Design and performance of a cryogenic apparatus for magnetically trapping ultracold neutrons

• We describe the cryogenic design and performance of an apparatus used to magnetically confine ultracold neutrons.
• The apparatus is capable of cooling 21 l of isotopically pure 4He to approximately 250 mK.
• The system is designed around a high-current, 1.5 m long, horizontally-oriented superconducting Ioffe-type magnetic trap.
• The apparatus was successfully cooled and remained cold for a period of approximately six months.

The cryogenic design and performance of an apparatus used to magnetically confine ultracold neutrons (UCN) is presented. The apparatus is part of an effort to measure the beta-decay lifetime of the free neutron and is comprised of a high-current superconducting magnetic trap that surrounds ∼21 l of isotopically pure 4He cooled to approximately 250 mK. A 0.89 nm neutron beam can enter the apparatus from one end of the magnetic trap and a light collection system allows visible light generated within the helium by decays to be transported to detectors at room temperature. Two cryocoolers are incorporated to reduce liquid helium consumption.