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.
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