Sources of Neutrons

Intense neutron beams cannot be produced in the laboratory. Currently, there are two methods for producing neutron beams of sufficient intensity for powder diffraction, namely, a nuclear reactor and a spallation source. The two methods are quite different and result in neutron beams with different characteristics. Both methods involve fission, though with a spallation source it is not necessary to use a naturally fissile material. The consequences of the two methods in terms of the design and function of neutron powder diffractometers are discussed below. 

At a spallation source a heavy-metal target, such as Pb, W, Ta or Hg, is bombarded with energetic particles, usually protons accelerated to energies of up to 1 GeV. Neutrons freshly released from an atomic nucleus have high energies, referred to as epithermal neutrons , and must be slowed down to be useful for powder diffraction experiments. This occurs by collisions between the neutrons and the moderator - such as liquid methane or water - placed in the path of the neutron beam, which cause the exchange of energy and a trend towards (partial) thermal equilibrium. 

Neutron sources include nuclear reactors, accelerators, and isotopic sources. Nuclear reactors are, by far, the most frequently used irradiation facilities. They provide high fluxes [upper limit 10 neutrons/(m s)] of mostly thermal neutrons E I eV). Fast neutrons in the keV range are also available, but at lower flux levels. 

When short-lived isotopes are involved, a higher activity is produced by irradiating the sample in a reactor that can be pulsed (see Lenihan et al.). Such a reactor producing a high flux of about 10 neutrons/(m s) for a short period of time (milliseconds) is the TRIGA reactor, marketed by General Atomic. 

Accelerators produce fast neutrons as products of charged-particle reactions. The most popular device is the so-called neutron generator, which operates on the reaction 

The cross section for this exothermic reaction peaks at a deuteron kinetic energy of about 120 keV with a value of about 5 b. The neutrons produced have an energy of about 14 MeV. (The neutron kinetic energy changes slightly with the direction of neutron emission.) The maximum neutron flux provided by a neutron generator is of the order of 10 neutrons/(m s). 

Neutrons with an average energy of about 2.5 MeV are produced by the (d, d) reaction 

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Polonium can be mixed or alloyed with beryllium to provide a source of neutrons. The element has been used in devices for eliminating static charges in textile mills, etc. however, beta sources are both more commonly used and less dangerous. It is also used on brushes for removing dust from photographic films. The polonium for these is carefully sealed and controlled, minimizing hazards to the user. 

In the startup of a reactor, it is necessary to have a source of neutrons other than those from fission. Otherwise, it might be possible for the critical condition to be reached without any visual or audible signal. Two types of sources are used to supply neutrons. The first, appHcable when fuel is fresh, is califomium-252 [13981-174-Jwhich undergoes fission spontaneously, emitting on average three neutrons, and has a half-life of 2.6 yr. The second, which is effective during operation, is a capsule of antimony and beryUium. Antimony-123 [14119-16-5] is continually made radioactive by neutron... 

Two types of sources are used. Originally developed in the 1940s, nuclear reactors provided the first neutrons for research. While reactors provide a continuous source of neutrons, recent developments in accelerator technology have made possible the construction of pulsed neutron sources, providing steady, intermittent neutron beams. 

During the red giant phase of stellar evolution, free neutrons are generated by reactions such as C(a,n) and Ne(a,n) Mg. (The (ot,n) notation signifies a nuclear reaction where an alpha particle combines with the first nucleus and a neutron is ejected to form the second nucleus.) The neutrons, having no charge, can interact with nuclei of any mass at the existing temperatures and can in principle build up the elements to Bi, the heaviest stable element. The steady source of neutrons in the interiors of stable, evolved stars produces what is known as the "s process," the buildup of heavy elements by the slow interaction with a low flux of neutrons. The more rapid "r process" occurs in... 

Almost every nuclide undergoes neutron capture if a source of neutrons is available. Unstable nuclides used in radiochemical applications are manufactured by neutron bombardment. A sample containing a suitable target nucleus is exposed to neutrons coming from a nuclear reactor (see Section 22-1). When a target nucleus captures a... 

Reactors are sources of neutrons, and thus most reactor-produced radionuclides are neutron-rich ft emitters. Reactor-produced radionuclides are of relatively low specific activity if the target nucleus is the same element as the product radionuclide, because the target and the product cannot then be chemically separated. 

Neutrons have no electrical charge and have nearly the same mass as a proton (a hydrogen atom nucleus). A neutron is hundreds of times larger than an electron, but one quarter the size of an alpha particle. The source of neutrons is primarily nuclear reactions, such as fission, but they are also produced from the decay of radioactive elements. Because of its size and lack of charge, the neutron is fairly difficult to stop, and has a relatively high penetrating power. 

Nuclides (i.e., 14C and 3H) formed by continuing natural nuclear transformations driven by cosmic rays, natural sources of neutrons, or energetic particles that are formed in the upper atmosphere by cosmic rays... 

Activation analysis requires the use of a powerful source of neutrons as the activator and is suitable only for elements which form an isotope whose half-life is longer than the isotopes of other elements which may be produced. 

Some materials have a spontaneous decay process that emits neutrons. Some shortlived fission products are in this class and are responsible for the delayed neutron emission from fission events. Another material in this class is Cf that has a spontaneous fission decay mode. Cf is probably the most useful material to use as a source of neutrons with a broad energy spectrum. 

There are no significant uses for actinium because of its scarcity and the expense of producing it. The only practical use for small amounts of actinium is as a tracer in medicine and industry. It is too difficult to produce in substantial quantities to make it useful. Actinium can be used as a source of neutrons to bombard other elements to produce isotopes of those elements, but other neutron sources are less expensive. 

Bear the formula in mind, however. The ingredients are metals and neutrons. Neutrons Here lies the whole difficulty of the matter, for free neutrons are unstable. How can they be liberated and made to react before they perish What is needed is a source of neutrons, a neutron-rich nucleus that will let one of its neutrons slip out from the folds of its robe. Neutron sources so far identified are few and far between. Indeed, we have only carbon-13 and neon-22. These are the only ones capable of supplying neutrons to nuclear reactions. And we are still worlds away from working out a detailed scenario for the explosive production of gold. 

American chemists and physicists Glenn T. Seaborg, Stanley Thompson, Albert Ghiorso, and Kenneth Street Produced by bombarding curium-242 with helium nuclei its isotope californium-252 is excellent source of neutrons useful in esearch. 

The element was discovered independently hy A. Dehierne and F. Giesel in 1899 and 1902, respectively. It is used in nuclear reactors as a source of neutrons. 

The isotope cahfomium-252 undergoes spontaneous fission generating neutrons. It serves as a convenient source of neutrons for neutron activation analysis, neutron moisture gages, and in the determination of water and oilbearing layers in well-logging. It is expected to have many other potential applications, including synthesis of other heavy isotopes. 

The vast majority of crystal structures published in the literature have been solved using X-ray diffraction. However, it is also possible to use neutron diffraction for crystallographic studies. It is a much less commonly used technique because very few sources of neutrons are available, whereas X-ray diffractometers can be housed in any laboratory. It does have advantages for certain structures, however. 

Sources of weak intensity have fluxes of a few million neutrons per second. They can be used to measure about 20 elements. A variation of this method consists of using a few pg of 252Cf (r = 2.6 years) as a rapid source of neutrons (2 MeV) that are slowed by collision with hydrogen atoms (2.4 x 106 neutrons s"1 pg 1). 

Small-scale, tabletop nuclear fusion devices, known as compact accelerator neutron generators, are routinely used as a source of neutron radiation. By design, however, these devices consume more energy than they release. The beam of neutrons generated by these devices can be used to identify the elemental composition of amaterial.The coal industry uses such beams to measure the sulfur content of coal in real time as the coal moves over conveyor belts. The cement industry similarly uses these beams to judge the quality of cement mixes. These fusiongenerated neutrons are also used to identify the elemental composition of nuclear wastes and for the detection and identification of explosives. 

Neutron diffraction is very similar in principle to X-ray diffraction. However, it differs in two important characteristics (1) Since neutrons are diffracted by the nuclei (rather than the electrons), one indeed locates the nuclei directly. (2) Furthermore, the hydrogen nucleus is a good scattered thus the hydrogen atoms can be located easily and precisely. The chief drawback of neutron diffraction is that one must have a source of neutrons, and so the method is expensive and not readily available. X-ray diffraction and neutron diffraction may be used to complement each other to obtain extremely useful results (cf. Fig. 12.24). 

Knowledge of fission and its consequences is important for the nuclear power industry and the related fields of nuclear waste management and environmental cleanup. From the point of view of basic research, fission is interesting in its own right as a large-scale collective motion of the nucleus, as an important exit channel for many nuclear reactions, and as a source of neutron-rich nuclei for nuclear structure studies and use as radioactive beams. 

Make an estimate of the neutron-to-proton ratio in the center of the sun if the only source of neutrons is thermal equilibrium of the weak interactions. 

Here on Earth, spallation facilities are being built, not just to produce specific nuclides, but to provide a source of neutrons. Typically, a lead or mercury target... 

The most important component of a neutron gauge is the source of neutrons. Radioisotopic neutron sources are radioactive isotopes which either in combination with other stable elements or by their own decay emit neutrons. The attractiveness of these encapsulated sources is their small size and weight and complete absence of operational problems. Until about 1970 the most widely used of such sources has been an... 

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