Neutron source

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

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

The number of neutrons emitted by the source per time interval divided by this time interval is called the source strength . In practice, the neutron flux density that can be achieved on a sample to be irradiated is often more important ; it depends on the strength of the source, on the geometry of the arrangement (extension of the source, extension of the sample and distance between them) and on whether the source emits the neutrons isotropically , ie evenly in all directions, or anisotropically . The radioactive neutron sources emit isotropically, the sources based on particle accelerators generally anisotropically. With nuclear reactors, both are possible depending on the choice of the irradiation site.

The sources described below require the level of safety that is always required when dealing with radioactivity . But they have the advantage of being small and easy to transport. The other sources described below are almost always fixed installations.

Alpha Beryllium Neutron Sources

A mixture of an alpha emitter and a material such as beryllium , which has a large cross-section for the (α, n) -nuclear reaction, represents a neutron source. The 9 Be-nucleus absorbs the α-particle, so that a 13 C- Compound core is created; this then decays into a 12 C nucleus and a neutron. The energy spectrum of the neutrons released is in the MeV range and depends in detail on the alpha emitter used. Mixtures of radium , polonium , plutonium or americium with beryllium are common. A few grams of the mixture are in a tightly sealed metal case. The exit of the alpha particles themselves is prevented by the housing, but the sources inevitably emit gamma radiation in addition to the neutrons .

Such sources were mainly used for experiments in the early stages of nuclear physics . They are still used, for example, for testing and calibrating neutron detectors , for activation with neutrons, in nuclear weapons and in nuclear reactors in order to generate a measurable neutron flux even when the (subcritical) reactor is switched off.

Radium-beryllium neutron sources have been produced up to source strengths of a few 10 7 neutrons per second. Because of their long half-life, they have the advantage of longer usability compared to other mobile neutron sources.

Spontaneous fission neutron sources

With a high-flux reactor, nuclides can be produced that decay through spontaneous fission , for example Californium 252 Cf with a half-life of 2.65 years. On average, around 3 neutrons are emitted per fission process. The energy spectrum of these neutrons is almost the same as that from the induced nuclear fission. Therefore, these sources are of particular importance in experiments on reactor physics . In nuclear reactors, 252 CF sources are used as "start-up sources".

Gamma beryllium neutron sources

A mixture of a gamma emitter and a material that has a large effective cross section for the (γ, n) -nuclear reaction represents a neutron source. A mixture of antimony (Sb) with beryllium (Be), which is used in nuclear reactors as a so-called secondary Neutron source is used. It is only when the reactor is in operation that γ-radiating 124 Sb is formed from 123 Sb, which releases neutrons in a (γ, n) nuclear reaction with 9 Be.

Nuclear reactors as neutron sources

Every nuclear reactor that is in operation is inevitably a strong source of neutrons, as fast free neutrons (mean energy about 2 MeV) are produced during nuclear fission . Reactors that are used as a source of neutrons and not for generating energy are called research reactors .

Generation of free neutrons with particle accelerators

General

In any nuclear reaction in which there is enough energy available, emission of neutrons is possible. The neutron flux densities that can be achieved in this way are - depending on the type of accelerator - greater than those from radioactive sources. The neutron energies can be varied and partially monoenergetic neutrons can be generated by a suitable choice of the reaction. Pulsing the accelerator beam allows time-of-flight measurements to determine the energy of the neutrons.

Examples of reactions used practically as a neutron source:

(p, n) reactions:

7 Li + p 7 Be + n${\ displaystyle \ rightarrow}$

(d, n) reactions:

2 H + 2 H 3 He + n (so-called dd reaction);${\ displaystyle \ rightarrow}$
2 H + 3 H 4 He + n (so-called dt reaction).${\ displaystyle \ rightarrow}$

Neutron generators based on the dt reaction deliver neutrons of relatively high energy (over 14 MeV ). They are therefore an important tool in experimental nuclear physics and research for nuclear fusion reactors , as these use the same nuclear reaction. dt neutron generators achieve source strengths of up to around 10 13 neutrons per second (SNEG-13 plant in Sergijew Posad , Russia).

(α, n) reactions:

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

Spallation neutron sources

As spallation refers to a nuclear reaction in which a high-energy particles (for example, a proton of 500 MeV) hits a core of it one or more first nucleons "knocks out" and in addition, the core "heats up". As a result of this heating, many more nucleons “evaporate” from the core. The neutron spectrum therefore shows a maximum at around 3 MeV and a less intense tail up to hundreds of MeV.

Spallation neutron sources represent a replacement for research reactors. They are more complicated and expensive than reactors because of the large accelerator required, but they have advantages in terms of being easy to switch on and off and in relation to radioactive waste. They are still nuclear facilities and the target is strongly activated.

One plant in operation is SINQ in Villigen (Switzerland). The European Spallation Spring in Lund (Sweden) is under construction .

Electron bremsstrahlung as a neutron source

Fast electrons generate when hitting matter Bremsstrahlung . With electron energies above about 10 MeV, the bremsstrahlung has energies above the binding energy of the neutrons in the target nuclei. Fast neutrons are then released via the reaction (γ, n), the nuclear photo effect . In heavy nuclei, photocleavage is also possible, which, like any nuclear fission, leads to the emission of neutrons.

Electron accelerators are not built specifically as neutron sources. However, neutron sources of this type are also operated on some electron accelerators that are already in existence. For example, such a source at the ELBE system generates up to 2 × 10 11 neutrons per second in short pulses with bremsstrahlung of 40 MeV electrons .

IFMIF

The planned International Fusion Materials Irradiation Facility (IFMIF) will use the reactions of deuterons accelerated to 40 MeV with lithium. Their neutron spectrum reaches up to about 50 MeV, the usable neutron flux density up to 10 15 cm −2 s −1 .

Pyroelectric fusion

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

Farnsworth-Hirsch-Fusor

The Farnsworth-Hirsch Fusor is a nuclear fusion device that is also used to generate neutrons. It is based on the principle of electrostatic plasma confinement ( English Inertial Electrostatic Confinement ). There are industrially usable neutron generators of this type.

literature

• H. Krieger: Radiation sources for technology and medicine . Springer, 2015, ISBN 978-3-8351-0019-0 , pp. 332-344.

Individual evidence

1. ^ KH Beckurts, K. Wirtz: Neutron Physics. Springer Verlag, 1964, pp. 28-29.
2. Ch. Segebade, H.-P. Weise, JL George: Photon Activation Analysis. Walter De Gruyter, 1987, ISBN 0-89925-305-9 .
3. M. Helm, P. Michel, M. Gensch and A. Wagner: Everything in the river. Physik Journal 15 (2016), No. 1, pp. 29–34.
4. ^ NSD-Fusion Technology. In: nsd-fusion.com. NSD-GRADEL-FUSION, accessed on February 5, 2018 .