Neutron howitzer

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A neutron howitzer is a neutron source that emits neutrons in a single direction. It was discovered in the 1930s that alpha radiation that strikes the beryllium nucleus would release neutrons. The high speed of the alpha is sufficient to overcome the relatively low Coulomb 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 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 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 wax or any other hydrogen-containing compound it ejected protons 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. Historically a variety of isotopes such as radium (Ra-226) were used, but in modern times the transuranic isotopes Am-241 and Pu-239 are almost exclusively used in AmBe resp. PuBe neutron sources. [1] 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, with one end open to allow the neutrons to shoot out in the direction of the open end, thus acting like a howitzer. [2] [3] [4]

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 discovery led to the development of the first nuclear reactor in 1942, and ultimately nuclear weapons in 1945.

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Alpha decay or α-decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms or 'decays' into a different atomic nucleus, with a mass number that is reduced by four and an atomic number that is reduced by two. An alpha particle is identical to the nucleus of a helium-4 atom, which consists of two protons and two neutrons. It has a charge of +2 e and a mass of 4 u. For example, uranium-238 decays to form thorium-234.

<span class="mw-page-title-main">Neutron</span> Subatomic particle

The neutron is a subatomic particle, symbol
n
 or
n0
, which has a neutral charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons behave similarly within the nucleus, and each has a mass of approximately one atomic mass unit, they are both referred to as nucleons. Their properties and interactions are described by nuclear physics.

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Neutron activation analysis

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<span class="mw-page-title-main">Radiation</span> Waves or particles moving through space

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Photodisintegration Disintegration of atomic nuclei from high-energy EM radiation

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<span class="mw-page-title-main">Alpha particle</span> Helium-4 nucleus; particle of two protons and two neutrons

Alpha particles, also called alpha rays or alpha radiation, consist of two protons and two neutrons bound together into a particle identical to a helium-4 nucleus. They are generally produced in the process of alpha decay, but may also be produced in other ways. Alpha particles are named after the first letter in the Greek alphabet, α. The symbol for the alpha particle is α or α2+. Because they are identical to helium nuclei, they are also sometimes written as He2+
 or 4
2He2+
 indicating a helium ion with a +2 charge. Once the ion gains electrons from its environment, the alpha particle becomes a normal helium atom 4
2He.

Discovery of the neutron Scientific background leading to the discovery of subatomic particles

The discovery of the neutron and its properties was central to the extraordinary developments in atomic physics in the first half of the 20th century. Early in the century, Ernest Rutherford developed a crude model of the atom, based on the gold foil experiment of Hans Geiger and Ernest Marsden. In this model, atoms had their mass and positive electric charge concentrated in a very small nucleus. By 1920 chemical isotopes had been discovered, the atomic masses had been determined to be (approximately) integer multiples of the mass of the hydrogen atom, and the atomic number had been identified as the charge on the nucleus. Throughout the 1920s, the nucleus was viewed as composed of combinations of protons and electrons, the two elementary particles known at the time, but that model presented several experimental and theoretical contradictions.

Discovery of nuclear fission 1938 achievement in physics

Nuclear fission was discovered in December 1938 by chemists Otto Hahn and Fritz Strassmann and physicists Lise Meitner and Otto Robert Frisch. Fission is a nuclear reaction or radioactive decay process in which the nucleus of an atom splits into two or more smaller, lighter nuclei and often other particles. The fission process often produces gamma rays and releases a very large amount of energy, even by the energetic standards of radioactive decay. Scientists already knew about alpha decay and beta decay, but fission assumed great importance because the discovery that a nuclear chain reaction was possible led to the development of nuclear power and nuclear weapons.

References

  1. ↑ Vega-Carillo, Héctor René; Manzanares-Acuña, Eduardo; Becerra-Ferreiro, Ana Maria; Carillo-Nuñez, Aureliano (2002). "Neutron and gamma-ray spectra of 239PuBe and 241AmBe". Applied Radiation and Isotopes. 57 (2): 167–170. doi:10.1016/S0969-8043(02)00083-0.
  2. ↑ Cox, A.J.; Francois, P.E.; Gatrell, R.P. (1968). "The design of neutron howitzers". The International Journal of Applied Radiation and Isotopes. 19 (6): 541–4. doi:10.1016/0020-708X(68)90009-4.
  3. ↑ Jha, D. K. (2004). Elements of Nuclear Reactors. ISBN   9788171418831.
  4. ↑ "Oregon Administrative Rules". 2001.
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