Neutron Activation Analysis

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Types of NAA

 

INAA – Instrumental neutron activation analysis

This method consists only of irradiation of samples and gamma-ray spectrometry measurement of the activity of the radionuclides formed without any chemical treatment. It is the simplest way of performing NAA, and for many types of matrices also the most frequently employed mode of NAA. INAA can be successfully applied if the activity of radionuclides produced by activation of matrix does not prevent determination of elements of interest. If the above condition is not fulfilled, usually when the trace and ultratrace amounts of elements are to be determined, the activity of matrix radionuclides must be eliminated. This can be performed using chemical processes (radiochemical separation, pre-iradiation separation), using physical means (selective activation, selective counting methods) or by a combination of both possible approaches (Fig.8). [40]

 

Fig.8 Types of NAA.

Fig.8 Types of NAA.

 

Chemical approaches of elimination of interfering radionuclides

 

RNAA – Radiochemical neutron activation analysis

This mode of NAA involves post-irradiation sample decomposition and separation of interfering elements (radionuclides) or of elements of interest using a suitable separation method, such as extraction, ion exchange, precipitation, distillation, etc. Inactive carries of elements can be added prior to the start of chemical processing to guarantee the regular behaviour of elements originally present at trace and ultratrace concentrations. Unlike classical analytical techniques, the yield of chemical separation can be determined using several approaches, such as employing the radiotracer technique, re-activation of the separated fraction with the added carrier (carrier budgeting), or using another analytical technique. There are, however, time limitation for performing radiochemical separation when separation of short-lived radionuclides is involved and there is also a higher radiation burden of personnel compared to INAA. [3, 4, 41]

 

MNAA – Molecular neutron activation analysis

The elements causing interferences in INAA or those to be determined can also be chemically separated prior to irradiation. In this approach, however, one of the main advantages of NAA for trace and ultratrace analysis – the essentially blank-free nature – may be lost. Also, there is a little control over the separation yield. On the other hand, there is also no time limitation for performing chemical separation and the radiation burden of personnel is even lower than in INAA. Another advantage is the possibility to perform speciation analysis. [3, 4]

 

Physical approaches of elimination of interfering radionuclides

 

CNAA – Cyclic neutron activation analysis

This method is useful for optimization of detection limit of element forming short-lived radionuclides (usually with T1/2 < 1 min.). In this mode of NAA the sample is irradiated and measured repeatedly for a short time and the gamma-ray spectra from individual runs are summed. This results in improving the counting statistics of peak areas of short-lived radionuclides. A special automated pneumatic transport system is required for this type of analysis. The repetition can continue till the accumulated activity of long-lived radionuclides is not too high. To avoid the accumulation of the activities of longer-lived radionuclides a series of fresh samples (aliquots) can be used in so-called pseudo-CNAA. [3]

 

PGNAA – Prompt gamma neutron activation analysis

This is a special method, where prompt-gamma radiation (emitted by the compound nucleus) is measured during irradiation in a neutron beam. It is necessary to have special equipment at the reactor – a neutron beam guide and adjacent gamma-ray detector assembly. PGNAA may provide information about contents of several elements which cannot be easily determined or cannot be assayed at all using conventional NAA, such as, H, B, C, N, P, S, Cd, Pb, and some rare earth elements, especially Sm and Gd. Most of these elements cannot easily be determined with normal NAA, so PGNAA is a complementary method. [42, 43]

 

Selective activation

The following methods are used to increase the probability of the required reaction between neutron and target nucleus of analyte (probability of the reaction is dependent on neutron energy) and to limit formation of undesired activities of matrix. [4, 44]

 

TNAA – Thermal neutron activation analysis

The sample is irradiated only by thermal neutrons, which are obtained by interaction of fission neutrons with moderator. It is therefore necessary to have a moderator block at a neutron source (for instance, a graphite block, so-called graphite column in a nuclear reactor). When neutron transits through this block it is decelerated up to the thermal energy and then it is used for activation.

 

ENAA – Epithermal neutron activation analysis

The sample is irradiated by neutrons with energies higher than 0.55 eV (cut-off energy for 1 mm thick Cd shielding). Under these circumstances the nuclei with high resonances for the radiation neutron capture with neutrons of the above energies are preferably activated compared with those, which follow the "1/v law". For this type of activation it is needed to shield off thermal neutrons. This can be achieved using filters made of cadmium or boron.

 

FNAA – Fast neutron activation analysis

This type of activation involves nuclear reaction with fast neutrons, such as (n, p), (n, 2n), (n, α), etc. The fast neutrons can be obtained from a nuclear reactor similarly as epithermal neutrons, i.e., by shielding off thermal neutrons or from other sources (the 14 MeV neutron generator, cyclotron-produced fast neutrons, etc.).

 

Selective counting methods

 

The undesired activities of matrix and gamma-ray spectrometric interferences can also be successfully eliminated using selective counting techniques, such as Compton suppression counting (CSC), coincidence counting, and measurement of delayed neutrons for the determination of fissile elements. [45, 46]

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