Particle Detection:
Measuring Momentum


The initial momentum of a charged particle can be calculated from the curvature of its track whilst it is moving in a magnetic field. This is because of the Lorentz force.

As the diagram of the Dě detector in section 3.2 shows, there is a solenoid around the tracking chamber. The magnetic field from the solenoid exerts the Lorentz Force on any charged particle within the detector, curving its path.

The magnetic field in Dě is 2 Tesla. In contrast, the magnetic field of the Earth is only about 0.05 milliTesla so this field is huge!

In the  Dě experiment the magnetic field is orthogonal to the direction of motion of the particles. Therefore the magnetic force exerted on a particle is given by;

If we equate this with the centripetal force of circular motion, and use p = mv, we can find the momentum of a particle in terms of its radius of curvature.

The greater the curvature of the path, the smaller the momentum of the particle, and vice-versa. Positively and negatively charged particles curve in opposite directions in the field.


Last modified Wed 23 January 2002 . View page history
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