19. Uses of Photon and Ion Nuclear Interactions — Characterization Techniques

The speaker, Michael Short, introduces the topic of the day, which is focused on practical applications of photon and ion interactions with matter. He mentions that the session will be less theoretical and more about what can be done with the knowledge acquired, referencing various analytical and materials characterization techniques. The audience is encouraged to explore MIT OpenCourseWare for further learning and support.

A review of different photon interactions, including the photoelectric effect, Compton scattering, and pair production, is presented. The energy relationships and cross-section dependencies on atomic number (z) and energy are discussed. The speaker emphasizes the importance of understanding these interactions for analytical techniques and how they can be applied based on the energy of photons and the atomic number of the material.

The paragraph delves into the three primary ways charged particles interact with matter: Bremsstrahlung, ionization, and Rutherford scattering. The discussion covers when elastic scattering off electrons is significant, particularly for low-energy electrons and other electrons. The concept of stopping power and the importance of mass in energy transfer during collisions are also explored.

Inelastic collisions, including Rutherford scattering and nuclear reactions like fusion, are explained. The formation of a compound nucleus during inelastic collisions and the subsequent emission of particles is described. The speaker uses the Janis database to illustrate when inelastic scattering becomes significant, noting that it typically requires higher energy collisions.

The speaker transitions to discussing various analytical techniques, emphasizing the application of the learned material. He introduces the ASM Handbook as a valuable resource for material science and metallurgy. Scanning electron microscopy (SEM) is highlighted, explaining its principles and the process of creating images using secondary electrons. The importance of understanding electron interactions with materials for effective SEM analysis is stressed.

The paragraph explains how SEM images achieve a 3D-like appearance due to the detection of secondary electrons and the influence of geometry and voltage on electron collection. Personal anecdotes from the speaker's experience with SEM imaging are shared, emphasizing the difference in resolution and clarity between optical and electron microscopy.

Energy-dispersive X-ray (EDX) analysis is introduced as a method for elemental identification in materials. The process involves focusing an electron beam on a sample and collecting characteristic X-rays emitted. The speaker discusses how the interaction of electrons with matter produces these X-rays and how understanding this interaction is crucial for accurate analysis. The challenges of elemental mapping and the need for correction factors are also mentioned.

The speaker identifies Bremsstrahlung as the cause of the broad background observed in X-ray spectra. The characteristic peak and tail-off curve of the Bremsstrahlung spectrum are explained, and the self-absorption of lower energy X-rays is discussed. The importance of correcting for self-absorption when performing elemental analysis is highlighted.

X-ray photoelectron spectroscopy (XPS) is introduced as a method to determine not only the elements present in a material but also their binding states. The precision of XPS allows for the identification of specific atomic shells and elements, providing insight into the chemical state of the material. The application of XPS in understanding oxide formation on stainless steel is discussed.

Secondary ion mass spectroscopy (SIMS) is explained as a technique that involves firing ions at a material to sputter away secondary ions, which are then analyzed by mass and charge. The speaker describes how SIMS can be used to create a 3D map of isotopes within a material with high precision. The technique's application in determining the order and nature of oxide formation on stainless steel in a lead-bismuth environment is highlighted.

The speaker summarizes the use of XPS and SIMS to understand the synergistic oxidation mechanism that protects stainless steel from corrosion in a lead-bismuth environment. The ability to determine the rate, order, and nature of oxide formation is emphasized, showcasing the practical applications of the principles learned in the course.