Radioactivity

Syllabus Statements :

What on earth is radioactivity?

Radioactivity is basically a way for the nuclei (plural of nucleus) of atoms to become more stable. These instabilities in the nucleus arise when there are too many or too little neutrons in the nucleus, usually in isotopes of atoms that have I high mass. There are 3 ways that nuclei can become more stable, Alpha, Beta and Gamma radiation (This is actually not true but you only need to know 3).

note: Whenever we're talking about radioactivity, forget about the electrons in the shells orbiting the nucleus, we're only interested in the Nucleus specifically. Electrons are still important though (Beta).

Alpha Radiation

Alpha radiation is when the nucleus of an atom spits out an alpha particle, an alpha particle being a collection of 2 protons and 2 neutrons, which is actually the same as the nucleus of a helium atom so you may hear it being referred to as a 'helium nucleus'. Because the alpha particle contains 2 protons, the charge of an alpha particle is 2+.

In all radiation equations the top number is the (relative) mass of the particle/nucleus and the bottom number is the atomic number or number of protons.

Alpha radiation can be written as a helium nucleus (He)

Or as an Alpha particle (α)

When a nucleus emits an Alpha particle, the bottom number (atomic number) reduces by 2, as the nucleus loses 2 protons. The top number (mass number) reduces by 4 as it has lost 4 particles that all have a mass of 1.

---

Beta Radiation

Beta radiation is an emission of an electron from the nucleus. What? An electron from the NUCLEUS? Yes. What actually happens is a neutron turn into a proton, releasing an electron. Therefore the product of the beta radiation will have the same mass but will have an extra proton (the atomic (bottom) number will go up by 1). You can use electron and 'beta particle' interchangeably when talking about radioactivity (only when talking about radioactivity!)

Beta radiation can be written as an electron (e)

Or as a Beta Particle (β)

---

Gamma Radiation

Gamma radiation is the emission of a gamma ray from the nucleus. It doesn't have any mass or charge because it is a wave (from the top end of the electromagnetic spectrum). If it doesn't change the mass or atomic number of the nucleus then why? Because giving off this gamma ray gets rid of a bunch of extra energy that was hanging around in the nucleus making it unstable. Gamma radiation is bound to be combined with other types of radiation in the exam because the equation for it is so easy, you literally start and end with the same thing.

Atomic nuclei spit out these particles in a completely random way, so you can never predict exactly when a decay will happen. They do however have a general rate of decay. Some isotopes will decay faster than others. You can see this on this PhET simulation. It's called Radioactive Dating Game but it actually has a lot more than that. If you look at the top it has tabs where you can select different simulations etc.

Play PhET simulation

For all radioactive decay graphs the curve is the same.

On the x axis we have number of half lives. This can sometimes be time, and they might ask you to figure out how long a half life is for that substance. The y axis here is amount of the isotope (in this case carbon 14) remaining. This can also be activity in Becquerels, 1 Becquerel being 1 radioactive decay a second. The time it takes for half of the radioactive isotope to decay is called a half life. This is essential! Don't ever forget this!

If one half life has been then you will have half the amount you originally had. The next (second) half life will halve the amount again, so if you had 5 grams after the first half life you will have 2.5 after the second half life. So basically it keeps halving.

Ionisation power and penetration

So radioactivity is basically when particles are spat out of the nucleus' of atoms right? Well there are some things you need to know about these types or radioactivity. The first thing is penetration power. Penetration power is basically how far the radiation can go before it is stopped. Alpha has the weakest penetration power, it is easily stopped by a hand or a piece of paper. This is because the alpha particle is big and bulky compared to the other types of radiation. Beta particles are smaller so they penetrate much further, and can only be stopped by a few millimetres of aluminium. Gamma meanwhile is an electromagnetic wave (imagine something seriously tiny) and can get through just about everything. The only way to stop it is with a few centimetres of Lead. Why lead? It's seriously dense.

Ionising power is another thing you need to know about. When radiation bumps into atoms, they knock off a few electrons, creating ions, hence the name 'ionisation power'. Ionisation power is how big of an impact the radiation has when it hits atoms. A low ionisation power means the radiation only knocks off a few electrons. A high ionisation power is seriously bad, and whacks off a lot of electrons in several places. It's a bit like this. Imagine hitting a pinata with a needle. it's not going to do much. Then imagine hitting the pinata with a cricket bat, that does a lot of damage. The cricket bat won't go very far, but the needle will go all the way through.

Alpha radiation has the highest ionisation power of all, followed by beta and gamma. If you look back in the newspapers, about 10 years ago a Russian guy was poisoned with an alpha radiation source. Outside the skin alpha radiation doesn't do much except give you skin cancer. Inside the body though, it wrecks important cells in your digestive system and can be fatal like it was for the Russian guy (remind me to search up his name later)

How do we detect radioactivity?

Well there is a very handy piece of equipment called a geiger counter that your teacher might have shown you and if you plug it into a speaker it clicks every time it detects ionising radiation. You can also use photographic film to detect it, but it isn't automatic like the geiger counter. People who work in environments with a lot of radioactivity wear a badge with photographic film inside it to track how much radiation they get.

The photographic film is put inside a light proof casing (because light just like radioactivity causes the film to darken). Inside the case there is a section where the photographic film is covered in lead. The lead prevents all radioactivity from entering, the section of photographic film covered in lead is used as a control.

 The section covered by aluminium stops beta radiation from getting through, thus the section of photographic film covered in aluminium measures the amount of gamma radiation. The remaining uncovered section measures the amount of alpha and beta radiation.

So The only place you'll find radioactivity is in a nuclear reactor right?

Nope! There is radiation coming from all sorts of places! It's just not enough to do us any harm. This radiation is called background radiation, it comes from radon gas that has come from the decay chain of uranium (uranium slowly decays into a bunch of other radioactive products one of which is radon gas), buildings, food and drink (some food like bananas have radioactive isotopes in them in very small amounts), Cosmic rays which are basically chunks of particles etc. chucked out by the sun and from medical uses which we will cover later! Just so you know you will need to remember a couple of sources of radiation for the exam.

So why is radiation so bad for us? And what can we use it for?

Radiation is bad for us because when radiation hits us it damages our cells and tissues. Also in some occasions the radiation can hit our DNA and cause mutations that can lead to, you guessed it, cancer.
Radiation is actually very useful to us though! If we have a blockage in a sewerage pipe we're not too keep to head on in there to find the leak (if it's even big enough for us!). To find out where the blockage is we can send a radioactive 'tracer' down there and track it's movement with a geiger muller tube and see where it stops. You can do the same in humans too, need to find out where the blockage is? Swallow a radioactive tracer! For tracers in the body, beta and gamma sources are chosen because these types of radiation can pass through your skin, and radioactive sources with short half lives are chosen so the radioactive tracer doesn't do too much damage to your body or stay in there too long. Radiation is also used in smoke detectors. Inside smoke detectors there is a small alpha radiation source. 

The alpha particles from the radioactive source hit and ionise the air. This means that current can flow, as the ions are free to move around. 

When smoke enters the smoke detector, the smoke absorbs some of the alpha radiation and instead of the air. This means because there are less ions in the air less current it conducted between the two plates causing the fire alarm to sound.

What else?

Well you can use radioactivity in chemotherapy or radiotherapy to (hopefully) kill cancer cells. In chemotherapy of let's say the thyroid, you would get a radioactive isotope of iodine (because iodine goes to the thyroid) and hopefully the radiation will destroy cancer cells there. Radiotherapy is when you direct radioactivity (gamma rays) at a certain target in the body and fire and cross your fingers that it gets some cancer cells. The problem with doing this though is that you're risking the chance of hitting and damaging other healthy cells too, possibly even making them cancerous too. 
One last use of radioactivity, carbon dating. So for every couple of hundred thousand carbon atoms there is one little atom of radioactive carbon 14. Carbon 14 is basically normal carbon but with 2 extra neutrons. The half life of carbon 14 is very very long. Let's say you need to find out how old a fossil is. First measure the radiation coming out of it (not including background radiation) and compare it to that of a young (not old) carbon sample and compare the radioactivity (in Becquerels!). From that you can figure out how old the fossil is by the amount of radioactivity it's giving out.

Phew that took a while! and that isn't even all of it!

That's all for now though, I'll do another post on Rutherford's gold foil experiment and what that shows about atoms, and I'll also do another post on fission of uranium in nuclear power plants

 I hope this was useful! Let me know down in the comments if not, I'm not sure how good my explanation of half life was, if it's bad let me know and I'll fix it!