Science 10 - Radiation Chapter Notes!
Vocabulary
1. electromagnetic spectrum / 9. alpha radiation / 17. half-life / 25. nuclear fusion2. visible spectrum / 10. beta radiation / 18. background radiation / 26. nuclear fission
3. electromagnetic wave / 11. gamma radiation / 19. ionizing radiation / 27. chain reaction
4. wavelength / 12. Geiger counter / 20. cancer / 28. nuclear reactor
5. frequency / 13. isotopes / 21. Gray (Gy) / 29. moderator
6. electromagnetic spectrum / 14. radioactive decay / 22. Sievert (Sv) / 30. critical mass
7. radioactive / 15. nuclear transmutation / 23. X-ray radiographs / 31. Albert Einstein
8. nuclear force / 16. Becquerel / 24. Marie Curie / *draw diagrams
· Radiation has numerous forms: e.g. sunlight, the heat from your hand, X-rays, and uranium bombs all produced different kinds of radiation.
· Electromagnetic Radiation is radiation that is in the form of electromagnetic waves. This includes the light we see with our eyes (called the visible spectrum, which includes all colours) plus some invisible types of radiation (radio & TV signals, microwaves, infrared (better known as heat), ultraviolet, X-rays, and cosmic rays).
· The energy of an electromagnetic wave is related to its frequency. As the frequency of the electromagnetic radiation increases, so does its energy. That is why X-rays and gamma rays, which have a high frequency and energy (and short wavelength) are dangerous to living things; their waves have so much energy they can penetrate deeply and can break molecules apart.
· Electromagnetic waves all move at the speed of light ("c" = 3.0 x 108 m/s (about 1.1 billion km/hour)
· The electromagnetic spectrum is shown below. The sun emits all of these waves, but we only see a small portion (called the visible spectrum)
ROYGBIVT.V./Radio / microwaves / infrared / visible / U.V. / X-Rays / gamma rays
¬longer wavelengths
¬lower frequency / 750 nm / 400 nm / shorter wavelengths®
higher frequency®
¬lower energy / higher energy®
Colour is simply different wavelengths of light:
RED / ORANGE / YELLOW / GREEN / BLUE / INDIGO / VIOLET400 nm / ~500 nm / 750 nm
"White Light" is all these colours put together.
Ever wonder why the sky is blue or rainbows form?
· The sky is blue because the sunlight (white light) gets the blue waves scattered more by the Earth's atmosphere. The scattered blue waves give the sky its blue colour. The sunlight looks yellow to us because white light minus the blue part = yellow.
· A rainbow occurs when white light is split up into its constituent colours by small drops of water in the air.
RADIATIONS FROM ATOMS
· Recall that atoms have in their nuclei positively charged protons and neutrons (with no charge).
· In 1896, Henri Becquerel (1852-1908) discovered that some atoms have nuclei that are unstable and give off several types of radiations. Atoms that do this are said to be radioactive.
· A radioactive element is unstable and will undergo radioactive decay, in which it will give off radiations from its nuclei. There are three types of radioactive emissions that can be emitted by a nucleus:
1. Alpha Radiation (a- rays): heaviest but least penetrating. Like a helium nucleus.
2. Beta Radiation (b-rays): high energy electrons traveling at high speed.
3. Gamma Radiation (d-rays): high energy electromagnetic rays (like x-rays but can have even more energy. Penetrate the furthest).
· Every time an alpha ray is released from an atomic nucleus, the element gets lighter and changes into another element.
· Decay will continue with alpha, beta, and gamma rays being emitted until finally an element is formed that is not radioactive.
· Radioactivity can be used to measure time because the rate of decay is very steady and happens at a constant rate, so it can be used like a clock (indeed, the most accurate clocks use radioactivity to measure time).
· The term HALF-LIFE refers to the time it takes for half of the atoms in a radioactive sample to convert to the stable end product.
· Radioactive decay can not be seen with the naked eye or normal microscopes since the radiations are so tiny or invisible, so we have a machine called a geiger counter that can detect radiation.
ISOTOPES
· The atomic number of an element is, by definition, the number of protons in its nucleus. If the number of protons changes (through alpha decay, for example), the atom will have a different number of protons, and will have different characteristics, and will therefore not be the same element any more. Therefore, the atomic number of an element can never change, because if it did, it would be a different element.
· Recall that neutrons have no electric charge. Some elements have different numbers of neutrons. Since the neutrons have no charge, they don't affect the basic characteristics of the element, though they contribute to its atomic mass. The Mass number of an element is the number of protons and neutrons in its nucleus. This number can change, as it is possible for the same element to have different numbers of neutrons. Atoms of the same element that have different numbers of neutrons (and therefore different mass numbers) are called isotopes.
For example, there are 3 isotopes of CARBON:
/ 6 protons, 6 neutrons/ 6 protons, 7 neutrons
/ 6 protons, 8 neutrons
· Carbon-12 is by far the most abundant of the carbon isotopes. It is stable and stays the same.
· Carbon-14 is radioactive and is used for finding the age of such things as Egyptian mummies. It undergoes radioactive decay to form nitrogen.
There are 3 isotopes of hydrogen:
("regular" hydrogen), ("Deuterium"), ("Tritium”). Deuterium is used in nuclear reactors.
Types of Radioactive Decay
· When an unstable nucleus emits radiation, it is said to have undergone radioactive decay.
· Again, there are 3 main types of emissions from nuclei, and so there are 3 types of radioactive decay.
1. Alpha Decay: the nucleus emits an alpha particle (two protons, two neutrons) and becomes a new, lighter element. e.g. Uranium-238 decays to Thorium-234 and Helium-4. A new nucleus (Thorium-234) is thus formed. This new nucleus is called the decay product. When a nucleus changes to another type of nucleus, it is called a nuclear transmutation.
2. Beta Decay: the nucleus emits a beta particle (high-energy electron). This happens when a neutron changes into a proton plus an electron (notice how the charges balance: neutrons can be thought of as being made of a proton and an electron). When the nucleus loses the electron, it loses one of its neutrons, but "gains" a proton, As a result, the atomic number increases by one. This also is a nuclear transmutation. This is what happens to carbon-14 when it decays:
3. Gamma Decay: Since gamma rays are not particles, when a parent nuclei undergoes gamma decay, it retains the same number of protons and neutrons. The nucleus just loses some energy, and therefore becomes more stable.
Radioactivitiy Changes With Time
· The activity of a sample is the number of nuclei in the sample that undergo radioactive decay each second. It is measured in becquerels (Bq) after Henri Becquerel. e.g. 10,000 decays in one second is equal to 10,000 Bq.
The Hazards of Radiation
· Your are always being bombarded by radiation. You have radioactive atoms in your body, for example, and ~250,000 of these decay in your body every minute. Likewise, you are being hit by radiation from space (cosmic rays) and in the water, soil and food you eat. The sum total of this "natural" radiation is known as natural background radiation. You can't avoid this!
· Alpha, beta, gamma radiation as well as UV light and X-Rays are hazardous. They can disrupt molecules by knocking electrons out of their orbits, forming ions (atoms that have lost or gained electrons). These types of radiation are therefore called ionizing radiation.
· Ionizing radiation hurts cells that are dividing the most (such as cells in the intestine, bone marrow, scalp). A large dose of radiation can cause "radiation sickness" by affecting these cells. Nausea, diarrhea, blood changes are some of the symptoms.
· If the radiation affects the cell's DNA (which contains the instructions for cell division in it, among other things), it can lead to cancer. This may not happen for years after the exposure. e.g. skin cancer from a severe sunburn may not develop until years after the sunburn.
· Ionizing radiation is measured by two units:
1. The Gray (Gy) measures how much energy is transferred to the object by radiation.
2. The Sievert (Sv) measures the effect of the radiation on living things.
· The sievert is related to the gray, but it takes into account the biological effect of the radiation. To compare grays to sieverts, you must know what type of radiation was absorbs, because they have different effects on living systems. For example, a one gray of beta, gamma, or X-ray radiation is equivalent to one sievert in its effect, but one gray of alpha radiation is equivalent to 20 Sv. Why do you think this is so?
· The average person should get no more than 0.005 Sv per year. A chest X-ray gives about 0.0001 Sv (i.e. 50 chest X rays would be give a year's worth of radiation).
· Under 0.25 Sv, there are no noticeable effects. Exposures above 1 Sv damages blood-forming cells. Exposures above 5 Sv (such as the radiation after an atomic bomb is dropped) will kill a person within a period of days or weeks.
· You should avoid radiation whenever possible. Use sunscreens to block UV. If you work around radiation, you must be protected by lead shields. Don't sit too close to your television (it actually produces small amounts of X-Rays!). Increasing your distance from a radioactive source will significantly reduce the amount you absorb.
Applications of Radiation
· X-Rays in medicine (X-Rays pass easily through flesh but not bone, so they will show where bones are on special photographic film).
· Radiography departments of hospitals produce X-ray radiographs.
· Nuclear medicine uses radioactive isotopes to trace events in the body, locate tumors or blockages in blood vessels.
· Radiation can be used to treat diseases (e.g. Iodine-131 can be used to treat a hyperactive thyroid gland). Some cancers can be treated with radiation (e.g. inject strontium-89 into blood; it gets absorbed by bone, especially by bone tumors. The radiation helps kill the bone tumors). X-Rays can also be shot at tumors to kill them. The radiation, however, harms normal cells, which is why radiation therapy usually makes the patient sick as well.
Nuclear Energy
· Nuclear fission and nuclear fusion reactions are two types of nuclear reactions that release a large amount of energy. In these reactions, small amounts of mass is converted into large amounts of energy according to Albert Einstein's famous equation: E = mc2 . (according to this equation, 1 kg of mass can be converted into 90,000,000,000,000,000 J of energy!).
· Nuclear fission occurs when a heavy atom splits apart into lighter atoms after being bombarded with neutrons. The particles that are released from the heavy nucleus weigh slightly less in total than the heavy nucleus. The difference in mass has been converted to energy. In addition, the reaction has produced more neutrons, which can go on to cause fission reactions in other heavy nuclei (a chain reaction). A chain reaction will only occur if there is enough of the radioactive material around (i.e. a "critical mass") and if there is something to slow down the fast moving electrons (a moderator) so they won't escape before hitting another nucleus.
· Nuclear fusion occurs when light elements combine to produce heavier ones. To stick nuclei together takes a lot of energy, since nuclei, having positive charges, will repel each other. The temperature necessary to get the nuclei to hit each other fast enough are very high -- several million degrees! However, when the nuclei do combine, the resulting single nucleus formed weighs slightly less than the original nuclei that came together. The "missing" energy again has been converted to energy according to E = mc2. This is how the sun and hydrogen bombs (the most powerful type of bomb known) produce their energy. Here is a typical fusion reaction.
· One gram of deuterium contains enough energy to run the average home for 40 years! Unfortunately, the temperatures required for fusion are so high that so far fusion reactors providing the world's energy needs from a few buckets of sea water remains an elusive dream for now. "Cold Fusion" has likewise been possible only in the imaginations of eager physicists.
· All nuclear reactors today are fission reactors. The first nuclear reactor was built in 1942. In a nuclear reactor, such as the Canadian-built CANDU (Canadian Deuterium Uranium) reactor, a controlled chain reaction takes place (as opposed to the uncontrolled chain reaction that occurs in atomic bombs). Uranium-235 from uranium oxide fuel rods undergoes fission, and the neutrons emitted by the reaction are slowed down by D2O ("heavy water") so that an uncontrolled chain reaction doesn't happen. The D2O acts as a "moderator" in the reaction (it slows down the neutrons to the right speed to cause fission reactions), and control rods which can absorb neutrons can be brought in between the fuel rods to increase or decrease the rate of fusion. Fission reactions produce huge amounts of heat, which is carried away by the D2O and then used heat water to produce steam to turn turbines and produce electrical energy. The CANDU reactor is one of the safest reactors because it can be safely "shut down" if it gets too hot simply by draining out the D2O, which will stop the chain reaction.