A neutron howitzer is a

neutron source A neutron source is any device that emits neutrons, irrespective of the mechanism used to produce the neutrons. Neutron sources are used in physics, engineering, medicine, nuclear weapons, petroleum exploration, biology, chemistry, and nuclear p ...

that emits neutrons in a single direction. It was discovered in the 1930s that

alpha radiation Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle (helium nucleus). The parent nucleus transforms or "decays" into a daughter product, with a mass number that is reduced by four and an atom ...

that strikes the

beryllium Beryllium is a chemical element; it has Symbol (chemistry), symbol Be and atomic number 4. It is a steel-gray, hard, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with ...

nucleus Nucleus (: nuclei) is a Latin word for the seed inside a fruit. It most often refers to: *Atomic nucleus, the very dense central region of an atom *Cell nucleus, a central organelle of a eukaryotic cell, containing most of the cell's DNA Nucleu ...

would release

neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...

s. The high speed of the alpha is sufficient to overcome the relatively low

Coulomb The coulomb (symbol: C) is the unit of electric charge in the International System of Units (SI). It is defined to be equal to the electric charge delivered by a 1 ampere current in 1 second, with the elementary charge ''e'' as a defining c ...

barrier of the beryllium nucleus, the repulsive force due to the positive charge of the nucleus, which contains only four protons, allowing for fusion of the two particles, releasing energetic neutrons. In 1930

Walther Bothe Walther Wilhelm Georg Bothe (; 8 January 1891 – 8 February 1957) was a German physicist who shared the 1954 Nobel Prize in Physics with Max Born "for the coincidence method and his discoveries made therewith". He served in the military durin ...

and Herbert Becker in

Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...

found that alpha particles striking light elements such as

boron Boron is a chemical element; it has symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the boron group it has three ...

, or

lithium Lithium (from , , ) is a chemical element; it has chemical symbol, symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard temperature and pressure, standard conditions, it is the least dense metal and the ...

would release a highly penetrating radiation, at first believed to be

gamma radiation 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 ...

, although it was more penetrating than any gamma rays known. The next important contribution was reported in 1932 by

Irène Joliot-Curie Irène Joliot-Curie (; ; 12 September 1897 – 17 March 1956) was a French chemist and physicist who received the 1935 Nobel Prize in Chemistry with her husband, Frédéric Joliot-Curie, for their discovery of induced radioactivity. They were ...

and Frédéric Joliot in

Paris Paris () is the Capital city, capital and List of communes in France with over 20,000 inhabitants, largest city of France. With an estimated population of 2,048,472 residents in January 2025 in an area of more than , Paris is the List of ci ...

, who showed that if this unknown radiation fell on

paraffin wax Paraffin wax (or petroleum wax) is a soft colorless solid derived from petroleum, coal, or oil shale that consists of a mixture of hydrocarbon molecules containing between 20 and 40 carbon atoms. It is solid at room temperature and melting poi ...

or any other

hydrogen Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

-containing compound it ejected

proton A proton is a stable subatomic particle, symbol , Hydron (chemistry), H+, or 1H+ with a positive electric charge of +1 ''e'' (elementary charge). Its mass is slightly less than the mass of a neutron and approximately times the mass of an e ...

s of very high energy. Finally, in 1932 the physicist

James Chadwick Sir James Chadwick (20 October 1891 – 24 July 1974) was an English nuclear physicist who received the Nobel Prize in Physics in 1935 for his discovery of the neutron. In 1941, he wrote the final draft of the MAUD Report, which inspired t ...

England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It is located on the island of Great Britain, of which it covers about 62%, and List of islands of England, more than 100 smaller adjacent islands. It ...

performed a series of experiments showing that the gamma ray hypothesis was untenable, and suggested that the new radiation consisted of uncharged particles of approximately the mass of the proton. He performed a series of experiments to verify this, these uncharged particles were eventually called "neutrons", and Chadwick is credited with this discovery. Any alpha-emitting

radioisotope A radionuclide (radioactive nuclide, radioisotope or radioactive isotope) is a nuclide that has excess numbers of either neutrons or protons, giving it excess nuclear energy, and making it unstable. This excess energy can be used in one of three ...

will suffice, but usually a high

specific activity Specific activity (symbol ''a'') is the activity per unit mass of a radionuclide and is a physical property of that radionuclide. It is usually given in units of becquerel per kilogram (Bq/kg), but another commonly used unit of specific activi ...

alpha-emitter is chosen. Historically a variety of isotopes such as

radium Radium is a chemical element; it has chemical symbol, symbol Ra and atomic number 88. It is the sixth element in alkaline earth metal, group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, ...

( Ra-226) were used, but in modern times the transuranic isotopes Am-241 and

Pu-239 Plutonium-239 ( or Pu-239) is an isotope of plutonium. Plutonium-239 is the primary fissile isotope used for the production of nuclear weapons, although uranium-235 is also used for that purpose. Plutonium-239 is also one of the three main iso ...

are almost exclusively used in AmBe and PuBe neutron sources, respectively. The alpha emitter and the beryllium are pulverized and mixed together in close intimate contact to ensure a high percentage of alpha-emitter and beryllium nuclei in close contact, since the alpha particle has a very short range through material, and would lose energy preventing reaction if sufficiently far away. This mixture of material is then packed into a suitable carrier with

radiation shielding Radiation protection, also known as radiological protection, is defined by the International Atomic Energy Agency (IAEA) as "The protection of people from harmful effects of exposure to ionizing radiation, and the means for achieving this". Exposu ...

, with one end open to allow the neutrons to shoot out in the direction of the open end, thus acting like a

howitzer The howitzer () is an artillery weapon that falls between a cannon (or field gun) and a mortar. It is capable of both low angle fire like a field gun and high angle fire like a mortar, given the distinction between low and high angle fire break ...

. Neutron howitzers were used by

Otto Hahn Otto Hahn (; 8 March 1879 – 28 July 1968) was a German chemist who was a pioneer in the field of radiochemistry. He is referred to as the father of nuclear chemistry and discoverer of nuclear fission, the science behind nuclear reactors and ...

, Fritz Strassman, and

Lise Meitner Elise Lise Meitner ( ; ; 7 November 1878 – 27 October 1968) was an Austrian-Swedish nuclear physicist who was instrumental in the discovery of nuclear fission. After completing her doctoral research in 1906, Meitner became the second woman ...

in 1938 to bombard

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 ...

nuclei with neutrons in the hopes of making

transuranic elements The transuranium (or transuranic) elements are the chemical elements with atomic number greater than 92, which is the atomic number of uranium. All of them are radioactively unstable and decay into other elements. Except for neptunium and pluton ...

. To their surprise, they found

barium Barium is a chemical element; it has symbol Ba and atomic number 56. It is the fifth element in group 2 and is a soft, silvery alkaline earth metal. Because of its high chemical reactivity, barium is never found in nature as a free element. Th ...

residue, a clear indication that they had instead fissioned uranium nuclei. This discovery led to the development of the first

nuclear reactor A nuclear reactor is a device used to initiate and control a Nuclear fission, fission nuclear chain reaction. They are used for Nuclear power, commercial electricity, nuclear marine propulsion, marine propulsion, Weapons-grade plutonium, weapons ...

in 1942, and ultimately

nuclear weapons A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either nuclear fission, fission (fission or atomic bomb) or a combination of fission and nuclear fusion, fusion reactions (thermonuclear weap ...

in 1945.

References

{{Reflist Neutron sources Neutron-related techniques