OCR Physics 21st Century (new) P6- 'Radioactive Materials'

Atoms consist of a nucleus plus orbiting electrons
The nucleus of an atom contains protons and neutrons. It makes up most of the mass of the atom, but takes up virtually no space -- it's tiny.

The electrons are really really small. They whizz around the outside of the atom. Their paths take up a lot of space, giving the atom it's overall size (though it's mostly empty space). 

The number of neutrons in an element isn't fixed 
Every atom of a particular element has the same number of protons in its nucleus, e.g. every carbon atom has 6 prorons in its nucleus, every nitrogen atom has 7 protons in its nucleus etc.
The  number of neutrons isn't fixed though. Many elements have a few isotopes- atoms with the same number of protons but different number of neutrons.
There are two common isotopes of carbon -- carbon-14 has two more neutrons than 'normal' carbon (carbon-12). Usually each element only has one or two stable isotopes -- like carbon-12.
The other isotopes tend to be radioactive - the nucleus is unstable, so it decays (breaks down) and emits radiation. Carbon-14 is an unstable isotope of carbon. 

Radioactive Elements Emit Ionising Radiation

Some elements emit ionising radiation all the time - these elements are radioactive. Radioactive atoms are unstable - they break up (decay) to make themselves more stable. Unstable atoms decay at random and you can't predict when it will happen - it's completely unaffected by physical conditions (like temperature) or chemical processes (e.g. bonding). When an atom does decay, it spits out one or more of three types of ionising radiation - alpha, beta and gamma. In the process, the atom often changes into a new element. Ionising radiation can transfer enough energy to break an atom or molecule into bits called ions - this is called ionisation. These ions can then go on to take part in other chemical reactions.

Alpha Radiation is Slow and Heavy

Alpha particales are realtively big and heavy and fairly slow- moving. So they don't penetrate far into materials - they're stopped quickly. Alpha particles are released by very heavy nuclei, e.g. uranium. An alpha particle is a helium nucleus (He) - made up of 2 protons and 2 neutrons. Alpha particles have a mass of 4 and a charge of +2. Alpha decay always changes the element of the atom that's decaying, since it loses it's protons. 

A typical alpha emission:

226 222      4
     Ra -->    Rn ---> He
88 86     2
Unstable isotope     New isotope    Alpha particle 

Beta Radiation is Lighter and More Penetrating

Beta particles move quite fast and they are quite small. They penetrate moderately into materials before they're stopped. Beta particles are released by nuclei that have too many neutrons. During beta decay, a neutron in the nucleus turns into a proton, so the element changes, and a beta particle is emitted. A beta particle is identical to an electron, with virtually no mass and a charge of -1. 

A typical beta emission:

14 14 0
    C  ----> N --> e
6 7 -1
Unstable isotope  New isotope Beta particle

Gamma radiation is an Electromagnetic Wave

After spitting out an alpha or beta particle, the nucleus might need to get rid of some extra energy. It does this by emitting a gamma ray -- a type of electromagnetic wave. They have no mass. They can penetrate a long way into materials without being stopped. Since a gamma ray is just energy, it doesn't change the element of the nucleus that emits it.

What blocks the three types

Alpha particles ----> Paper
Beta particles -----> Thin Aluminium 
Gamma rays -------> thick lead 

The Radioactivity of a Sample Always Decreases Over Time

Each time an unstable nucleus decays and emits radiation, that means one more radioactive nucleus isn't there to decay later. As more unstable nuclei decay, the radioactivity of the source as a whole decreases - so the older a radioactive source is, the less radioaction it emits. How quickly the activity decreases varies a lot. For some isotopes it takes just a few seconds before nearly all the unstable nuclei have decayed. For others it can take millions of years. The problem with trying to measure this is that the activity never reaches zero, which is why we have to use the idea of half-life to measure how quickly the activity decreases.

HALF-LIFE IS THE TIME TAKEN FOR HALF THE RADIOACTIVE NUCLEI NOW PRESENT TO DECAY. 

A short half-life means the activity falls quickly, because lots of the nuclei decay in a short time.
A long half-life means the activity falls more slowly because most of the nuclei don't decay for a long time. 

Rutherford's Scattering showed that atoms have a positive nucleus

In 1909, Ernest Rutherford and his merry men, Hans Geiger and Ernest Marsden tried firing alpha particles - which are positively charged - at thin gold foil.
Most of the alpha particles just went straight through, but the odd one came straight back at them.

Conclusion:

• Most of the mass of a gold atom was concentrated at the centre in a tiny nucleus. The rest of the atom must be mainly empty space- as most of the alpha particles went straight through the foil. 
• The nucleus had to have a positive charge - otherwise the positively charged alpha particles wouldn't be repelled bythe nucleus and wouldn't scatter. 

The nucleus is held together by the strong force
The nucleus contains positively charged proton particles, which repel each other. The nucleus doesn't fly apart because it's held together by an attractive force much greater than tghe repulsive electrostatic force between the protons. This is called the strong force. The strong force only has a very short range - it can only hold protons and neutrons together when they're seperated by tiny distances. At larger seperations, the strong force is so weak that it effectively disappears.

Nuclei have to be brought close together to fuse
Two nuclei can combine (fuse) to create  a larger nucleus, releasing energy when they do - this is called nuclear fusion. For example, hydrogen nuclei fuse together to make helium nuclei. Nuclei can only fuse if they overcome the repulsive electrostatic force and get close enough for the strong force (see above) to hold them together. For that you need lots of energy - which means a high temperature. 

Albert Einstein reckoned that mass is a form of energy. So, mass can be converted into other forms of energy.
This idea is summed up by his famous equation:

       2
E= mc 

When nuclei undergo nuclear fusion and fission they lose mass and energy is released. You can calculate how much energy is released during fusion or fission using E= mc^2

Where:
E= amount of energy released
m= amount of mass lost
c= speed of light in a vacuum. 

Nuclear Power Stations release energy by spitting atoms
A nuclear fuel e.g. uranium or plutonium, releases large amounts of energy when its nuclei split apart. This process is called nuclear fission and starts when neutrons are fired at the fuel, causing some of its large, unstable nuclei to split into two smaller nuclei of roughly equal size  Each split nucleus also releases 2 or 3 more neutrons and lots of energy. 

Nuclear Fission releases a lot of energy...
Nuclear reactions release a lot more energy than chemical reactions (like burning). Splitting a gram of unranium releases over 10, 000 times more energy than burning a gram of oil. You can calculate just how much eneergy is released using E = mc^2. 

... So it needs to be carefully controlled
In nuclear reactors, a chain reaction is set up. A neutron splits a nucleus releasing more neutrons. These can then go on to split more nuclei and releases more neutrons, and so on.. The unranium (or plutonium) fuel used in nuclear reactors is contained in fuel rods. These fuel rods capture the neutrons, and emit neutrons when nuclei in the rod split. The chain reaction in the reactor has to be controlled, or the reactor would overheat. Control rods absorb some of the neutrons and slow down the reaction. They can be moved futher into and out of the reactor to absorb more or less neutrons. Coolant, e.g. water, is used to take away the heat produced by the fission process. This heat is used to produce steam to drive a turbine and generator.. and we get electricity. 

The waste from nuclear power stations is hard to deal with 

• Most waste from power stations (or medical use)is 'low level' This waste can be disposed of by burying it in secure landfill sites.
• Intermediate level waste includes the metal cases of used fuel rods and some waste from hospitals. It's quite radioactive and it can stay that way for thousands of years. It usually sealed into concrete blocks then put in steel canisters for storage.
• High level waste from nuclear power stations is so radioactive that it generates  a lot of heat. This waste is sealed in glass and steel, then cooled for about 50 years before it's moved to more permanent storage.
• The canisters of intermediate and high level wastes could then be buried deep underground. The site has to be geologically  stable (e.g. not suffer earthquakes), since big movements in the rock could break the canisters and radioactive material could leak out.
• Even when geologists do find suitable sites, people who live nearby often object. So, at the moment, most intermediate and high level waste is kept 'on-site' at nuclear power stations. 

Ionising Radiation can damage living cells 

Alpha, beta and gamma radiation are all ionising radiation - they can break up molecules into smaller bits called ions. Ions can be very chemically reactive, so they go off and react with things and generally make nuisances of themselves.
In humans, ionisation can cause serious damage to cells in the body. A high does of radiation tends to kill cells outright, causing radiation sicknees. Lower doses tend to damage cells without killing them, which can cause cancer.

Radioactive materials put people at risk through either:

• IRRADIATION- being exposed to radiation without ooming into contact with the source. The damage to your body stops as soon as you leave the radioactive area.
• CONTAMINATION - picking up some radioactive material, e.g. by breathing it in, drinking contaminated water or getting it on your skin. You'll still be exposed to the radiation once you've left the radioactive area.

How likely you are to suffer damage to your body if you're exposed to radiation depends on the radiation does.
Radiation dose is measured in sieverts (Sv) or more usually millisieverts (mSv). It takes into account the type and amount of radiation you've been exposed to.

Categories of people who are at higher risk of radiation exposure include:

• uranium miners and processors
• workers in nuclear power plants
• airline staff (the radiation comes from cosmic rays)
• miners (many rocks  are naturally radioactive)
• some medical staff (e.g. radiographers)
• nuclear researchers

In Britain, people at a higher risk have to have their radiation doeses carefully monitored and have regular check -ups to make sure they're not getting sick because of the radiation they're exposed to at work. 

Background Radiation is everywhere all the time
There's a low-level background radiation all around us that is constantly irradiating and contaminating us. It comes from radioactive materials:

• Natural  radioactive elements in the air, in soil, in living things, in the rocks under our feet ....
• Space (cosmic rays) - these come ostly from the Sun.
• Human Activity - e.g. from nuclear explosions or waste from nuclear power plants

Radioactive sources are considered to be "safe" when the radiation they are emitting is at about the same level  as the background radiation. The half-life of the source gives an idea of how long it will take for this to happen. E.g. strontium-90 has a half-life of 29 years so a sample emitting 1000 cpm will take four half-lives, 116 years, to reach roughly the background count of 60 cpm.  

Ionising Radiation can be very useful for...

• ... Treating Cancer
  Since high doses of gamma rays will kill all living cells, they can be used to treat cancers. The gamma rays have to be directed carefully and at just the right dosage so as to kill cancer cells, without damaging too many normal cells. However, a fair bit of damage is inevitably done to normal cells, which makes the patient feel very ill. But if the cancer is successfully killed off in the end, then it's worth it.
• ...Sterilising Medical Equipment
  Gamma rays are used to sterilise medical instruments by killing all the microbes. This is better than trying to boil plastic instruments, which might be damaged by high temperatures. You need to use a strongly radioactive source that has a long half-life, so that it doesn;t need replacing too often.

• .. Sterilising Food

Food can be sterilised in the same way as medical instruments - again killing all the microbes. This keeps the food fresh for longer, without having to freeze or cook it or preserve it some other way. The food is not radioactive afterwards, so it's safe to eat. 

• Detecting Diseases using tracers

Tracers are radioactive molecules that can be injected into people. Their progress around the body is followed using an external detector. They can detect cancer or whether an organ is working properly. Isotopes used as tracers must be gamma or beta emitters so the radiation passes out of the body. They should have a short half-life so that the radioactivity inside the patient quickly disappears.