Physics
Cambridge IGCSE
Cambridge IGCSE
TOPIC 5: NUCLEAR PHYSICS
The rock shown here is radioactive. It contains uranium-238, a radioactive isotope.
But what makes it 'radioactive'? Many atoms are unstable as discussed in the last section. Eventually the uranium nuclei will split apart, emitting some particles. Uranium decays very slowly and this rock will be radioactive for millions of years. The particles emitted can be extremely dangerous to human health. They 'ionise' other atoms, knocking outer electrons away and this can change the chemical bonds of any molecules they hit.
Figure 1: A rock containing uranium
Public Domain-wikimedia
Radioactivity, and the radiation emitted is said to be:
One of the most interesting early discoveries of nuclear physics was that we are constantly being exposed to small doses of radiation. It is all around us, and called background radiation. The Earth contains a significant quantity of radioactive uranium and thorium. Some isotopes of these will last for billions of years. As they slowly decay, they produce gamma rays which can penetrate through the rocks in the ground. Also, the decay products of uranium and thorium include an isotope of the gas radon which is also radioactive. This gas seeps through the ground and is present in the atmosphere in small quantities. There are some regions in the world with quite high concentrations of this radon isotope.
Additionally, we are constantly exposed to radiation from space, in the form of cosmic rays. This includes gamma radiation and other fast-moving particles.
The chart shown in figure 2 lists the world average for causes of this background radiation:
Figure 2: Causes of background radiation in the world population
(Source: World Health Organisation)
A 'Geiger-Muller tube and counter' (often called a Geiger counter) is a detecting device that can measure nuclear radiation that enter the tube. The standard method is to count how many events there are over a period of time, typically one second (counts/s). This is the 'count rate' recorded. The count rate might also be measured over a longer period such as a minute (counts/min).
See the video in the next section below to watch a detector and counter being used to record the count rate.
When atomic nuclei decay, they can split into many different fragments. However, there are three very common types of decay that we will cover in this course - alpha, beta and gamma decay:
An alpha particle (symbol 'α') consists of 2 neutrons and 2 protons, and is identical to a helium nucleus. The mass of an α-particle is 4 and it has a charge of +2. Many atoms decay by losing this 'clump' of 4 particles, as shown in figure 3.
Alpha particles are extremely ionising as they are more massive than beta and gamma. The positive charge also pulls on nearby electrons causing ionisation.
Figure 3: Alpha decay
Public Domain-wikimedia
Beta particles (symbol 'β') are very fast moving electrons, emitted by the nucleus. (How this happens is discussed in section 5.2b). As they are electrons, they have a charge of -1 and (effectively) no mass. They can still ionise other atoms, but not as strongly as α-particles.
Gamma rays (symbol 'γ') are part of the electromagnetic spectrum. (See section 3.3). They therefore have no mass and no charge. When gamma decay occurs, the nucleus emits this high energy ray, but no particles. It loses energy as a result, making the nucleus more stable. Gamma rays are only weakly ionising as they have no mass and no charge.
There is a standard investigation into the penetrating powers of alpha, beta and gamma. This might be performed by your teacher as a demonstration:
Apparatus:
Method:
Place the Geiger tube in front of one of the radioactive isotopes. Then place absorbing materials in between the detector and the isotope, starting with thin tissue of paper. Try thicker materials until the count rate drops significantly. You could simply observe the effect for a general pattern, or record the total count over 30 seconds for a more detailed conclusion.
As with any use of radioactive isotopes, this experiment is extremely dangerous and you should not be performing this on your own! There are also some excellent simulations that your school may have available.
Watch the video here to see this experiment being performed:
You tube video: Radioactive penetration
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Results:
The table here summarises the key features of alpha, beta and gamma radiation, which you need to learn:
| Type | description | mass | charge | stopped by.. | range in air | ionising? |
|---|---|---|---|---|---|---|
| α | 2 protons, 2 neutrons (A helium nucleus) |
4 | +2 | paper or tissue | a few cm | very |
| β | a fast-moving electron | 0 | -1 | thin aluminium | a few metres |
some |
| γ | an electromagnetic wave | 0 | 0 | thick lead | several hundred metres | little |
Questions:
1. A student is researching the nature of alpha, beta and gamma radiation. Which of these three:
a) Alpha particles have the largest mass.
b) Gamma radiation has no charge.
c) Alpha can only pass through a few cm of air.
2. A radioactive isotope is thought to be a beta emitter, with no alpha or gamma being emitted. Describe an experiment to prove this result.
You need a Geiger counter to use as a detector.
First, place paper / tissue in front of the source. If alpha radiation is present, the detected count rate will drop significantly. If there is no drop, the radiation is beta or gamma.
Second, place a thin aluminium sheet or two in front of the source. If the count rate drops, the source is a beta emitter. Gamma rays will not be significantly affected by the aluminium barrier.
(Note that you need to do both tests, to rule out alpha and gamma. All three will be stopped by thick lead so this test does not help in this situation).
Extension:
Why is it that alpha radiation has such a short range in air and low penetrating power, even though it has a mass of 4 and is strongly ionising?
Alpha is the most massive particle and also has a strong positive charge. This means it is highly likely to ionise any atoms it passes near to. When an atom is ionised, the alpha particle loses some energy. As it smashes into one atom after another, it quickly loses it's kinetic energy and stops moving through the substance. Hence it has a very short range through matter.
All of the three types of radiation discussed above can ionise atoms. But why is it that alpha is more ionising than beta and gamma?
An alpha particle ejected from a nucleus has a mass of 4 units. The very high mass means the kinetic energy is very high, and is therefore much more likely to remove the outer electrons in other atoms, causing ionisation. A beta particle (a fast moving electron) has a very small mass and so low kinetic energy, and gamma radiation has no mass at all. Therefore, the higher the kinetic energy, the more ionising the radiation.
In addition, alpha particles have a charge of +2, and so will attract electrons towards them This also causes the electrons of atoms in their path to be dislodged and therefore ionise atoms easily. Beta particles are electrons and with therefore repel other electrons. As gamma radiation has no charge, it is only the energy in the radiation that can affect electrons with - effectively - a direct impact with electrons in its path. The higher the charge, the more ionising the radiation.
An electric field is a region of space where charged particles experience a force. An electric field can be shown just like magnetic field lines (see section 4.1). However instead of the field lines going from north to south poles like magnetic fields, they are drawn from positive to negative charges. This is because the field lines will show the direction of force acting on a small positive charge placed in the field, which would be repelled from the positive charge(s) and attracted to the negative charge(s). In the example below (figure 4) the field lines are all parallel, and go from the positive plate at the top to the negatively charged plate at the bottom.
Figure 4: Electric field lines between charged parallel plates
If alpha, beta and gamma radiation passes into this field, what do you think would happen? The answer is that:
These paths are shown below in figure 5:
Figure 5: The path of ionising radiation through an electric field
In a magnetic field, the motion is much more complicated. We learned in section 4.5a about the motor effect - the force acting on a current carrying conductor in a magnetic field. The direction of the force is given by Fleming's left-hand rule.
However, a current is simply a flow of charges. So if a current experiences a force, then so will any flow of charge through a field. We can then use Fleming's left hand rule to predict the direction of force on a 'flow' of positive charge. Using the rule, the direction of force on the positive alpha particle shown in figure 6 below will be downwards. For the beta particle, as it is a negative charge, it will be pushed in the opposite direction! One way to think about this is that a flow of negative charge into the field is the same as a flow of positive charge out of the field, and then use Fleming's left-hand rule to show the direction.
Note that a gamma ray has no charge and so is not deflected by the field.
Figure 6: The path of ionising radiation through a magnetic field
Again, typically we would expect the beta particle to change direction rapidly compared to the alpha particle, as it has a low mass.
What would happen if the alpha and beta particles remained in a much larger field? In this situation, because the force is always at right-angles to the direction of the motion, it would make the particles move in a circle, with positive charges rotating in one direction, and negative charges rotating the other. As they lose kinetic energy, the radius of the circular path gets smaller, forming a spiral.
Particle physicists use special 'bubble' or 'cloud' chambers to show the tracks of particles. In figure 7 below you can see both positive and negatively charged particles in a magnetic field with spiral paths. There are also some straight paths, from uncharged particles or waves - possibly gamma radiation.
Figure 7: Particle paths through a magnetic field in a bubble chamber
There are more practice questions on this topic after the next section on decay equations (5.2b).
