Towards a high precision measurement of the free neutron lifetime with tauSPECT

Abstract

The β decay of the neutron is an interesting quantity both in particle physics as well as in astoparticle physics: A precise determination of the free neutron lifetime τ helps understanding Big Bang Nucleosynthesis, during which first light elements were produced. Furthermore, it can be used to calculate the Vud element of the CKM matrix, which is responsible for the mixing of weak eigenstates of quarks. Currently, two methods to determine the neutron lifetime exist. While beam experiments measure the decay products (’counting the dead’), bottle type experiments store very low energetic neutrons (so-called ’ultracold’ neutrons, UCN) in a vessel and count the remaining neutrons after a certain storage time (’counting the survivors’). Both methods yield results with a precision of ∆τ/τ = 3 × 10^(-3) or better, but their extracted values for the neutron lifetime differ by 3.9σ. The difficulty with both techniques is to take all systematic effects into account. In bottle type experiments such effects are e.g. interactions of UCN with the walls of the storage vessel. By replacing the material walls by strong magnetic fields, magnetic storage experiments attempt to eliminate this systematic effect and thus allow for measurements of τ with higher precision. The τSPECT experiment at the TRIGA research reactor in Mainz, Germany, is the first neutron lifetime experiment which employs only magnetic fields for the storage of UCN of energies E < 47 neV. The trap consists of a longitudinal field generated by two cylindrical superconducting coils, as well as a Halbach type octupole made from permanent magnets; the superposition of both creates a low field region, which is surrounded by strong magnetic fields. UCN are filled into the trap by an adiabatic fast passage spin flipper, which generates a rotating magnetic field B1 and transforms low energetic neutrons in the high-field seeking state to storable low-field seekers. They are stored inside the trap and after varying storage times the surviving neutrons are detected. The neutron lifetime can be extracted from an exponential fit to the counted number of neutrons depending on the storage time. First storage measurements of UCN in September 2019 were successful but yielded only low statistics, so that the target uncertainty in the neutron lifetime of ∆τ = 1.0 s would have been unachievable in a reasonable amount of measurement time. In this work, the filling process was therefore fully understood, simulated and optimised. The initially used spin flipper (SF) in form of a birdcage resonator was replaced by a double saddle coil, so that due to its high flexibility in finding the appropriate SF-position at different resonance conditions an increase in statistics by a factor of ∼ 4 was achieved. Additionally, a modified filling technique involving two double saddle coils was successfully demonstrated as proof of concept. One of the remaining systematic effects in the experiment is due to marginally trapped neutrons, which poses a loss mechanism additional to β decay. These neutrons have to be removed (’cleaned’) from the spectrum before the storage time begins. First optimisations are presented in this work together with recent storage curve measurements (preliminary results).