A low-temperature proton detector for a neutron lifetime experiment

摘要

The measurement of the neutron lifetime has always been an active area in fundamental physics. The experimental value, together with one correlation between the products of the neutron β decay, protons, electrons and antineutrinos, allows to address one of the main problems in the standard model of particle physics, the unitarity of the Cabibbo-Kobayashi-Maskawa matrix. The most promising experimental method is the storage of ultra-cold neutrons (UCN) (kinetic energies below 250 neV!). These very slow neutrons have the pleasant property to be reflected when hitting walls from suitable materials. Hence they may be stored in bottles and the lifetime may be determined by varying the storage time and observing the number of surviving neutrons. The value adopted in 2006 by the Particle Data Group (PDG), τ{sub}n = 885.7 ± 0.8s, is mainly determined by one measurement with UCN stored in such a material bottle. A new experiment [1], however, using the same method, but cooling the walls, arrived at τ{sub}n = 878.5 ± 0.8 s, a value 6σ off the PDG value. Therefore it seems mandatory to scrutinize the results again by conducting a new and complementary measurement. The possibility to perform such an experiment comes from the interaction of the neutron magnetic moment with a strong magnetic field. Neutrons with the right orientation of this magnetic moment are repelled by an increasing field. B = 2T is sufficient to trap UCN with kinetic energies below 110 neV. This allows loss-less storage without wall contacts.