Instrumentation
E. A. Schweikert Center for Trace Characterization Dept. of Chemistry Texas A & M University College Station, Tex. 7 7 8 4 3
Charged Particle Activation Analysis When an ion beam interacts with matter, many different types of interactions occur. They can be at the atomic or nuclear levels, and some of these interactions result in the emission of signals which are element- or isotope-specific. Figure 1 summarizes the products resulting from ion bombardment and some of the ion beam techniques used for materials characterization. The probability of the different interactions varies with the type and the energy of the bombarding ions. If we consider only ion beams of energies ^ 1 MeV/nucleon (e.g., energies of at least 1 MeV for protons or 4 MeV for 4 He ions), there are three types of interactions that can be exploited for chemical analysis: • Ion atom collisions creating inner shell vacancies, which result in the emission of characteristic K, L, or M X-rays. This is the basis for the particle-induced X-ray emission technique or "PIXE" (/); • Elastic scattering of the bombarding ions, which allows measurement of the masses of the target atoms
present. This technique is usually referred to as Rutherford backscattering spectrometry, RBS (2); • Nuclear reactions, particularly those yielding radioisotopes. The characteristics and amounts of the radioactivities are used to identify and quantify the parent isotopes present in the bombarded material. The best known of these techniques is neutron activation analysis, which uses neutrons as the bombarding species. The neutrons can be replaced with protons 0H+), deuterons (2H+), tritons (3H+), helium-3 ions ( 3 He + ), or helium-4 ions or alphas ( 4 He + ). In this case the technique is termed charged particle activation analysis, CPAA. The present discussion is devoted to CPAA. Scope The reasons for using CPAA stem from its unusual features. These in turn determine the technique's performance, viz. accuracy, sample size probed, sensitivity, and range of elements that can be detected. A major feature of CPAA is that dif-
ferent radionuclides can be obtained from a given target isotope, depending on the nature and energy of the bombarding particle. For protons, deuterons, and He-3 ions the energies used range from 5 to 20 MeV and from 20 to 45 MeV for He-4 ions. With these beams numerous nuclear reactions are possible (Figure 2). The multiple reaction pathways enhance the prospects of having for each stable isotope a reaction combining high yield and specificity. Conversely, with a greater number of activation reactions, the chances of obtaining the same radionuclide from isotopes belonging to different elements increase. Interferences multiply with rising bombarding energies. In practice a compromise must be found; sensitivity increases with bombarding energy, and selectivity sets an upper limit on the bombarding energy. In CPAA the sample depth probed is limited since the interaction of an ion beam with matter is accompanied by a rapid loss of energy. The penetration depth or range depends on the incident energy and nature of the bom-
X-rays PIXE Scattered Ions RBS
Electrons IIAES y-rays
Incident Ion
emission Visible Photons IILE
y-rays CE
Neutron and/or C h a r g e d Particles from Nuclear Reactions PRA
Figure 1.
CPAA
Ion Bombardment: products and selected analytical techniques
(CE = coulomb excitation; CPAA = charged particle activation analysis; IIAES = ion-induced Auger emission spectroscopy; IILE PIXE = particle-induced X-ray emission; PRA = prompt radiation analysis; RBS = Rutherford backscattering) 0003-2700/80/0351-827A$01.00/0 © 1980 American Chemical Society
ion-induced light emission;
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Figure 2. Products of charged particle reactions induced on a target nucleus (black field). Z = atomic number, N = atomic mass
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Target Thickness (/xm) Figure 3. and lead
Ranges in yum for 20 MeV protons (—) and He-3 ions (
barding ion as well as the atomic number of the target. For example, in silicon the range varies from ~ 50 Mm for 8 MeV 3 He ions to ~ 2400 »im for 20 MeV protons. A graphical illustration is given in Figure 3. In practice one deals with a useful or effective range that is somewhat smaller than that corresponding to complete stopping of the beam, because nuclear reactions induced by charged particles have a positive energy threshold. Sample volume irradiated will be on the order of 10 - 2 to 10 _1 cm3 assuming a 1 cm2 surface is exposed to the beam. Given the small volumes involved, proper selection of the fraction of sample to be assayed is critically important. It must
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) in silicon
further be noted that the cross section is energy-dependent, hence the activation is nonuniform along the depth axis and this must be taken into account in quantitative calculations. Originally the interest in CPAA was largely due to its unique potential for determining light elements (e.g., boron, carbon, oxygen) at the subppm level. The nuclear reactions of interest feature fairly large cross sections (typically in the hundreds of millibarns). These, coupled with high intensity particle beams (1012 to 1014 particles/s), yield radioactive products with specific activities adequate for detection at the ppm to ppb level. Although the major thrust of CPAA re-
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mains in the field of light element analysis, highly sensitive CPAA procedures have now been devised for most stable elements. Detection limits reported for 72 elements are assembled in Figure 4 (3-14). The data must be taken as "limits for qualitative detection" (15). The actual performance of CPAA is dependent on the major, minor, and/or trace element makeup of each sample. Experimental Considerations
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