In ion optics, there is a concept called Magnetic Rigidity. When an charged ion passing through a magnetic field, it will bend and rotate. The magnetic rigidity is the $B\rho$, where $B$ is the magnetic field strength in Tesla, and $\rho$ is the rotation radius.

using basics physics, we have the centripetal force equals to the Lorentz force,

$\displaystyle \frac{mv^2}{\rho} = Q \vec{v}\times \vec{B}$

Simplify,

$B\rho = mv/Q$

The above is non-relativistic, and I always assume the $mv$ can be replaced by relativistic momentum $p = m \gamma \beta$. Now I give a prove.

$\displaystyle \vec {F} = m \gamma \vec{a_\perp} = - m \gamma\frac{ v^2}{\rho} \hat{\rho}$

Thus,

$\displaystyle m \gamma \frac{v^2}{\rho} = QvB$

$\displaystyle m \gamma \beta = p = cQB \rho$

where $c$ is speed of light. In the last equation, the unit of $p$ is MeV/c, $c = 299.792458$ mm/ns, and $\rho$ in meter.