Neutron capture

Neutron capture is a kind of nuclear reaction in which an atomic nucleus collides with one or more neutrons and they merge to form a heavier nucleus. Since neutrons have no electric charge they can enter a nucleus more easily than positively charged protons, which are repelled electrostatically.

Neutron capture plays an important role in the cosmic nucleosynthesis of heavy elements. In stars it can proceed in two ways - as a rapid process (an r-process) or a slow process (an s-process). Nuclei of masses greater than 56 cannot be formed by thermonuclear reactions (i.e. by nuclear fusion), but can be formed by neutron capture.

Neutron capture at small neutron flux

At small neutron flux, as in a nuclear reactor, a single neutron is captured by a nucleus. For example, when natural gold (¹⁹⁷Au) is irradiated by neutrons, the isotope ¹⁹⁸Au is formed in a highly excited state which then quickly decays to the ground state of ¹⁹⁸Au by the emission of γ rays. In this process, the mass number (the number of nucleons - both protons and neutrons) increases by one. In terms of a formula, this is written ¹⁹⁷Au(n,γ)¹⁹⁸Au. If thermal neutrons are used, the process is called thermal capture.

The isotope ¹⁹⁸Au is a beta emitter that decays into the mercury isotope ¹⁹⁸Hg (see decay scheme of this process). In this process, the atomic number (the number of protons in the nucleus) rises by one.

More important ¹⁹⁶Hg +n → ¹⁹⁷Hg or ¹⁹⁸Hg - n → ¹⁹⁷Hg ß- → ¹⁹⁷Au.

The s-process mentioned above happens in the same way, but inside of stars.

Neutron capture at high neutron flux

The r-process happens inside stars if the neutron flux density is so high that the atomic nucleus has no time to decay via beta emission in between neutron captures. The mass number therefore rises by a large amount while the atomic number (i.e., the element) stays the same. Only afterwards, the highly unstable nuclei decay via many β^- decays to stable or unstable nuclei of high atomic number.

Capture cross section

The absorption neutron cross-section of an isotope of a chemical element is the effective cross sectional area that an atom of that isotope presents to absorption, and is a measure of the probability of neutron capture. It is usually measured in barns (b).

Absorption cross section is often highly dependent on neutron energy. Two of the most commonly specified measures are the cross-section for thermal neutron absorption, and resonance integral which considers the contribution of absorption peaks at certain neutron energies specific to a particular nuclide, usually above the thermal range, but encountered as neutron moderation slows the neutron down from an original high energy.

The thermal energy of the nucleus also has an effect; as temperatures rise, Doppler broadening increases the chance of catching a resonance peak. In particular, the increase in uranium-238's ability to absorb neutrons at higher temperatures (and to do so without fissioning) is a negative feedback mechanism that helps keep nuclear reactors under control.

Uses

Neutron activation analysis can be used to remotely detect the chemical composition of materials. This is because different elements release different characteristic radiation when they absorb neutrons. This makes it useful in many fields related to mineral exploration and security.

Neutron absorbers

Main article: Neutron poison

The most prolific neutron absorbers are the radioactive isotopes of elements that happen to become (nearly) stable by absorbing one neutron. An example of these is xenon-135 (half life about 9.1 hours), which absorbs a neutron to become the stable isotope xenon-136. Xenon-135 is formed in nuclear reactors when the splitting of uranium-235, uranium-233, or plutonium-239, in a nuclear chain reaction commonly leads to the production of some iodine-135. Iodine-135 soon undergoes nuclear decay, by emitting a beta particle - with a quite short half-life - to produce xenon-135.

The most important neutron absorber is boron as B₄C in control rods, or boric acid as a coolant water additive in PWRs. Other important neutron absorbers that are used in nuclear reactors are cadmium, hafnium, gadolinium, cobalt, samarium, titanium, dysprosium, erbium, europium, and new suggested by Kay Uwe Böhm molybdenum and ytterbium all of which usually consist of mixtures of various isotopes—some of which are excellent neutron-absorbers. For example the Advanced CANDU Reactor uses a soluion of gadolinium nitrate to shutdown. ⁵⁹Co is used in reactors for producing ⁶⁰Co used for x-ray.

Neutron cross section of boron (top curve is for ¹⁰B and bottom curve for ¹¹B)

Hafnium, one of the last stable elements to be discovered, presents an interesting case. Even though hafnium is a heaver element, its electron configuration makes it practically identical with the element zirconium, and they are always found in the same ores. However, their nuclear properties are different in a profound way. Hafnium absorbs neutrons avidly (Hf absorbs 600 times better than Zr), and it can be used in reactor control rods, whereas natural zirconium is practically transparent to neutrons. Thus, zirconium is very desirable for use in the construction of reactors, including in such parts as the metal cladding of the fuel rods which contain either uranium, plutonium, or an alloy of the two metals.

Hence, it is quite important to be able to separate the zirconium from the hafnium in their naturally-occurring alloy. This can only be done inexpensively by using modern chemical ion-exchange resins. Similar resins are also used in reprocessing nuclear fuel rods, when it is necessary to separate uranium and plutonium, and sometimes thorium.

See also

• Beta decay
• Induced radioactivity
• List of particles
• Neutron emission
• Radioactive decay
• Rays: α — β — γ — δ — ε
• p-process (proton capture)

External link

• Thermal Neutron Capture Data