Activation Analysis with Charged Particles

It has been stated that the bulk of the published work on activation analysis has been concerned with the more general technique of neutron activation. A low probability of neutron activation or an overlong or overshort half-life of the activated species produced can, however, result in poor sensitivity for the method. This is the case with a number of the lighter elements such as beryllium, aluminum, carbon, nitrogen, and oiQTgen. In a number of these cases a satisfactory alternative may be found in activation by charged-particle bombardment (65). 

The simultaneous occurrence of several different nuclear reactions which lead to a variety of products is a feature of activation by charged particles. This may be exemplified by the following reactions which may occur in deuteron bombardment  

For a given energy of bombardment all these reactions tend to take place to a certain extent. At low energies the (d,w) reaction frequently predominates, while the (d,p), (d, ), (d,t), and (d,2n.) reactions become increasingly important as the deuteron energy is increased. This complexity compares unfavorably with thermal-neutron activation where frequently one reaction occurs to the virtual exclusion of all others. 

The protons, deuterons, or alpha particles used are obtained from accelerating machines, although lower fluxes of alpha particles may also be obtained by the decay of suitable nuclides. The particle flux obtainable from an accelerating machine may be estimated from the ion beam currents quoted in Table II. One microampere of protons or deuterons will correspond to 6.3 X 10 ions per sec, while the same beam current of alpha particles will contain one-half of this number. 

The large amount of energy released in charged-particle bombardment tends to exclude this method where thermally labile organic or biological materials are employed as targets. However, Odeblad (54) was able to determine sodium in urine and other elements in biological materials by means of the weak source of alpha particles (160 millicuries) derived from the decay of Po . A similar source of alpha particles has been used to determine aluminum by the reaction AP (a,n)P —half-life 2.5 min (66). 

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G.J. McCallum, Determination of Trace Elements by Activation Analysis With Charged Particles , JSci 12 (3), 470-5 (1969) CA 70, 97975 (1969) 52) Anon, Analysis of... 

E3. Englemann, C., Activation analysis with charged particles and gamma-photons. Rapp. Comm. Energie Atom. Paris) R4072 (1970). 

Activation Analysis with Charged Particles, NAS-NS-3110, 1.00 Applications of Computers to Nuclear and Radlochemlstry. NAS-NS-3107, 2.50 Application of Distillation Techniques to Radiochemical Sei>aratlons, NAS-NS-... 

Proc. 2nd Int. Conf. on Practical Aspects of Activation Analysis with Charged Particles, 1967, Brussels, Euratom, 1968, p. 31... 

Activation analysis with charged particles, although essentially a surface technique, has been used to determine carbon, nitrogen, and oxygen, especially in high-purity silicon, and offers excellent sensitivity and good reliability when done carefully. 

Some relevant terms for activation analysis are activation analysis, neutron activation analysis (NAA), instrumental neutron activation analysis (INAA), neutron activation analysis with radiochemical separation (RNAA), photon activation, neutron capture prompt gamma activation analysis (PGAA), charged particle activation, autoradiography, liquid scintillation counting, nuclear microprobe analysis, radiocarbon (and other element) dating, radioimmunoassay, nuclear track technique, other nuclear and radiochemical methods. Briefly, the salient features of some of the more popular techniques are as follows ... 

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 ... 

Radiochemical Methods in Analytical Chemistry. Activation Analysis with Gamma-Ray Photons and Charged Particles. Chimica 21, No. 3, 116 (1967). 

The bulk of the published work on activation analysis has been concerned with neutron activation and this, the more general technique, wall be dealt with in some detail. A tivation with charged particles will be considered in Section VII. 

Although NAA and PAA have a few characteristics in common with charged-particle activation analysis (CPAA), there are fundamental differences, owing to the electric charge of the bombarding particle. 

Accurate results were also obtained using the methylene blue method as described by Meier et al. (20). However, as it is the case for most of the photometric methods critical examination is required to avoid errors due to small alterations in the operational conditions (8). Also the correction for blanks can be a problem, especially when the method is used near its detection limit. This was demonstrated within the above BCR round robin a first result of (0.30 0.12) g/g was obtained for the same aluminium as above. When a purer aluminium sample, accurately characterized with charged particle activation analysis, was used to determine the blank, a value of (1.27 + 0.08) tig/g was obtained. 

The oxygen concentration in titanium, zirconium and their alloys is usually fairly high (several hundred to several thousand Mg/g). Activation analysis with 14 MeV neutrons can thus be used for an accurate determination of oxygen in those metals. Less attention has therefore been given to the determination of oxygen by charged particle activation analysis. 

Classical methods such as reducing fusion or 14 MeV neutron activation analysis are only able, under optimal circumstances, to carry out routine control analyses. In most cases this means "YES/NO-analyses" with a sensitivity of approximately 0.5 ig/g. Activation analysis with photons or charged particles allows to determine concentrations of 0.1 tg/g or below. These developments led... 

Some 60 elements can be identified and quantified after having been transformed into radioisotopes by bombardment with particles. This particular labelling technique, whose sensitivity varies with the element (1000 ppm to ppb), is part of the field of multi-elemental nuclear analysis. Neutron activation is the most widely used of these techniques because it does not involve charged particles. 

Similar analysis can be made for particles with an arbitrary initial charge multiplicity Z. If, in particular, a particle is originally neutral, tunneling will occur in an attractive electric field. It can be readily seen that, in the general case, the activation energy is ET + (Z+ 1)A.FT. The assumption that the molecular affinity EA is independent of the electric field of charged particles is quite reasonable, because at Z < 3 this field is still considerably weaker than the local electric fields associated with the chemisorption process. 

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 contrast to neutrons, the penetration depth of charged particles is relatively small, with the result that only the surface layers of thicker samples are activated. Furthermore, the energy of charged particles decreases drastically with the penetration depth, and consequently the cross section varies with the penetration depth. On the other hand, these properties of charged particles offer the possibility of surface analysis. 

Activation analysis has been proposed to obtain more reliable analytical information.73 The sample is irradiated with neutrons, photons (gamma radiation), or charged particles the radioactivity generated is measured by sensing the entire gamma spectrum with a semiconductor detector. The most reliable variant is neutron activation analysis (NAA).73... 

NAA, XRF, and isotope dilution analysis, by listing the parameters analytes determinable, concentration ranges, selectivity, analysis time and cost. Vandecasteele (1991) presents a review of recent developments and trends in instrumental and radiochemical neutron activation analysis as well as charged particle activation analysis and performance comparison with ICP-MS. 

INAA with isotope neutron sources) and Vandecasteele (1986) (Applications of charged particle 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. 

In activation analysis advantage is taken of the fact that the decay properties such as the half-life and the mode and energy of radioactive decay of a particular nuclide serve to identify uniquely that nuclide. The analysis is achieved by the formation of radioactivity through irradiation of the sample either by neutrons or charged particles. Neutron irradiation is by far the more common technique, and hence this method is often referred to as neutron activation analysis, NAA. A major advantage in activation analysis is that it can be used for the simultaneous determination of a number of elements and complex samples. If the counting analysis of the sample is conducted with a Ge-detector and a multichannel analyzer, as many as a dozen or more elements can be measured quantitatively and simultaneously (instrumental NAA, or INAA). 

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