Activation analysis charged particle reactions

Charged-particle reactions are also used in activation analysis. Their disadvantage over neutron reactions is that charged-particle reactions are mostly endothermic, i.e., they have a threshold. Table 15.2 gives several examples of such reactions. 

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. 

Charged particle activation analysis (CPAA) is based on charged particle induced nuclear reactions producing radionuclides that are identified and quantified by their characteristic decay radiation. CPAA allows trace element determination in the bulk of a solid sample as well characterization of a thin surface layer. 

Activation analysis is based on the production of radioactive nuclides by means of induced nuclear reactions on naturally occurring isotopes of the element to be determined in the sample. Although irradiations with charged particles and photons have been used in special cases, irradiation with reactor thermal neutrons or 14 MeV neutrons produced by Cockcroft-Walton type accelerators are most commonly used because of their availability and their high probability of nuclear reaction (cross section). The fundamental equation of activation analysis is given below ... 

Neutron activation analysis is based upon the production of radioisotopes by nuclear reactions resulting from neutron bombardment, followed by identification and measurement of the different radioisotopes formed. Element activation can also be carried out by bombardment with high-energy charged particles, X-rays or gamma rays (5). 

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 potential value of high-energy electron-producing machines such as the linear accelerator for activation analysis must not be overlooked. Here photonuclear reactions y,n) can be used, either to produce a high neutron flux (as most charged-particle machines can by choice of a suitable reaction), or directly on samples. This may well be valuable, particularly for some light element determinations. 

Photon activation complements neutron and charged-particle activation. Photons are better than neutrons in certain cases. For example, photons are preferred if the product of the neutron activation is an isotope that has a very short half-life or emits only low-energy betas or low-energy X-rays. The cross sections for photonuclear reactions are generally smaller than those for neutrons and charged particles. Table 15.3 gives several photonuclear reactions that have been used in activation analysis. 

Prompt activation analysis (Erdtmann and Petri, 1986 Alfassi, 1990) uses the prompt radiation accompanying a nuclear reaction for determining elemental or isotopic concentrations. The variety of prompt methods is large because a sample can be irradiated with various particles - neutrons, charged particles or gamma-rays. Prompt activation analysis permits the determination of several elements - about 17 elements in environmental matrices (Germani et al., 1980) - but most analysis are used for the determination of light elements (H, He, Li, B, C, N, Si, S, Cl) as well of Cd and Gd. 

For most charged particle-induced reactions the atomic number of the radionucHde B is different from that of the analyte element A. This is the case for (p,n), (p,a), (d,n), (d,a), ( He,n), ( He,d), (o,n), and (a,d) reactions, but not for (p,d) and ( He,a) reactions. The radiochemical separation to be developed for CPAA is thus different from that for all non-nuclear analytical methods and for some other methods based on activation analysis, such as thermal- and fast-NAA (using the (n,y) and (n,2n) reactions, respectively) and PAA (using the ( y,n) reaction). Also, in principle, it is not necessary to separate the matrix, but rather the radionuclide(s) formed out of the matrix element(s). Again, the atomic number of the radionuclide(s) is usually different from that of the matrix element(s). Owing to the chemical separation involved, CPAA is considered to be an independent analytical method, not subject to the same systematic errors as other analytical methods. 

In classical activation analysis a material to be analyzed is bombarded with neutrons, charged particles, or photons (gamma-rays). By this bombardment radionuclides are produced from elements of the target material. These radionuclides can be analyzed qualitatively and quantitatively by radioassay methods. From the results and the knowledge of the nuclear reactions during the bombardment an analytical determination of the target material can be achieved. The radioactive products of the bombardment can be measured after the irradiation and the emitted types, energies, and half-lives provide information used for qualitative analysis and the radiation intensity supplies data for the quantitative composition of the material to be analyzed. 

See also Activation Anaiysis Neutron Activation Charged-Particle Activation Photon Activation. Atomic Emission Spectrometry Inductively Coupled Plasma. Atomic Mass Spectrometry Inductively Coupled Plasma. Mass Spectrometry Overview. Surface Analysis Particle-Induced X-Ray Emission Auger Electron Spectroscopy Ion Scattering Nuclear Reaction Analysis and Elastic Recoil Detection. X-Ray Fluorescence and Emission Wavelength Dispersive X-Ray Fluorescence Energy Dispersive X-Ray Fluorescence. 

Activation analysis is another technique that has found some applications. In these methods, the desired stable isotope is made to undergo a suitable nuclear reaction. The parent isotope is determined by measurement of the resulting nuclide. As an example, the 0 content of water samples as small as 1.S jul has been determined by charged particle activation [142]. 

If high fast-neutron fluxes are available, the sensitivity of Be and B analysis using reactions (2) and (4) is good and, in the absence of facilities for charged particle activation, may be of value. However, the fast-neutron fluxes generally available at the rabbit irradiation positions of many small reactors is too low to be of use. 

Charged Particle Activation.—Charged particle activation analysis is most often applied to the measurement of the light elements Be to F using p, d, He, or a-particles. The various reactions used for the determination of these elements are subject to many mutual interferences. Table 3 shows the various reactions possible for the determination of O using charged particles. 

IS satisfactory. The precision obtained by the N(p,a) C reaction (ca 3 %) is however significantly better. The latter procedure (49) is therefore recommended for the determination of nitrogen in zirconium and zircaloy by charged particle activation analysis. 

CHARGED PARTICLE ACTIVATION ANALYSIS 3.1 NUCLEAR REACTIONS... 

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