Radiation and the Universe

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					                        07/04/2010




Radiation and the Universe
      Electromagnetic Radiation
                                                      07/04/2010



E-M radiation is basically a movement of energy in the form of
a wave. Some examples:
  The Electromagnetic Spectrum
                                                                        07/04/2010



Each type of radiation shown in the electromagnetic spectrum has a
different wavelength and a different frequency:

High frequency,                                           Low frequency, _____
_____ wavelength                                               (high) wavelength
 Gamma      X-rays   Ultra violet   Visible   Infra red    Microwaves    Radio/TV
  rays                               light

   γ
Each of these types travels at the same speed through a _______
(300,000,000m/s), and different wavelengths are absorbed by different
surfaces (e.g. infra red is absorbed very well by ___________ surfaces).
This absorption may heat the material up (like infra red and _______) or
cause an alternating current (like in a __ _______).

   Words – black, microwaves, long, short, TV aerial, vacuum
 The Electromagnetic Spectrum
                                                  07/04/2010



Type of radiation         Uses               Dangers


  Gamma rays        Treating cancer,      Cell mutation
                      sterilisation
     X rays              Medical          Cell mutation

  Ultra violet          Sun beds           Skin cancer

  Visible light       Seeing things      None (unless you
                                         look at the sun)
   Infra red        Remote controls,         Sunburn
                     heat transfer
  Microwaves        Satellites, phones      Very few

    TV/radio         Communications         Very few
         Transmitting information
                                                                  07/04/2010



Although E-M radiation travels in straight lines, we can send infra-red and
light signals around a curved path using an optical fibre:




Optical fibres have two main advantages: they can send more information
compared to electrical cables of the same diameter and with less signal
weakening.


Microwaves are used by satellites
because they can pass through the
Earth’s atmosphere:

Microwaves are also used in
mobile phone networks.
          The Wave Equation
                                                 07/04/2010




All E-M waves obey the Wave Equation:



    Wave speed (v) = frequency (f) x wavelength ()
         in m/s         in Hz             in m




                         V


                    f           
      Analogue vs. Digital Signals
                                                                  07/04/2010



Analogue signals (like talking or           +
music) continually vary in
amplitude and/or frequency
                                            -
 1                            Digital signals, however, are either
                              off or on, and the information is sent
 0                            in a series of pulses

There are two main advantages of digital:
1) More information can be sent down the same cable
2) Better quality, because a digital signal can be amplified without
   amplifying the extra noise:
The structure of the atom
                               07/04/2010




   ELECTRON –
  negative, mass
  nearly nothing




                     PROTON –
                   positive, same
NEUTRON –             mass as
neutral, same       neutron (“1”)
   mass as
 proton (“1”)
The structure of the atom
                                        07/04/2010




Particle   Relative Mass    Relative Charge
 Proton           1                +1
Neutron         1                  0
Electron        0                  -1


             MASS NUMBER = number of
             protons + number of neutrons

                      SYMBOL


            PROTON NUMBER = number of
                protons (obviously)
                      Isotopes                         07/04/2010


An isotope is an atom with a different number of neutrons:
 Notice that the mass number is different. How many
 neutrons does each isotope have?




    Each isotope has 8 protons – if it didn’t then it just
                 wouldn’t be oxygen any more.
A “radioisotope” is simply an isotope that is radioactive –
e.g. carbon 14, which is used in carbon dating.
   Introduction to Radioactivity
                                                       07/04/2010



Some substances are classed as “radioactive” – this means that
they are unstable and continuously give out radiation:




                            Radiation




The nucleus is more stable after emitting some radiation – this
is called “radioactice decay”.
               Types of radiation                                07/04/2010




                                  1) Alpha () – an atom decays into a new
                                  atom and emits an alpha particle (2
                                  protons and 2 ______ – the nucleus of a
Unstable    New        Alpha      ______ atom)
 nucleus   nucleus    particle

                                 2) Beta () – an atom decays into a new
                                 atom by changing a neutron into a
                                 _______ and electron. The fast moving,
                         Beta high energy electron is called a _____
                        particle particle.
Unstable    New
 nucleus   nucleus                3) Gamma – after  or  decay surplus
                                  ______ is sometimes emitted. This is
                                  called gamma radiation and has a very
                                  high ______ with short wavelength.
                                  The atom is not changed.

Unstable     New       Gamma           Words – frequency, proton,
 nucleus    nucleus   radiation        energy, neutrons, helium, beta
                       Ionisation
                                                         07/04/2010



Radiation is dangerous because it “ionises” atoms – in other
words, it turns them into ions by “knocking off” electrons:




Alpha radiation is the most ionising (basically, because it’s the
biggest). Ionisation causes cells in living tissue to mutate,
usually causing cancer.
                 Blocking Radiation
                                                                07/04/2010



Each type of radiation can be blocked by different materials:




    
    
    

                 Sheet of         Few mm of            Few cm of
                  paper           aluminium              lead
   Deflection by Magnetic Fields
                                                                07/04/2010




                                +               2 protons, 2 neutrons,
Alpha and beta particles                +       therefore charge = +2
have a charge:                                  1 electron, therefore
                                    -
                                                      charge = -1


Because of this charge, they will be deflected by electric and
magnetic fields:
                                            +




                                            -
      Background Radiation
                                  07/04/2010



 13% are
man-made


                      Radon gas
                      Food
                      Cosmic rays
                      Gamma rays
                      Medical
                      Nuclear power
           Uses of radioactivity
                                                       07/04/2010


1) Medical uses – gamma rays can be used to destroy
cancerous cells or to sterilise medical instruments
2) Tracers – a tracer is a small amount of radioactive material
used to detect things, e.g. a leak in a pipe:



  Gamma
  source

The radiation from the radioactive source is picked up above
the ground, enabling the leak in the pipe to be detected.

Tracers can also be used to develop
better plant fertilisers and in
medicine to detect tumours:
    Uses of radioactivity 2
                                    07/04/2010



                          Beta
                        detector

Rollers                            Paper




               Beta
              emitter
        Dangers of radioactivity
                                                07/04/2010




                 Radiation will ionise atoms in living
Alpha            cells – this can damage them and
                 cause cancer or leukaemia.
Beta


Gamma

                      OUTSIDE the body  and  are
                      more dangerous as  radiation
                      is blocked by the skin.
                      INSIDE the body an  source
                      causes the most damage
                      because it is the most ionising.
                         Half life
                                                      07/04/2010




 The decay of radioisotopes can be used to measure the
 material’s age. The HALF-LIFE of an atom is the time
 taken for HALF of the radioisotopes in a sample to decay…

        = radioisotope              = new atom formed




                 After 1 half    After 2 half    After 3 half
  At start      life half have   lives another   lives another
there are 16        decayed         half have        2 have
radioisotopes      (that’s 8)     decayed (12     decayed (14
                                  altogether)     altogether)
    A radioactive decay graph
                            07/04/2010




Count




                             Time
        1 half life
   Dating materials using half-lives                              07/04/2010



 Question: Uranium decays into lead. The half life of uranium is
 4,000,000,000 years. A sample of radioactive rock contains 7 times as
 much lead as it does uranium. Calculate the age of the sample.

 Answer: The sample was originally completely uranium…

         1 half life         1 half life          1 half life
         later…              later…               later…

    8                   4                   2                     1
    8                   8                   8                     8
  …of the       Now only 4/8 of       Now only 2/8 of       Now only 1/8 of
sample was         the uranium        uranium remains       uranium remains
  uranium         remains – the       – the other 6/8       – the other 7/8
                other 4/8 is lead          is lead               is lead

 So it must have taken 3 half lives for the sample to decay until only 1/8
 remained (which means that there is 7 times as much lead). Each half
 life is 4,000,000,000 years so the sample is 12,000,000,000 years old.
           An exam question…
                                                       07/04/2010




Potassium decays into argon. The half life of potassium is
1.3 billion years. A sample of rock from Mars is found to
contain three argon atoms for every atom of potassium.
How old is the rock?
                                              (3 marks)




    The rock must be 2 half lives old – 2.6 billion years
                           07/04/2010

Evidence about the origins of
       the universe…
                    07/04/2010




Source of
  light     “Spectra”
                                            07/04/2010
  If you pass the light through a gas something
                different is seen…




             helium




Some wavelengths of light
are absorbed by the gas –
an “absorption spectrum”.
                                                       07/04/2010
If the light source is moving away the absorption
spectra look a little different…


                                               Before


                helium
                 helium




                                               After
                                                       07/04/2010
The absorption lines have all been “shifted”
towards the longer wavelength end (red end)…
                                         Before
        This is called red
        shift. The faster
        the light source
        moves the further
        its light will be
        “shifted”



                                         After
               A similar effect happens with sound –
                this is called “The Doppler Effect”

                       Hear Doppler Effect
                                      07/04/2010
Light from different stars and from the edge
of the universe also shows this “red-shift”.
This suggests that everything in the universe
is moving away from a single point.




  This is the BIG
  BANG theory
            Red shift summary
                                                      07/04/2010




Light from other galaxies has a longer _________ than
expected. This shows that these galaxies are moving ____
from us very quickly. This effect is seen to a greater
extent in galaxies that are _______ away from us. This
indicates that the further away the galaxy is, the ______
it is moving.


This evidence seems to suggest that everything in the
universe is moving away from a single point, and that this
process started around 15 _____ years ago. This is the
____ ________ Theory.

         Words to use – faster, away, big bang,
             billion, wavelength, further
         Observing the Universe
                                                     07/04/2010



Consider different types of telescope:




 Ground-based telescopes           Space-based telescopes


What are the advantages and disadvantages of each?