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alpha partices are helium nuclei 2p+2n ejected from the nucleus at high energy. This form of nuclear decay occurs in large mass nuclei.

beta particles are high energy electrons ejected from the nucleus. This decay occurs in either small or large nuclei. It reslts in no change in mass number but an increase in one atomic number (one neutron breaks down to give a proton and an electron, which is then ejected)

For info on the mass spectrometer with animationclick here