THE NATURE OF BETA RADIATION

At the time, Rutherford had classified radiation based only on its penetrating power. However, he soon found that an electric or magnetic field could split such emissions into three types of beams. Based on the direction in which the rays were deflected, it was found that alpha rays carried a positive charge, beta rays carried a negative charge, and gamma rays were neutral (Figure 64). Based on how much each was deflected by the same field, it was discovered that alpha particles were much more massive than beta particles.²⁰ Other experiments showed that beta radiation and cathode rays are both streams of electrons. Rutherford also found that gamma radiation and X-rays are both high-energy electromagnetic radiation.

Figure 64 A magnetic field can split the radiation from a combined alpha, beta, and gamma source into three beams. Because of their positive charge, alpha rays are deflected to one side of the gamma beam, and negatively charged betas to the other side. Alpha particles are deflected much less than beta particles by the same beam, because the former are much more massive than the latter. Gamma rays are not deflected at all, because they are chargeless photons.

Carrying out the magnetic separation experiment with beta particles is easy. Figure 65 shows our setup, which consists of a ⁹⁰Sr disk source of beta particles, two copper washers to collimate the beam, and a GM tube placed at 90° to the beta particle beam. A sufficiently strong magnetic field (around 800 Gauss = 0.08 Tesla) provided by a permanent magnet bends the beam so much that it may even be detected at a right angle. When you conduct this experiment be careful not to get the magnet too close to the GM tube, especially if you are using one that is also sensitive to alpha particles, since any components inside the tube that the magnet may move could damage it.

Figure 65 You can bend a collimated beam of beta particles using a strong magnet. We use a ^90Sr disk source of beta particles, two copper washers to collimate the beam, and a rare-earth magnet to bend the beam.