Velocity Selector
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A Wien filter also known as a velocity selector is a device consisting of perpendicular
electric Electricity is the set of physical phenomena associated with the presence and motion of matter possessing an electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described by Maxwel ...
and
magnetic Magnetism is the class of physical attributes that occur through a magnetic field, which allows objects to attract or repel each other. Because both electric currents and magnetic moments of elementary particles give rise to a magnetic field, m ...
fields that can be used as a velocity filter for charged particles, for example in
electron microscope An electron microscope is a microscope that uses a beam of electrons as a source of illumination. It uses electron optics that are analogous to the glass lenses of an optical light microscope to control the electron beam, for instance focusing it ...
s and spectrometers. It is used in
accelerator mass spectrometry Accelerator mass spectrometry (AMS) is a form of mass spectrometry that accelerates ions to extraordinarily high kinetic energies before mass analysis. The special strength of AMS among the different methods of mass spectrometry is its ability t ...
to select particles based on their speed. The device is composed of
orthogonal In mathematics, orthogonality (mathematics), orthogonality is the generalization of the geometric notion of ''perpendicularity''. Although many authors use the two terms ''perpendicular'' and ''orthogonal'' interchangeably, the term ''perpendic ...
electric and magnetic fields, such that particles with the correct speed will be unaffected while other particles will be deflected. It is named for Wilhelm Wien who developed it in 1898 for the study of anode rays. It can be configured as a charged particle energy analyzer,
monochromator A monochromator is an optics, optical device that transmits a mechanically selectable narrow band of wavelengths of light or other radiation chosen from a wider range of wavelengths available at the input. The name is . Uses A device that can ...
, or
mass spectrometer Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a '' mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is us ...
.


Theory

Any charged particle in an electric field will feel a force proportional to the charge and field strength such that \vec = q \vec, where ''F'' is force, ''q'' is charge, and ''E'' is electric field strength. 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 = q \vec \times \vec, 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 mathematics, the cross product or vector product (occasionally directed area product, to emphasize its geometric significance) is a binary operation on two vectors in a three-dimensional oriented Euclidean vector space (named here E), and ...
. 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 = qvB in the direction described by the cross product. Setting the two forces to equal magnitude in opposite directions it can be shown that \frac = v. Which means that any combination of electric (\vec) and magnetic (\vec) fields will allow charged particles with only velocity \vec through.


See also

* Neutron-velocity selector


References

Mass spectrometry Electron microscopy {{Electromagnetism-stub