Irradiation, neutron activation analysis

Neutron Activation Analysis Few samples of interest are naturally radioactive. For many elements, however, radioactivity may be induced by irradiating the sample with neutrons in a process called neutron activation analysis (NAA). The radioactive element formed by neutron activation decays to a stable isotope by emitting gamma rays and, if necessary, other nuclear particles. The rate of gamma-ray emission is proportional to the analyte s initial concentration in the sample. For example, when a sample containing nonradioactive 13AI is placed in a nuclear reactor and irradiated with neutrons, the following nuclear reaction results. 

The concentration of Mn in steel can be determined by a neutron activation analysis using the method of external standards. A 1.000-g sample of an unknown steel sample and a 0.950-g sample of a standard steel known to contain 0.463% w/w Mn, are irradiated with neutrons in a nuclear reactor for 10 h. After a 40-min cooling period, the activities for gamma-ray emission were found to be 2542 cpm (counts per minute) for the unknown and 1984 cpm for the standard. What is the %w/w Mn in the unknown steel sample ... 

The HFBR at Brookhaven National Laboratory is a heavy water moderated and cooled reactor designed to provide an intense beam of neutrons to the experimental area. In addition using thimbles i oiitaincd within the vessel, it provides isotopic production, neutron activation analysis, ami muiemi irradiations. It began operation in 1965 at a power of 40 MW to be upgraded to 60 MW m 19S2. 

This section will deal briefly with some aspects of expls safety peculiar to neutron activation analysis expts. We are concerned here with a) the possible effect of the ionizing radiation dose on the energetic material which will cause it to be more sensitive or hazardous to normal handling as an expl, and b) the potential direct expl hazards involved in the physical and mechanical transportation of samples to and horn the irradiation source and in a nuclear counting system... 

The apphed pretreatment techniques were digestion with a combination of acids in the pressurized or atmospheric mode, programmed dry ashing, microwave digestion and irradiation with thermal neutrons. The analytical methods of final determination, at least four different for each element, covered all modern plasma techniques, various AAS modes, voltammetry, instrumental and radiochemical neutron activation analysis and isotope dilution MS. Each participating laboratory was requested to make a minimum of five independent rephcate determinations of each element on at least two different bottles on different days. Moreover, a series of different steps was undertaken in order to ensure that no substantial systematic errors were left undetected. 

Byrne AR, and Kucera J (1991) Radiochemical neutron activation analysis of traces of vanadium in biological samples A comparison of prior dry ashing with post-irradiation wet ashing. Fresenius f Anal Chem 340 48-52. 

Principles and Characteristics In neutron activation analysis (NAA) the sample is irradiated by neutrons. The principal reaction in NAA is ... 

Neutron activation analysis (NAA) is a technique for the qualitative and/or quantitative determination of atoms possessing certain types of nuclei. Bombarding a sample with neutrons transforms some stable isotopes into radioactive isotopes measuring the energy and/or intensity of the gamma rays emitted from the radioactive isotopes created as a result of the irradiation reveals information on the nature of the elements in the sample. NAA Is widely used to characterize such archaeological materials as pottery, obsidian, chert, basalt, and limestone (Keisch 2003). 

Cerium was included in a list of 14 elements determined by Lee et al. [627] in seawater using neutron activation analysis. The metals were first precon-centraed on a mixture of Chelex 100 and glass powder. The elements were desorbed from the column by 4 M nitric acid, and aqueous solution was irradiated for 3 days and subjected to y-ray spectrometry method with a Ge(Ii) detector coupled to a 4000-channel analyser. Cerium was found to be present to the extent of 16.7 xg/l in water taken from the Kwangyang Bay (South Korea). 

Holzbecker and Ryan [825] determined these elements in seawater by neutron activation analysis after coprecipitation with lead phosphate. Lead phosphate gives no intense activities on irradiation, so it is a suitable matrix for trace metal determinations by neutron activation analysis. Precipitation of lead phosphate also brings down quantitatively the insoluble phosphates of silver (I), cadmium (II), chromium (III), copper (II), manganese (II), thorium (IV), uranium (VI), and zirconium (IV). Detection limits for each of these are given, and thorium and uranium determinations are described in detail. Gamma activity from 204Pb makes a useful internal standard to correct for geometry differences between samples, which for the lowest detection limits are counted close to the detector. 

The y particle is emitted virtually instantaneously on the capture of the neutron, and is known as a prompt y - it can be used analytically, in a technique known as prompt gamma neutron activation analysis (PGNAA), but only if such y s can be measured in the reactor during irradiation. Under the conditions normally used it would be lost within the nuclear reactor. In this reaction, no other prompt particle is emitted. The isotope of sodium formed (24Na) is radioactively unstable, decaying by beta emission to the element magnesium (the product nucleus in Figure 2.13), as follows ... 

The number of protons is unique to the element but most elements can exist with two or more different numbers of neutrons in their nucleus, giving rise to different isotopes of the same element. Some isotopes are stable, but some (numerically the majority) have nuclei which change spontaneously - that is, they are radioactive. Following the discovery of naturally radioactive isotopes around 1900 (see Section 10.3) it was soon found that many elements could be artificially induced to become radioactive by irradiating with neutrons (activation analysis). This observation led to the development of a precise and sensitive method for chemical analysis. 

A technique for the determination of Tc amounts as little as 4 x 10 g by neutron activation analysis has been described by Foti et al. . Tc in triply distilled water is irradiated in a thermal neutron flux of 5 x 10 neutrons per cm and per second to produce °°Tc. Other radionuclides are removed by co-precipi-tation with Fe(OH)j. Then, °°Tc is co-precipitated twice with tetraphenylarsonium perrhenate which can be removed by sublimation. The chemical purification of °°Tc requires 40-45 s and the technetium yield is about 53%. 

Trace amounts of Tc are also determined in filter paper and vegetable samples by neutron activation analysis The procedure consists of the following major steps separation of technetium from the sample, thermal neutron irradiation of the Tc fraction to produce °°Tc, post-irradiation separation and purification of °°Tc from other activated nuclides, and counting of the 16 s Tc in a low-background P counter. The estimated detection limits for Tc in this procedure are 5 x 10 g in filter paper and 9 x 10 g in vegetable samples. 

With analytical methods such as x-ray fluorescence (XRF), proton-induced x-ray emission (PIXE), and instrumental neutron activation analysis (INAA), many metals can be simultaneously analyzed without destroying the sample matrix. Of these, XRF and PEXE have good sensitivity and are frequently used to analyze nickel in environmental samples containing low levels of nickel such as rain, snow, and air (Hansson et al. 1988 Landsberger et al. 1983 Schroeder et al. 1987 Wiersema et al. 1984). The Texas Air Control Board, which uses XRF in its network of air monitors, reported a mean minimum detectable value of 6 ng nickel/m (Wiersema et al. 1984). A detection limit of 30 ng/L was obtained using PIXE with a nonselective preconcentration step (Hansson et al. 1988). In these techniques, the sample (e.g., air particulates collected on a filter) is irradiated with a source of x-ray photons or protons. The excited atoms emit their own characteristic energy spectrum, which is detected with an x-ray detector and multichannel analyzer. INAA and neutron activation analysis (NAA) with prior nickel separation and concentration have poor sensitivity and are rarely used (Schroeder et al. 1987 Stoeppler 1984). 

All impactor and filter samples were analyzed for up to 45 elements by instrumental neutron activation analysis (INAA) as described by Heft ( ). Samples were irradiated simultaneously with standard flux monitors in the 3-MW Livermore pool reactor. The x-ray spectra of the radioactive species were taken with large-volume, high-resolution Ge(Li) spectrometer systems. The spectral data were transferred to a GDC 7600 computer and analyzed with the GAMANAL code (1 ), which incorporates a background-smoothing routine and fits the peaks with Gaussian and exponential functions. 

The concentration of silver nanoparticles and ions in solntions was determined by neutron activation analysis [15]. Samples were irradiated in the nuclear reactor at the Institute of Nuclear Physics, Tashkent, Uzbekistan. The product of nuclear reaction ° Ag(n,y)" Ag has the half-life Tj j=253 days. The silver concentration was determined by measnring the intensity of gamma radiation with the energy of 0.657 MeV and 0.884 MeV emitted by "" Ag. A Ge(Li) detector with a resolution of about 1.9 keV at 1.33 MeV and a 6,144-channel analyzer were used for recording gamma-ray quanta. 

During the late 1960s and early 1970s, neutron activation analysis provided a new way to measure bulk chemical composition. Neutron activation analysis utilizes (n,y) reactions to identify elements. A sample is placed in a nuclear reactor where thermal neutrons are captured by atoms in the sample and become radioactive. When they decay, the radioactive isotopes emit characteristic y-rays that are measured to determine abundances. Approximately 35 elements are routinely measured by neutron activation analysis. A number of others produce radioactive isotopes that emit y-rays, but their half-lives are too short to be useful. Unfortunately, silicon is one of these elements. Other elements do not produce y-ray-emitting isotopes when irradiated with neutrons. There are two methods of using neutron activation to determine bulk compositions, instrumental neutron activation analysis (INAA) and radiochemical neutron activation analysis (RNAA). 

Using the Chart of the Nuclides as a guide, estimate the sensitivity (minimum quantity that can be detected) of neutron activation analysis for europium using a thermal neutron flux of 3 x 1012 n/cm2-s. Assume no irradiation may last more than 1 h and the minimum detectable activity is 10 dpm. 

Instrumental neutron activation analysis was conducted at the University of Missouri Research Reactor (MURR). Samples of approximately 50-100 mg were subjected to long and short irradiations using the same methodology used on pottery and other materials with appropriate reference standards (23). The analysis provided abundances of 33 elements for 72 samples. 

Scott WL Jr. 1967. Silver uptake in brains of chronically gamma-irradiated rats A study by neutron activation analysis. Radiat Res 31 522-528. 

Neutron Activation Spectrometry. Another instrumental technique which has applicability to a wide range of elements is neutron activation analysis. In this method the sample (which could be orange juice without any prior sample treatment) is irradiated with a strong neutron flux. The elements of analytical interest are thus converted to unstable isotopes which decay with characteristic energies and thus measurement of the intensities results in analytical values for the elements of interest. There are some serious drawbacks to this method, however. The matrix can cause severe background effects especially when the sample contains large amounts of an element, like potassium, which is the situation with orange juice. In this event tedious chemical separations must be carried out to achieve adequate selectivity, accuracy... 

Taylor, D. Neutron Irradiation and Activation Analysis. London Newnes 1964. 

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