Protons possessing a KE of 5.00MeV are circulating around the way of the protons in the space of magnetic field which  is circular, and energy K, which is related to its momentum by K =p2/2mp. ... an electron orbiting a proton and maintained in a fixed circular path of radius ...we know that once the proton enters the region of magnetic field it will follow a circular path of radius r, and once it leaves the region it will once again move in a straight line path, tangential to the circular path in the field region. We will need to use some geometry to relate α to the given distance x and the radius of the circular path r.The path of the protons in the region of magnetic field is circular, and described by a radius r. The protons will move through the region of magnetic field across its lateral distance d, and this will define an angle α, carving out an arc of a circle of radius r.