Delayed neutron activation analysis

Gonzales ER, Gladney ES, Boyd HA, et al. 1988. Determination of U inhuman tissues by delayed neutron activation analysis. Health Phys 55(6) 927-932. 

Although most of the elements have been determined by XRF (21), some other methods were used. The fluorometric method for selenium uses diaminonaphthalene (32). The colorimetric method for molybdenum uses potassium thiocyanate (33). The uranium analyses were done by delayed neutron activation analysis (34). For the XRF analyses of the oil and water, a blank value implies that there were no x-rays above background for that element. Two elements conspicuously missing from Table IV are cadmium and mercury. Preliminary analyses for these two elements have not yielded reproducible results. Further work is needed before we can make definitive statements about cadmium and mercury. 

DNAA Delayed neutron activation analysis (analytical method)... 

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.)...

The majority of the applications of 14 MeV neutron activation analysis involve the use of short-lived indicator radionuclides. Therefore, it is essential that the sample be returned quickly to the counting station following irradiation. Pneumatic sample transfer systems employing compressed nitrogen, or a vacuum are most commonly used 34-35>. An inexpensive system may be constructed from ordinary low density polyethylene tubing 18>. Irradiation, delay and counting times are ordinarily controlled by means of preset timing circuits. Completely automated control and transfer systems are available commercially. 

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],... 

Of the five stable Isotopes of zinc, only three ( Zn, Zn, and Zn) can be measured with (delayed gamma) Neutron Activation Analysis and hlgh-resolutlon gamma spectrometry. The required methodology for the measurement of Zn is the most stringent of the three isotopes and the necessary Radiochemical Neutron Activation Analysis has been developed and described in detail elsewhere (.8), The salient features of neutron activation of these three Isotopes are tabulated in Table li and from these, two points emerge First, with the availahility of thermal neutron... 

The fundamental principle behind analysis by activation analysis is activation or excitation of an atomic nucleus by exposure to radiation such as neutrons, protons or high-energy photons with subsequent measurement of emitted sub-atomic particles or radiation. The most common aspect of the technique involves activation with neutrons in a nuclear reactor and measurement of delayed emitted gamma rays, denoted neutron activation analysis, either instrumental neutron activation analysis (INAA) or neutron activation followed by radiochemical separation (RNAA) in which the element of interest is chemically separated from the matrix after irradiation to provide for better, unimpeded counting. 

The history of neutron activation analysis goes back to the middle of the 1930s when it was first described by G. Hevesy and H. Levi at the Niels Bohr Institute in Copenhagen. The principle of the technique is that elements can be made radioactive by exposure to neutron irradiation. Two types of physiological processes are associated with this activation one prompt and one delayed. Classically, neutron activation analysis is based on the detection of the delayed event, viz. the characteristic radiation emitted during the decay -with a specific half-life (ti/a) - of the unstable nuclei formed by (n,y) reaction. 

An additional hindrance to the use of neutron activation analysis - especially in case of clinical applications - is the long delay between the collection of a sample and the final calculation of the result. Indeed, in extreme analysis - more particularly when long-lived radioisotopes are involved - Irradiation periods of several days, even up to a couple of weeks, and measuring periods of tens of hours per sample are often required to attain the desired radioactivity. During the irradiation, matrix elements or major components become intensely radioactive so that long cooling periods - weeks - may be necessary to allow undesirable isotopes to decay. Fortunately, for some elements, there are several alternatives e.g., selenium can not only be quantified via its Se (ti/2 = 119.770 d) (Versieck et al.. 1977) but also via its 86 (ti/2 = 57.3 min) (Heydorn and Damsgaard, 1973) or, even, via its radioisotope (ti/2 = 17.5 s) (Dickson and Tomlinson, 1967 Hahn et... 

Hgure 9 A totally automated system, based on pneumatic transfer, for neutron activation analysis with y-ray spectrometry and delayed neutron counting. (Reprinted with the permission from Atomic Energy of Canada Limited (AECL) Research, Canada.)... 

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). 

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

With respect to the time of measurement, NAA falls into two categories (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. 

DGNAA Delayed gamma-ray neutron activation analysis... 

Moxham, R. M., Senftle, F. E., Boynton, G. R. Borehole Activation Analysis by Delayed and Capture Gamma Rays Using a 252Cf Neutron Source. Econ. Geol. 67, 579-591... 

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. 

Thermal-hydraulics design of the clad failure detection system. This system is used to detect possible clad failures in the fuel subassemblies while the plant is operating. Clad failure causes a release of fission products emitting delayed neutrons, that are transported to the hot plenum and to the detectors. In SPXl the detector itself is placed outside the hot plenum with a continuous poped sampling system for analysis of the primary sodium was set up other systems in which neutron detectors are placed near the intermediate heat exchanger inlets and enabled activity to be measured directly, have been studied. In both cases thermal-hydraulics studies were necessary to measure the hydraulic transfer functions between the various core... 

Fissile Elements. Reactor activation of fissile elements and counting of the delayed neutrons emitted in the decay of short-lived fission products provides a very specific method of analysis. Unfortunately, in order to provide discrimination between the different fissile isotopes (or elements) it is necessary to irradiate the samples in two different flux spectra thus utilizing the differing rates of reaction of fast and thermal fission. Decay curve analysis is impractical because of the similarity of the half-lives of delayed neutron groups from all the fissile isotopes. 

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