Neutron howitzer
Encyclopedia
The name "Neutron Howitzer" was given to a neutron source
that was developed after it was discovered in the 1930s that alpha radiation that strikes the Beryllium
nucleus
would release neutron
s. The high speed of the alpha is sufficient to overcome the relatively low Coulomb barrier of the Beryllium nucleus, which contains only four protons, allowing for fusion
of the two particles, releasing energetic neutrons.
In 1930 Walther Bothe
and Herbert Becker in Germany
found that alpha particles striking light elements such as beryllium
, boron
, or lithium
would release a highly penetrating radiation, at first believed to be a gamma radiation although it was more penetrating than any gamma rays known. The next important contribution was reported in 1932 by Irène Joliot-Curie
and Frédéric Joliot in Paris
, who showed that if this unknown radiation fell on paraffin
or any other hydrogen
-containing compound it ejected proton
s of very high energy. Finally, in 1932 the physicist James Chadwick
in England
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 will suffice, but usually a high specific activity
alpha-emitter is chosen, such as Radium
(Ra-226). The alpha emitter and the Beryllium are pulverized and mixed together in close intimate contact to insure a high percentage of alpha-emitter and beryllium nuclei in close contact, since the alpha 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, with one end open to allow the neutrons to shoot out, thus acting like a howitzer
.
Neutron howitzers were used by Otto Hahn
, Fritz Strassman and Lise Meitner
in 1938 to bombard uranium
nuclei with neutrons in the hopes of making transuranic elements. To their surprise, they found barium
residue, a clear indication that they had instead fissioned
uranium nuclei. This result led immediately to the development of the first nuclear reactor
in 1942, and ultimately nuclear weapons in 1945.
Neutron source
A Neutron source is a device that emits neutrons. There is a wide variety of different sources, ranging from hand-held radioactive sources to neutron research facilities operating research reactors and spallation sources...
that was developed after it was discovered in the 1930s that alpha radiation that strikes the Beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
nucleus
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
would release neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s. The high speed of the alpha is sufficient to overcome the relatively low Coulomb barrier of the Beryllium nucleus, which contains only four protons, allowing for fusion
Nuclear fusion
Nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy...
of the two particles, releasing energetic neutrons.
In 1930 Walther Bothe
Walther Bothe
Walther Wilhelm Georg Bothe was a German nuclear physicist, who shared the Nobel Prize in Physics in 1954 with Max Born....
and Herbert Becker in Germany
Germany
Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...
found that alpha particles striking light elements such as beryllium
Beryllium
Beryllium is the chemical element with the symbol Be and atomic number 4. It is a divalent element which occurs naturally only in combination with other elements in minerals. Notable gemstones which contain beryllium include beryl and chrysoberyl...
, boron
Boron
Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...
, or lithium
Lithium
Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly...
would release a highly penetrating radiation, at first believed to be a gamma radiation 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
Irène Joliot-Curie was a French scientist, the daughter of Marie Skłodowska-Curie and Pierre Curie and the wife of Frédéric Joliot-Curie. Jointly with her husband, Joliot-Curie was awarded the Nobel Prize for chemistry in 1935 for their discovery of artificial radioactivity. This made the Curies...
and Frédéric Joliot in Paris
Paris
Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...
, who showed that if this unknown radiation fell on paraffin
Paraffin
In chemistry, paraffin is a term that can be used synonymously with "alkane", indicating hydrocarbons with the general formula CnH2n+2. Paraffin wax refers to a mixture of alkanes that falls within the 20 ≤ n ≤ 40 range; they are found in the solid state at room temperature and begin to enter the...
or any other hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
-containing compound it ejected proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s of very high energy. Finally, in 1932 the physicist James Chadwick
James Chadwick
Sir James Chadwick CH FRS was an English Nobel laureate in physics awarded for his discovery of the neutron....
in England
England
England is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west, with the North Sea to the east and the English Channel to the south separating it from continental...
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 will suffice, but usually a high specific activity
Specific activity
In nuclear sciences and technologies, "activity" is the SI quantity related to the phenomenon of natural and artificial radioactivity. The SI unit of "activity" is becquerel, Bq, while that of "specific activity" is Bq/kg. The old unit of "activity" was curie, Ci, while that of "specific activity"...
alpha-emitter is chosen, such as Radium
Radium
Radium is a chemical element with atomic number 88, represented by the symbol Ra. Radium is an almost pure-white alkaline earth metal, but it readily oxidizes on exposure to air, becoming black in color. All isotopes of radium are highly radioactive, with the most stable isotope being radium-226,...
(Ra-226). The alpha emitter and the Beryllium are pulverized and mixed together in close intimate contact to insure a high percentage of alpha-emitter and beryllium nuclei in close contact, since the alpha 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, with one end open to allow the neutrons to shoot out, thus acting like a howitzer
Howitzer
A howitzer is a type of artillery piece characterized by a relatively short barrel and the use of comparatively small propellant charges to propel projectiles at relatively high trajectories, with a steep angle of descent...
.
Neutron howitzers were used by Otto Hahn
Otto Hahn
Otto Hahn FRS was a German chemist and Nobel laureate, a pioneer in the fields of radioactivity and radiochemistry. He is regarded as "the father of nuclear chemistry". Hahn was a courageous opposer of Jewish persecution by the Nazis and after World War II he became a passionate campaigner...
, Fritz Strassman and Lise Meitner
Lise Meitner
Lise Meitner FRS was an Austrian-born, later Swedish, physicist who worked on radioactivity and nuclear physics. Meitner was part of the team that discovered nuclear fission, an achievement for which her colleague Otto Hahn was awarded the Nobel Prize...
in 1938 to bombard uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
nuclei with neutrons in the hopes of making transuranic elements. To their surprise, they found barium
Barium
Barium is a chemical element with the symbol Ba and atomic number 56. It is the fifth element in Group 2, a soft silvery metallic alkaline earth metal. Barium is never found in nature in its pure form due to its reactivity with air. Its oxide is historically known as baryta but it reacts with...
residue, a clear indication that they had instead fissioned
Nuclear fission
In nuclear physics and nuclear chemistry, nuclear fission is a nuclear reaction in which the nucleus of an atom splits into smaller parts , often producing free neutrons and photons , and releasing a tremendous amount of energy...
uranium nuclei. This result led immediately to the development of the first nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
in 1942, and ultimately nuclear weapons in 1945.