Investigation of titanium and polyethylene as UCN absorber materials with AbEx

Marginally trapped neutrons are a major source of systematic errors in storage experiments with ultracold neutrons (UCN): their energies slightly exceed the trapping potential and their storage lifetimes are of the same order of magnitude as the neutron beta-decay lifetime to be measured. Hence, they have to be removed before the actual neutron storage period starts. For the magneto-gravitational neutron-lifetime experiment PENeLOPE, a novel absorber scheme was proposed; its efficiency to reduce the systematic influence of marginally trapped UCN on the extracted beta-decay lifetime value had to be investigated. To this end, the cryogenic material-storage experiment AbEx (Ab sorber Ex periment) was conducted at ILL, Grenoble; neutron-optical properties of storage and absorption materials were investigated. Storage lifetimes shorter than 10s could be reached for high-energy UCN with the proposed scheme. This translates to a systematic effect on the neutron-lifetime measurement with PENeLOPE of Delta tau(n) < 0.03 s. Polyethylene (PE) and titanium were tested is absorber materials. The temperature dependence of their UCN absorbing efficiency was determined to be rather small and connected not with the upscattering cross-section, but probably with surface contaminations. (C) 2009 Elsevier B.V. All rights reserved.