# Synopsis: Slow Neutrons Are Dense

Liquid helium-$4$ is integral in a source of slow-moving neutrons, with a five-fold increase in yield in density.

Some aspects of fundamental physics that are studied in high-energy accelerator labs can also be investigated within less-expensive neutron facilities. With a mass but no charge, neutrons are particularly useful probes as they lack atomic structure and are thus immune to, for example, van der Waals and Casimir forces.

Ultracold neutrons (UCN)—slow-moving neutrons that can be trapped in containment devices with the help of effects such as geometrical constraints and gravity—can be captured for hundreds of seconds, enabling precise studies of the neutron’s static and decay properties.

Traditional UCN sources, based on liquid deuterium, made densities of about $10$ per ${\text{cm}}^{3}$ available for experiments. In their paper in Physical Review Letters, Oliver Zimmer at the Institut Laue-Langevin, France, and colleagues detail progress with a technology they published in Physical Review Letters in 2007. The authors now report a new source that produces UCN density of about $55$ per ${\text{cm}}^{3}$—a fivefold increase. This source is based on superfluid helium-$4$, which has no neutron absorption cross section or excitations below $1\phantom{\rule{0.333em}{0ex}}\text{meV}$ that can scatter the UCN. Advances in the storage time and density should prove useful to various user groups, particularly in their long-standing search for the neutron electric dipole moment. – Sami Mitra

More Features »

### Announcements

More Announcements »

Astrophysics

Read More »

## Next Synopsis

Materials Science

Read More »

## Related Articles

Atomic and Molecular Physics

### Viewpoint: Trapped Ions Test Fundamental Particle Physics

New precision experiments using trapped molecular ions provide an alternative method for determining if the electron has an electric dipole moment. Read More »

Particles and Fields

### Synopsis: Dark Photon Conjecture Fizzles

The lack of so-called “dark photons” in electron-positron collision data rules out scenarios in which these hypothetical particles explain the muon’s magnetic moment. Read More »

Nuclear Physics

### Viewpoint: Watching the Hoyle State Fall Apart

Two experiments provide the most precise picture to date of how an excited state of carbon decays into three helium nuclei. Read More »