Neutrons can be produced in different ways:

• In neutron sources.

• In neutron generators.

• In nuclear reactors.

• By nuclear spallation.

In neutron sources, neutrons are mostly produced by (a, n) nuclear reactions (as discussed in Section 6.2.3). The alpha particles are obtained from an alpha emitter radioactive isotope such as Ra-226, Pu-239, or Po-210. These isotopes are mixed with a light element (the binding energy of neutron is relatively low), mainly by beryllium. The neutrons are produced in the reaction as follows:

9Be(a, n)12C (5.100)

The neutron yield of these neutron sources is 106—108 neutrons/s.

The radium—beryllium (RaBe) neutron source has undesirably high gamma radi­ation, and therefore it is no longer used.

Neutrons can be produced by the spontaneous fission of 252Cf. The yield of the commercial 252Cf neutron sources is about 107—109 neutrons/s.

Neutrons can be produced by (Y, n) nuclear reactions (see Section 6.2.2). Gamma photons can initiate nuclear reactions if their energy is higher than the binding energy of the target nucleus. For example, the 24Na isotope has high-energy gamma photons. The gamma photons can initiate nuclear reactions with deuterium, lithium, beryllium, and boron. For example:

Be(Y, n)8Be (5.101)

2H 1 y! n 1 p1 (5.102)

Therefore, when a salt containing an 24Na isotope is dissolved in heavy water (D2O), a mobile neutron source can be produced (as described in Section 6.2.2).

In neutron generators, the isotopes of hydrogen are used in nuclear reactions. Mostly deuterium, tritium nuclei, or the mixture of these nuclei are accelerated in linear accelerators, and the metal hydride target containing deuterium, tritium, or both is bombarded by the accelerated nuclei. The nuclear reactions (described fur­ther in Section 6.2.4) are:

2H 13H! 4He 1 n (5.103)

2H 12H! 3He 1 n (5.104)

The energy of the neutrons produced in neutron generators is about 14 MeV. The yield of the neutron is about 108—109 neutrons/s.

Neutrons can be produced in cyclotrons by (p, n) nuclear reactions. For this reac­tion, lithium or beryllium is used as target material.

The neutron production in nuclear reactors will be discussed in detail in Section 6.2.1 and Chapter 7. Thus, it is not detailed here; we will just mention that in the fission reaction, high-energy gamma photons are also produced, which initi­ate the reaction (5.102). This reaction produces extra neutrons, which affects the neutron balance of the nuclear reactors.

The greatest neutron yields can be obtained by nuclear spallation. Spallation is a nuclear reaction in which photons or particles with high energy (e. g., protons with GeV) hit a nucleus, resulting in the emission of many other particles (such as neu­trons or light nuclei) or photons. The target is a heavy element (e. g., mercury, tung­sten, or lead). Recently, there are only a few spallation neutron sources all over the world.

The lifetime of free neutrons is short; they transform into protons, beta particles with 0.728 MeV, and antineutrinos.

0n mo, ! 1p 1 в0.782 MeV 1 0V (5Л°5)

10.25 min

The half-life of the reaction is 10.25 min.