It is a beta particle that is being emitted when the radioactive nucleus of 131 / 53 I decays to form the stable isotope of xenon 131 / 54 Xe. It is written as 0/-1 e-. It is a high speed and high energy positron or electron during beta decay. An unstable nucleus containing an excess of neutrons would unergo a beta decay wherein the neutron is transformed into a proton, and electron and an antiparticle. The electrons emitted are not the electrons from the shells but are being produced when a neutron splits firming a proton and an electron. These particles would be able to pass in paper but can be blocked by an aluminum foil.