Singly charged positive ions pass straight, without bending, through a region where an electric field of $2.0 \times 10^4\ \text{V/m}$ and a magnetic field of $0.2\ \text{T}$ are perpendicular to each other and to the ions' path. The ions then enter a region with only a uniform magnetic field of $0.5\ \text{T}$, perpendicular to their velocity, where they move in a circle of radius $4.15\ \text{cm}$. Take $e = 1.6 \times 10^{-19}\ \text{C}$ and $1\ \text{u} = 1.66 \times 10^{-27}\ \text{kg}$. The mass of each ion, in atomic mass units, is ____.