Particle beam

A particle beam is a directed stream of particles prepared for transport, interaction or detection. A molecular beam can contain neutral chemical atoms or molecules; an accelerator beam can contain charged electrons, protons or ions. Its direction, energy spread, spatial distribution and particle flux describe different aspects of the preparation.

A source, direction-selecting apertures or focusing elements, a transport region and a detector perform separate tasks. For a charged beam, electric fields can change its energy and magnetic fields can bend its trajectory through the Lorentz force. A neutral atomic beam requires interactions appropriate to its neutral constituents. A vacuum reduces unwanted collisions along either transport path, but it does not itself make every particle's speed equal.

An electron beam is a directed population of electrons with specified energy, current and spatial distribution. Electric fields accelerate or focus it, while magnetic fields bend trajectories through the Lorentz force.

In an electron microscope, the beam's interaction with a specimen produces signals carrying structural information. Its brightness, energy spread and angular spread matter independently of total current. A vacuum reduces collisions with background gas, and electron-electron repulsion can limit transport at high current. Treating it as a single trajectory omits this collective and statistical structure.

A particle beam dominated by neutrinos. Accelerator beams can use particle decay of focused charged pions and kaons, followed by shielding that suppresses other beam components.

A decay tunnel provides a low-density flight region where selected unstable particles decay before remaining charged particles are removed or absorbed. For a pion-produced neutrino beam, its length balances pion decay probability against backgrounds from longer-lived daughters.

A magnetic horn uses a pulsed magnetic field around a high-current conductor to focus charged secondary pions and kaons of a selected sign. Their decays then create a directed neutrino beam. The field acts on the charged parents, not on the neutral neutrinos.

Here particle flux means the number of particles crossing a surface per unit area and unit time, with the area taken perpendicular to the incident particle beam. Multiplying by the target interaction cross section gives an event rate per target particle in the ideal independent-target model.

The particle flux of neutrinos through a specified surface; an incident-energy distribution may also be specified.

An ion source produces ions for a beam or another experiment. Ionization changes the charge state of an atom or molecule; an applied electric field can then extract charged particles into a transport region. The resulting particle beam depends on the ionization mechanism, extraction geometry and the distribution of initial positions and velocities.

One arrangement uses an electron beam to ionize a gas and an electrode voltage to extract the products. Surface ionization is another mechanism: suitable atoms can leave a hot surface as ions, as in the surface ionization detector. Producing an ion and identifying its species are separate operations, so a source may need downstream mass spectrometry or other selection when several charged species are present.

A stream of positive ions in a gas discharge. Historical canal rays pass through holes in the cathode; collisions can also produce moving neutral chemical atoms.