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Travelling throguh matter, neutron undergoes a series of "elastic" collisions on nuclei. It transmits its energy to the nuclei recoiling under the shocks (the path of the neutron, which does not interact between collisions, is invisible). This slowdown is the fastest in a hydrogenated matter where recoil nuclei are protons with a mass equal to the neutron. These electrically charged recoil nuclei are the one that ionize and produce side effects. 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, a fission.