Physics - topic 6

Physics - topic 6

1. Background radiation and half life

   • Background radiation is the low-level radiation that's around us all the time. It comes from: Radioactivity of naturally occuring unstable isotopes which are all around us. Radiation from space which is knwon as cosmic rays. Radiation due to human activity.
     • The level of background radiation changes depending on where you are: Certain underground rocks can cause higher levels of background radiation at the surface, especially if they release radiactive radon gas, which tends to get trapped inside people's houses.
   • Each time a radioactive nucleus dacays and radiation is emitted, one more radioactive nucelus disappears. As the unstable nuclei all steadily disappear, the activity as a whole will decrease.
   • The half-life of a radioactive isotope is the time taken for half of the undecayed nuclei to decay. A long half-life means the activity falls more slowly because most of the nuclei don't decay for a long time.

2. Calculating Half-life

   • e.g. initial count 640 (/2) after one half life 320 (/2) after 2 half-lives 160,etc.
     • The half-life is found from the graph, by finding the time interval on the bottom axis corresponding to a halving of the activity on the vertical axis.

3. Uses of radioactivity

   • Alpha radiation: Household fire alarms - A weak source of alpha radiation is placed in a smoke detector, close to 2 electrodes. The source causes ionisation, and a current flows. If there's a fire then smoke will absorb the radiation and the current stops and the alarm sounds.
     • Gamma rays: Sterilisation of food and equipment - Food can be irradiated with a high dose of gamma rays which will kill all microbes. So food doesn't go bad as quickly as it would do. Medical equipment can be sterilised using gamma rays instead of being boiled. Irradiated doesn't involve high temperatures so things can be sterilised without being damaged. The radioactive source used for this needs to be a very strong emitter of gamma rays with a reasonably long half-life so it doesn't replacing too often. To treat cancer - High doses of gamma rays will kill all living cells. Radiotherapists have to be very careful when treating a patient to direct gamma rays right at the cancerous cells, so they can minimise the damage done to healthy cells.
       • Radiation is used in tracers and thickness gauges: Certain radioactive isotopes can be used as tracers. A medical tracer is injected into a patient and its progress around the body is followed using an external detector. A computer uses the reading from the detector to produce an image to follow the flow of the injected isotope. This methd can be used to treat and diagnose medical conditions. All isotopes which are taken intot he bidy must be beta or gamma emitters so radiation passes out of the body.
         • Gamma emitter tracers are used in industry to detect leaks in underground pipes. Beta radiation is used in thickness control.

4. Dangers of radioactivity

   • Ionising radiation can cause tissue damage and cell mutation: lower doses ten to cause minor damage wihtout killing the cell. This can give rise to mutant cells which divide uncontrollably. This is cancer. Higher doses tend to kill calls completely which causes radiation sickness. The extent of harmful effects depend on: 1) How much exposure and 2) The energy and peentration.
     • Outside the body beta and gamma sources are the most dnagerous. This is because beta and gamma can get inside to the delicate organs, whereas alpha is much less dangerous because it can't penertrat the skin. Inside the body, an alpha source is the most dangerous. Alpha sources od all their damage in a very localised area. Beta amd gamma sources on the other hand are less dangerous inside the boyd because they mostly pass straight out without doing much damage.
       • You should protect yourself in the laboratory: 1) Never allow skin contact with a source. always handle with tongs. 2) keep the source at arm's length. 3) Kepep the source pointing away from the body. 4) always put the source back in a labelled lead box.
         • If you work with nuclear radiation: 1) Full protective suits. 2) lead lined suits, lead/concrete barriers and thick lead screens. 3) Workers are remote controlled robot arms.

5. Nuclear power

   • Nuclear waste is a big problem for the nuclaer industry: 1) Nuclear fission creates radioactive waste prodcts that can't just be thrown away. The radioactive waste can be really dangerous. Nuclear power stations usually deal with the most dangerous nuclear waste by vitrification (the liquid glass is sealed and buried deep underground or put nuclear waste into thick metal containers and bury waste in a deep hole topped with tons of concrete.
   • Pros of using nuclear power: pretty safe way of generating electricity. Very reliable energy resource. Nuclear power doesn't release greenhouse gases so it's a cleav source of energy. Huge amounts of energy can be generated from a small amount of nuclear material.
     • Cons of using nuclear power: Public perception of nuclear power is very negative - they worry that the waste isn't disposed of safely (it could leak out and pollute land, rivers and oceans.) Nuclear power carries the risk of leaks from the power station or a major catastrophe like Chernobyl. The overall cost of nuclear power is high.