Our experiment using gravitationally trapped ultracold neutrons (UCN) to measure the neutron lifetime is reviewed. Ultracold neutrons were trapped in a material bottle covered with perfluoropolyether. The neutron lifetime was deduced from comparison of UCN losses in the traps with different surface-to-volume ratios. The precise value of the neutron lifetime is of fundamental importance to particle physics and cosmology. In this experiment, the UCN storage time is brought closer to the neutron lifetime than in any experiments before: the probability of UCN losses from the trap was only 1% of that for neutron $\beta$ decay. The neutron lifetime obtained, $878.5\pm 0.7_{\mathrm{stat}}\pm 0.3_{\mathrm{sys}}$ s, is the most accurate experimental measurement to date.

• Received 11 February 2008

DOI:https://doi.org/10.1103/PhysRevC.78.035505