An international team of researchers used a neutron beam to conduct pendellösung interferometry on silicon, offering both measurements of maximum accuracy with this method to date and a snapshot of the characteristics of both the silicon crystals and the neuron itself.
Image Credit: North Carolina State University.
The distinct characteristics of neuron beam also make it suitable for potentially detecting the presence of a “fifth force,” which evades the forces connected to some of the mysteries of physics, like the existence of dark or extra spatial dimensions.
Pendellösung interferometry is an optical method used to quantify crystal material characteristics — specifically of silicon, such as its mechanical and thermal properties and the way the forces binding it together, i.e., the atoms within it, are organized.
Even though silicon is ubiquitous, we are still learning about its most basic properties. The neutron, because it has no charge, is excellent to use as a probe because it doesn’t interact strongly with electrons inside the material. X-rays have some drawbacks when measuring atomic forces within a material due to their interaction with electrons.
Albert Young, Study Co-Author and Professor of Physics, North Carolina State University
The research team employed a neutron beam at the National Institute of Standards and Technology’s (NIST) Center for Neutron Research to perform pendellösung interferometry on an ideal silicon crystal. The research team was led by Benjamin Heacock while he was a graduate student at NC State and worked closely with NIST instrument scientist, Michael Huber.
Although crystal structure studies are typically conducted with X-rays, a neutron beam is orders of magnitude with lower sensitivity to atomic electron density than X-rays, which makes it an optimum probe of the vibrational characteristics of the silicon crystal.
A perfect crystal is one in which the sheet of atoms in each interior plane are identical, and their spacing and orientation repeat throughout the whole crystal. Neutrons interact with these planes as a quantum wave, and when the planes get to organize in a perfect crystal, the neuron beam gains the ability to reflect repeatedly from the crystal planes.
Every reflection bounces neutrons off the atoms in the crystal, and the reflecting wave encodes these interactions. As the neuron wave finally escapes from the crystal, the bounce history offers data on the way atomic forces are arranged and the way they impact the neurons.
The silicon crystal was organized very accurately to select a specific reflection condition. The crystal was then rotated around a perpendicular axis to generate the pattern of waves, providing the team the information regarding the arrangement of atomic forces inside the crystals.
Before this study, pendellösung had been conducted on only one alignment of the beam concerning the crystal planes, which is called a Bragg condition. In this research, two new Bragg conditions were measured with neutrons and the accuracy was enhanced by about a factor of four.
Unlike previous pendellösung measurements, our technique gives you the properties of and forces within the crystal super precisely — including the neutron charge radius and short-range forces — which add to our understanding of not just silicon, but of neutrons themselves.
Albert Young, Study Co-Author and Professor of Physics, North Carolina State University
Short-range forces are noted to be undiscovered forces that physicists have theorized to exist but which have not been noticed. This can be connected to dark energy or new theories of gravity with additional spatial dimensions.
Young hopes that the investigation of short-range forces could provide more information to scientists regarding these questions. His research also offers new tools and a new community of scientists who can address these questions using small-scale experiments.
The great thing about this work is not only the precision—we can hone in on specific observables in the crystal — but also that we can do it with a tabletop experiment, not a large collider. Making these small-scale, precise measurements could make progress on some of the most challenging questions for fundamental physics.
Albert Young, Study Co-Author and Professor of Physics, North Carolina State University
The research was supported by the U.S. Department of Energy, the National Science Foundation, the Natural Sciences and Engineering Council of Canada Discovery program, and the Canada First Research Excellence Fund.
Heacock is the first author. Co-investigators of the study include researchers from NIST; Nagoya University, Japan; Tulane University; the University of Waterloo, Canada; RIKEN Center for Advanced Photonics, Hirosawa, Japan; Triangle Universities Nuclear Laboratory, NC; and the High Energy Accelerator Research Organization, Tsukuba, Japan. Paul Huffman, professor of physics at NC State, also contributed to the study.
Journal Reference:
Heacock, B., et al. (2021) Pendellösung interferometry probes the neutron charge radius, lattice dynamics, and fifth forces. Science. doi.org/10.1126/science.abc2794.