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Understand Nuclear Decay

Worksheet Overview

QUESTION 1 of 10

In 2011, an earthquake hit Japan registering as a 9 on the Richter scale – the largest earthquake in Japan's history and the fourth biggest in the world. This created a tsunami six metres high and killed over 15,000 people. One of the places hit by the earthquake was a town called Fukushima where they had a nuclear reactor. Unfortunately,  the damage caused by the tsunami stopped the reactors from working correctly and three of them suffered meltdowns, spewing radioactive material into the environment. People were evacuated from their homes and a five km exclusion zone was set up to protect people from the damage of the radiation. But what is radiation and how is it caused? This is what we will be looking at in this activity.

 

We are constantly exposed to radiation. It is made by the rocks we build our houses from, it comes from space and it is even in our fruit! We call it background radiation – and there is nothing that we can do to avoid it.

 

So, if there is so much radiation around us all the time, why are we not all being killed? Well, that is because the level of radiation that we are exposed to is quite low. We measure radiation using a device called a Geiger Muller tube (GM tube), and it measures ionisation events inside the tube. The more ionisation events, the more radiation there is. The GM tube will give us a ‘count rate’ telling us how many ionisation events there are.

 

The count rate is not the same as the activity of a sample, however! The activity of a sample is defined by a unit called a becquerel (Bq), where one Bq is one nucleus undergoing nuclear decay every second – decay is when a particle shoots out some radiation. Are you feeling overwhelmed by all the new words yet? We have made a handy list of words and their meanings for you at the bottom of this explanation to help you out!

 

When the nuclear commission was helping the clean-up from Fukushima, they took a range of readings looking for all of the different types of radiation. They needed to do this because different types of radiation cause more or less damage depending on where they are. Let’s look at the different types of radiation in detail:

 

Alpha (α):

Alpha radiation causes the most ionisation, but only travels a very short distance before it stops (about 15 cm). It can be stopped by the skin and so it is safe outside your body – but if you inhale it or drink it, then it can be deadly. A Russian spy, Litvinenko, was killed by a VERY small amount of alpha radiation being put in his tea.

 

Alpha radiation is made of 2 protons and 2 neutrons – it is a helium nucleus. This gives it a charge of +2, meaning that it will rip electrons off things it comes near, causing damage to them – but it can only do this twice.

 

 a helium nucleus

 

 

Beta (β):

This is moderately ionising and can travel up to about 1 metre. It is not really safe inside or outside your body, so it’s best to stay away from it, full stop – but unlike alpha, it is less likely to kill you unless you have a high dosage of it.

It is made of a fast-moving electron usually spat out from the inside of a nucleus. It causes damage by hitting other electrons and removing them from their atoms.

 

an electron

 

 

Gamma (γ):

This is a strange one because it is a wave, not really a particle (although it is carried by photons). It is the least ionising of all of the three that we have looked at so far because it needs to hit stuff at just the right speed to cause damage. So, as a result, it normally just passes through everything. Safe on the inside and the outside, unless there is a lot of it.

 

a gamma wave

 

 

Neutrons (N):

These are the ones that you need to be afraid of and the ones that they were most concerned were being released from Fukushima. They are made of a neutron and they are so dangerous because if they collide with a nucleus, then they will make the nucleus itself radioactive, causing it to release alpha, beta and gamma radiation. This can then, in turn, lead to more neutrons being released. The long and short of it is that you don’t want to be anywhere near these things unless they are bound to your nuclei!

 

a neutron

 

 

Alpha, beta, gamma and neutron radiation come from the nucleus of a radioactive atom. Alpha can be stopped by a thin piece of paper, beta penetrates the paper but can be stopped by a 3 mm thick piece of aluminium, and gamma can penetrate a 3 m thick lead block and concrete. The diagram shows this in more detail:

 

Penetrating abilities of alpha, beta and gamma.

 

As you can see, α doesn't go through the paper, β will go through the paper but not the aluminium, and γ will go through everything and is only reduced by the lead.

 

Alright? You got it? Good! Here is that handy list of keywords that we were talking about earlier.

 

Keyword Meaning
Ionisation When an atom gains or loses an electron
Event When  decay or ionisation happens
Decay When a nucleus emits a particle of radiation
Count rate How many ionisation events happen in a set amount of time
Activity Have many decays a radioactive element is doing per second
Becquerel The unit of activity
GM tube A device used to measure ionisation - gives results in count rate

 

Let's try some questions now.

Select the correct symbols for the different types of radiation. 

N

γ

α


β

Ω

What is the definition of  nuclear decay?  

 

[1]

Some uranium emits an alpha particle.

 

What does an alpha particle consist of?

2 electrons and 2 protons

2 protons and 2 neutrons

2 neutrons and 2 electrons

Choose the correct shielding to protect against beta radiation.

Paper

Skin

Plastic

Aluminium

Thin lead

Thick lead

Which of the following can travel the furthest in air? 

α

β

γ

N

An atom of uranium has 92 protons and decays into an atom of thorium by alpha decay.

 

How many protons does thorium have? 

88

89

90

91

92

Thorium 90 is a source of radiation. It is an α emitter and needs to be stored for safe transportation.

 

Choose the safest way to store the thorium during transportation.

Paper bag

Plastic box

Plastic box with aluminium lining

Lead box

What is the unit of activity? 

Describe the differences between the required shielding and penetrating depths in air for α, β and γ. 

 

[4]

Describe the difference between activity and count rate.  

 

[2]

  • Question 1

Select the correct symbols for the different types of radiation. 

CORRECT ANSWER
N
γ
α
β
EDDIE SAYS
A tricky question to start with! Questions in the exams tend to refer to them as alpha, beta, gamma and neutrons or α, β, γ and N. You need to be familiar with both of these ways of writing them - and this will only come from practice. Get out some old exam papers or take a look at our exam papers and practise the nuclear decay questions!
  • Question 2

What is the definition of  nuclear decay?  

 

[1]

CORRECT ANSWER
EDDIE SAYS
Decay happens when the nucleus cannot stay in the state that is in. The nucleus becomes unstable and emits energy in the form of radiation. This could be in the form of alpha, beta or gamma particles or even neutrons, and the nucleus is changed forever.
  • Question 3

Some uranium emits an alpha particle.

 

What does an alpha particle consist of?

CORRECT ANSWER
2 protons and 2 neutrons
EDDIE SAYS
This is the most highly charged of all of the types of radiation, which is why it can cause so much damage - but it only travels a really short distance. Having 2 protons and 2 neutrons, it is one of the most highly charged objects for its mass in the universe.
  • Question 4

Choose the correct shielding to protect against beta radiation.

CORRECT ANSWER
Aluminium
EDDIE SAYS
There were a lot to choose from - did you make the correct choice? If this had been asking about alpha radiation, then you could have chosen both paper and skin but beta rays will pass through both of those. However, you would need only some thin aluminium to stop beta radiation. This is because it will lose all of its energy if it collides with a nucleus or another electron. In aluminium, there are a lot of nuclei and other electrons just flying around. This means that it is more likely to have a collision event.
  • Question 5

Which of the following can travel the furthest in air? 

CORRECT ANSWER
γ
EDDIE SAYS
Gamma can travel across 100 light years of lead if it gets lucky - it can get spat out by the Sun and then travel straight through the earth. Impressive, right? This has the added effect of letting it travel through the air forever. At least, we think it is forever.
  • Question 6

An atom of uranium has 92 protons and decays into an atom of thorium by alpha decay.

 

How many protons does thorium have? 

CORRECT ANSWER
90
EDDIE SAYS
Don't panic when confronted with a seemingly impossible question! It must be possible to work this out or you wouldn't be being asked it! Let's look at all the information you know. An alpha particle is made of 2 protons and 2 neutrons. These particles are not created from thin air, they have to come from somewhere - that place is the nucleus of the atom they came from. In this case, uranium has 92 protons, but would have lost 2 during the decay (as well as 2 neutrons) - so now it would only have 90 protons (92 - 2 = 90).
  • Question 7

Thorium 90 is a source of radiation. It is an α emitter and needs to be stored for safe transportation.

 

Choose the safest way to store the thorium during transportation.

CORRECT ANSWER
Plastic box with aluminium lining
EDDIE SAYS
When it comes to safety, you always overestimate the safety needs you have rather than underestimating them. Let's go through all the different options, one at a time. A paper bag could rip and spill the contents - this would not be very safe if people breathed it in. A plastic container would be much safer, but might also be prone to breaking or melting if it got hot. The plastic container with the metal inside is much safer as the metal provides an extra security level. The lead box is heavy, and lead is also soft, so things could damage it and it could damage the thorium. Not a good idea in general.
  • Question 8

What is the unit of activity? 

CORRECT ANSWER
Bq
Becquerel
EDDIE SAYS
Did you remember it? Either Bq or Becquerel would be acceptable but only if you got the capitalisation right - a capital B and a small q! It is really important that you get the capitals correct or you could lose marks.
  • Question 9

Describe the differences between the required shielding and penetrating depths in air for α, β and γ. 

 

[4]

CORRECT ANSWER
EDDIE SAYS
You need to know your penetration depths! It's not that difficult if you can remember the order of thickness of the materials and match them with the intensity of the radiation. Paper, aluminium and lead can be matched with alpha, beta and gamma. Well done if you got them all.
  • Question 10

Describe the difference between activity and count rate.  

 

[2]

CORRECT ANSWER
EDDIE SAYS
This was a hard question so don't despair if you found it tricky. There are a lot of different measurements of radiation because there are a lot of different ways that radiation can be created. The two most common measurements are count rate (this tells us the average amount of radiation that is in the environment) and activity (which is specific to that one sample of radiation). Well done for completing this activity. If you're feeling more confident with this topic, why not have a look at the activity on half-life which will go into it in more detail.
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