What is the difference between electron scattering and neutron scattering?
85 MW neutron flux graph for the High Flux Isotope Reactor.
“Electrons, when they were first discovered, behaved exactly like particles or bullets, very simply. Further research showed, from electron diffraction experiments for example, that they behaved like waves. As time went on there was a growing confusion about how these things really behaved —- waves or particles, particles or waves? Everything looked like both.” -Richard P. Feynman, The Character of Physical Law
The examination of matter at the atomic and molecular levels has fundamentally evolved through scattering techniques, primarily electron and neutron scattering. Though both methods aim to extract details of material properties. They differ in interaction mechanisms. In addition, applicable domains, and the nature of information they provide.
Moreover, Richard P. Feynman revolutionized our understanding of particle behavior, thus indirectly contributing to the way scattering experiments become understood today.
Did Feynman ever meet Einstein?
Electron Scattering
Feynman diagram of electron/positron annihilation.
Electron scattering operates based on the principles of quantum electrodynamics (QED). A theory notably developed and popularized by Richard P. Feynman. In this process, accelerated electrons interact with a target sample. The interactions are largely electromagnetic. As electrons are charged particles. Their sensitivity to the electronic structure of atoms and molecules makes them particularly useful for mapping out charge distributions.
Neutron Scattering
DNA Image, in png format Inspired by Figure XVIII-1 of Johns HE The Physics of Radiology, Charles C Thomas 3rd edition (1978). Page 676. Also See Hall, E. Radiobiology for the Radiologist 5th edition (2000) for similar drawings. Created using Open office.
Neutrons, being neutral, do not become influenced by electromagnetic forces. Additionally, interact mainly through the strong nuclear force. This interaction allows neutrons to penetrate deeper into materials. Neutron scattering provides insights into isotopic variations, as well as nuclear and magnetic structures. Feynman’s work in quantum mechanics offers a framework for understanding the probabilistic nature of these scattering events.
Interaction Mechanisms
Representation of the electric field vector of a wave of circularly polarized electromagnetic radiation.
Electromagnetic Force in Electron Scattering
The primary mechanism of interaction in electron scattering is the electromagnetic force. This interaction offers high spatial resolution and allows detailed mapping of electronic structures. However, it limits penetration depth, making it more suitable for surface or thin-layer analyses.
Strong Nuclear Force in Neutron Scattering
Schematic of NIST’s “eSANS” (electrochemical Small-Angle Neutron Scattering) cell. A highly porous, sponge-like carbon electrode maximizes surface area for electrochemical reactions while structural details like particle size and configuration are measured using neutron scattering (image at left). See also www.nist.gov/mml/polymers/esans-030612.cfm Credit: Prabhu/NIST
Relativistic electromagnetism (fig. 5): A horizontal wire carrying a current, represented by evenly spaced positive charges moving to the right whilst an equal number of negative charges remain at rest, with a positively charged particle outside the wire and traveling (initially) in a direction parallel to the current.
Neutrons bypass the electromagnetic forces that electrons experience. This lack of charge allows them to interact less discriminately among atoms, making them powerful probes for complex systems where an understanding of bulk properties is required.
Advantages and Limitations
Surface Sensitivity and High Resolution
A 7 nm long part of a single-walled carbon nanotube.
Electron scattering, due to its strong interaction with matter, is highly surface-sensitive. Techniques such as Scanning Tunneling Microscopy (STM) and Transmission Electron Microscopy (TEM) offer nanometer or even sub-nanometer resolution.
Isotopic and Magnetic Sensitivity
Neutron scattering can distinguish between isotopes and is valuable in magnetic studies. Feynman’s quantum theories give a deeper understanding of how these particles interact at a fundamental level, providing valuable mathematical models that help in data interpretation.
Bertram Brockhouse, Nobel laureate in physics, for his work with neutron scattering.
Bertram Brockhouse was instrumental in the development and popularization of neutron scattering techniques. He was particularly involved in the development of inelastic neutron scattering methods for probing dynamic properties, which won him the Nobel Prize in Physics in 1994.
The interaction in neutron scattering governed by strong nuclear forces. Thus, enables deep penetration into materials. Brockhouse’s contributions include the development of triple-axis crystal spectrometry techniques in inelastic neutron scattering, which enables highly detailed studies of phonon dispersion and magnetic excitations in materials. Neutron scattering, as Brockhouse showed, can probe the dynamics and structural complexities within bulk materials, even allowing for isotopic discrimination and detailed magnetic studies.
Generic layout of an inelastic neutron scattering experiment
Neutron scattering’s ability to distinguish between isotopes like hydrogen and deuterium makes it highly valuable in biological studies, an area that has increasingly benefited from Brockhouse’s methodological advancements.
In conclusion, Electron and neutron scattering are seminal techniques for probing matter but are characterized by different interaction mechanisms, advantages, and limitations. And of course you will need some serious high performance xy stages to observe any of this.
Electron scattering is highly sensitive to electronic structures and is more accessible but lacks the isotopic and magnetic sensitivity that neutron scattering offers. Neutron scattering provides a deeper understanding of complex systems but is limited by its requirement for specialized facilities.
Neutron diffraction, used along with molecular simulations, revealed that an ion channel’s voltage sensing domain (red, yellow and blue molecule at center) perturbs the two-layered cell membrane that surrounds it (yellow surfaces), causing the membrane to thin slightly.
