Radioactive materials emit radiations of characteristic wave lengths from the study of which it is possible to identify them and from the measurement of intensity of these radiations the concentration of radio-active material present in the sample can be determined.

Radio-activity is property of the nuclei of an element and hence methods based on induction of radioactivity followed by study and measurement of the induced activity are referred to as Nuclear methods. There are usually two main types of nuclear methods which are usually used in the process of environmental monitoring.

1. Neutron activation analyses.

2 Radioactive tracer technique.

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Neutron Activation Analyses:

Neutron activation analyses is one of the most sensitive technique of trace analyses. The sensitivity can be as high as 10~12 moles per litre. It is a non-destructive method based on measurement of nuclear properties of an element after it has been irradiated with slow neutrons which cause the element to acquire radioactivity. The induced activity is given by the expression

Where N is the number of target nuclei,/is the neutron flux or the number of neutrons per cm2 per second used to irradiate the sample, a is the nuclear cross-section, t is the period of irradiation in seconds and T is the half life in seconds. The number of target nuclei can be expressed as:

The induced isotope emit characteristic radiations and has characteristic mode of decay which permits their qualitative identification. From the measurement of induced activity and the knowledge of neutron flux used, the nuclear cross-section, the duration of bombardment by neutrons and half life of the nuclides the weight of the target nuclei can be calculated. Thus, both qualitative as well quantitative determinations may be carried out with a high degree of precision and accuracy.

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The quantitative estimation can be performed by another method which uses the familier technique of comparision of the unknown sample with a known standard sample. Simultaneosly with the unknown sample another sample containing a known amount of the element to be estimated is bombarded with slow neutrons. Thus, radioactivity is induced in both the known and the unknown sample. From the ratio of measured activities of the standard and the unknown sample the weight of the element present can’b?determined:

Advantages of Neutron Activation Analyses

These are two main advantages of neutron activation analyses. These are:

1. Neutron activation analyses is a simple non-destructive technique of analyses which does not require any separation or reaction steps. The sample is just placed in a beam of slow neutrons for activation which is followed by measurement of induced activity.

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2 It is highly sensitive method which permits quantitative determinations of elements in so small traces which all other methods fail to detect.

Limitations of Neutron activation analyses

Like all other methods, neutron activation analyses has its own disadvantages as well which include:

1. For neutron activation analyses we require reactor facilities which are available only at a few places.

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2 Almost all elements become radioactive to some degree after neutron irradiation. But half life of some of the radioactive nuclides thus formed is so short, a few seconds or a fraction of second that it is practically not possible to remove the sample from the reactor pile and subject it to analyses. For example, He, Li, B, W, O and Ne, produce nuclides which have half lives in seconds. Such elements cannot be put to neutron activation analyses.

3. During activation of the element when it is bombarded with neutrons it is usually subjected to very high temperatures generated in the nuclear reactor. Therefore, the sample should be able to withstand these temperatures. It should be almost invariably enclosed in a suitable container in order to avoid contaminating the reactor.

4. The sample should not have too high neutron absorption capacity during the irradiation. Cadmium, for example, could stop the chain reaction in the same way as control rods of cadmium do in an atomic pile.

5. Sometimes the radioactive nuclides which have to be used as a measure for a particular element may be formed from a number of other elements (transmutation). For example, estimation of 33As75 depends upon the formation of 33As76 by the following reaction: