The k 0 -International Scientific Committee was originally mainly involved in the organization of the k 0 -Workshops and the accepting and reviewing of the papers presented there. The mandate of the k 0 -ISC is to promote the development and application of the k 0 -method of neutron activation analysis, k 0 -NAA.
George de Hevesy — received the Nobel Prize for Chemistry in for his work on the use of isotopes as tracers in the study of chemical processes. The Hevesy Medal as illustrated in Figure 2 is the premier international award of excellence honoring outstanding achievements in radioanalytical and nuclear chemistry as illustrated. Table 1 presents the list of the laureates of George de Hevesy award during the period — [ 10 ]. George de Hevesy — who received the Nobel Prize for Chemistry in List of the laureates of George de Hevesy award — [ 10 ].
The development of k0-NAA method is one of the most remarkable advances in the history of neutron activation analysis Table 2. The k 0 -NAA method includes comprehensive and accurate models of the neutron activation, radionuclide decay, and gamma-ray detection processes.
During the period —, Prof F. Currently, this method is an inactive use in numerous laboratories all over the world [ 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 ]. List of the past international conferences on modern trends in activation analysis, MTAA — [ 10 ].
The authors F. De Corte and A. Simonits published in , the recommended nuclear data for use in the k 0 -standardization of neutron activation analysis [ 3 ]. Javimovic et al. More than half of the operational RRs worldwide performing neutron activation analysis distributed over 59 member states [ 31 ].
The highest roles of NAA have been identified as the most suitable opportunity for research, education and training, and commercialization of RR services. For that, NAA groups focused their efforts on the development and modernization of neutron activation analysis process including irradiation devices, gamma-ray spectrometers, and data analyzing instruments [ 31 , 32 , 33 , 34 , 35 ].
Interesting paper published by P. Bode, the opportunities for innovation in NAA gives an overview by focused position-sensitive detection of elements in large samples, Monte-Carlo calculations replacing the use of standards, use of scintillator detectors, and new deconvolution techniques for increasing the sensitivity are examples of challenging new roads in NAA [ 30 ].
Menezes et al. Neutron activation analysis has traditionally been used mainly for the characterization of geological, environmental, and biological materials. However, other analysis techniques have emerged to replace NAA for many of those applications, and NAA now thrives mainly because of its unique advantage, the high penetrating power of neutrons and gamma rays, leading to ease of use in many instances where no sample preparation is required.
In addition, NAA practitioners have innovated to provide fast, accurate, and reliable analyses of various matrices posing great difficulties for other techniques. Some of the applications made possible by these innovations are listed here. In the medical field, it has often been hypothesized that a lack of selenium in the body may lead to increased risk for certain cancers. Several NAA laboratories [ 37 , 38 , 39 ] have measured Se in toenails using the very short-lived Sem, half-life This required the development of fast pneumatic sample transfer systems with accurate control of irradiation and decay times as well as accurate correction of counting losses when the count-rate is changing during the counting period.
The results of studies [ 37 , 38 , 39 ] using thousands of samples sometimes revealed an association between lower Se levels and increased cancer risk, while in others, no significant difference in Se level was observed between the controls and subjects who developed certain forms of cancer.
A knowledge of the levels of trace elements in plastics may be useful from several points of view, as it may reveal the presence of toxic elements such as Cd [ 36 ], or information on the production process through the presence of catalyst residues, it may permit differentiating new plastics from those containing recycled material with flame retardants Br and Sb, and it may help decide the suitability of plastics for insulation of high-voltage electrical cables [ 37 ] or cables used in the nuclear industry [ 38 ].
Plastic samples of mg mass are appropriate for some applications but several NAA laboratories have innovated to be able to analyze routinely and quickly samples up to 4 g mass which, for certain trace elements, are more representative of the original material, and in special cases, even larger samples may be analyzed using large-sample NAA. The problem of the growth of fungus or mold on wood or paper products used in humid environments, wood for windows, cardboard on wallboard used in basements, and wrappers for bars of soap stored in bathrooms, has been remediated by the application of fungicides containing heavy elements like copper and iodine.
Regular quality control measurements are needed to ensure that the right amount of fungicide has been applied. NAA is ideally suited for this as there is no sample preparation and methods have been developed [ 39 ] for large representative samples, at least 6 cm 2 for wood samples cut from the surface of the board and 60 cm 2 for paper, and the NAA results are independent of penetration depth.
To successfully provide a fast and reliable service for industry, the reactor and staff must be available when needed; the service must be given high priority.
They should be able to control the counting equipment 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, therefore, a multi-disciplinary team could run the NAA system well. Step 1: sample preparation Figure 3 means in most cases only heating or freeze drying, crushing or pulverization, fractionating or pelletizing, evaporation or pre-concentration, put through a sieve, homogenising, weighing, washing, check of impurities blank test , encapsulation 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 often 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 important, and standards must be well chosen depending on the nature of the samples.
Some instruments and materials used for the sample preparation. Step 2: irradiation of samples can be taken from the various types of neutron sources according 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 calibration of the irradiation positions has been carried out to implement the k 0 -NAA in our laboratory. Step 3: after the irradiation the measurement is performed after a suitable cooling time t c. In NAA, nearly exclusively the energy of the gamma radiation is measured because of its higher penetrating power of this type of radiation, and the selectivity that can be obtained from distinct energies of the photons - differently from beta radiation which is a continuous energy distribution.
The interaction of gamma- and X-radiation with matter results, among others, in ionization processes and subsequent generation of electrical signals currents that can be detected and recorded. Pneumatic system for short irradiations using a thermal neutron flux at Es-Salam research reactor.
Although HPGe detectors come in many different shapes and sizes, the most common shape is coaxial. These detectors are very useful for measurement of gamma rays with energies in the range from about 60 keV to 3.
The two most important characteristics a HPGe detector are its resolution and efficiency. Other characteristics to consider are peak shape, peak-to-Compton ratio, pulse rise time, crystal dimensions or shape, and price.
For most NAA applications, a detector with 0. Detector efficiency for a given detector depends on gamma-ray energy and the sample and detector geometry, i. Of course, a larger volume detector will have a higher efficiency.
A radiation detector therefore consists of an absorbing material in which at least part of the radiation energy is converted into detectable products, and a system for the detection of these products. Figure 5 illustrates Gamma-ray spectroscopy systems. The detectors are kept at liquid nitrogen temperatures dewers under cave. The boxes in the left and in the right of the computer are the Lynx Digital Spectrometer Processing.
Step 5: Measurement, evaluation and calculation involve taking the gamma spectra and the calculating trace element concentrations of the sample and preparation of the NAA report. In this part of work, Peter bode describes clearly in his paper [ 1 ] the analysis procedure of gamma-spectrum to the determination of the amount of element in sample. The acquisition of gamma spectrum Fig. The spectrum analysis starts with the determination of the location of the centroids of the peaks.
Secondly, the peaks are fitted to obtain their precise positions and net peak areas. Gamma-ray spectrum showing several short-lived elements measured in a CRM-DSD standard irradiated at Es-salam research reactor for 30 seconds, decayed for The positions — often expressed as channel numbers of the memory of a multi-channel pulse height analyzer — can be converted into the energies of the radiation emitted; this is the basis for the identification of the radioactive nuclei.
On basis of knowledge of possible nuclear reactions upon neutron activation, the stable element composition is derived. The values of the net peak areas can be used to calculate the amounts of radioactivity of the radionuclides using the full energy photopeak efficiency of the detector. The amounts mass of the elements may then be determined if the neutron fluence rate and cross sections are known.
In the practice, however, the masses of the elements are determined from the net peak areas by comparison with the induced radioactivity of the same neutron activation produced radionuclides from known amounts of the element of interest. The combination of energy of emitted radiation, relative intensities if photons of different energies are emitted and the half life of the radionuclide is unique for each radionuclide, and forms the basis of the qualitative information in NAA.
The amount of the radiation is directly proportional to the number of radioactive nuclei produced and decaying , and thus with the number of nuclei of the stable isotope that underwent the nuclear reaction.
It provides the quantitative information in NAA. Gamma-ray spectrum a from 0 to keV, b from to keV and c from to keV: showing medium- and long-lived elements measured in a sample of CRM-GSD standard irradiated at Es-salam research reactor for 4 hours, decayed for 5 days, and counted for 90 minutes on a HPGe detector.
The measured in NAA — the quantity intended to be measured — is the total mass of a given element in a test portion of a sample of a given matrix in all physico-chemical states.
The measurement results in the number of counts in a given period of time, from which the disintegration rate and the number of disintegrating nuclei is calculated; the latter number is directly proportional to the number of nuclei of the stable isotope subject to the nuclear reaction, and thus to the number of nuclei of the element, which finally provides information on the mass and amount of substance of that element see Eq.
An example of typical ranges of experimental conditions is given in Table 3 [1]. In practice, our laboratory proceeds in the treatment of spectra and calculation of elemental concentrations of analyzed samples according the approach illustrated in figure 8.
In Eq. In short, these characteristics are:. In nuclear research reactors — which are intense sources of neutrons — three types of neutrons can be distinguished. The neutron flux distribution can be divided into three components see Figure 9 :. Fission or fast neutrons released in the fission of U. These neutrons are slowed down by interaction with a moderator, e. H2O, to enhance the probability of them causing a fission chain reaction in the U. The epithermal neutron component consists of neutrons energies from 0.
A cadmium foil 1 mm thick absorbs all thermal neutrons but will allow epithermal and fast neutrons above 0. The thermal neutron component consists of low-energy neutrons energies below 0. At room temperature, the energy spectrum of thermal neutrons is best described by a Maxwell-Boltzmann distribution with a mean energy of 0. A typical reactor neutron energy spectrum showing the various components used to describe the neutron energy regions. Relation between neutron cross section and neutron energy for major actinides n, capture.
The disintegration rate of the produced radionuclide at the end of the irradiation time ti follows from:. The dependence of the activation cross section and neutron fluence rate to the neutron energy can be taken into account in Eq.
The integral in Eq. The second term is re-formulated in terms of neutron energy rather than neutron velocity and the infinite dilution resonance integral I 0 — which effectively is also a cross section m 2 — is introduced:. It simplifies the Eq. This reaction rate applies to infinite thin objects. In objects of defined dimensions, the inside part will experience a lower neutron fluence rate than the outside part because neutrons are removed by absorption.
The nuclear transformations are established by measurement of the number of nuclear decays. The number of activated nuclei N t i ,t d present at the start of the measurement is given by:. Additional correction resulting from high counting rates may be necessary depending upon the gamma-ray spectrometer hardware used as illustrated in chapter 2 [1]. I is the gamma-ray abundance, i. Although the photons emitted have energies ranging from tens of keVs to MeVs and have high penetrating powers, they still can be absorbed or scattered in the sample itself depending on the sample size, composition and photon energy.
This effect is called gamma-ray self-attenuation. Also, two or more photons may be detected simultaneously within the time resolution of the detector; this effect is called summation. Standardization is based on the determination of the proportionality factors F that relate the net peak areas in the gamma-ray spectrum to the amounts of the elements present in the sample under given experimental conditions:.
Since the various parameters were often achieved via independent methods, their individual uncertainties will add up in the combined uncertainty of measurement of the elemental amounts, leading to a relatively large combined standard uncertainty. Moreover, the metrological traceability of the values of the physical constants is not known for all radionuclides. The unknown sample is irradiated together with a calibrator containing a known amount of the element s of interest. The calibrator is measured under the same conditions as the sample sample-to-detector distance, equivalent sample size and if possible equivalent in composition.
From comparison of the net peak areas in the two measured spectra the mass of the element of interest can be calculated:. In this procedure many of the experimental parameters - such as neutron fluence rate, cross section and photopeak efficiency cancel out at the calculation of the mass and the remaining parameters are all known. This calibration procedure is used if the highest degree of accuracy is required.
The relative calibration on basis of element calibrators is not immediately suitable for laboratories aiming at the full multi-element powers of INAA. It takes considerable effort to prepare multi-element calibrators for all 70 elements measurable via NAA with adequate degree of accuracy in a volume closely matching the size and the shape of the samples. Single comparator method Multi-element INAA on basis of the relative calibration method is feasible when performed according to the principles of the single comparator method.
Assuming stability in time of all relevant experimental conditions, calibrators for all elements are co-irradiated each in turn with the chosen single comparator element.
Once the sensitivity for all elements relative to the comparator element has been determined expressed as the so-called k-factor, see below , only the comparator element has to be used in routine measurements instead of individual calibrators for each element.
The single comparator method for multi-element INAA was based on the ratio of proportionality factors of the element of interest and of the comparator element after correction for saturation, decay, counting and sample weights defined the k-factor for each element i as:. Masses for each element i then can be calculated from these k i factors; for an element determined via a directly produced radionuclide the mass m x unk follows from:.
These experimentally determined k-factors are often more accurate than when calculated on basis of literature data as in the absolute calibration method. However, the k-factors are only valid for a specific detector, a specific counting geometry and irradiation facility, and remain valid only as long as the neutron fluence rate parameters of the irradiation facility remain stable. The single comparator method requires laborious calibrations in advance, and finally yield relatively compared to the direct comparator method higher uncertainties of the measured values.
Moreover, it requires experimental determination of the photopeak efficiencies of the detector. Metrological traceability of the measured values to the S. The k 0 -based neutron activation analysis k 0 -NAA technique, developed in s, is being increasingly used for multielement analysis in a variety of matrices using reactor neutrons [ 4 - 10 ].
In the k 0-based neutron activation analysis the evaluation of the analytical result is based on the so-called k 0 - factors that are associated with each gamma-line in the gamma-spectrum of the activated sample. These factors replace nuclear constants, such as cross sections and gamma-emission probabilities, and are determined in specialized NAA laboratories. This technique has been reported to be flexible with respect to changes in irradiation and measuring conditions, to be simpler than the relative comparator technique in terms of experiments but involves more complex formulae and calculations, and to eliminate the need for using multielement standards.
The parameters from 1 to 4 are dependent on each irradiation facility and the parameter 5 is dependent on each counting facility. The neutron field in a nuclear reactor contains an epithermal component that contributes to the sample neutron activation [ 12 ].
These two formalisms should be taken into account in order to preserve the accuracy of k 0 -method. The k 0 -NAA method is at present capable of tackling a large variety of analytical problems when it comes to the multi-element determination in many practical samples.
During the three last decades Frans de Corte and his co-workers focused their investigations to develop a method based on co-irradiation of a sample and a neutron flux monitor, such as gold and the use of a composite nuclear constant called k 0 -factor [ 3 , 16 ]. In addition, this method allows to analyze the sample without use the reference standard like INAA method. The k-factors have been defined as independent of neutron fluence rate parameters as well as of spectrometer characteristics.
Compared with relative method k 0 -NAA is experimentally simpler it eliminates the need for multi-element standards [ 3 , 18 ], but requires more complicated calculations [ 19 ].
The k 0 -method requires tedious characterizations of the irradiation and measurement conditions and results, like the single comparator method, in relatively high uncertainties of the measured values of the masses. Moreover, metrological traceability of the currently existing k 0 values and associated parameters to the S. Summarizing, relative calibration by the direct comparator method renders the lowest uncertainties of the measured values whereas metrological traceability of these values to the S.
As such, this approach is often preferred from a metrological viewpoint. The concentration of an element can be determined as:. Where: the indices x and Au refer to the sample and the monitor, respectively; W Au and W x represent the mass of the gold monitor and the sample in g ; N p is the measured peak area, corrected for dead time and true coincidence; S, D, C are the saturation, decay and counting factors, respectively; tm is the measuring time; G th and G e are the correction factors for thermal and epithermal neutron self shielding, respectively.
Many publications reported in literature [ 20 - 25 ] treat the concept of evaluation of uncertainties in large range of analytical techniques. We can give in this part of chapter, the evaluation of uncertainties for neutron activation analysis measurements. Among the techniques of standardization the comparator method for which the individual uncertainty components associated with measurements made with neutron activation analysis NAA using the comparator method of standardization calibration , as well as methods to evaluate each one of these uncertainty components [ 1 ].
This description assumes basic knowledge of the NAA method, and that experimental parameters including sample and standard masses, as well as activation, decay, and counting times have been optimized for each measurement. It also assumes that the neutron irradiation facilities and gamma-ray spectrometry systems have been characterized and optimized appropriately, and that the choice of irradiation facility and detection system is appropriate for the measurement performed.
Careful and thoughtful experimental design is often the best means of reducing uncertainties. The comparator method involves irradiating and counting a known amount of each element under investigation using the same or very similar conditions as used for the unknown samples. The measurement equation can be further simplified, by substituting:.
Note that the R values are normally very close to unity, and all units are either SI-based or dimensionless ratios. Thus an uncertainty budget can be developed using only SI units and dimensionless ratios for an NAA measurement by evaluating the uncertainties for each of the terms in Eqs.
Uncertainties for some of the terms in Eq. If we sub-divide the uncertainty for each term in the above equations into individual components, add terms for potential corrections, and separate into the four stages of the measurement process, including: pre-irradiation sample preparation ; irradiation; post-irradiation gamma-ray spectrometry , and radiochemistry, we arrive at the complete list of individual uncertainty components for NAA listed below in Table 4.
Only uncertainties from the first three stages should be considered for instrumental neutron activation analysis INAA measurements, while all four stages should be considered for radiochemical neutron activation analysis RNAA measurements.
More details are given in chapter 2 of reference [ 1 ] for each subsection of uncertainty component. Complete list of individual uncertainty components for NAA measurements using the comparator method of standardization; line numbers in this table represent subsections. The detection limit represents the ability of a given NAA procedure to determine the minimum amounts of an element reliably. The detection limit depends on the irradiation, the decay and the counting conditions.
It also depends on the interference situation including such things as the ambient background, the Compton continuum from higher energy-rays, as well as any-ray spectrum interferences from such factors as the blank from pre-irradiation treatment and from packing materials. The detection limit is often calculated using Currie's formula:.
This relation is valid only when the gamma-ray background counting statistical error is the major interference. The amount of material to be irradiated and to be counted. This is often set by availability, sample encapsulation aspects and safety limits both related to irradiation irradiation containers and counting e.
For these reasons practically the sample mass is often limited to approximately mg. The duration of the irradiation time. This is set by practical aspects, such as the limitations in total irradiation dose of the plastic containers because of radiation damage. The total induced radioactivity that can be measured is set by the state-of-the-art of counting and signal processing equipment, with additional radiation dose and shielding considerations.
As an example, the maximum activity at the moment of counting may have to be limited to approximately kBq. 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 applicable to elements with extremely high neutron capture cross-sections B, Cd, Sm, and Gd ; elements which decay too rapidly to be measured by DGAA ; elements that produce only stable isotopes e.
The technique is flexible with respect to time such that the sensitivity for a long-lived radionuclide that suffers from an 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 irradiation to minimize the interference of long-lived isotopes.
Healthy diet? Click on the image to know how INAA was used to quantify selenium in cereal crops. With the use of automated sample handling e. The application of purely instrumental procedures is commonly called instrumental neutron activation analysis INAA and is one of NAA 's most important advantages over other analytical techniques, especially in the multi-element analysis.
If chemical separations are done to samples after irradiation to remove interferences or to concentrate the radioisotope of interest, the technique is called radiochemical neutron activation analysis RNAA.
The latter technique is performed infrequently due to its high labor cost. Muon research Muons, multi-faceted elementary particles - find out more here. News and media News from NMI3 and the neutron and muon worlds.
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