Three of the remaining steps of the series, all with short half-lives, also produce beta. Uranium similarly has a very long half-life but the next two decays in its series are quick and both produce beta.
Beta particles are high speed electrons formed when neutrons decay.
They can be stopped by thin plastic shielding. On this page the Geiger counter was positioned for detecting beta particles in addition to gamma radiation. The detector is calibrated only for gamma but the presence of beta is determined by an increase in the counts when the slotted side of the instrument is aimed at the mineral or other radioactive source. When the Geiger counter is aimed straight at the sample the plastic case blocks the beta particles from reaching the detector tube but there are slots on one side to allow beta detection.