Photofission is a process in which a nucleus, after absorbing a

gamma ray A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...

, undergoes

nuclear fission Nuclear fission is a reaction in which the nucleus of an atom splits into two or more smaller nuclei. The fission process often produces gamma photons, and releases a very large amount of energy even by the energetic standards of radioactiv ...

and splits into two or more fragments. The reaction was discovered in 1940 by a small team of engineers and scientists operating the Westinghouse Atom Smasher at the company's Research Laboratories in Forest Hills, Pennsylvania. They used a 5 MeV

proton beam A charged particle beam is a spatially localized group of electrically charged particles that have approximately the same position, kinetic energy (resulting in the same velocity), and direction. The kinetic energies of the particles are much la ...

to bombard

fluorine Fluorine is a chemical element; it has Chemical symbol, symbol F and atomic number 9. It is the lightest halogen and exists at Standard temperature and pressure, standard conditions as pale yellow Diatomic molecule, diatomic gas. Fluorine is extre ...

and generate high-energy

photons A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that ...

, which then irradiated samples of

uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...

and

thorium Thorium is a chemical element; it has symbol Th and atomic number 90. Thorium is a weakly radioactive light silver metal which tarnishes olive grey when it is exposed to air, forming thorium dioxide; it is moderately soft, malleable, and ha ...

. Gamma radiation of modest energies, in the low tens of MeV, can induce fission in traditionally fissile elements such as the

actinide The actinide () or actinoid () series encompasses at least the 14 metallic chemical elements in the 5f series, with atomic numbers from 89 to 102, actinium through nobelium. Number 103, lawrencium, is also generally included despite being part ...

plutonium Plutonium is a chemical element; it has symbol Pu and atomic number 94. It is a silvery-gray actinide metal that tarnishes when exposed to air, and forms a dull coating when oxidized. The element normally exhibits six allotropes and four ...

, and

neptunium Neptunium is a chemical element; it has chemical symbol, symbol Np and atomic number 93. A radioactivity, radioactive actinide metal, neptunium is the first transuranic element. It is named after Neptune, the planet beyond Uranus in the Solar Syste ...

. Experiments have been conducted with much higher energy gamma rays, finding that the photofission cross section varies little within ranges in the low GeV range. Baldwin ''et al'' made measurements of the yields of photo-fission in uranium and

together with a search for photo-fission in other heavy elements, using continuous

x-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

s from a 100-MeV betatron. Fission was detected in the presence of an intense background of x-rays by a differential ionization chamber and linear amplifier, the substance investigated being coated on an electrode of one chamber. They deduced the maximum cross section being of the order of 5×10⁻²⁶ cm² for uranium and half that for thorium. In the other elements studied, the cross section must be below 10⁻²⁹ cm².

Photodisintegration

Photodisintegration Photodisintegration (also called phototransmutation, or a photonuclear reaction) is a nuclear process in which an atomic nucleus absorbs a high-energy gamma ray, enters an excited state, and immediately decays by emitting a subatomic particle. The ...

(also called phototransmutation) is a similar but different physical process, in which an extremely high energy

interacts with an

atomic nucleus The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford at the Department_of_Physics_and_Astronomy,_University_of_Manchester , University of Manchester ...

and causes it to enter an

excited state In quantum mechanics Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Add ...

, which immediately decays by emitting a

subatomic particle In physics, a subatomic particle is a particle smaller than an atom. According to the Standard Model of particle physics, a subatomic particle can be either a composite particle, which is composed of other particles (for example, a baryon, lik ...

References

{{Nuclear processes Nuclear physics Nuclear chemistry Nucleosynthesis Nuclear fission