My watch list

Velocity selector

A velocity selector is used in accelerator mass spectroscopy to select particles based on their speed. The velocity selector is composed of orthogonal electric and magnetic fields, such that particles with the correct charge to mass ratio and speed will be unaffected, and other particles will be deflected.

Additional recommended knowledge

Correct Test Weight Handling Guide: 12 Practical Tips

How to quickly check pipettes?

Daily Visual Balance Check

Theory

Any charged particle in an electric field will feel a force proportional to the charge and field strength such that $\vec{F} = q \vec{E}$ , where F is force, q is charge, and E is electric field. Similarly, any particle moving in a magnetic field will feel a force proportional to the velocity and charge of the particle. The force felt by any particle is then equal to $\vec{F} = q \vec{v} \times \vec{B}$ , where F is force, q is the charge on the particle, v is the velocity of the particle, B is the strength of the magnetic field, and $\times$ is the cross product. In the case of a velocity selector, the magnetic field is always at 90 degrees to the velocity and the force is simplified to $F = q v B$ in the direction described be the cross product.

Setting the two forces to equal magnitude in opposite directions it can be shown that $\frac{E}{B} = v$ . Which means that any combination of electric ( $\vec{E}$ ) and magnetic ( $\vec{B}$ ) fields will allowed charged particles with only velocity $\vec{v}$ through.

Applications

Accelerator mass spectroscopy

References

Tuniz, C., et al., "Accelerator Mass Spectrometry: Ultrasensitive Analysis for Global Science", CRC Press, (1998) ISBN 0-8493-4538-3
Muzikar, P., et al., "Accelerator Mass Spectrometry in Geologic Research", Geological Society of America Bulletin v. 115 (2003) p. 643 - 654.

Category: Spectroscopy