Page 246 - Physics Coursebook 2015 (A level)

Cambridge International AS Level Physics
Figure 16.16 shows the effect of a magnetic field. In this case, the deflecting force on the particles is at right angles to their motion.
magnetic field into paper in this region
α
γ
β−
Figure 16.16 A magnetic field may also be used to separate α-, β−- and γ- radiations. The deflection of the α-radiation has been greatly exaggerated here.
QUESTION
234
15 a
b
Some radioactive substances emit α-particles having two different speeds. Draw a diagram similar to Figure 16.16 to show how these particles would move in a uniform electric field. Label your diagram to show the tracks of the faster and slower α-particles.
A β−-emitting radioactive substance emits β−-particles with a range of speeds. Add to the diagram you drew in a to show how these particles would behave in a uniform electric field.
Figure 16.17 Alpha-particle tracks show up in this photograph of a cloud chamber. Notice that all the particles travel roughly the same distance through the air, indicating that they all have roughly the same initial kinetic energy.
Alpha-particles can also be detected by a solid-state detector, or by a Geiger–Müller (GM) tube with a thin end-window (Figure 16.18) connected to an electronic counter. By moving the source back and forth in front of the detector, it is simple to show that the particles only penetrate 5 or 6 cm of air. Similarly, with the source close to the detector, it can be shown that a single sheet of paper is adequate to absorb all of the α-radiation.
Radiation penetration
Safety note
When working with radioactive sources, it is essential to follow the relevant safety regulations, which your teacher will explain to you.
Alpha-radiation
Because α-radiation is highly ionising, it cannot penetrate very far into matter. A cloud chamber can be used to show the tracks of α-particles in air (Figure 16.17). The tracks are very dense, because of the dense concentration of ions produced, and they extend for only a few centimetres
into the air. By the time the α-particles have travelled
this far, they have lost virtually all of their kinetic energy. The α-particle, which is a nucleus of helium-4, grabs two drifting electrons in the air and becomes a neutral atom of helium gas.
paper
GM tube
α-source
Figure 16.18 Alpha-radiation can be absorbed by a single
sheet of paper.
Beta-radiation
A Geiger–Müller tube can detect β−-radiation. The source is placed close to the tube, and different materials are positioned between source and tube. Paper has little effect; a denser material such as aluminium or lead is a more effective absorber. A few millimetres of aluminium will almost completely absorb β−-radiation.