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Science 10 - Radiation Chapter Notes

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					                     Science 10 - Radiation Chapter Notes!
Vocabulary
1.    electromagnetic spectrum   9.    alpha radiation         17.   half-life               25.   nuclear fusion
2.    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).
    Energy can be transferred from place to place in the form of disturbances called waves. There are
     different types of waves, but all waves have some things in common.
    AMPLITUDE: the distance from the midpoint of the wave          crest           wavelength

     to the trough or crest
    WAVELENGTH (): the distance between two successive                  amplitude

     crests
    FREQUENCY: the number of crests that pass a given
     point per unit time (equals the number of "cycles" per                 trough
     second). Measured in Hertz (Hz)                                                               wavelength

    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.
    WAVE VELOCITY: the speed at which the wave crest appears to move. The speed of a wave will remain
     constant unless the medium changes. These terms are related by the following formula:
                               WAVE VELOCITY = WAVELENGTH X FREQUENCY
                                        v = f          = v/f        f = v/
      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)

                                     R O Y G B I V
T.V./Radi microwav         infrared           visible           U.V.                          X-Rays       gamma
    o            es                                                                                         rays
longer wavelengths                  750 nm           400 nm                                  shorter wavelengths
lower frequency                                                                                 higher frequency
lower energy                                                                                       higher energy
Colour is simply different wavelengths of light:
   RED          ORANGE         YELLOW        GREEN          BLUE                            INDIGO         VIOLET

700 nm                                         ~500 nm                                                        400 nm
"White Light" is all these colours put together.




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Ever wonder why the sky is blue or rainbows form?
   The sky is blue because the sunlight (white light) gets the blue waves are 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.
   Electromagnetic waves are called                   varying electric field

    "electromagnetic" because they have both
    an electric part and a magnetic part, moving
    at right angles to each other. The electric part
    induces the magnetic part, and vice-versa!


                                                              varying magnetic field

RADIATIONS FROM ATOMS

    Recall that atoms have in their nuclei positively charged protons and neutrons (with no charge). Why
     don't the positive charges repel each other and cause the nucleus to break up? The reason is that
     the nucleus is held together by a force called the STRONG NUCLEAR FORCE, which is very powerful
     but only exerts its effect over a very short range (i.e. the size of the nucleus). There is also a force
     called the WEAK NUCLEAR FORCE (which is actually a form of electromagnetic force) that is involved
     in the radioactive decay of some atoms.
    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 (- rays): heaviest but least penetrating. Like a
                                    helium nucleus.
                       alpha ray
                                    2.     Beta Radiation (-rays): high energy electrons traveling at high
                        gamma ray   speed.
                                    3.     Gamma Radiation (-rays): high energy electromagnetic rays (like
                        beta ray
                                    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.
                                       238           4            234
                                             U          He            Th
                                     92        2         90


 Decay will continue with alpha, beta, and gamma rays being emitted until finally an element is formed that
   is not radioactive. This stable element is lead (Pb), which has an atomic weight of 206.
 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.



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       This graph shows the decay of a 16 g sample
of Thorium-234. It has a half-life of about 24 days.                     Radioactive Decay of Thorium-234
So in about 24 days, if you started with 16 grams,
                                                                16
you would have 8 grams. In 48 days you would have
                                                                14
4 grams, and so on.                                             12
 Radioactive decay can not be seen with the                   10
    naked eye or normal microscopes since the           Grams Th 8
    radiations are so tiny or invisible, so we have a            6
    machine called a GEIGER COUNTER that can                     4
    detect radiation.                                            2
                                                                 0
ISOTOPES                                                             0            50          100           150
                                                                                       Days
  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:
     12
      6   C
                             6 protons, 6 neutrons
     13
      6   C
                             6 protons, 7 neutrons
     14
      6   C
                             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:
1                         2                  3
1 H ("regular" hydrogen), 1 H ("Deuterium"), 1 H ("Tritium”). Deuterium is used in nuclear reactors.




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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:
                                        14
                                         6   C  -1e + 14 N
                                                  0
                                                        7
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. 56
                           137
                                 Ba*  137 Ba + gamma radiation
                                        56

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.

 Do Review 7.2 on pp 144 - 145 #1 - 8




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