Visualising strong magnetic fields with neutrons
Researchers at the Paul Scherrer Institute PSI have developed a new method with which strong magnetic fields can be precisely measured. They use neutrons obtained from the SINQ spallation source. In the future, it will therefore be possible to measure the fields of magnets that are already installed in devices and thus are inaccessible by other probing techniques. The researchers have now published their results in the journal Nature Communications.
Neutrons are, as their name suggests, electrically neutral and are the building blocks of almost all atomic nuclei. Neutrons interact with magnetic fields due to their so-called spin. Researchers at the Paul Scherrer Institute PSI have now shown that this property can be used to visualise magnetic fields. They used polarised neutrons, which means that all of the neutrons have the same spin orientation.
If beams of polarised neutrons pass through a magnetic field, a refraction of the neutron beam can be detected behind this field. From the refraction pattern, the magnetic field and in particular the differences in field strengths can be reconstructed. For the first time this method, also known as polarised neutron grating interferometry (pnGI), has been used to measure magnetic fields.
One million times stronger than Earth's magnetic field
pnGI can be used to measure very strong magnetic fields with a so-called gradient strength in the order of 1 Tesla per centimeter. "This allows us to move in orders of magnitude about one million times stronger than Earth's magnetic field", says Christian Grünzweig, a neutron researcher at the Paul Scherrer Institute PSI. Until now, neutrons could only be used to measure significantly weaker magnetic fields.
From alternators to MRI systems
Numerous applications are conceivable for the new method, above all because neutrons penetrate most materials non-destructively. "We can also probe magnetic fields that are difficult to access because they are already built into an apparatus", explains Jacopo Valsecchi, first author of the study and a doctoral candidate working at PSI. "Applications range from alternators in car engines to many components of the energy supply system to magnetic fields from magnetic resonance tomography systems used in medicine."
The researchers proved that their method works by using computer models to simulate the expected results of the measurement. They then checked whether comparable results could actually be achieved with a real measurement. "The results from the simulations and the actual measurement results agree very well", says Grünzweig.
With the new method, fluctuations in the magnetic field can also be detected. For example, even permanent magnets, such as those familiar from magnetic stickers for refrigerator doors, do not have a homogeneous magnetic field. "We can now detect possible gradients, even if the magnetic field is very strong", says physicist Valsecchi.
The researchers have now published their results in the journal Nature Communications.
Text: Paul Scherrer Institute/Sebastian Jutzi
The Paul Scherrer Institute PSI develops, builds and operates large, complex research facilities and makes them available to the national and international research community. The institute's own key research priorities are in the fields of matter and materials, energy and environment and human health. PSI is committed to the training of future generations. Therefore about one quarter of our staff are post-docs, post-graduates or apprentices. Altogether PSI employs 2100 people, thus being the largest research institute in Switzerland. The annual budget amounts to approximately CHF 407 million. PSI is part of the ETH Domain, with the other members being the two Swiss Federal Institutes of Technology, ETH Zurich and EPFL Lausanne, as well as Eawag (Swiss Federal Institute of Aquatic Science and Technology), Empa (Swiss Federal Laboratories for Materials Science and Technology) and WSL (Swiss Federal Institute for Forest, Snow and Landscape Research). (Last updated in May 2019)