Charge and mass decide how a field bends each emission

1. Fields exert forces on charges, so the way an emission behaves in a field follows directly from the charge it carries.
2. An alpha particle carries $+2$, a beta particle carries $-1$, and gamma radiation carries no charge at all.
3. That gives three predictions that hold in both kinds of field: alpha and beta deflect in opposite directions, and gamma is not deflected at all.
4. How far each one bends is a separate question, and that is settled by mass rather than by charge.
5. An alpha particle has roughly $7000$ times the mass of a beta particle, so the same size of force barely alters its path while it swings the beta particle sharply aside.

In an electric field, alpha and beta bend towards opposite plates

1. The standard arrangement is a pair of parallel plates, one charged positive and one charged negative, with the radiation passing through the gap between them.
2. The alpha particle is positive, so it is repelled by the positive plate and attracted to the negative one.
3. The beta particle is negative, so it bends the other way, towards the positive plate.
4. Gamma radiation passes straight through, because an uncharged wave feels no force from the field.
5. The beta particle is deflected much more than the alpha particle, even though the alpha carries twice the charge, because the beta's tiny mass makes it far easier to push off course.

In a magnetic field, the paths curve at right angles to the field

1. A moving charge in a magnetic field experiences a force that is at right angles both to the field and to the direction of travel.
2. Because the force keeps turning as the particle turns, the path is a curve rather than a straight-line drift to one side.
3. Alpha and beta particles carry opposite charges, so in the same field they curve in opposite directions.
4. Gamma radiation is again undeflected and carries on in a straight line through the field.
5. The beta particle follows a much tighter curve than the alpha particle, for the same reason as before: far less mass to turn.
6. Seeing all three emissions at once, a magnetic field splits the beam into three: a gentle curve one way, a tight curve the other way, and one path straight ahead.

Why alpha ionises most: bigger charge, and energy spent quickly

1. Ionising an atom means pulling one of its electrons away, which takes both an electrostatic force to act on the electron and some energy to break it free.
2. The alpha particle's charge of $+2$ is twice the size of the beta particle's, so it pulls on the electrons of every atom it passes with a much stronger force.
3. It is also the slowest of the three, so it lingers near each atom for longer, giving that force more time to act.
4. The result is a huge number of ionisations packed into a short distance, and each one takes a share of the particle's kinetic energy.
5. So the alpha particle runs out of kinetic energy after a few centimetres, which is exactly why the most ionising emission is also the least penetrating.
6. The beta particle has half the size of charge and travels close to the speed of light, so it passes each atom too quickly to interact strongly and ionises far fewer atoms per centimetre.
7. Losing its kinetic energy more gradually, it keeps going for perhaps a metre of air or a few millimetres of aluminium.
8. Gamma radiation has no charge, so it exerts no electrostatic pull as it goes past and only ionises on the rare occasions it interacts with an electron directly.
9. Ionising almost nothing means losing almost no energy, which is why gamma travels furthest and needs dense shielding such as lead to reduce it.