Background

The successful development and deployment of advanced technologies requires solutions to be found for many material related problems and challenges.

These solutions can range from the ability to design, engineer and manufacture suitable candidate materials, to the reliable prediction of their service life performance from accelerated testing conditions, to the provision of suitable analytical tools and instruments for materials analysis.

Nuclear analytical techniques are extremely useful tools for the analysis of materials and are able to support and provide significant contributions to many of these areas. While nuclear analytical techniques are often complementary to non-nuclear techniques, nuclear analytical techniques can, in many cases, provide information that cannot be obtained through alternative methods. Hence, the continued and expanded use of nuclear analytical techniques will be required in the material characterization community.

The nuclear analytical technique ion beam analysis (IBA) uses energetic ions to probe matter. IBA encompasses a large set of different techniques, which together can probe the compositional depth profile and structure of samples of all types, provenances and purposes, from fundamental science to technologically advanced materials.

Taken as a whole, IBA is sensitive to all the elements in the periodic table.

The accessible depth varies from the surface-most atomic layer to about a tenth of a millimetre. The sensitivity of the analysis is often around one in a thousand, but it can also reach part per million levels in favourable cases, and even part per billion levels in extreme cases. Quantification is usually based on first principles, without the need for standards. In fact, many analytical techniques, both nuclear and non-nuclear, use IBA to characterize standard samples that are required for quantification.

The reliable determination and quantification of light elements in materials (Z < 14) is of importance in the development of advanced materials, both for energy and non-energy applications. Heavy ion beams from accelerators (principally, chlorine, bromine and iodine) provide a tool capable of analysing light and heavy elements simultaneously in thin films and on surfaces of materials;

heavy ion beams do not suffer from many drawbacks encountered with other light element spectrometries. Heavy ion elastic recoil detection analysis (ERDA) is being called on to analyse increasingly complex materials, particularly those composed of light elements such as glass and ceramics. However, new materials

can pose new problems and challenges for data analysis and interpretations, which must be adequately addressed.

Many advanced materials derive their functional properties from compounds containing light elements. Applications abound for thin films tailored for electrical, electronic, optical and magnetic properties. Advanced engineering materials pose many bulk and surface related material challenges. High temperatures, high stresses, and corrosive and oxidizing environments all place high demands on material properties and performance, spawning the development of carbon and oxide ceramic materials. Carbon based materials (e.g. tungsten carbide, titanium carbide, silicon carbide) have very high resistance to high heat loads and can be extremely hard, making them useful for high temperature heat pipes, superhard cutting tools and plasma facing materials. Alumina, zirconia and magnesium alloys are being developed for oxidation and corrosion resistant protective coatings.

Light elements are important components in many materials of nuclear interest, with oxide ceramic and glass matrices for nuclear waste immobilization being of particular relevance and importance. Many studies are being undertaken to investigate the chemical durability of the surfaces of those materials against radiation induced and chemical induced damage, surface behaviour being an important issue in the qualification of nuclear matrices.

IBA provides very efficient investigative tools for evaluations, in which heavy ion beams from accelerators are applied to analyse surface alterations in a wide range of matrices. However, an issue affecting all of the above cited application areas is that the reliability, quantification and interpretation of data obtained by heavy ion beams from accelerators is not satisfactory and is constrained by inadequacies and inconsistencies in the analytical software codes and basic ion beam data.

Practitioners of IBA are dependent on the availability of suitable analytical software and its ability to provide reliable and correct results. Erroneous results or misinterpretations of a material’s structure and composition can result from inadequate science in the analysis software, insufficient accuracy in basic ion beam data, or inadequate documentation and guidance for people to extract the correct information. The need for a comparison and validation of ion beam analytical codes has long been recognized by the IBA and materials science communities. This need has been discussed at several IAEA meetings, resulting in various recommendations to undertake a comparison and validation exercise (see Refs [1, 2]). As a result of these recommendations, resources were made available to conduct an IAEA exercise implemented through two technical meetings on the intercomparison and evaluation of software for accelerator based nuclear techniques of analysis (September 2004, Vienna, and October 2005,

entitled International Atomic Energy Agency Intercomparison of Ion Beam Analysis Software and presented as an invited talk at the 18th International Ion Beam Analysis Conference (Hyderabad, India, 2007) [3].

Accurate quantification also depends on the basic data used by the ion beam data evaluation codes, such as scattering cross-sections and stopping powers. To meet the nuclear data needs of the IBA community, the coordinated research project (CRP) entitled Development of a Reference Database for Ion Beam Analysis was initiated by the IAEA in 2005 and was concluded in 2010.

On the basis of new data and new evaluations made under the framework of that project, as well as on the data available from the literature, the evaluation of the most widely used differential scattering cross-sections was performed.

The data measured were incorporated into the IBANDL database¹; the evaluated cross-sections were made available to the community through the on-line calculator SigmaCalc², both supported by the IAEA. Through the continued inclusion of new data, and through the development of new evaluations, as well as improvements and extensions of existing ones, the need of the IBA community for scattering cross-sections has been effectively met.

Marie Curie noted, in 1900, that “Alpha rays are material projectiles capable of losing velocity when crossing matter” (translation from French)³ [4]. This is the first reference to what today is called ‘stopping power’, and it constitutes the basis for the depth sensitivity of IBA techniques. For instance, in Rutherford backscattering, particles backscattered at a deeper layer lose more energy than particles backscattered at a surface layer. By measuring the energy of the backscattered ions, it is possible to determine the depth from which they come, provided the energy loss (i.e. the stopping power) is known.

Theoretical calculation of stopping powers from first principles has seen strong progress over the decades, but an equivalent to SigmaCalc — where extremely reliable values are calculated for practically any system in a very fast manner — does not yet exist. It is not likely that this goal will be reached in the foreseeable future, and in practice, semiempirical interpolative schemes are used. The most widely used method is called the stopping and range of ions in matter (SRIM)⁴. SRIM is based on theoretical formulations and semiempirical models, and it includes a host of adjustable parameters that come from adjusting the models to existing experimental data. This poses a strong constraint for heavy ions because there are about as many stopping power experiments published

1 www-nds.iaea.org/ibandl/

2 www-nds.iaea.org/sigmacalc/

3 In the French original: “les rayons alpha sont des projectiles materiels susceptibles de perdre de leur vitesse en traversant la matière” [4].

for hydrogen and helium as for all other ions taken together. The database of stopping powers for heavy ions is very sparse, and there are no data at all for many systems, including a number that are essential for technologically relevant materials. As a consequence, the average accuracy of SRIM was found to be, for instance, 3% for ⁴He in the 1–10 MeV/nucleon range. For ions with an atomic number Z from 19 to 92 in solid compounds, the average accuracy was found to be 10.7% [5].

As an example, time of flight–ERDA (TOF–ERDA) spectra were collected at the Ruđer Bošković Institute, Croatia [6], for the National Institute of Standards and Technology standard reference material 2136, which consists of eight thin chromium layers (30 nm) separated by chromium oxide layers (two to three monolayers thick). The total areal density of the sample is certified to be 175.3 ± 6.4 µg/cm² and can, therefore, be used to test how well experimental spectra can be simulated using heavy ion SRIM stopping power data. Measurements were performed with different incident ions (³⁵Cl, ⁸¹Br, ¹²⁷I and ¹⁹⁷Au). The data for ⁸¹Br are shown in Fig. 1, together with the results of state of the art Monte Carlo simulations, marked as ‘CORTEO’ in the figure. There are strong discrepancies between the experimental and simulated data using SRIM stopping power values, and the discrepancy is different for each recoil; that is, the accuracy of SRIM stopping powers is not the same for all the ions. Clearly, more measurements of heavy ion stopping powers are needed to improve the reliability of heavy IBA experiments.