A beta particle.β
Looking at the particles before and after for the decay, you'll see that there is a total of 214 neutrons and protons both before and after the radioactive decay. But you'll also see that before the decay, there were 82 protons and after the decay, there are 83 protons. So a neutron was converted to a proton which means that a negative charge had to be emitted, so it's a beta decay where a neutron emits a beta particle (an electron). The beta particle can be represented as a greek beta β followed by a minus sign, giving β-, or as a "e" followed by a minus sign, so e-
