tSPECT: Determination of the free neutron lifetime
The tSPECT experiment aims to measure the free neutron lifetime by suspending very low-energy neutrons, so-called ultracold neutrons (UCNs), in a fully magnetic trap without collisions with material walls. Based on the interaction of the neutron magnetic moment with a magnetic field gradient, we suspend UCNs in free space. UCNs that are minimizing their potential energy in a low magnetic field region can be stored there to measure their decay rate through neutron β-decay. With a combination of permanent and superconducting magnets, we create a suitable trapping region inside the cold bore of a cryostat.
In this magnetic field environment, the potential energy of the UCNs depends on the orientation of their spin relative to the direction of the external magnetic field. UCNs that are repelled from high-field regions are called low-field-seeking UCNs (LFS UCN). UCNs that get attracted towards stronger magnetic fields are called high-field-seeking UCNs.
We convert UCNs between these two states with fast adiabatic spin flippers to turn a repulsive potential into a trapping potential.
We store UCNs in the three-dimensional trap for various periods of time, and then count the number of surviving neutrons at the end of the storage cycle. In this way we obtain a storage curve, characterizing the storage time constant of the bottle for UCNs. To extract the lifetime of the free neutron, we have to correct for
a large number of systematic bias effects.
The tSPECT experiment was developed at JGU's source for ultracold neutrons. Here we demonstrated for the very first how one can load spin polarized UCNs in a magnetic trap by flipping their spin orientation.
The experimental setup at JGU is shown before the experiment was relocated to the UCN source at the Paul Scherrer Institute, Switzerland, with the team in 2022.
The tSPECT experiment is now in full operation at the UCN source at PSI since 2023 to gather a data set, that allows us to determine the free neutron lifetime with 0.3 s accuracy. At PSI, a much higher UCN intensity is available.
