What is a neutron?
Neutrons are within nuclei of atoms. They belong to the group of the fundamental particles, the smallest known composites of the matter. They do not possess any electric charge, they are neutral. That is where the name neutrons comes from, which was invented by their discoverer James Chadwick (Discovery of the neutrons). The neutron's spin is ½, their mass 1,67 * 10-24 gramms. Free neutrons being outside of the nucleus can live for 886 seconds, almost 15 minutes - this is quite long for a fundamental particle. Then they decay by emission of an electron and an electron antineutrino to become a proton.
Free neutrons interact with fundamental particles within the nucleus, if they hit an atom. Thus, their effect on the atom depends on the number of neutrons and protons within the nucleus. Nuclei with a certain number of neutrons and protons are called nuclide. Nuclides with the same number of protons, but a different number of neutrons, are called isotopes (of an element). The interaction of X-rays with matter depends on the electrons of an atom and therefore all isotopes of an element interact the same way. But free neutrons interact with the particles within the nucleus causing different interactions for different isotopes of the same element (Why neutrons?).