To prepare ultra cold neutrons we start with fast neutrons and slow them down. The source we are building at TRIUMF will produce fast neutrons by stopping a beam of protons in a block of tungsten. The tungsten nuclei break up into smaller pieces. Among the fragments are large numbers of fast neutrons. The neutrons then pass through sucessivley colder materials, slowing down.
In the animation to the right, neutrons from the spallation target cool to 300 K in room temperature heavy water, then 20 K - 80 K in frozen heavy water or liquid deuterum. Finally they loose energy by phonon scattering in superfluid Helium and become ultra cold.
Overview of UCN Source and Neutron EDM Experiment:
Ultra-cold neutrons leave the UCN source and travel through the rotary valve up to the EDM cell. The door valve closes and they spend time in the cell, during which measurements are made. The door valve on the EDM cell then opens, and the neutrons fall down through the redirected rotary valve into the detector.