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Now that we’ve discussed the particles that make up the atom, we’ll have to look at something that doesn’t appear to make sense: Why doesn’t the nucleus explode? Positive charges repel each other so why don’t the protons in the nucleus fly away?
In this part of the booklet we’ll have a look at the atoms in which this actually happens. These atoms are called unstable and they become stable again by expelling a piece from the nucleus. The stream of particles that has been expelled is what nuclear radiation is.
We’ll have a look at:
· Whether an isotope is stable or unstable
· Which types of radiation there are, what the properties are and how we recognise them.
· What happens when radiation meets other atoms and what dangers that poses.
· How we can use radiation (safely).
First let’s have a look at stable and unstable.
In the first part of the simulation (atom) mark the checkbox next to “stable/unstable” so the program tells you the stability of the atom. Also, make sure you can see the mass number.
Assignments
49. Build the atoms hydrogen to fluorine and add neutrons until they are stable. Then add more neutron until they become unstable. Write down the mass numbers of the isotopes that are stable. (So for hydrogen the stable mass numbers are 1 and 2. For helium 3 and 4.)
When you have done this for the first 9 elements, have a look at your numbers and answer these questions.
50. Would you now be able to say with certainty which isotopes of Neon are stable?
51. What is the “rule” for the amount of protons and neutrons regarding stability?
Now build the atom Neon and check whether you were right.
Nuclear radiation
In your blood, you have red blood cells and they contain haemoglobin. It is the haemoglobin that binds oxygen for transport to your cells. At the centre of a haemoglobin molecule, there are four iron atoms, probably iron-56. That isotopes or iron is stable, so stable that we can say for sure that the very iron atom in that molecule of haemoglobin is older than the earth. It’s also older than the sun, it even predates the nebula from which our solar system started to form 4,6 billion years ago. It was created inside one of the stars of a previous generations of stars. Not all iron is stable though, iron-59 is an unstable isotope.
When an atom or ion is unstable, it has the tendency to do whatever is necessary to become stable again (hence the terms stable and unstable). In the previous chapter, you’ve discovered that whether a nucleus is stable or unstable has to do with the contents of that nucleus. It is therefore perfectly logical that an unstable nucleus must change its contents to become stable and it can do that ejecting particles (or energy), this action is called decay. It is that which has been ejected from the nucleus that we call Nuclear Radiation. Because an unstable isotope produces radiation, we call it a radioisotope.
Assignment
Watch the video and answer the following questions:
52. What are α-particles made of?
53. What is β-radiation made of?
54. What is γ-radiation made of?
55. Sometime β-particles are written like β- particles, why would there be a minus-sign?
Penetration
Radiation can be absorbed by substances in its path. For example, alpha radiation travels only a few centimetres in air, beta radiation travels tens of centimetres in air, while gamma radiation travels many meters. Also, all types of radiation become less intense the further the distance from the radioactive material, as the particles or rays spread out more.
The thicker the substance, the more the radiation is absorbed. The three types of radiation penetrate materials in different ways.
Alpha radiation can be stopped by a sheet of paper.
Beta radiation can penetrate air and paper, but is stopped by a sheet of aluminium.
Gamma radiation can only be stopped by a few centimetres of lead, or many meters of concrete.
Assignments
If you know what materials (and in what quantity) can stop different kinds of radiation, you can use that knowledge to determine what kind of radiation you are dealing with.
56. When a piece of radioactive material is covered with a piece of aluminium, no more radiation is being measured. What kinds of radiation could this material produce?
57. The piece of aluminium is replaced with a paper towel, there is still no radiation being measured. What kind of radiation does the material produce?
Ionisation
Radiation itself doesn’t do much, except for traveling away from the source. It is when the particle or ray collides with another atom that damage is being done.
Virtually all materials, including living tissue, are made up of atoms. That means that they don’t have a charge, otherwise they would be called ions. When this atom is being hit by a particle or ray, the energy of that collision is great enough to knock one or more electron away from the atom. As you’ve learned already, the atom is now an ion, it has been ionised.
This is why we call the radiation from a radioactive source: Ionising Radiation. (not radioactive radiation; the source is radioactive, the radiation is not)
Ionisation (in the UK it is commonly written as ionisation but with a z is correct as well) has some major consequences, the chemical properties have changed. Chemical reactions that were impossible with atoms can suddenly occur and other reactions which needed atoms suddenly become impossible. It is because of this change in chemical properties that ionising radiation is harmful to living organisms.
The ionising ability of radiation is of course determined by what makes up the radiation. An α-particle is about 8000 times as heavy as a β-particle so you can imagine the damage done by an α-particle is quite a bit more serious. The size of the α-particle also means it doesn’t get very far before it collides with an atom, so when the particle is large, the ionising ability is high but the penetration is low.
When the alpha or beta particle has hit a few atoms (and ionised them), the particle has slowed down but of course, it doesn’t disappear.
Remember, β-particles are electrons. So when the particle has slowed down enough by bouncing of atoms and knocking away other electrons, it can join an atom, ionising his last victim by adding an electron.
When an α-particle slows down it picks up a couple of electrons (because of its charge). It is now a Helium-atom! All the helium gas you find in carnival balloons is made up of α-particles that were created and slowed down in the crust of the earth and pumped up together with natural gas.
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