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. Details about the first demonstration of the neutron loading process are reported here.
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
numerous 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.
In 2023, the tSPECT experiment was relocated to the UCN source at the Paul Scherrer Institute, Villigen, Switzerland. There we profit from the 24/7 operation of the UCN source and the higher per-shot number of stored neutrons compared to the UCN facility at JGU.
Within about three months, the team turned an empty experimental site again into a running experiment.
The current version of the tSPECT experiment shall allow us to determine the free neutron lifetime with 0.3 s accuracy.
The team (as of September 2025) got together at the experimental site during the PSI 2025 conference.
An overview presentation and four posters about the status of the experiment, the normalization detector, the Monte-Carlo simulation framework, and a spatially resolving UCN detector can be found here.