Neutron source

Neutron sources for the production of free neutrons for research or application purposes. They are mostly based on nuclear reactions, but in some cases to spontaneous fission.

The released neutrons are initially always fast neutrons with kinetic energies of at least several hundred keV. If thermal neutrons needed, the source is combined with a moderator.

The number of emitted by the source per unit time is called neutron source strength. Is often more practical importance, the neutron flux density which can be achieved on a sample to be irradiated; it depends on the source strength, the geometry of the array ( the source expansion, expansion of the sample and the distance between them ), and whether the source is isotropic neutron, ie equally in all directions, or anisotropic Agreement. The radioactive neutron sources emit isotropic, anisotropic those based on particle sources in general. In nuclear reactors, both are possible depending on the choice of the irradiation site.

• 3.1 General
• 3.2 spallation neutron sources
• 3.3 electron bremsstrahlung as a neutron source
• 3.4 IFMIF

Radioactive neutron sources

The sources described below require the security effort, which is always needed when handling radioactivity. But you have the advantage of being small and easily transportable. The other, described below sources are almost always fixed installations.

Alpha -beryllium neutron sources

A mixture of an alpha emitter, and a material which has a large cross section for the ( α, n) - nuclear reaction, is a neutron source; the energy spectrum of neutrons is released in the MeV range and will depend in detail on the nuclides used. Commonly used are mixtures of radium, polonium, plutonium or americium with beryllium. Some grams of the mixture are in a hermetically sealed metal case. The exit of the alpha particle itself is prevented by the housing, however, give the sources in addition to the inevitable neutron and gamma radiation.

Such sources were mainly used in the initial phase of the nuclear physics experiments. They are used as before for example, for testing and calibration of neutron detectors for activation with neutrons in nuclear weapons and in nuclear reactors to produce a measurable neutron flux even when the ( sub-critical ) reactor.

Radium -beryllium neutron sources have been made ​​to source strengths of some 107 neutrons per second. Because of the long half-life, they have the advantage of long usability over other mobile neutron sources.

Spontaneous fission neutron sources

With a high-flux reactor nuclides can be produced that decay by spontaneous fission, for example, californium 252Cf with a half-life of 2.65 years. An average of three neutrons are emitted each splitting process. The energy spectrum of the neutrons is substantially equal to that of the induced fission. Therefore, these sources are of particular importance in experiments on reactor physics. In nuclear reactors, 252Cf sources are used as the primary neutron sources.

Gamma -beryllium neutron sources

A mixture of a gamma radiator and a material which has a large cross section for the ( γ, n) - nuclear reaction, provides a neutron source dar. Commonly used is a mixture of antimony with beryllium, which is used in nuclear reactors as a so-called secondary neutron source. Only in the operation of the reactor arises from 123Sb the γ -radiative 124Sb, which releases in a ( γ, n) nuclear reaction 9Be with neutrons.

Nuclear reactors as neutron sources

Each nuclear reactor in operation is inevitably a strong neutron source, as in nuclear fission fast free neutrons arise ( mean energy of about 2 MeV). Reactors used as a neutron source and not for energy, hot research reactors.

Production of free neutrons with particle accelerators

Generally

Wherein each core reaction in which sufficient energy is available, emission of neutrons is also possible. The so- recoverable neutron flux densities are - depending on the accelerator type - larger than the radioactive sources. By suitable choice of the reaction, the neutron energy can be varied, and some produce monoenergetic neutrons. A pulsing of the accelerator beam allows flight measurements for the purpose of determining the neutron energy.

Examples of practically used as a neutron source reactions:

(p, n) reaction:

(d, n ) reactions:

Neutron generators based on the reaction dt deliver relatively high energy neutrons ( about 14 MeV). They are therefore an important tool in experimental nuclear physics and research for nuclear fusion reactors, since they use the same nuclear reaction. dt neutron generators achieve source strengths up to about 1013 neutrons per second (Appendix SNEG -13 in Sergiev Posad, Russia).

( α, n) reaction:

All reactions of the above radioactive sources are also possible with alpha particles from an accelerator.

Spallation neutron sources

As spallation is called a nuclear reaction (example: protons of 500 MeV) in the high-energy particles strike a core, first knock out of him one or more nucleons and in addition the core " heat up ". As a result of this heating " evaporate " from the core of many other nucleons. The energy spectrum therefore shows a maximum at about 3 MeV and a less intense foothills up to hundreds of MeV.

Spallation neutron sources provide a replacement for research reactors; they are more complicated and costly than reactors due to the necessary large-scale accelerator, but have advantages in terms of easy entry and shutdown and in relation to radioactive waste. They are still nuclear facilities and the target is strongly activated.

Electron bremsstrahlung as a neutron source

Fast electrons produce when hitting matter bremsstrahlung. At electron energies greater than 10 MeV, the bremsstrahlung energies above the binding energy of the neutrons in the target nuclei. About the reaction ( γ, n), the nuclear photo effect, then fast neutrons are released. In heavy nuclei also photocleavage is possible that results as each nuclear fission in the emission of neutrons.

Electron accelerator correspondingly high energy are not specifically built as neutron sources. However, be operated in addition to some already existing electron accelerators, neutron sources of this type.

IFMIF

The proposed International Fusion Materials Irradiation Facility ( IFMIF ) is to use the reactions of accelerated to 40 MeV deuterons with lithium. Your neutron spectrum extends up to about 50 MeV, the usable neutron flux density to 1015 cm -2s -1.

Pyroelectric fusion

When Pyroelectric fusion, the above already mentioned nuclear reaction D ( d, n) He-3 is triggered by pyroelectric crystals. This method is suitable as a portable neutron source.

Farnsworth - Hirsch Fusor

The Farnsworth - Hirsch Fusor is a nuclear fusion apparatus that is also used for neutron production. It is based on the principle of electrostatic plasma confinement (English Inertial Electrostatic Confinement ). There are industrially applicable neutron generators of this type