18. The mass spectrometer. In 1910, W. Aston proposed and constructed an experiment measuring themass of a charged particle. The key elements of a mass spectrometer are shown in Figure 4. Acollimated beam enters the velocity selector. Upon emerging from slit C, these particles of velocity unow enter a region with only a magnetic induction B0perpendicular to the plane of the diagram.These particles experiencing the Lorentz force traverse a semicircular path (with radius R) until theystrike a screen. Find the resulting radius R .