A Wien filter also known as velocity selector is a device consisting of perpendicular
electric
Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described b ...
and
magnetic
Magnetism is the class of physical attributes that are mediated by a magnetic field, which refers to the capacity to induce attractive and repulsive phenomena in other entities. Electric currents and the magnetic moments of elementary particles ...
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 accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a ...
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 mass spectrometric methods is its power to separate a r ...
to select particles based on their speed. The device is composed of orthogonal
In mathematics, orthogonality is the generalization of the geometric notion of '' perpendicularity''.
By extension, orthogonality is also used to refer to the separation of specific features of a system. The term also has specialized meanings in ...
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
Wilhelm Carl Werner Otto Fritz Franz Wien (; 13 January 1864 – 30 August 1928) was a German physicist who, in 1893, used theories about heat and electromagnetism to deduce Wien's displacement law, which calculates the emission of a blackbody ...
who developed it in 1898 for the study of anode rays
An anode ray (also positive ray or canal ray) is a beam of positive ions that is created by certain types of gas-discharge tubes. They were first observed in Crookes tubes during experiments by the German scientist Eugen Goldstein, in 1886. Lat ...
. It can be configured as a charged particle energy analyzer, monochromator
A monochromator is an 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 from the Greek roots ''mono-'', "si ...
, 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 u ...
.
Theory
Any charged particle in an electric field will feel a force proportional to the charge and field strength such that , 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 , 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 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 i ...
. In the case of a velocity selector, the magnetic field is always at 90 degrees to the velocity and the force is simplified to in the direction described by the cross product.
Setting the two forces to equal magnitude in opposite directions it can be shown that . Which means that any combination of electric () and magnetic () fields will allow charged particles with only velocity through.
See also
*Neutron velocity selector
A neutron-velocity selector is a device that allows neutrons of defined velocity to pass while absorbing all other neutrons, to produce a monochromatic neutron beam.
It has the appearance of a many-bladed turbine. The blades are coated with a st ...
References
Mass spectrometry
Electron microscopy
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