Abstract
Modern precision experiments trapping low-energy particles require detailed simulations of particle trajectories and spin precession to determine systematic measurement limitations and apparatus deficiencies. We developed PENTrack, a tool that allows to simulate trajectories of ultracold neutrons and their decay products—protons and electrons—and the precession of their spins in complex geometries and electromagnetic fields. The interaction of ultracold neutrons with matter is implemented with the Fermi-potential formalism and diffuse scattering using Lambert and microroughness models. The results of several benchmark simulations agree with STARucn v1.2, uncovered several flaws in Geant4 v10.2.2, and agree with experimental data. Experiment geometry and electromagnetic fields can be imported from commercial computer-aided-design and finite-element software. All simulation parameters are defined in simple text files allowing quick changes. The simulation code is written in C++ and is freely available at github.com/wschreyer/PENTrack.git.
Highlights
Neutron-lifetime experiments storing ultracold neutrons (UCNs) in material bottles recently have suffered from poorly understood apparatus effects [1, 2], e.g. unaccounted losses of UCNs at the bottle walls, and their results often deviate beyond the quoted uncertainties [3]
I depends on the loss cross sections σl,i for a given velocity vn. This cross section is the sum of absorption and inelastic-scattering cross sections, since inelastic scattering increases the energy of a UCN so far above the storage potential that it can be considered lost
If the electric field is inverted, this shift is inverted and a small phase difference between spins in opposite electric fields is accumulated over the free-precession time
Summary
Precision experiments with particles at low energies require an excellent understanding of particle trajectories. Neutron-lifetime experiments storing ultracold neutrons (UCNs) in material bottles recently have suffered from poorly understood apparatus effects [1, 2], e.g. unaccounted losses of UCNs at the bottle walls, and their results often deviate beyond the quoted uncertainties [3]. The implemented physics processes cover UCN transport, UCN storage in material bottles and magnetic traps, spin precession of neutrons and co-magnetometer atoms, and tracking of protons and electrons in electromagnetic fields. It provides a flexible configuration interface and allows to load complex electromagnetic fields and experiment geometries directly from finite-element (FEM) and computer-aided-design (CAD) software. We describe the underlying physics and algorithms, compare our results to experiments and February 21, 2017 other simulation tools, and provide examples for the optimization of experiments and the estimation of false results due to apparatus effects
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