In a fixed-target experiment,
a charged particle such as an electron or a
proton is accelerated by an electric field and collides
with a target, which can be a solid, liquid, or gas.
A detector determines the
charge, momentum, mass, etc. of the resulting particles.
An example of this process is
Rutherford's gold foil experiment, in which the
radioactive source provided high-energy alpha particles, which collided with
the fixed target of the gold foil. The detector was the zinc sulfide screen.