An experiment to measure the free neutron lifetime using magnetically trapped neutrons is described. Neutrons are loaded into a 1.1 T deep superconducting Ioffe-type trap by scattering an 0.89 nm neutron beam in an isotopically pure superfluid 4He bath cooled to below 300 mK. The neutron decays are detected by the scintillation light produced in the helium by the neutron decay products. The 0.89 nm neutron beam is produced by diffracting a cold neutron beam from a potassium intercalated graphite monochromator.
The measured trap lifetime at 300 mK and with no ameliorative magnetic ramping is substantially shorter than the free neutron lifetime. This is attributed to the presence of neutrons with an energy higher than the magnetic potential of the trap. Magnetic field ramping is implemented to eliminate these neutrons, resulting in an $833^{+74}_{-63}$s trap lifetime, that is consistent with the currently accepted value of the free neutron lifetime.
An increase in the thermal upscattering rate is observed in the temperature range 0.5 K ≤ T ≤ 0.85 K. A number of other systematic effects are studied.