Simulations and Component Characterizations for the Neutron Lifetime Experiment PENeLOPE

Abstract

The neutron lifetime is an essential parameter in Big Bang nucleosynthesis, and to determine Vud of the CKM matrix. Currently, a 4σ discrepancy exists between beam and trap lifetime experiments. The PENeLOPE experiment aims to measure the neutron lifetime using ultracold neutrons (UCNs) with a precision of 0.1 s. Two butterfly shutters were tested for PENeLOPE and had average relative leak rates of 2.23 ± 1.10 % and 1.01 ± 0.85 %,which proved them to be inadequate. PENeLOPE simulations showed the precision goal is met with a stainless-steel spin-flip probability (βSS) of 10−5, resulting in a 0.0451 ± 0.0022 s systematic shift created by marginally trapped UCNs. Seven runs yielded a statistical precision of 0.096 ± 0.0012 s, using the neutron yield from the planned UCN source at FRM II. Higher βSS values caused prohibitively large systematic shifts, which could be mitigated by nickel-phosphorous coating to reduce β.