  CrSBr is an air-stable 2D van der Waals semiconducting magnet with great
technological promise, but its atomic-scale magnetic interactions -- crucial
information for high-frequency switching -- are poorly understood. We present
an experimental study to determine the CrSBr magnetic exchange Hamiltonian and
bulk magnon spectrum. We confirm the $A$-type antiferromagnetic order using
single crystal neutron diffraction. We also measure the magnon dispersions
using inelastic neutron scattering and rigorously fit the excitation modes to a
spin wave model. The magnon spectrum is well described by an intra-plane
ferromagnetic Heisenberg exchange model with seven nearest in-plane exchanges.
This fitted exchange Hamiltonian enables theoretical predictions of CrSBr
behavior: as one example, we use the fitted Hamiltonian to predict the presence
of chiral magnon edge modes with a spin-orbit enhanced CrSBr heterostructure.
