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Radioactivity

Physics (GCSE) Revision (Radioactivity)

StatementResponseComment
Some substances give out radiation all the time, whatever is done to them. They are said to be . . . radioactive. Radioactivity is a random process - we cannot predict when a given atom will emit radiation.
There are three types of radiation emitted by radioactive sources . . . alpha, beta and gamma. Other types of particle can be emitted but we need not concern ourselves with them at the moment.
Alpha radiation is easily absorbed by . . . thin sheets of paper or a few cm of air. They are easily stopped by your skin or clothing.
Alpha particles are actually composed of . . . helium nuclei. 2 protons plus 2 neutrons.
Beta particles are actually . . . electrons. They are emitted from the nucleus but they are the same as those in orbit round it.
Gamma rays are . . . a form of electromagnetic radiation. It has very short wavelength and travels at the same speed as light.
Beta particles travel at nearly the speed of light but are mostly absorbed by . . . a few mm of metal. Aluminium will absorb them but lead is better.
Gamma rays are very penetrating and can only be absorbed by . . . many cm of lead or metres of concrete. They do not interact very strongly with matter so are difficult to stop.
There are radioactive substances all around us, particularly in . . . the ground, air, in building materials and food. It is known as background radiation. We grow up in a radioactive environment - perhaps that is what drives evolution?
Radiation which reaches us from space is known as . . . cosmic rays. They are produced in supernova explosions.
When radiation from radioactive materials collides with neutral atoms or molecules, they may become . . . ionised (charged) The term 'ionising radiation' is often used.
When radiation ionises molecules in living cells it can cause damage, including . . . cancer. The larger the dose, the greater the risk.
Higher doses of radiation are used to . . . kill cancer cells and harmful micro-organisms. Sterilisation of surgical instruments.
As radiation passes through a material it can be . . . absorbed. The greater the thickness, the more is absorbed.
The absorption of radiation can be used to . . . monitor or control the thickness of materials. For example, paper or aluminium 'bacofoil'
When sources of radiation are outside the body, which types are most dangerous? Beta and gamma because they can penetrate the skin to reach the cells of organs and may be absorbed by them.
Outside the body, which type of radiation is least dangerous? Alpha because it cannot penetrate the outer layers of clothing or skin.
Workers who are at risk from radiation wear a film badge. Why? If the film turns black when developed, they have been exposed to too much radiation. Different layers of absorbers on top of the film can tell us what type of radiation has caused the exposure.
When sources of radiation are inside the body, which are most dangerous? Alpha because it is strongly absorbed by cells.
Inside the body which types of radiation are least dangerous? Beta and gamma because they can escape the body and are less likely to be absorbed.
The half life of a radioactive substance is . . . the time taken for half the number of radioactive atoms in a sample to decay. This is an average time - decay is a random process.
The half life of a substance can also be . . . the time taken for the count rate to halve. The rate of decay is proportional to the number of radioactive atoms in a sample.
The nucleus which emits radiation is referred to as a parent. What is its decay product called? A daughter That's politically correct at least!
Radioactive isotopes are used as tracers. What is a tracer? A substance which is introduced into a person or a water supply to monitor the movement of blood or water through the system. Beta and gamma radiation are usually used for this.
What type of radiation would be most suitable for use as a biological tracer in a patient? Gamma because it is not easily absorbed by the cells and will cause little damage.
Atoms have a small nucleus made up of . . . protons and neutrons Protons are positively charged. Neutrons have zero charge
Electrons are negatively charged and are usually found . . . moving around the nucleus They are NOT like planets going around the Sun !
What task did Ernest Rutherford set to his students, Geiger and Marsden? To fire alpha particles at a sheet of gold foil. Why gold foil ? (The foil can be made very thin so that the alpha particles only made single collisions with gold atoms.)
What did Rutherford's experiment prove? The nucleus is tiny, very massive and positively charged. JJThomson thought it was like a plum pudding - how daft !
In Rutherford's experiment, what were the main observations? Most of the alpha particles missed the nucleus, but a few bounced straight back. Like a cannonball reflecting from tissue paper !
All atoms of the same element have the same number of . . . Protons I don't care about the electrons !!
In a neutral atom, there must be the same number of . . . Protons and electrons Now I care !
Isotopes are . . . atoms of the same element which have different numbers of neutrons in the nucleus. You can forget the electrons now.
The total number of protons and neutrons in an atom is called its . . . mass or nucleon number. See - no electrons even mentioned.
Radioactivity occurs because of . . . changes in the nuclei of atoms. Electrons are NOT involved in nuclear Physics.
Radioactive isotopes are also called . . . radioisotopes or radionuclides They have UNSTABLE nuclei which split up and emit radiation.
When an unstable nucleus decays it . . . becomes a different nucleus with a different number of protons and neutrons. The electrons just hang around until something better comes along.
When a gamma ray is emitted by a radioactive nucleus . . . there is no change to the structure of the nucleus - it just cools down a bit. Gamma rays have no mass or charge - they just carry energy away from the nucleus.
Alpha decay results in the nucleus losing . . . 2 protons and 2 neutrons An alpha particle is just a helium nucleus - they exist inside heavy nuclei in clusters.
A beta decay occurs when . . . a neutron changes into a proton - it emits an electron which takes away negative charge. Bit like a sex change !
The older a radioactive material . . . the less radiation it emits. Can be used to date materials and rocks.
Nuclear reactors use a process called . . . nuclear fission. A heavy nucleus (uranium) absorbs a slow neutron and splits ( fissions) into two lighter nuclei plus neutrons.
The products of nuclear fission are themselves radioactive. This causes problems because . . . they are hard to dispose of properly. They must be stored carefully for hundreds of years.
The energy released during nuclear fission is . . . very large compared with chemical reactions. Those involving electrons !
Uranium isotopes which have a long half life, decay by a series of short-lived isotopes to produce . . . stable isotopes of lead. Lead has more stable isotopes than any other element.
The age of a rock can be found by comparing . . . the relative proportions of uranium and lead in the rock. Igneous rock can be dated this way because its initial composition is fixed when it solidifies from lava. After this radioactive minerals start to decay - the clock is ticking.
The radioisotope potassium-40 can be used to date igneous rocks because . . . its stable decay product argon is unable to escape from the rock. Apply the idea of half life to the ratio of what remains to what was there initially.
If an igneous rock is found to contain 3 atoms of argon for every 1 of potassium-40, how old is the rock? Two half lives because only 1/4 of the original potassium remains.
Carbon-14 is a radioactive isotope of carbon-12. How is it used? To date organic remains. Living things breathe in C14 while alive, but after death the C14 decays to Nitrogen.
How much of radioactive material X remains after 3 half lives 1/8 1/2 x 1/2 x 1/2 = 1/8
Created by: J Thomson on 2005-04-25



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