Quasi-elastic scattering of ultracold neutrons
Ultracold neutrons (UCN) are neutrons of very low energies, enough to undergo total reflection from a given surface at any angle of incidence. As a result, they can be confined to a material trap when their kinetic energies are smaller than the Fermi potential of the trap material. However, they can be lost from the trap via nuclear capture reaction and thermal inelastic scattering from the trap surface. They can also be lost by quasi-elastic scattering when the energy gain is sufficient to exceed the Fermi potential. ^ Possible models of UCN quasi-elastic scattering are interactions with hydrogen diffusing in an impurity surface layer or with surface waves of a liquid. Using two different experimental methods we investigated both the energy-loss and the energy-gain side of quasi-elastic scattering from a Fomblin grease surface. Fomblin is a perfluorinated polymer. We also measured the up-scattering effects for various Fomblin liquids as a function of temperature and energy transfer. For a new Fomblin oil, the measured quasi-elastic up-scattering probability tends to be negligible at low temperature T < 150 K. ^ We performed storage lifetime measurements using UCN traps coated with the new oil. We achieved the longest storage lifetime τs = 872 ± 2 s at T 110 K, which is only 1.5% away from the current accepted mean neutron lifetime value τn = 885 ± 0.9 s. The corresponding UCN loss factor is η = 5 × 10−6, which is the lowest value so far. Thus, we emphasize that this method promises to improve the measured τn-value with the precision below 0.1%. This is required in particle physics and astrophysics to test various theories. ^
"Quasi-elastic scattering of ultracold neutrons"
Dissertations and Master's Theses (Campus Access).