I wanted to buy a scintillation crystal to study its light yield at cryogenic temperatures. It had to be large enough to fully absorb 2.6 MeV gamma-rays and had to be small enough so that I could afford it. This was when Geant4 came into play. Plots shown in this blog are tracks of particles left in the air by three types of incident particles, alpha, beta and gamma-rays. One can clearly see how far they can go in air. The plots were generated using a Geant4 based program, gears. It was written by me and one of my undergraduate students. One can easily change the target material from air to a certain scintillation crystal in Geant4 to study the penetration length of the incident particle in the crystal. The size of the crystal can then be determined.