A Neutron Elevator to Increase Neutron Loading Efficiency and Improve the Neutron Lifetime Precision in the UCNτ+ Experiment*

Measurement of the neutron lifetime τ_n to a precision of 0.1 s is key to calculate the CKM matrix element V_ud, the probability of up-to-down quark transition in charge-current weak decays, accurately enough to test the unitarity of the CKM matrix and investigate physics beyond the Standard Model. The UCNτ experiment at Los Alamos National Laboratory (LANL) traps ultra-cold neutrons (UCNs) in a bowl-shaped Halbach array using gravity. Sheets of cleaner materials then remove neutrons with energy >38 meV via both up-scattering and capture. The resulting population of neutrons in the trap, measured via coincidence events between two PMTs over some time period, is then fitted to an exponential decay curve to determine τ_n. UCNτ has measured a value of τ_n = 877.75 +- 0.28_stat + 0.22/- 0.16_syst s, the most precise τ_n measured so far.

The UCNτ+ upgrade increases loading efficiency via an elevator to lower UCNs into the trap: higher-energy neutrons will transfer energy to the elevator by colliding with its front wall, resulting in a lower-energy population of UCNs. This minimizes error by reducing the number of higher-energy neutrons that escape the trap and maximizing the number of UCNs cold enough to remain after cleaning. It is critical that the elevator moves smoothly: any vibrations will transfer energy from the elevator back to the neutrons, heating them up. This poster will describe the UCNτ+ procedure and present accelerometer data on the elevator’s vibrations that demonstrates the repeatability of the motion.