both beta and positron emission are observed for artifically produced radioisotopes of low atomic number bot only beta emission is observed with naturally occuring radioisotopes of high atomic number
Notice that as you proceed from low to high atomic numbers, the number of neutrons in the nucleus increases faster than the number of protons. Therefore, the high atomic number elements have too many neutrons so the probability is higher to get rid of neutron by converting that to a beta particle + a proton. At the low end of the periodic table there is a greater chance of either occurring (Beta^- or Beta^+).
