Ionisation measurements show that a particular nuclear particle carries a double charge and is moving with a speed of . It follows a circular path of radius 4.72 m in a magnetic field of 1.33 T. We have to find the mass of the particle and identify it.

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For a relativistic charged particle, charge q, moving in a circular orbit, radius r, in magnetic field, B, its momentum, p, in terms of q, B, and r is

In our problem

Therefore,

For a relativistic particle

Speed of the particle

Therefore, the mass of the charged particle is

In units of mass of a proton, the mass of the charged particle of charge is

Therefore, the nuclear particle is an alpha particle.