In 1909, Ernest Rutherford, Hans Geiger and Ernest Marsden tried firing positively charged alpha particles at thin foil. Most of these particles just went straight through but the odd one came straight back at them.
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 by the nucleus and wouldn’t scatter.
The nucleus contains positively charged proton particles, which repel each other. The nucleus doesn’t fly apart because it is held together by an attractive force much greater than the repulsive electrostatic force between the protons. This force is called strong force.
The strong force only has a very short range – it can only hold protons and neutrons together when they’re separated by tiny distances. At large separations, the force is so weak that it effectively disappears.
Two nuclei can combine (fuse) to create larger nucleus, releasing energy when they do – this is called nuclear fusion. Nuclei can only fuse if they overcome the repulsive electrostatic force and get close enough for the strong force to hold them together.
Albert Einstein reckoned that mass is a form of energy, meaning that mass can be converted into other forms of energy.
When nuclei undergo nuclear fusion or fission they lose mass and energy is released.
A nuclear fuel (uranium or plutonium) releases large amounts of energy when its nuclei spilt 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 spilt nucleus also releases 2 or 3 more neutrons and lots of energy.
Release of Energy from Nuclear Fission
Nuclear reactions release a lot more energy than chemical reactions. Splitting a gram of uranium releases over 10,000times more energy than burning a gram of oil. You can calculate just how much energy is released by using E = mc2.
Nuclear Power Stations
In nuclear reactors, a chain reaction is set up. A neutron splits a nucleus, realising more neutrons. These can then go on to split more nuclei and release more neutrons and so on.
The uranium 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 will overheat.
Control rods absorb some of the neutrons and slow down the reaction. They can be moved further into and out of the reactor to absorb more or less neutrons. Coolant 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 meaning we get electricity.
The main problem with nuclear power is that it produces a vast amount of radioactive waste.
Low level waste: Paper, gloves – slightly radioactive – disposed of by burying it in secure landfill sites.
Intermediate level waste: metal cases of used fuel rods – quite radioactive – some will stay that way for tens of thousands of years. Often sealed in concrete blocks and put into steel canisters for storage.
High level waste: waste from nuclear power stations – highly radioactive – waste is sealed in glass and steel, the cooled for about 50years before it is moved to a more permanent storage.
Danger from Radiation
Alpha, beta and gamma radiation are all ionising radiation – they can break up molecules into smaller bits of ions. Ions can be very chemically reactive, so they go off and react with thing and generally make nuisances of themselves.
In humans, ionisation can cause serious damage to the cells in the body. A high doses of radiation tends to kill cells outright, causing radiation sickness. Lower doses tend to damage cells without killing them, which can cause cancer.
Irradiation: being exposed to radiation without coming 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.
Effects of Radiation
|Typical background radiation experienced by everyone
|Exposure to airline crew flying the New York to Tokyo polar route
|Current limit for nuclear industry employees and uranium miners
|Lowest level at which an increase in cancer is clearly evident. Above this, the probability of cancer occurrence increases with dose.
|Cause (temporary) radiation sickness such as nausea and decreased white blood cell count, but not usually death. Above this, severity of illness increases with dose.
|Would kill about half of those receiving it within a month.
|10 000 mSv/dose
|Fatal within a few weeks
Using Ionising Radiation
There’s low level background radiation all around us that is constantly irradiating and contaminating us. It comes from radioactive materials:
- Natural radioactive elements in air, in soil, in living things, in the rocks under our feet.
- Space (cosmic rays) – these come mostly from the sun.
- Human activity – 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.
Ionising radiation can be useful for…
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 the cancer cells, without damaging too many 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.
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 it 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.