NEUTRINOS
    In 1930 Wolfgang Pauli gave a hypothesis of the subsistence of small, neutral (uncharged) particles. They were to participate in radioactive disintegration. Neutrinos - that's how they are called - have no charge nor rest mass (that haven't been absolutely confirmed yet - new observation confirm that neutrinos have got very small rest mass). They can easily go through matter. It is estimated that to be sure that neutrino collide with a matter particle we have to construct a lead shield which should be about a light year thick. Luckily, if we have the source which emits many, many neutrinos and we want to observe a collision, we don't have to construct such a shield. Some of this neutrinos certainly collide with matter particles even if the shield we have is thin.
    Neutrino (which later was named the electron neutrino) was discovered in 1959 by F. Reines and C.L. Cowan.
    As the source of neutrinos (in fact it was the source of antineutrinos) they used a nuclear reactor. As the shield they used a huge ladle with cadmium dichloride. Some of the antineutrinos collided with the protons and caused inverse beta disintegration. During this kind of disintegration positron and neutron are emitted. Positron meets electron and they just annihilated. During the annihilation two photons are emitted. These photons and the neutron can be easily observed by scientists. And this observation was the first experimental evidence of existing of the neutrinos.

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