Introduction


Radiation and radioactivity makes some people feel uneasy. Possibly, the reason for this is that alpha, beta and gamma radiations can cause cancers and health problems and even death. However, hospitals use exactly the same radiations to look inside people's bodies, diagnose health problems and to treat and kill cancers when they form. They are used to check for underground leaks in pipes and to check for cracks inside jet engines. They sterilise fruit and stop bacteria turning food off.
To understand why radiation can be dangerous and helpful at the same time, you will need to understand what the three types of radiation are – what they are made up of. Because these radiations come from the middle of atoms – the nucleus – we will begin with this: the nucleus of the atom.


The nucleus of the atom


The middle of the atom is called the nucleus (just like the nucleus of a cell is in the 'middle'). The nucleus is not a thing, it is a place. In this place are a clump of particles: protons and neutrons.

Protons

Protons have:
  • an atomic mass of 1
  • an electric charge of +1

Neutrons

Neutrons have:
  • an atomic mass of 1
  • an electric charge of zero

Atomic Mass

The mass of an atom is almost totally down to the mass of the protons and neutrons in the nucleus
  • Atomic mass = mass of protons + mass of neutrons

Atomic Number

The periodic table is ordered according to how many protons the atoms have in their nucleus.
This just happens to be the number of electrons, too, so another way of saying the same thing is
  • Atomic number = number of protons or number of electrons in an atom

atom.jpg
How protons neutrons and electrons are arranged in a carbon atom


Example: Helium has two protons and two neutrons and 2 electrons and has the letter symbol He
He.PNG


Summary Table of particles in an atom:
particle
mass
electric charge
proton
1
+1
neutron
1
0
electron
1
-1

Isotopes

Isotope: the name given to an atom of the same element that has more or less neutrons. It has exactly the same number of protons, so it has the same atomic number. Because of the different number of neutrons, it will have a different atomic mass.

Take oxygen.... The one you'll find in the periodic table looks like this: 168O. This means there are 8 protons and (16-8) = 8 neutrons. There is more than one type of oxygen though. There is also 178O and 188O. These are called
isotopes of oxygen:
168O 8 protons 16-8 = 8 neutrons
178O 8 protons 17-8 = 9 neutrons
188O 8 protons 18-8 = 10 neutrons


Here's the periodic table of elements
periodic_table_of_elements1.jpg

Here are two videos to explain isotopes. The second one covers all the material so far. I kinda like both, even though the second is a bit fuzzy in places. See which one works for you:


Unstable nuclei and radioactive decay


The middle bit of most atoms (the 'nucleus') is not always stable. Bits 'come off'. Here's a simple reason for it: the 'mix' of protons and neutrons isn't quite right. So, bits come off, to make the 'mix' a bit better.


The three types of radiation


Alpha Radiation (α)
Alpha radiation is a lump of two protons and two neutrons. It is sometimes called an alpha particle, although it is actually four particles clumped together.
Alpha particles are given a mass number and an atomic number. The mass is the same as the mass of 2 protons and 2 neutrons added together: 4. The atomic number is the same as the number of protons in an alpha particle: 2. It is written like this:
42α+2 or sometimes, because this is exactly what a helium nucleus looks like, 42He2+. The 2+ bit means that it has an electric charge of 2+ from its two protons.
Beta Radiation (β)
Beta radiation is actually an electron. This is what happens to make it: To even up the mix in an unstable nucleus with too many neutrons, a neutron changes into a proton.
Beta particles are given a mass number and an atomic number. The mass is the same as the mass of zero protons and zero neutrons added together: zero. The atomic number is the same as the number of electrons in a beta particle: -1 (yeah, I know, it is an electron, but its atomic number is -1, its because of the charge). It is written like this:
0-1β- or sometimes, because this is just an electron, we use 0-1e- instead.
Gamma Radiation (γ)
Gamma radiation is not a particle, it is very high energy light - it is an electromagnetic wave. They are shorter wavelengths even than X-rays, and in other topics this gamma radiation is called gamma rays. Either is fine for us.


Activity
The number per second of alpha, beta or gamma that is shot out from unstable nuclei is called the ACTIVITY. 1 'count' of radiation per second is called a Becquerel.


Background Radiation

'Background' here means 'it's just around us all the time, and there's not a fat lot you can do about it'. Or, another way of saying the same thing is this: 'Even if I don't stand in front of something giving off radiation, I am going to breathe in, eat, drink and be hit by stuff called radiation, because small amounts of it are everywhere'. Don't be too worried though, because we have evolved with this background radiation.

What you need to be able to do: We find natural (and in the last century, human-caused) sources of radiation in many places.
You've got to say where this comes from, so get a pen, and pop down where it comes from:


Radon Gas
Radon gas is the biggest factor to most people's background radiation. Radioactive radon gas is produced by radioactive rocks. Because different rocks under the ground are more (or less) radioactive, the amount of radon gas you have in your background is different all over the UK:
uk-radon-map.png
The darker the brown, the more radioactive radon gas there is around

radon-penetration_1.jpg
How radon gas gets into homes

Cosmic radiation
Radiation from space is very intense. (Very) fortunately, the Earth's magnetic field deflects a lot of it, so not much gets to the ground where we live.
soho-solwind_angled_lines.jpg
The earth's magnetic field protects us from solar radiation


Ionisation

What is ionisation?

When radiation gets shot out of unstable nuclei, it carries on until it hits something. This something is other materials and their atoms. It could for instance be the material that you happen to call “me”. The radiation will hit the outside of atoms. If it does and it knocks out an electron, the atom that was hit becomes an ion. The name we give to 'atoms having their electrons knocked off by radiation' is ionisation. Radiation that is big and has electric charge ionise atoms more. So, alpha ionises more than beta, which ionises more than gamma.

Why is it dangerous?
Ions can undergo 'unpredictable' chemical reactions. Inside living tissue, this is not so good. Cells can start doing things they normally wouldn't - it depends on the amount of radiation received (the 'dose').
Low doses vs high doses of radiation
High doses of ionising radiation can kill cells entirely. Lower doses can cause mutations in the cell DNA and lead to tumours and cancers. In a neat turnaround, very high doses are used in medicine to kill tumour cells (that may or may not have been caused by lower doses of radiation).

Penetration Distance


Radiation hitting other atoms eventually slows down and stops. How far it goes before it stops is called the penetration distance. The more a type of radiation hits other atoms, the less distance it goes. So, bigger radiation that has more chance of hitting stuff ends up not going as far as smaller radiation that doesn't hit other atoms as much. This means that gamma goes further than beta which goes further than alpha.


Summary of Alpha, Beta and Gamma radiations types20of20radioac_0.JPG

Smoke Alarms




Half Life

Decay curves
The amount of alpha beta or gamma radiation that comes out of a lump of radioactive 'stuff' can be measured. A useful way of measuring how radioactive something is see how many alpha, beta or gamma comes out every minute. Or every hour (if the radioactive stuff doesn't decay as quickly). Or, even counts per year (or hundreds of years etc - you get the idea). Now, when we do this and plot how much radioactive stuff is left every minute (or hour, or year etc etc) you get a graph with a shape called a decay curve.
Activity vs time
Activity is the amount of alpha, beta or gamma coming from some radioactive stuff. It's measured in units like counts per minute (cpm) or counts per hour (cph)...
Half Life
The amount of time that passes while the amount of radioactive stuff left goes down by half is called the half-life. This means the same amount of time will pass before the radioactivity drops from
  • 100% to 50% or
  • 80% to 40% or
  • 30% to 15% and so on...it doesn't matter which values you look at - it will go down by a half in a period of time called one half life
Radioactive dating
Most everyday objects contaions atoms whose nuclei are unstable. maybe not a lot of them are unstable and radioactive, but some of them will be. They won't stay radioactive forever. They will give off radiationa and become more stable. What this means is that most things are getting less radioactive as they get older. How much radioactivity is left in things can tell us how long ago it was made. This is how we know that rocks in some parts of the world are very old, and the same 'type' of rock in other parts of the world are a lot younger - they have less radioactivity.
Radiocarbon dating
Living things like plants and animals can be radioactive. Carbon-14 is a radioactive element that is present in all animals and plants. Animals and planst that live on the surface of the earth away from radioactive rocks and coal beds always have a certain % of carbon-14 in them when they are alive. As soon as they die though, the % of carbon-14 begins to drop. We can use the % of carbon-14 left in a dead thing to work out how long ago it died. Here's how:

Half life calculation from graph


The age of the Earth
How do we know how old the Earth is? We measure it:

The basic idea is this: the earth is made up of rocks, and these rocks contain isotopes of elements that are unstable - they are radioactive. Once the rocks are formed, the amount of radioactivity will start to decrease over time. Now if we know (or have measured) the half life of these elements that are in the rock that are radioactive, we can figure out how long ago the rock was made. How? Well, by comparing how much radioactive stuff there is in the rock compared to how much radioactive stuff would be in the rock when it was formed. Essentially the same as for radioactive carbon dating, but with the elements in the rock having much longer half lives, we can date things much, much further back in time.

Radiation in medicine

<coming soon>
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