A precision neutron lifetime measurement using Ultracold neutrons in a magneto-gravitational trap.

UCNtau Experiment





Meetings & talks



Design Features:

  • Permanent magnet (PM) configuration along trap floor repels spin polarized neutrons (low field seekers), avoiding UCN loss on material surfaces by magnetic levitation.
  • Trap asymmetry and B ripple cause quasi-trapped, super-barrier neutrons to quickly evolve into escape trajectories, minimizing errors in extracting the beta-decay lifetime.
  • Toroidal current configuration generates non-zero holding B field throughout the trap, minimizing additional UCN loss from spin depolarization by crossing field-zeros.

Magneto-Gravitational Trap:

(Left) IU undergraduate, Bailey Slaughter, demonstrates the working of the gravitational trap, and serves as a field-mapping robot. (Right) The Halbach array was delivered to Los Alamos Jan. 2013, picked up with a crane in the mid-air, and inserted into the vacuum chamber.


The trap door actuation (iu box) (video download)

A Cu gasket dropping on the surface of the Halbach array (video1, video2)

The actuation of a UCN cleaner (video)

Portable UCN (video1, video2)


Aerial view in area B (2013) with the UCNtau apparatus installed in the UCN beamline. The UCNA spectrometer can be seen in the background.

The V-detector package on top of the vacuum chamber. In the Feb. 2013 run, only two NaI detectors were used.

A side view of the UCNtau apparatus.


last updated by C.-Y. Liu, 04/16/2014