This would be the relevant segment:
So a nucleon breaks down and emits an electron at high speed out of the nucleus, and this forms the beta decay that is used to produce the light we see in the vial. It does make sense that the energy being emitted by the electron when it slows down as it enters some materials could be converted into a type of gamma-ray, because momentum is a different kind of energy and there was a lot of it imparted to the electron when the nucleon separated. The reason it’s not a concern is that it’s a type of “soft gamma-ray”. It vibrates at the same frequency, but its at a much lower energy state (in eV) than what you would conventionally consider gamma radiation. Also, that process won’t always occur because it’s based on a variety of factors.
Look into it yourself, but I wouldn’t worry about it.