Upscattering of Ultracold Neutrons from Gases

We present measurements of the upscattering cross sections of ultracold neutrons (UCNs) from room-temperature hydrogen, deuterium, neon, argon, xenon, C4H10, CF4, and air. The values of these cross sections are important for estimating the loss rate of trapped neutrons due to residual gas and are therefore of importance for neutron lifetime measurements using UCNs. Cross sections were obtained from a combined analysis of the UCN attenuation in a gas cell and direct measurement of the neutrons upscattered in the cell. The effects of the UCN velocity and path-length distributions were accounted for using a Monte Carlo transport code. Results are compared with measurements at higher neutron energy as well as with calculations.