About UCN

What Are Ultra-Cold Neutrons

Ultra-Cold Neutrons (UCN) are moving so slowly that they bounce off solid materials and can be contained the same way you would contain a gas in, say, a metal or glass bottle. A typical UCN velocity is less than 7 m/s, corresponding to a temperature of 3 mK (0.003 K) or a kinetic energy of 300 neV.^‡

Unlike ultra-cold neutrons, fast neutrons are hard to stop. If fast neutrons hit a 4 mm thick nickel plate, about 9 out of 10 will go straight through without any interaction^†. The fast neutron has to make a "direct hit" on a nucleus. This is a rare event because the solid-looking nickel is actually small nickel nuclei separated by essentially empty space. The inter-atomic spacing in solid nickel is 250,000 fm, whereas the nickel nuclei are only about 10 fm in diameter. To a fast neutron this looks like like golf balls spaced 1 km apart. As the speed goes down, the neutron becomes more and more likely to interact. Eventually, a speed is reached where the material looks like a solid wall and neutrons bounce back.  Ultra-cold neutrons have such a long wavelength that they interact with hundreds of nuclei at a time.

For each material there is a critical neutron velocity below which that material looks like a "solid wall". The table below shows values for some common materials.

‡ In UCN literature, you will find temperature, speed, and energy used interchangeably.
† Assuming a 3 barn total cross section.