Question: Is alpha radiation the most powerful only because it is so much bigger than the other two, and therefore has a higher chance of hitting something (and is stopped more, so is less penatrating), or is there somehow another reason it's more powerful?
Bridget. This is an approximate answer, so don’t take is gospel. Radiation causes harm through a process called “ionisation”. Essentially the radiation removes an electron from an atom, which changes its chemical properties (typically it then reacts with other nearby molecules). This can damage DNA, or start cancers, or just damage parts of the body.
To remove an electron, a gamma ray (which is just a high energy photon) needs to almost directly hit an electron. In the process it transfers most of its energy to the electron, so that’s the end of the gamma ray.
An alpha particle bashes its way along, transferring a bit of energy to every electron it comes across. That’s why it slows down and can be stopped quicker, but in the process it has hit lots of electrons.
Think of sending a truck through a crowded mall versus sending a cricket ball. The cricket ball has got more chance of making it to the other side without hitting anything, and if it does hit somebody, that’s one person hit. The truck is going to be stopped, but a lot of people will be hurt along the way.
Alpha radiation is the most dangerous to living things because it transfers its energy to the cells in a different way to beta and gamma. Beta and gamma go along, ionising atoms in the cell every now and then (only every millionth of a metre!) whereas alpha is a lot more trigger happy, zapping the atoms every billionth of a metre. So a thousand times more! Imagine a man chopping down one in every thousand trees in a wood, and another chopping down every tree! That’s how much more powerful it is!! On the bright side, it’s so big that sticking even a big piece of paper in the way stops it, so don’t worry!