Neutrons, like protons, are nucleons—that is, constituent particles of the atomic nucleus. Neutrons are found in the nucleus of every atom except ordinary hydrogen (1 1H), whose nucleus contains only a single proton. Although stable inside the nucleus, once outside, a free neutron becomes unstable and decays with a half-life of 10.24 minutes (614.8 seconds) into a proton, beta particle, and electron-antineutrino. Equation 4.28 describes the nuclear reaction for free neutron decay.
There are no significant naturally occurring, neutron-emitting radioactive sources on Earth, so scientists have devised a variety of clever ways to make neutrons available for research and practical applications. The neutrons commonly encountered in modern nuclear technology come primarily from the cores of nuclear reactors. For example, more than a million million (1012) neutrons cross a representative square-centimeter area each second in the core of a typical low-power-level (i.e., less than one megawattthermal) research reactor. However, neutrons can also come from special (α, n) mixtures of radionuclides, from the decay of californium-252 (252 98Cf)—a human-made transuranium radionuclide—or from accelerators that bombard special target materials with beams of charged particles.
Chadwick used the classic (α, n) nuclear reaction described in equation 4.29 to experimentally confirm the existence of the neutron in 1932. Since his discovery, nuclear scientists have fabricated small, self-contained neutron sources by mixing alpha-emitting radionuclides, such as polonium210, radium-226, and (later) plutonium-239, with a suitable target material, such as beryllium (Be). While these sealed sources are not as efficient at producing neutrons as nuclear reactors are, the family of (α, n) sources has proven convenient for use in laboratory research and nuclear science education. The name (α, n) source derives from the fact that a small fraction of the alpha particles emitted by the decay of the radioisotope hit the nuclei of the target material atoms (such as beryllium) in just the right way to cause the emission of neutrons through a nuclear reaction like the one presented in equation 4.29. A typical plutonium-239 and beryllium (239Pu/Be) neutron source uses the emission of 5.14 MeV alpha particles to create neutrons at a rate of approximately 60 neutrons per 1 million primary alpha particles. Similarly, a typical 210Po/Be neutron source uses the 5.30 MeV alpha particle emitted by polonium-210 to provide neutrons at a rate of approximately 70 neutrons per 1 million primary alpha particles.
Californium-252 has a half-life of 2.65 years and decays by both alpha emission and spontaneous fission. The spontaneous fission of californium252 is almost always accompanied by the release of a few neutrons. The phenomenon of nuclear fission, including spontaneous fission, will be discussed shortly. For now, it is sufficient to recognize that californium-252, properly packaged and sealed, represents a compact, relatively intense neutron source. A typical microgram sample of californium-252 yields approximately 2.3 million neutrons per second with energies generally ranging between 0.5 and 1.0 MeV, but a few spontaneous fission neutrons can reach energies as high as 8 or 10 MeV.
Scientists also use accelerators to produce neutrons. Two of the most common accelerator-induced nuclear reactions for making neutrons are the D-T reaction and the D-D reaction. Deuterium (D or 2 1H) is the heavy, but not radioactive, isotope of hydrogen, and tritium (T or 3 1H) is the radioactive isotope of hydrogen. The D-T nuclear reaction is described in equation 4.9. The D-D nuclear reaction appears in equation 4.30. To produce neutrons with either the D-D or the D-T nuclear fusion reaction, scientists use an accelerator to give incident deuterons (2 1H nuclei) sufficient energy to overcome the coulomb barrier as they strike special targets made of light nuclei, such as deuterium or tritium. For example, a 1 milliampere (mA) beam of deuterons will make about 109 neutrons per second from a thick deuterium target and about 1011 neutrons per second from a tritium target. All of the neutrons emerging from the accelerator target will have approximately the same energy—about 3 MeV for the D-D reaction and about 14 MeV for the D-T reaction.
Beta (β) Decay
