Localization and detection of molecules with the use of radioactive radiations and produce a photograph on the photographic emulsion is known as autoradiography and the film obtained is called autoradiography.

(1)    Alpha rays which consist of 2 neutrons and 2 protons and in fact charged helium atoms.  Polonium-210 is an alpha rays emitter.
(2)    Beta rays are the electrons emitted by nuclei. These are emitted from C14, P32
(3)    Gamma rays are electromagnetic rays which resemble X-rays.  CO60 is a gamma emitter and it has the highest penetration power.
When this β-particle or $\gamma$-rays passed through photographic emulsion the silver ions of the emulsion are converted to silver atoms.
Direct autoradiography-In this sample is placed with intimate contact with the film and produce black spots on the photographic film. Although this technique is not used for high energy particles like $\beta$ -rays and $\gamma$-rays because they pass through the film so they are detected by the indirect method.
Indirect autoradiography-In this technique emitted energy is converted to light by use of a scintillator or by fluorography. Scintillators are the molecules which have the capability to luminescence by any ionizing radiation. So every sample used in this process is coated with the scintillator. The radioactive emissions transfer their energy to the scintillator molecules, which generates photon and they are exposed to photographic emulsion.
Two General methods by which autoradiography is done which are in-vivo autoradiography. In first molecules are radiolabeled inside the living tissues and tissue is removed, processed, and visualized. In-vitro autoradiography tissues are isolated from specimen and slide-mounted and then incubated with radioligand and visualized. Any emissions passing through the photographic emulsion are absorbed by the screen and converted to light, effectively superimposing a photographic image upon the direct autoradiographic films.
A single hit by an alpha particle or gamma rays can produce hundreds of silver atoms, but a single hit by a photon of light produces only a single silver atom. Measurement of radioactivity is done by Geiger Muller counter and a Scintillation counter.
Application
Localization of a radioactive substance, which are either bound to a receptor or enzyme or hybridized with the nucleic acid.
PET and SPECT for three dimensional localization of the radiation source.
Quantitative analysis of macroautographs.
Analyze the length and number of DNA.