What is the source of beryllium

In normal radioactive decay there are no free neutrons. Free neutrons must therefore be generated in nuclear reactions. There are numerous ways to do this:

Radioactive neutron sources

Radioactive alpha emitters and light nuclides with a small atomic number are used as targets. Deuterium and beryllium are favorable. Alpha beryllium sources (a mixture of typically radium, polonium or americium and beryllium), which are produced via the reaction, are therefore common

about highly excited 13C generates neutrons, which, however, vary in energy; Disadvantage: high gamma background!

Such sources were mainly used in the early stages of nuclear physics. Today they are used to calibrate neutron detectors, to activate them with neutrons and for safety investigations.

After at high flow reactors starting from the isotope 238If transuranic elements could be produced, plutonium-beryllium sources have established themselves as neutron sources. The 239Pu emits an alpha particle that is captured by beryllium. An alloy of plutonium and beryllium is used for the source. Also with americium (241A neutron source is possible on the).

Spontaneous fission neutron sources

Here, too, a high-flux reactor is used to produce nuclides that are subject to spontaneous fission, for example californium252Cf with a half-life of 2.65 years. On average, 3 neutrons are emitted per fission process. The energy spectrum of these neutrons is almost the same as that from the induced nuclear fission.

Generation of free neutrons with particle accelerators

In principle, every nuclear reaction in which sufficient energy is available can be expected to emit neutrons. The intensities that can be achieved are greater than those of radioactive sources; In addition, the neutron energies can be varied and, in some cases, monoenergetic neutrons can be generated by a suitable choice of reactions. Pulsing the accelerator beam allows time-of-flight measurements (energy determination of the neutrons possible).

Possible reactions and examples are

(p, n) reactions: 7Li + p 7Be + n

(d, n) reactions: 2H + 2H 3He + n (so-called dd reaction); 2H + 3H4He + n (so-called dt reaction)

(α, n) reactions: All reactions of the radioactive sources are also possible with alpha particles from an accelerator.

Neutron release through bremsstrahlung and heavy ion reactions

If electrons are accelerated to a few 100 MeV, they generate bremsstrahlung when they hit a target. This has energies that are greater than the binding energies of the neutrons in the target nuclei. Neutrons are then released via the reaction (gamma, n), the nuclear photo effect. With heavy nuclei, photocleavage is also possible with subsequent neutron emission.

In heavy ion reactions (for example at the Society for Heavy Ion Research GSI in Darmstadt), highly excited heavy nuclei are formed that emit neutrons and gamma radiation.

Nuclear reactors as neutron sources

During nuclear fission, fast free neutrons are created which, through moderation, can become thermal neutrons.

Spallation Neutron Sources

Spallation is a nuclear reaction in which high-energy particles (example: protons of 500 MeV) hit a nucleus, first knock out one or more nucleons and additionally heat the nucleus. As a result of this heating, further nucleons “evaporate” from the nucleus. Advantage over a reactor: can be switched off; less radioactive waste.

Pyroelectric fusion

In pyroelectric fusion, the nuclear reaction D (d, n) He-3 is triggered with the help of pyroelectric crystals, which produces neutrons. Although this method is unsuitable as an energy source, it is suitable as a portable neutron source.

See also:Pyroelectric fusion in the English Wikipedia


The Farnsworth-Hirsch Fusor is a nuclear fusion device that is also used not to generate energy, but to generate neutrons.

Category: nuclear physics