
Radioactive disintegration occurs at a very high rate, so a scaling circuit is used which counts the original pulse. Here n is the number of scaling units or the scaling factor. The number of counts is indicated on a series of glow lamps, with a mechanical counter measuring only the scaled down signals. Electric timers may be incorporated in the scaling unit which automatically start and stop for the counting intervals.
Method # 7. Pulse Height Discriminators:
If the amplitude of the pulse is proportional to the energy of the original ionizing particles, the intensity of the pulse height may be useful in detecting and identifying the nature of the radiation. Only fixed amplitude signals may be easily counted.
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Now a days a multiple channel analyzers are used by which suitable circuitry can sort out the pulses on the basis of size and send each pulse into a particular counting device that counts only those pulses of a given range of energy. Thus the whole spectrum is obtained at once.
The three methods for radiometric analysis are as follows:
1. Nuclear Activation Analysis:
(N.A.A.) is a non-destructive technique. It is a sensitive and is one of the most useful methods of trace analysis. A large number of elements (about 70) are present either singly or in a combination can be analysed by this method at one time.
Principle:
The principle involved in N.A.A. is bombardment of an element with neutrons (emitted by Vande Graff Accelerator). Capture of neutrons enlarges the atomic nuclei (for e.g., Na) according to the following nuclear reactions.
2311Na + 10n →2411 Na + ү
The newly formed isotope of sodium 2311Na is unstable and undergoes spontaneous decomposition accompanied by the emission of ү rays. The radioactive isotopes formed in this way differ widely in half-life values and can be identified by determination of this constant along with other relevant information.
For quantitative work the measurement of radiation is made with a scintillation counter a instrument which counts the intensity of radiations.
Application:
1. N.A.A. is used in environmental research as a minimum quantity of the sample is needed for the treatment, prior to irradiation.
2. This technique is very sensitive and traces of elements present in the sample can be determined easily with great accuracy.
3. It is non-destructive technique.
4. The analysis of trace elements present in the rocks, soils, and sandy materials can be done easily by this method.
5. This method is generally used in utilizing the geographic origin from specimen of pottery.
6. Trace elements present in moon and meteorites have been analysed by this technique.
2. Isotopic Dilution Analysis:
According to this method a pure and radioactive form of the substance is mixed with the sample in known amount. After equilibration, a fraction of component under examination is isolated and its activity is determined for e.g., suppose glycine is mixed with amino acids and we have to determine glycine, this glycine contains an atom of C14 which is found one in every million of its molecule and that can be synthesized easily.
A 0.800 gm portion of this active preparation whose specific activity is 25000 counts per min. per gm. is mixed with unknown substances. Suppose 250 gm of pure glycine having an activity of 1250 counts per min is obtained from mix. The specific activity of pure glycine is 150 counts per 5 min.
The data can be summarized as follows:
m = 0.800 gm,
S0 = 25000 cpm per gm.
A = mS0 = 0.800 × 25000 = 20000 cpm
B = 1250/15 – 150/5
= 83.333 – 30 = 53.33 cpm
W = 0.250
M = WA/B – m/1
= 0.250 × 20000/53.33 – 0.800/1
= 5000/53.33 – 0.800/1
= 93.755859 – 0.800
= 92.955859 gm.
Thus is this way the glycine can be calculated from the sample.
Application:
1. This method is very useful, as through this method we can determine the volume of the blood in the animal or human being. A sample is taken out from the donor and its activity per milli litre of blood is determined and finally sample aliquot is introduced.
After a certain time the sample from recipient is examined for radioactivity. From the dilution of activity it is possible to calculate the volume of blood in the recipient.
2. By this method one can determine the number of amino acids present in the protein hydrolysis.
3. As we know that the solubility of water in benzene or other hydrocarbons is so small that it cannot be determined or measured easily by physical and chemical means, hence this technique is used.
4. This method is used by biologist for determining the elements and compounds of hydrogen, carbon and nitrogen.
5. This method is also used in the analysis of Cobalt in steel and other alloys, by gravimetric method, and in other metallurgical operations.
6. By this method one can determine the number of isotope present in a given sample.
3. Radioactive Analysis:
According to this method a radioactive substance is used indirectly to determine the quantity of an inactive substance. The most important method for determining the substance is radiometric titrations. Thus we can determine the end point easily which can tell the amount of radioactivity carried out.
The amount of radioactivity is proportional to the concentration of reacting species which shows the changes in the radioactivity, e.g., Formation of AgCl by the reaction of radiometric or radioactive AgNO3 with HCl.
Application:
1. The radioactive titrations gives a sharp and accurate end points.
2. In the case of radiometric tritrations weighing is not required and the purified chemicals are not considered very important.
3. Quantities required for estimation are usually very small.
4. These reactions can be carried out in the medium which can be non-aqueous solvents, turbid, coloured, corrosive and heterogeneous.
5. The radioactive C-14 is generally used for the carbon dating and discovering the historical backgrounds of the dead animals and plants. Such an experiment can be carried out during the excavations of the Mohenjodaro and Harappa for determining the years, at which civilization established.
6. Similarly radio-hydrogen in used in radioactive dating in meteorology, hydrology, and also studying and verifying the cosmic rays.