A magneto-gravitational trap for absolute measurement of the ultra-cold neutron lifetime

We present a conceptual design for an experiment to measure the neutron lifetime with a relative accuracy of 10^{- 4}. The measurement will be performed by observing the decay rate of a sample of ultra-cold neutrons (UCN) confined in vacuum in a magneto-gravitational trap. The UCN sources under development are expected to produce stored UCN densities of the order of 10 cm^{- 3}. Such densities make possible determination of the neutron lifetime with an improved level of accuracy using novel neutron traps. The measurement involves no fundamentally new technologies beyond the UCN source. We discuss the trap design and methodology for the measurement. The slow loss of neutrons in the trap volume that have stable orbits, but are not energetically trapped, would produce a systematic uncertainty in the measurement. This is a potential systematic source of error in lifetime measurements with earlier magnetic and magneto-gravitational traps. We discuss our approach to eliminating this problem by a combination of trap asymmetry and field ripple to induce chaotic behavior in the orbits of neutrons with enough energy to escape the trap.