Abstract
The Kassiopeia software package was originally developed to simulate electromagnetic fields and charged particle trajectories for neutrino mass measurement experiments. Recent additions to Kassiopeia also allow it to simulate neutral particle trajectories in magnetic fields based on their magnetic moments. Two different methods were implemented: an exact method that can work for arbitrary fields and an adiabatic method that is limited to slowly-varying fields but is much faster for large precession frequencies. Additional interactions to simulate reflection of ultracold neutrons (UCNs) from material walls and to allow spin–flip pulses were also added. These tools were used to simulate neutron precession in a room temperature neutron electric dipole moment experiment and predict the values of the longitudinal and transverse relaxation times as well as the trapping lifetime. All three parameters are found to closely match the experimentally determined values when simulated with both the exact and adiabatic methods, confirming that Kassiopeia is able to accurately simulate neutral particles. This opens the door for future uses of Kassiopeia to prototype the next generation of atomic traps and UCN experiments.
Highlights
At sufficiently low energies, neutrons can be reflected from material walls by the coherent strong interaction [1, 2]
We demonstrate the ability of Kassiopeia [16], a software package originally developed for neutrino experiments, to accurately simulate ultracold neutron storage
Since the adiabatic equation of motion (6b) for fully aligned or anti-aligned spins always gives a zero derivative of the aligned spin, our simulations instead started spins at 1◦ or 179◦ to the magnetic field, though we found that any angle up to a few degrees led to indistinguishable results, as the dominant contribution to T1 came from reflection depolarization
Summary
Neutrons can be reflected from material walls by the coherent strong interaction [1, 2] Such neutrons can be stored in material traps, and are termed “ultracold neutrons” (UCNs). The ability of UCNs to be stored for prolonged periods makes them of interest in several experimental areas. Several software packages have previously been used to simulate ultracold neutrons, including Geant4 [11], PENTrack [12], STARucn [13] and MCUCN [14]. Some comparisons between these are made in [12, 15].
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