Handbook of Modern Coating Technologies

Local microanalysis with the use of a scanning nuclear microprobe

Methods for local 3D microanalysis of thick samples by application of focused charged parti­cle beams are considered in terms of the requirements for the "ideal” spatial location of atoms and their identity as chemical elements. The lack of such methods to date dictates the need for estimating the existing microanalytic techniques from the standpoint of their com­pliance with the "ideal” requirements. In this context, each method is evaluated by three parameters, namely, spatial resolution, detection limit, and sensitivity. The spatial resolution is determined by the size of the region from which the secondary products of interactions between beam particles and atoms of the sample are released. The detection limit charac­terizes a minimal content of a certain element detectable with a given reliability; it depends on the possibility of extracting a desired signal from the totality of detected events. The sensi­tivity of a method is understood as its capability of discriminating between close concentra­tions of the atoms of a given element. Sensitivity is a function of the cross section of the secondary product yield and the number of particles in the probe. Sensitivity and resolution of most methods are interrelated parameters, because a maintenance of the necessary sensi­tivity requires a rise in the number of particles or beam current, which, in turn, implies an increase in the probe size. An important characteristic of any method is also the possibility of making the quantitative analysis of the level of elements contained in the region of the sample being studied. The most popular methods for local microanalysis of thick samples with the use of SNMPs are listed in Table 5.1.

The first four methods collated in Table 5.1 allow in the aggregate local quantitative microanalysis of all elements.

• Method of characteristic X-ray emission induced by beam ions

The PIXE method is based on atomic ionization in the sample. The observed X-ray spectrum consists of the continuous spectrum induced by bremsstrahlung radiation of secondary elec­trons and the linear spectrum produced by recombination of ionized atoms and the filling of K, L, and M electron shells. The PIXE method is currently well established; as mentioned above, its advantages are due to the relatively low bremsstrahlung background compared with electron beams in EPMA. Fig. 5—7 represents X-ray spectra obtained for a thick sample using EPMA and PIXE. The comparison shows that the high bremsstrahlung background in EPMA (Fig. 5—7A) prevents detection of small concentrations of certain elements that are readily identified in the PIXE spectrum (Fig. 5—7B).

Several software packages for quantitative analysis of PIXE spectra are available. These programs were tested independently under the auspices of the IAEA;the results are pub­lished in Ref. [83].

Worthy of special mention is the GeoPIXE software package [83] that allows not only quantitative analysis but also two-dimensional mapping of the location of elements for SNMPs, when the data are acquired during scanning in an event by-event mode and each event is marked by three parameters, that is, beam energy and beam positions (x, y) of the PIXE yield. Modification of the PIXE method for the improvement of locality of the analysis implies diminishing the beam spot size on the surface of the object under study. However, it leads to a substantial decrease in the beam current and, therefore, the number of atom ioni­zation events.

(B)

2.5-MeV protons

Fe

Cu

Zn

u

I_______ I_______ I                       I

FIGURE 5-7 X-ray spectra induced by different types of charged particle beams: (A) electrons, 20 keV and (B) protons, 2.5 MeV [80,81]. Optical image (C) and tomogram (D) of green algae Euglena gracilis. The surface reconstructed from the STIM data is compared with the high-phosphorus region identified from the results of using the PIXE method [82]. PIXE, Particle-induced X-ray emission; STIM, scanning transmission ion microscopy.

The maintenance of the PIXE yield by increasing current density alone when using high­brightness ion sources is somewhat limited by radiation damage to the initial material and defects incorporated into the original sample. Another approach, that is, increasing the solid angle of the detector by enlarging its area, is inefficient because it lowers detector resolution and enhances the superposition effects of event records within a narrow time interval.

One of the ways to solve this problem is the development of matrix detectors with an appropriate controller for synchronizing a set of all events and improving the sensitivity of the PIXE method up to several hundred ppb. The development of such matrix detectors is discussed in Refs. [84,85]; their controllers govern parallel processes and have the following characteristics: a solid angle of 1.2 sr, energy resolution of ~184 eV (Mn Ka), and peak-to- background ratio of «10³. A new parallel data acquisition method permits loading up to 10⁸ events per second and practically preventing event overlaps during detection.

The high sensitivity of the PIXE method coupled with submicron spatial resolution of SNMPs is widely used in medical applications, for example, for the elucidation of factors responsible for various diseases and the analysis of the distribution of chemical elements present at low concentrations in arterial walls [86]. The role of iron in the pathogenesis of Parkinson's disease, the subject of a growing interest among neurochemists, was considered in Ref. [87]. It was demonstrated that iron accumulates in abnormally high quantities in the gray matter of Parkinson's patients. The combination of PIXE and STIM makes it possible to reconstruct 3D tomograms illustrating the spatial distribution of microelements. Fig. 5—7C and D shows a cell of the green algae Euglena gracilis for which such a 3D tomogram was obtained.