Nuclear Reaction Analysis NRA

Nuclear reaction analysis is exacdy that a beam of nuclei (e.g. deuterium ions) is accelerated towards the sample and undergoes a nuclear reaction with the nuclei in 

The protons have a kinetic energy which is characteristic of this particular nuclear reaction (as opposed to the reaction D+(l MeV) + C + H , for example). 

Particle-induced X-ray emission (PKE, this acronym is also sometimes referred to just as proton-induced X-ray emission) is in many ways analogous to XRF (see Sec. 4.4.), but has distinct advantages that justify the greater technical effort. A comparison of the principles of XRF (Fig. 5) and PDCE (Fig. 9) reveals that both are essentially based on the creation of holes in the core shells and the emission of the same characteristic X-ray radiation upon filling of that hole with an upper shell electron. The decisive difference between the two techniques is in the way the incident particles (electrons vs fight ions) interact with the sample. In the case of an electron beam. 

As nuclear reactions are isotope specific, NRA may be used, for example, to distinguish the distribution of binary blends of polymers in a polymer film, where one of the polymers is labelled with deuterium. The depth distribution of the deuterium atoms can be established and hence that of the labelled polymers. 

Because the cross-sections for nuclear reaction are usually lower than the cross-sections for elastic scattering of projectiles used in RBS or in elastic recoil detection analysis (ERDA), higher currents must be used to obtain comparably high intensity in 

In most nuclear reactions A(a,b) B the emitted radiation b used for analysis consists of cliarged particles, e. g. o -particles and protons. Tills is why NRA is covered in tliis section on ion detection. Tlie emitted radiation can, how- 

NRA as in RBS or ERDA, and possible modification of the target composition as a result of irradiation must be considered. Nuclear reaction cross-sections are also usually not available in analytical form for direct evaluation of measured data. Concentrations are, therefore, often obtained by comparison of the measured data with results from standard samples of known concentration. 

Eor analysis of emitted particles, solid state surface barrier detectors (SBD) are used inside the scattering chamber to measure the number and energy of the reaction products. Stopper foils are used to prevent scattered projectiles from reaching the detector. Depth profiles can be obtained from the energy spectra, because reaction products emitted in deeper layers have less energy than reaction products emitted from the surface. The concentration in the corresponding layer can be determined from the intensity of reaction products with a certain energy. 

The cross-section curve a(E) gives the dependence of the nuclear cross-section on the projectile energy, E. The measured energy spectra of emitted particles or the excitation curve N(Eq) wiU depend on the depth profile N(x) of the analyzed isotope and on the cross-section curve (t(E(x)), where E(x) gives the energy of the projectiles at a depth x. Evaluation of the depth profile N (x) from measured energy spectra or excitation curves often requires a tedious evaluation procedure if the cross-section curve has a complex structure. It is simplified for two special types of behavior of the cross-section curve  

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This overview covers the major teclnhques used in materials analysis with MeV ion beams Rutherford backscattering, chaimelling, resonance scattering, forward recoil scattering, PIXE and microbeams. We have not covered nuclear reaction analysis (NRA), because it applies to special incident-ion-target-atom combinations and is a topic of its own [1, 2]. 

All the techniques discussed here involve the atomic nucleus. Three use neutrons, generated either in nuclear reactors or very high energy proton ajccelerators (spallation sources), as the probe beam. They are Neutron Diffraction, Neutron Reflectivity, NR, and Neutron Activation Analysis, NAA. The fourth. Nuclear Reaction Analysis, NRA, uses charged particles from an ion accelerator to produce nuclear reactions. The nature and energy of the resulting products identify the atoms present. Since NRA is performed in RBS apparatus, it could have been included in Chapter 9. We include it here instead because nuclear reactions are involved. 

Nuclear reaction analysis (NRA) is used to determine the concentration and depth distribution of light elements in the near sur ce (the first few lm) of solids. Because this method relies on nuclear reactions, it is insensitive to solid state matrix effects. Hence, it is easily made quantitative without reference to standard samples. NRA is isotope specific, making it ideal for isotopic tracer experiments. This characteristic also makes NRA less vulnerable than some other methods to interference effects that may overwhelm signals from low abundance elements. In addition, measurements are rapid and nondestructive. 

Nuclear reaction analysis (NRA) also identifies emitted particles which are different from the incident ones. In order to avoid permanent radioactivity, the energy of the projectile is maintained below 6 MeV, so that it is used primarily to determine the concentration and depth of light elements (Z < 9) in the near surface of solids. 

Mitchell et al. (390) using nuclear reaction analysis (NRA), found 6 = 0.25 for the saturation adsorption of C2H4 on Pt(lll) at 100 K (also see 391). This result has been confirmed by a combined study done with NRA and XPES (392) and by STM (393-395). Furthermore, the value of 8 = 0.25 for C2H4 saturation coverage at low temperature is in agreement with a Monte Carlo simulation of QH4 adsorption on Pt(lll) by Windham et al. (396), who showed that an ensemble of four Pt surface atoms is required to absorb one C2H4 molecule. 

Nuclear reaction analysis (NRA). Based on the detection of charged particles emitted during nuclear reaction, NRA can be considered as an inelastic counterpart of RBS. NRA is useful in the reverse case as for RBS, namely the depth profiling light elements in a sample composed of heavy elements, (e.g. corroded layers on metallic samples containing O, C, and N). Incident ions are protons ( H) or deuterons (2H). 

In the following, those ion beam analysis techniques that allow for fluorine detection will be presented. By far, the most important technique in this respect is nuclear reaction analysis (NRA). Although it can be rather complex to perform, it is the most often applied technique for fluorine trace element studies, due to a number of convenient and prolific resonant nuclear reactions which make it very sensitive to fluorine in most host matrices. NRA is often combined with particle-induced X-ray emission (PIXE) which allows for simultaneous determination of the sample bulk composition and concentrations of heavier trace elements. By focusing and deflecting the ion beam in a microprobe, the mentioned techniques can be used for two- or even three-dimensional multi-elemental imaging. 

K. Noll, M. Dobeli, L. Tobler, D. Grambole, U. Krahenbuhl, Fluorine profiles in ac-hondrites and chondrites from Antarctica by nuclear reaction analysis (NRA), Meteor. Planet. Sci. 32 (1997) A101. 

Particle-Induced X-ray Emission, PIXE Nuclear Reaction Analysis, NRA Hydrogen Mass Spectrometry, HMS Noble Gas Mass Spectrometry, NGMS... 

For all these specialty polymers, deuterium can be used as a label on one or the other monomer. Deuterium labeling allows the use of techniques based on ion detection such as forward recoil spectrometry (FRES), nuclear reaction analysis (NRA) or secondary ion mass spectrometry (SIMS). If a high-resolution depth profile of the interfacial region is needed, neutron reflectivity can also be used. The main drawback of that approach is the cost of the deuterated polymers while deuterated styrene and methyl methacrylate are expensive but commercially available, other monomers need to be synthesized and the cost can be quite prohibitive. 

Nuclear reaction analysis (NRA) - See Techniques for Materials Characterization, page 12-1. 

Acronyms RNRA (resonant nuclear reaction analysis) NRA (nuclear reaction analysis)... 

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