Foundations and trends of high resolution energy dispersive PIXE (HiRED-PIXE)

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Abstract

High Resolution Particle Induced X-ray Emission, for short HR-PIXE, dates back to 1977 and is, therefore, nearly as old as standard PIXE itself. Until roughly ten years ago, High Resolution PIXE (HR-PIXE) work used only wavelength dispersive spectrometers (WDS). The installation, in 2008, of an X-ray Microcalorimeter Spectrometer, XMS at CTN (ITN at the time) 3MV tandetron, altered this situation and, as can be recognised today, will lead to major changes in PIXE, and open up many possibilities that have been hindered so far. In this work, motives, developments, important breakthrough results obtained until now, and a case study displaying speciation data and complementary information from nuclear magnetic relaxometry, are presented and discussed, showing the full quantitative potential of High Resolution Energy Dispersion PIXE (HiRED-PIXE), in particularly the opening up of the possibility to establish fast and efficient methods for quantitative elemental speciation in unknown samples.

Introduction

High resolution PIXE emerged promptly after the 1970s PIXE foundational papers [1], [2], using wavelength dispersive spectrometers [3]. Since then, this type of work was never abandoned, although it remained for a long time limited to a few laboratories around the world, and oriented mostly towards fundamental problems. Comprehensive reviews of the work of various laboratories in this context were published by Terasawa, Torök and Petukhov in the 1990s [4], [5], [6]. After the turn of the century, using WDS high resolution PIXE for applications became more frequent. The works of Maeda [7], Hasegawa [8], Kavčič [9], Tada [10] and Woo [11] are good examples of this.

In the beginning of the 21st century, Transition Edge Sensor (TES) based X-ray Microcalorimeter Spectrometers (XMS) emerged, leading to works as that of Li on the analysis of airborne particles using a TES high resolution energy dispersive spectrometer (EDS) coupled to a Scanning Electron Microscopy (an XMS-SEM in fact), published in 2009 [12], as well as to the installation of the first XMS based PIXE system at Instituto Tecnológico e Nuclear, in 2008 [13], [14] (presently C2TN-IST/ULisboa), and more recently to the installation of a second generation XMS-PIXE system at the Universty of Jyväskylä [15].

After more than ten years passed over the installation of the XMS-PIXE system at C2TN, we are now in a position to state that a new field of High Resolution Energy Dispersive PIXE (HiRED-PIXE) has been launched. New software tools have been developed [16] (although not yet fully operational for use by non-specialists) and it seems the appropriate time to present some perspectives for the future use and capabilities of HiRED-PIXE technique, even if based on preliminary data.

All the work done on WDS-PIXE since the 1970s’ decade is a solid background for HiRED-PIXE. Still, the fact that the energy region covered in a single HiRED-PIXE spectrum (tens of keV) is much broader than in a single WDS-PIXE spectrum (sometimes only a tenth of a keV), allows to address many problems using HiRED-PIXE, which would be difficult or even impossible to study properly using WDS-PIXE.

Extending high resolution over a wide energy region in a single spectrum, provides more that just a quantitative step relative to WDS-PIXE. Once resolution is improved relative to the standard solid-state detectors, as happens in WDS-PIXE, fine details in the ionization and de-ionization processes become observable. In HiRED-PIXE this remains true even when details are far apart in energy, which opens up the possibility of studying many processes hardly at reach of WDS-PIXE work.

One situation where WDS-PIXE and HiRED-PIXE are similar is multiple ionization [17] studies, because the satellite transitions involved have energies close to that of the parent line.

Charged particle ionization is nevertheless a process more complex than photoionization by X-rays, as illustrated in Fig. 1. Cross sections for ionization of outer shells by particles are larger than those for more inner shells, which makes multiple ionization conditions more frequent in PIXE than when X-ray ionization is faced. Although optimized WDS-PIXE systems [9] do still have better resolution than HiRED-PIXE systems[18], which can be an advantage in some cases, HiRED-PIXE systems can observe multiple ionization transitions for more than a single transition and for more than a single element, in a single spectrum, which may also be an advantage. Which system is better depends on the problem being studied, therefore HiRED systems are not a replacement for WDS systems, but rather a new and complementary tool.

Beyond multiple ionization, the added complexity of charged particle ionization, is present even in the simpler case of a single ionization. In this case the final state of the ion electron cloud presents a complex fine structure due to the coupling of the angular momentum of the vacancy (in fact the result of the response of all remaining electrons to the lack of the removed one) to other non-paired angular momenta in the vicinity, including non-paired valence electrons of the ion itself. These couplings can generate high angular momentum ionization states that lead to satellite transitions that are present relatively far apart from the parent, as was recently pointed out [14], [19]. Because these transitions originate in high angular momentum ionization states, they will be favoured by charged particle ionization, and not by X-ray ionization. These strange satellite transition are still not much explored, one of the reasons being that although they are accessible to HiRED-PIXE, their study using WDS-PIXE is at least very difficult, if not impossible, given the large energy separation between the satellite energy and its parent transition.

Furthermore, as shown by Reis et al. in 2005 [20], the number of X-ray lines per 100 eV, is of the order of 25 for energies below 5 keV, of the order of 10 for energies between 5 and 20 keV, and below 3 for energies above 20 keV. Therefore, once a relative resolution of 1% or better is attained, lines are separable over the whole X-ray spectra, even in the most dense region below 5 keV, and details become observable. The present state-of-the-art XMS may be tuned to achieve energy resolution levels of 1.5 eV at Mn 5.89 keV, well below the 1% level, thus assuring a complete separation of lines. Still, as mentioned above and shown in a case study below, the fact that HiRED-PIXE spectra cover wide energy regions, provides capabilities that far exceed a mere separation of X-ray lines.

In this work, the composition of two Fe–Mn crusts samples, displayed in the photograph of Fig. 2(a), was studied using proton and heavy ions PIXE and their porous structure was accessed by nuclear magnetic relaxometry, a commonly used methodology to study rapidly, non-invasively and non-destructively porous materials [21], [22].

In the case of heavy ions PIXE, line energy shifts emerging from the use of 6 MeV O3+ beams were determined, and compared to calculations by Verma [17]. In the case of proton PIXE, HiRED-PIXE results provide speciation data by comparison to spectra of known Fe compounds. Finally, nuclear magnetic relaxometry data provides an insight regarding the porous nature of each of the samples.

Section snippets

Materials and methods

HiRED-PIXE experiments were carried out using the High Resolution High X-ray Energy (HRHE)-PIXE setup [23] endstation of the CTN 3.0 MV Tandetron. X-rays were collected using both an Amptek Peltier Cooled 3×3×1 mm3 CdTe detector having a 250 μm Beryllium window, placed at about 25 mm from target and at 145° relative to the beam direction, and the TES high resolution EDS Vericold Tech. GmbH Polaris XMS referred here as the C2TN-XMS, set at 90° to beam direction. More details about the HRHE-PIXE

Results and discussion

Fig. 2(b) displays the CdTe detector spectra for sample 1 and spot S1 of sample 2, for the energy region covering from 3 to 22 keV, where the K lines from elements having atomic number, Z, between 25  Z  42 are present. Overlaps are observable in the Co/Ni/Cu/Zn energy region and doubts emerge regarding the presence of each of these elements. Fig. 2(c) shows the corresponding spectra for the energy region from 15 to 45 keV, where the K X-rays of Zr and rare earth elements, REE, are present.

A K

Conclusions

In this work we presented a preliminary and exploratory first study of two geological samples from the Portuguese continental shelf. HiRED-PIXE spectra were obtained using the C2TN HRHE-PIXE setup and the combined use of the CdTe and XMS detectors. Spectra data were collected from 1.5 keV to 120 keV. O3+ HiRED-PIXE data made possible the determination of K line shifts from Mg to Fe. H+ HiRED-PIXE spectra provided speciation of Fe. A detailed analysis based on NMR data and details of the

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was made possible by the partial financial support of the IAEA through Research contract No. 18357 in the frame of the Coordinated Research Project F11019 on Development of Molecular Concentration Mapping Techniques using MeV Focused Ion Beams and of the Portuguese Foundation for Science and Technology, FCT, fellowship SFRH/BPD/76733/2011 and UID/Multi/04349/2013 project.

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