Delayed NAA

As a result of slow (thermal) neutron irradiation, a sample composed of stable atoms of a variety of elements will produce several radioactive isotopes of these activated elements. For a nuclear reaction to be useful analytically in the delayed NAA mode the element of interest must be capable of undergoing a nuclear reaction of some sort, the product of which must be radioactively unstable. The daughter nucleus must have a half-life of the order of days or months (so that it can be conveniently measured), and it should emit a particle which has a characteristic energy and is free from interference from other particles which may be produced by other elements within the sample. The induced radioactivity is complex as it comprises a summation of all the active species present. Individual species are identified by computer-aided de-convolution of the data. Parry (1991 42-9) and Glascock (1998) summarize the relevant decay schemes, and Alfassi (1990 3) and Glascock (1991 Table 3) list y ray energy spectra and percentage abundances for a number of isotopes useful in NAA. 

PGNAA involves the measurement of the prompt y rays emitted when the nuclei first absorb the neutrons. By energy and intensity analysis of the prompt y spectrum it is possible to reveal the elements present and determine their concentrations. Because the analysis is carried out during the irradiation, the method is quicker than delayed NAA, but it needs special equipment on site at the reactor to record the y spectrum whilst the sample is being irradiated (Glascock et al. 1984, Glascock 1994). It can, however, measure some elements more efficiently than delayed NAA (e.g., Pb), since it does not require the daughter nucleus to be radioactively unstable. 

Delayed or prompt NAA. As explained above, y-rays are measured at some time after the end of the irradiation (delayed-NAA). This is the useful method for the vast majority of elements that produce radioactive nuclides. Yet, in some cases, for elements which decay too rapidly during neutron irradiation or for elements that produce only stable isotopes, y-rays are measured during neutron irradiation. This is called the prompt-NAA. 

Theoretically, every element can be neutron activated. Yet, some conditions are required. First, the element must have an isotope with a high cross-section that is able to fix the incoming neutrons. Second, if delayed-NAA is used, the half-life of the isotope has to be long enough so the amount of activity is measurable. Third, the isotope itself must be relatively easy to get in sizeable quantities. Lastly, y-rays must be produced that are reasonably intense and in a limited energy range. 

In terms of atomic spectrometry, NAA is a method combining excitation by nuclear reaction with delayed de-excitation of the radioactive atoms produced by emission of ionising radiation (fi, y, X-ray). Measurement of delayed particles or radiations from the decay of a radioactive product of a neutron-induced nuclear reaction is known as simple or delayed-gamma NAA, and may be purely instrumental (INAA). The y-ray energies are characteristic of specific indicator radionuclides, and their intensities are proportional to the amounts of the various target nuclides in the sample. NAA can thus... 

Figure 2.13 Schematic diagram of the nuclear processes involved in neutron activation analysis. Prompt gamma neutron activation analysis (PGNAA) occurs within the reactor delayed gamma NAA (DGNAA) occurs at some remote site. (After Glascock, 1994 Fig. 1. John Wiley Sons Limited. Reproduced with permission.)...

Figure 17. Snapshots of wavepacket propagation during the picosecond pump-probe excitation for Naa for selected delay times between pump and probe pulse [15].

The cost per sample of NAA analysis is high, involving additional personnel and reactor, and detector and processing systems. Additionally, with NAA, reactor accessibility, sample workup and analysis do not lend themselves to rapid throughput, especially where a heavy case load is involved. Reactor accessibility is limited, posing still further delays. 

In PIGE the y-emission is usually prompt. If very low amounts of trace elements have to be detected it can be advantageous to use a delayed decay. In this case, the technique is called charged particle activation (CPA) and is an analogue to neutron activation analysis (NAA). It has the advantage that the prompt background from interfering reactions is completely removed as irradiation and analysis are completely separated in time. This also allows to remove external contaminants in the short time between irradiation and measurement which further improves detection limits. A comprehensive description of the technique can be found in the ion beam analysis handbook [2], For 19F CPA is conceivable in special cases via the 19F(d,dn)18F reaction. However, we have found only one application in the literature [64],... 

The NAA method can be divided into NAA (Instrumental NAA) and RNAA (Radiochemical NAA). In the latter, the various neutron-induced products are separated chemically to minimize interferences. There are several comprehensive review papers on INAA published in the literature 1,2,3,4). Briefly, the basic parameters controlling sensitivity for a multi-element determination are neutron flux, irradiation time, delay interval prior to counting, half-life and gamma-ray energy of the induced activity, and eflBciency and resolution of the detector. Table I outlines the irradiation parameters used for each of the two sequential irradiations. The final count occurring 40-50 days after the second irradiation is performed on an anti-coincidence-shielded Ge(Li) system developed recently in our laboratory. 

Hypersensitivity, Delayed Type, Pages 559-561, Leigh Ann Bums-Naas... 

Figure 9.1-2 Comparison of developmental landmarks in the developing irnmime systems of the rodent (A) and human (B). Data have been normahzed to three key developmental phases—gestation, lactation, and maturation to young adulthood, and clearly demonstrate that the development of the immune system of the rodent is delayed compared to that of the human, a fact that should be considered in the design and interpretation of DIT studies. Adapted from Holsapple et al. (2003) and Birrns-Naas et al. (2008).

Neutron activation analysts (NAA) can be accomplished by measurement of the prompt gamma rays during irradiation or by measurement of the delayed... 

Why is delayed gamma-ray emission most often used in NAA ... 

In infants who have suffered perinatal hypoxic-ischaemic brain injury (birth asphyxia), P spectra obtained within a few hours of birth show no abnormalities. However, in many cases a delayed secondary energy failure (SEP) develops within 24 h. The Pi signal increases, accompanied by reduced PCr and, in severe cases, low NTP. Furthermore, in contrast to the profound acidosis seen during acute hypoxia-ischemia, an intracellular alkalosis may be detected. Additionally, localized spectroscopy shows elevated lactate levels for several weeks following SEE. Both PCr/Pi and lactate/N-acetylaspartate (Lac/NAA) ratios have proved to be extremely useful indices of the severity of hypoxic-ischaemic injury, with strong prognostic capabilities. 

Neutron activation analysis (NAA) is the general term used to describe a nuclear-based technique in which a solid or liquid sample is irradiated with neutrons. Capture or absorption of a neutron excites the nuclide that returns (promptly or after a delay) to ground state by emission of an energetic photon (gamma ray) and/or other particles from the nucleus (Figure 1.20). 

The term PCNAA is used when preconcentration precedes the neutron activation while if epithermal neutrons are used to excite the sample the acronym given is ENAA. The monitoring of the delayed neutrons emitted after excitation is termed DNAA. All these NAA procedures are nondestructive techniques used for characterizing solid (and in some cases also liquid) samples. However, a neutron source and a suitable detector are required and the sample can become quite radioactive after irradiation. The sensitivity of NAA techniques varies widely among different elements and sample preparation and post-irradiation methods employed. Several specific examples of NAA application for analysis of uranium in different matrices will be presented in the appropriate chapters. 

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