**2.1. Basis principles**

Nowadays, there are many elemental analysis methods that use chemical, physical and nu‐ clear characteristics. However, a particular method may be favoured for a specific task, de‐ pending on the purpose. Neutron activation analysis (NAA) is very useful as sensitive analytical technique for performing both qualitative and quantitative multielemental analy‐ sis of major, minor and traces components in variety of terrestrial samples and extra-terres‐ trial materials. In addition, because of its accuracy and reliability, NAA is generally recognized as the "referee method" of choice when new procedures are being developed or when other methods yield results that do not agree. It is usually used as an important refer‐ ence for other analysis methods. Worldwide application of NAA is so widespread it is esti‐

142 Imaging and Radioanalytical Techniques in Interdisciplinary Research - Fundamentals and Cutting Edge Applications

The method is based on conversion of stable atomic nuclei into radioactive nuclei by irra‐ diation with neutrons and subsequent detection of the radiation emitted by the radioac‐ tive nuclei and its identification. The basic essentials required to carry out an analysis of samples by NAA are a source of neutrons, instrumentation suitable for detecting gamma rays, and a detailed knowledge of the reactions that occur when neutrons interact with target nuclei. Brief descriptions of the NAA method, reactor neutron sources, and gamma-

This chapter describes in the first part the basic essentials of the neutron activation analysis such as the principles of the NAA method with reference to neutron induced reactions, neu‐ tron capture cross-sections, production and decay of radioactive isotopes, and nuclear decay and the detection of radiation. In the second part we illustrated the equipment requirements neutron sources followed by a brief description of Es-Salam research reactor, gamma-ray de‐ tectors, and multi-channel analysers. In addition, the preparation of samples for neutron ir‐ radiation, the instrumental neutron activation analysis techniques, calculations, and systematic errors are given below. Some schemes of irradiation facilities, equipment and

Finally, a great attention will be directed towards the most recent applications of the INAA and k0-NAA techniques applied in our laboratory. Examples of such samples, within a se‐ lected group of disciplines are milk, milk formulae and salt (nutrition), human hair and me‐ dicinal seeds (biomedicine), cigarette tobacco (environmental and health related fields) and

All steps of work were performed using NAA facilities while starting with the prepara‐ tion of samples in the laboratory. The activation of samples depends of neutron fluence rate in irradiation channels of the Algerian Es-Salam research reactor. The radioactivity in‐ duced is measured by gamma spectrometers consist of germanium based semiconductor detectors connected to a computer used as a multichannel analyser for spectra evaluation and calculation. Sustainable developments of advanced equipment, facilities and manpow‐ er have been implemented to establish a state of the art measurement capability, to imple‐

mated that approximately 100,000 samples undergo analysis each year.

ray detection are given below.

materials are given as examples in this section.

iron ores (exploration and mining).

ment several applications, etc.

The sequence of events occurring during the most common type of nuclear reaction used for NAA, namely the neutron capture or (n, gamma) reaction, is illustrated in Figure 1. Creation of a compound nucleus forms in an excited state when a neutron interacts with the target nucleus via a non-elastic collision. The excitation energy of the compound nucleus is due to the binding energy of the neutron with the nucleus. The compound nucleus will almost in‐ stantaneously de-excite into a more stable configuration through emission of one or more characteristic prompt gamma rays. In many cases, this new configuration yields a radioac‐ tive nucleus which also de-excites (or decays) by emission of one or more characteristic de‐ layed gamma rays, but at a much lower rate according to the unique half-life of the radioactive nucleus. Depending upon the particular radioactive species, half-lives can range from fractions of a second to several years.

In principle, therefore, with respect to the time of measurement, NAA falls into two catego‐ ries: (1) prompt gamma-ray neutron activation analysis (PGNAA), where measurements take place during irradiation, or (2) delayed gamma-ray neutron activation analysis (DGNAA), where the measurements follow radioactive decay. The latter operational mode is more common; thus, when one mentions NAA it is generally assumed that measurement of the delayed gamma rays is intended. About 70% of the elements have properties suitable for measurement by NAA.

The PGAA technique is generally performed by using a beam of neutrons extracted through a reactor beam port. Fluxes on samples irradiated in beams are in the order of one million times lower than on samples inside a reactor but detectors can be placed very close to the sample compensating for much of the loss in sensitivity due to flux. The PGAA technique is most appli‐ cable to elements with extremely high neutron capture cross-sections (B, Cd, Sm, and Gd); ele‐ ments which decay too rapidly to be measured by DGAA; elements that produce only stable isotopes (e.g. light elements); or elements with weak decay gamma-ray intensities. 2D, 3Danalysis of (main) elements distribution in the samples can be performed by PGAA.

DGNAA (sometimes called conventional NAA) is useful for the vast majority of elements that produce radioactive nuclides. The technique is flexible with respect to time such that the sensitivity for a long-lived radionuclide that suffers from interference by a shorter-lived radionuclide can be improved by waiting for the short-lived radionuclide to decay or quite the contrary, the sensitivity for short-lived isotopes can be improved by reducing the time

**Figure 1.** Diagram illustrating the process of neutron capture by a target nucleus followed by the emission of gamma rays.

irradiation to minimize the interference of long-lived isotopes. This selectivity is a key ad‐ vantage of DGNAA over other analytical methods.

In most cases, the radioactive isotopes decay and emit beta particles accompanied by gam‐ ma quanta of characteristic energies, and the radiation can be used both to identify and ac‐ curately quantify the elements of the sample. Subsequent to irradiation, the samples can be measured instrumentally by a high resolution semiconductor detector, or for better sensitiv‐ ity, chemical separations can also be applied to reduce interferences. The qualitative charac‐ teristics are: the energy of the emitted gamma quanta (Eγ) and the half life of the nuclide (T½). The quantitative characteristic is: the Iγ intensity, which is the number of gamma quan‐ ta of energy Eγ measured per unit time.

The n-gamma reaction is the fundamental reaction for neutron activation analysis. For ex‐ ample, consider the following reaction:

58Fe +<sup>1</sup> n →<sup>59</sup> Fe + Beta- + gamma rays

58Fe is a stable isotope of iron while 59Fe is a radioactive isotope. The gamma rays emitted during the decay of the 59Fe nucleus have energies of 142.4, 1099.2, and 1291.6 KeV, and these gamma ray energies are characteristic for this nuclide (see figure 2) [2]. The probability of a neutron interacting with a nucleus is a function of the neutron energy. This probability is referred to as the capture cross-section, and each nuclide has its own neutron energy-cap‐ ture cross-section relationship. For many nuclides, the capture cross-section is greatest for low energy neutrons (referred to as thermal neutrons). Some nuclides have greater capture cross-sections for higher energy neutrons (epithermal neutrons). For routine neutron activa‐ tion analysis we are generally looking at nuclides that are activated by thermal neutrons.

probability that a reaction will take place, and can be strongly different for different reaction types, elements and energy distributions of the bombarding neutrons. Some nuclei, like 235U are fissionable by neutron capture and the reaction is denoted as (n,f), yielding fission

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**•** Isotopic neutron sources, like 226Ra(Be), 124Sb(Be), 241Am(Be), 252Cf. The neutrons have dif‐ ferent energy distributions with a maximum in the order of 3–4 MeV; the total output is

s -1 GBq-1 or, for 252Cf, 2.2 1012 s-1g-1.

products and fast (highly energetic) neutrons [1].

Neutrons are produced via

**Figure 2.** Decay scheme of 59Fe.

–107

typically 105

The most common reaction occurring in NAA is the (n,γ) reaction, but also reactions such as (n,p), (n,α), (n,n′) and (n,2n) are important. The neutron cross section, σ, is a measure for the Concepts, Instrumentation and Techniques of Neutron Activation Analysis http://dx.doi.org/10.5772/53686 145

**Figure 2.** Decay scheme of 59Fe.

irradiation to minimize the interference of long-lived isotopes. This selectivity is a key ad‐

**Figure 1.** Diagram illustrating the process of neutron capture by a target nucleus followed by the emission of gamma rays.

144 Imaging and Radioanalytical Techniques in Interdisciplinary Research - Fundamentals and Cutting Edge Applications

In most cases, the radioactive isotopes decay and emit beta particles accompanied by gam‐ ma quanta of characteristic energies, and the radiation can be used both to identify and ac‐ curately quantify the elements of the sample. Subsequent to irradiation, the samples can be measured instrumentally by a high resolution semiconductor detector, or for better sensitiv‐ ity, chemical separations can also be applied to reduce interferences. The qualitative charac‐ teristics are: the energy of the emitted gamma quanta (Eγ) and the half life of the nuclide (T½). The quantitative characteristic is: the Iγ intensity, which is the number of gamma quan‐

The n-gamma reaction is the fundamental reaction for neutron activation analysis. For ex‐

58Fe is a stable isotope of iron while 59Fe is a radioactive isotope. The gamma rays emitted during the decay of the 59Fe nucleus have energies of 142.4, 1099.2, and 1291.6 KeV, and these gamma ray energies are characteristic for this nuclide (see figure 2) [2]. The probability of a neutron interacting with a nucleus is a function of the neutron energy. This probability is referred to as the capture cross-section, and each nuclide has its own neutron energy-cap‐ ture cross-section relationship. For many nuclides, the capture cross-section is greatest for low energy neutrons (referred to as thermal neutrons). Some nuclides have greater capture cross-sections for higher energy neutrons (epithermal neutrons). For routine neutron activa‐ tion analysis we are generally looking at nuclides that are activated by thermal neutrons.

The most common reaction occurring in NAA is the (n,γ) reaction, but also reactions such as (n,p), (n,α), (n,n′) and (n,2n) are important. The neutron cross section, σ, is a measure for the

vantage of DGNAA over other analytical methods.

ta of energy Eγ measured per unit time.

ample, consider the following reaction:

+ gamma rays

58Fe +<sup>1</sup> n →<sup>59</sup> Fe + Beta-

probability that a reaction will take place, and can be strongly different for different reaction types, elements and energy distributions of the bombarding neutrons. Some nuclei, like 235U are fissionable by neutron capture and the reaction is denoted as (n,f), yielding fission products and fast (highly energetic) neutrons [1].

#### Neutrons are produced via

**•** Isotopic neutron sources, like 226Ra(Be), 124Sb(Be), 241Am(Be), 252Cf. The neutrons have dif‐ ferent energy distributions with a maximum in the order of 3–4 MeV; the total output is typically 105 –107 s -1 GBq-1 or, for 252Cf, 2.2 1012 s-1g-1.

**•** Particle accelerators or neutron generators. The most common types are based on the ac‐ celeration of deuterium ions towards a target containing either deuterium or tritium, re‐ sulting in the reactions 2H(2H,n)3He and 3H(2H,n)4He, respectively. The first reaction, often denoted as (D,D), yields monoenergetic neutrons of 2.5 MeV and typical outputs in the order of 108 –1010 s−1; the second reaction (D,T) results in monoenergetic neutrons of 14.7 MeV and outputs of 109 –1011 s−1.

and lower reflector. The core is reflected laterally by heavy water maintained in aluminium

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In the majority of INAA procedures thermal reactor neutrons are used for the activation: neutrons in thermal equilibrium with their environment. Sometimes activation with epither‐ mal reactor neutrons (neutrons in the process of slowing down after their formation from fission of 235U) is preferred to enhance the activation of elements with a high ratio of reso‐ nance neutron cross section over thermal neutron cross section relatively to the activation of elements with a lower such a ratio. In principle materials can be activated in any physical state, viz. solid, liquid or gaseous. There is no fundamental necessity to convert solid materi‐ al into a solution prior to activation; INAA is essentially considered to be a non-destructive method although under certain conditions some material damage may occur due to thermal heating, radiolysis and radiation tracks by e.g. fission fragments and α-radiation emitting nuclei. It is essential to have more than two or three qualified full-time member of the staff with responsibility for the NAA facilities. They should be able to control the counting equip‐ ment and have good knowledge of basic principles of the technique. In addition, the facility users and the operators must establish a good channel of communication. Other support staff will be required to maintain and improve the equipment and facility. It seems, there‐

**Step 1:** sample preparation (Figure 3) means in most cases only heating or freeze drying, crush‐ ing or pulverization, fractionating or pelletizing, evaporation or pre-concentration, put through a sieve, homogenising, weighing, washing, check of impurities (blank test), encapsu‐ lation and sealing irradiation vial, as well as the selection of the best analytical process and the preparation of the standards. The laboratory ambiance is also important for preservation and storage of the samples. Standardization is the basis for good accuracy of analytical tools and of‐ ten depends on particular technology, facility and personnel. For production of accurate data, careful attention to all possible errors in preparing single or multi-element standards is impor‐

**Step 2:** irradiation of samples can be taken from the various types of neutron sources ac‐ cording to need and availability. For the INAA, one pneumatic transfer system installed in the horizontal channel at Es-Salam research reactor for short irradiation of samples (Figure 4). In addition, two vertical channels located in different sites of the heavy water moderator and the graphite reflector have been used for long irradiations. The neutron spectrum pa‐ rameters at different irradiation channels such as alpha, f, Tn, etc are experimentally deter‐ mined using cadmium ratio, cadmium cover, bare triple monitor and bi-isotopic methods using HΦgdhal convention and Westcott formalism Table 1 and Table 2. The calibration of the irradiation positions has been carried out to implement the k0-NAA in our laboratory.

tant, and standards must be well chosen depending on the nature of the samples.

tank followed by the graphite.

**2.2. Neutron activation analysis procedure**

fore, a multi-disciplinary team could run the NAA system well.

The analytical procedure is based on four steps:

**•** Nuclear research reactors. The neutron energy distribution depends on design of the reac‐ tor and its irradiation facilities. An example of an energy distribution in a light water moderated reactor is given in Fig. 2.3 from which it can be seen that the major part of the neutrons has a much lower energy distribution that in isotopic sources and neutron gen‐ erators. The neutron output of research reactors is often quoted as neutron fluence rate in an irradiation facility and varies, depending on reactor design and reactor power, be‐ tween 1015 and 1018 m-2 s-1.

Owing to the high neutron flux, experimental nuclear reactors operating in the maximum thermal power region of 100 kW -10 MW with a maximum thermal neutron flux of 1012-1014 neutrons cm-2 s-1 are the most efficient neutron sources for high sensitivity activation analy‐ sis induced by epithermal and thermal neutrons. The reason for the high sensitivity is that the cross section of neutron activation is high in the thermal region for the majority of the elements. There is a wide distribution of neutron energy in a reactor and, therefore, interfer‐ ing reactions must be considered. In order to take these reactions into account, the neutron spectrum in the channels of irradiation should be known exactly. E.g. if thermal neutron ir‐ radiations are required, the most thermalized channels should be chosen.

Although there are several types of neutron sources (reactors, accelerators, and radioisotopic neutron emitters) one can use for NAA, nuclear reactors with their high fluxes of neutrons from uranium fission offer the highest available sensitivities for most elements. Different types of reactors and different positions within a reactor can vary considerably with regard to their neutron energy distributions and fluxes due to the materials used to moderate (or reduce the energies of) the primary fission neutrons. This is further elaborated in the title "Derivation of the measurement equation". In our case, the NAA method is based on the use of neutron flux in several irradiation channels of Es-Salam Research reactor. In 2011, Ha‐ midatou L et Al., reported "Experimental and MCNP calculations of neutron flux parame‐ ters in irradiation channel at Es-Salam reactor" the core modelling to calculate neutron spectra using experimental and MCNP approaches. The Es-Salam reactor was designed for a thermal power output of 15 Mw, with 72 cylindrical cluster fuel elements; each fuel element consists of 12 cylindrical rods of low enriched UO2. In addition the both of fuel throttle tube of the cluster and fuel element tube encloses heavy water as moderator and coolant. The fuel elements are arranged on a heavy water square lattice. The core of the reactor is constituted by a grid containing 72 fuel elements, 12 rods for reactivity control and two experimental channels.

There is also a heavy water in the middle of the core including five experimental channels called inner reflector, In addition, all fuel elements have a reflector at each end called upper and lower reflector. The core is reflected laterally by heavy water maintained in aluminium tank followed by the graphite.
