Charged particles are affected by electric fields. An electric field is the area around a charged object. it forms a 'landscape' for charged objects rather like hills and vales in the 'charge dimension'. This makes charges accelerate as they move in a potential gradient, just like a ball would accelerate if it rolled down a hill.

If two parallel plates (one negative and one positive) form an electric field that particles from radioactive decay are made to travel through the particles that are charged will accelerate towards the plate with opposite charge. An alpha particle will therefore accelerate towards the negative plate and the beta (minus) particle towards the positive plate.

The gamma ray has no charge so it is not affected by this 'electric landscape' and will just continue on its straight path.

A beta- particle (β- - a high energy, fast moving electron) has much less mass than an alpha particle (a helium nucleus) - it has mass of 1/7300 of an alpha particle. A typical velocity for an alpha particle is about 16 km/s , whereas the typical speed of a beta particle is about 270 km/s - about 17 times the speed. But despite the greater speed, the larger mass makes the alpha particle have a lot more momentum than the beta particle. Therefore an alpha particle is deflected less that an beta particle in a given electric field because of its higher momentum.

Explanation:

Alpha particles are positively charged, beta particles are negatively charged, and gamma radiation is electrically neutral . This means that alpha and beta radiation can be deflected by electric fields , but gamma radiation cannot.