Activation analysis, RNAA

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). 

Destructive method (radiochemical neutron activation analysis, RNAA) based on the chemical separation of radioelements into fractions, each of which contains some radionuclides [17-20]. 

In radiochemical neutron activation analysis (RNAA), separations are applied in instrumental neutron activation analysis (INAA) no chemistry is involved - samples are irradiated and counted over a period of time to obtain the desired information about the elements of interest. For most elements, spectral interference-free gamma rays are available and where there are two isotopic species at the same energy, their contribution can generally be computed. 

After the cooling period, the sample is either counted directly or some chemical manipulation is performed before counting. The first procedure is known as instrumental neutron activation analysis (INAA), whereas the latter is referred to as radiochemical neutron activation analysis (RNAA). In RNAA a stable carrier for the element to be determined may be added to the sample after irradiation. The carrier is equilibrated with the element in the sample (often by fusing it with Na202, or treating it with strong acid). Then the element of interest is separated along with the carrier. The chemical yield of the separation is determined from the amount of carrier recovered, and this correction is applied to the measured activity. 

Neutron activation analysis is a sensitive and versatile method of rock analysis, chiefly applicable to trace elements and capable of determining a large number of elements simultaneously without necessarily destroying the sample. There are two approaches. Instrumental neutron activation analysis (INAA) employs a powdered rock or mineral sample radiochemical neutron activation analysis (RNAA) involves the chemical separation of selected elements. The range of elements analysed is given in Table 1.5 and the methods are described in detail by Muecke (1980). 

To derive actual levels and establish quality measurement for trace elements in foodstuffs, materials such as milk powder (A-11) and representative regional dietary blends (eg, USDIET-1) have been provided (8.9 ). Consistency of interlaboratory results among "independent" analytical procedures makes possible the certification of such materials or the establishment of dietary intakes. The nature of the initial measurement problem is illustrated by the results of the milk powder (A-11) intercomparlson, where the means, relative ranges (max/min, dimensionless) and medians of 17 atomic absorption spectroscopy [AAS] and 7 radiochemical neutron activation analysis [RNAA] laboratory results for manganese were as follows. 

Electrothermal vaporization isotopa dilution inductively coupled plasma mass spectrometry (ETV-ID-ICP-MS) has been utilized for the analysis of cadmium in fish samples. Radiochemical neutron activation analysis (RNAA), differential pulse anodic stripping voltametry (ASV) and the calorimetric dithizone method may also be employed. The AAS techniques appear to be most sensitive, with cadmium recoveries ranging from 94 to 109% (Koplan, 1999). 

The second purpose is to extend the scope of the radiochemical neutron activation analysis (RNAA) method. Preirradiation of inorganic samples to induce fission products prior to oxygen flask ignition and solvent extraction can be used to distinguish ionic and nonionic form. The use of double irradiation in RNAA allows the determination of nuclides with short, medium, and long lives in a single experiment. This enables simultaneous determination of 13 1, and " Hg. 

The nondestructive instrumental (INAA) and destructive radiochemical (radiochemical neutron activation analysis — RNAA) procedures of this method are discussed later in this chapter. Specific problems, such as chain decays, cyclic activation, and interferences, as well as typical appKcations have recently been discussed by Alfassi (2001) and the references therein. 

Accelerator mass spectrometry (AMS) is useful to measure extremely low-abundance nuclides (isotope ratio of 10 to 10 relative to its stable isotope), such as Be, C, A1, C1, " Ca, and I, in natural samples. Small amounts of C and T can be measured by AMS on mg size samples of carbon and iodine extracted from 500-ml seawater samples (Povinec et al. 2000). Neutron activation analysis (NAA), radiochemical neutron activation analysis (RNAA), and inductively coupled plasma mass spectrometry (ICP-MS) are useful for the determination of ultra-trace Th and U in geological and cosmochemical samples, and for determination of the concentration of Pu and Pu. Reference marine-biological samples are necessary to test the performance of the analytical methods employed in surveying and monitoring radioactive materials in the sea. An ocean shellfish composite material containing 0.1% w/w Irish Sea mussel, 12% w/w White Sea mussel, and 87.9% w/w Japan Sea oyster has been prepared as the NIST SRM 4358 (The National Institute of Standards and Technology, SRM) in the natural-matrix, environmental-level radioactive SRM series (Altzitzoglou 2000). This NIST SRM 4358 sample will be useful for the determination of the activity of K, Cs, Pb, Ra, Th, and Am. 

Elements that cannot be found by straightforward activation spectrometry (INAA) because of other dominant or interfering activity may be determined by radiochemical neutron activation analysis (RNAA). 

The uranium activity ratio in water and fish samples from pit lakes in Kazakhstan and Tajikistan was determined by ICPMS, alpha spectrometry, and radiochemical neutron activation analysis (RNAA) (Stromman et al. 2013). The samples were collected from lakes that were formed in open pits in abandoned uranium mines. Fish in those lakes are occasionally consumed by the local population and domestic animals drink water from those lakes that contain high uranium levels (1 and 3 mg L ). The activity ratio was found to increase from about 1 to 1.5 a few kilome-... 

The second step in neutron activation analysis is the measurement of the induced radioactivity. A number of different techniques have evolved over the years to keep pace with changes in technology. The techniques can be divided into two groups radiochemical neutron activation analysis (RNAA), a destructive technique in which various elements are separated after the irradiation, and instrumental neutron activation analysis (INAA), a non-destructive technique in which the activity in the sample is measured directly, relying on differences in y-ray energy and half-life to discriminate the various nuclides. 

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