Neutrons Effects

The neutron slows down by collisions and capture ...

[Neutrons are neutral particles only sensitive to nuclear forces. Therefore they travel readily and untouched through the electron cloud of atoms encountered. However they may interact strongly with their nuclei. But the nuclei are very small and the neutron radiation is more penetrating than gamma radiation. The most common interaction is a simple collision. However; during these collisions the neutron can be captured and then cause a nuclear reaction.

Vacuum for neutrons
When it travels through matter, a neutron sees only the nuclei of atoms. The electronic clouds are transparent to it. To make the figure understandable, the neutron and the nuclei were enlarged, but at atom scale they look punctual. The neutron almost only encounters vacuum, which explains why the neutron is particularly penetrating.

Neutrons slow down when colliding with nuclei that ionize the surrounding matter while recoiling. The slowdown is fast in a moderator environment rich in hydrogen where the neutron can lose sometims practically all its energy in a single collision(*). The result of this series of collisions is an erratic path that can be long in air.

A free neutron is an unstable particle, with a 10.2 minutes period. For this reason it is absent in our environment. In a dense medium, a neutron is captured long before it decays, sometimes triggering a nuclear reaction when the formed nucleus is unstable.

Often, the neutron does not get out from the nucleus : it is captured and the nucleus reacts by emitting a radiative gamma. The probability - called cross section - of this capture varies greatly with the nucleus and the neutron energy. At characteristic energies (which corresponds to energy levels of the nucleus) the neutron resonates as it passes nearby and finds itsself absorbed. It is as if the nucleus became suddenly very big. The phenomenon reminds of what happens in a musical instrument that resonates at certain frequencies. The neutron energy plays here the role of frequency.

A billiards game ...
A neutron in the material undergoes a series of collisions called "elastic" on nuclei. It transmits its energy to the nuclei that recoil under the shock (the path of the neutron, which does not interact between two collisions, should be represented in dotted lines). This slowdown is the fastest in a hydrogenated matter where recoil nuclei are protons with a mass equal to the neutron. These recoil nuclei are the one that ionize and produce side effects because they carry electric charges. The neutron is generally finally captured by a nucleus that can become radioactive: the material is activated. In special cases such as uranium-235 or plutonium-239, the capture triggers the explosion of the nucleus. This process is called fission.

In a conventional nuclear reactor, one must not lose inadvertently neutrons. During the slowdown, these resonant energies should be avoided and nuclei which have too much appetite for them as well. The choice of materials becomes critical.

The neutron flux is abundant and dangerous near the core of nuclear reactors. The phenomenon of resonant capture is then welcomed for radioprotection. Nuclei, such as, boron-10 or cadmium fond of slow neutrons are intentionally included in control rods and shielding materials. Similarly, one controls the reactor reactivity when the nuclear fuel is new by adding boron in the moderator water. This "boric" water tempers the fission reactions. The amount of boron-10 decreases during operation, together with the reactor, whose fuel becomes depleted, has less and less need to be bridled.

Neutron radiation effects remain after a capture. An isotope that contains an extra neutron is, moreover, generally unstable. Irradiated support structures in reactors become radioactive. Neutron capturs also generate in the fuel radionuclides heavier than uranium, like plutonium and actinides. Radionuclides useful for applications are also extracted from reactors.

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