Radioactivity

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Radioactivity

• The Idiots Guide
• By Jess Jones Laura McManus

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What is Radiation?

• The breaking down of unstable atomic nuclei
• As the nucleus of an unstable atom breaks down,
  it gives out rays and particles called emissions.
• The breaking down of unstable nuclei happens
  spontaneously it is unaffected by heat,
  pressure, or whether the element is solid, liquid
  or gas.

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What is Radiation?

• Isotopes (atoms of the same element with
  differing numbers of neutrons) whose nuclei break
  down at random are referred to as radioactive.
• They are also known as radioisotopes.
• There are three different types of atomic
  radiation alpha (a), beta (ß), or gamma (?).

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Atomic Structure

• The Nucleus This is the centre of the atom. It
  contains protons and neutrons. The mass of the
  atom is concentrated in the nucleus.
• Protons These have a positive charge and a mass
  of 1.
• Neutrons These dont have a charge and have a
  mass of 1.
• Electrons These move around the nucleus. They
  have virtually no mass and have a negative
  charge.

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Alpha Radiation

• 2 protons and 2 neutrons ( helium nucleus)
  ejected from the nucleus
• Positive charge of 2
• Very high ionising power this means it collides
  with lots of atoms and knocks electrons off them,
  making them ions
• Short range in air a few centimetres
• Stopped by a piece of paper.

238U ? 234Th 4He 92 90 2
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Beta Radiation

• An neutron that breaks down into a proton and an
  electron
• The electron is ejected from the nucleus the
  atomic number of the atom changes because there
  is an extra proton in the nucleus
• Negative charge of 1
• Low ionising power
• Stopped by a piece of aluminium foil
• Travels several metres in air

14C ? 14N 0e 6 7 -1
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Gamma Radiation

• Not made of protons or electrons
• A high-energy electromagnetic wave
• Emitted from nuclei changing from a high energy
  level to a lower one
• Frequently accompanies a and ß emissions
• No charge, so very low ionisation power
• The most penetrating atomic radiation can
  travel huge distance through air
• Stopped by several feet of lead

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Important Experiments

• Wilhelm Conrad Roentgen(1845-1923) On November
  8th 1895 Roentgen discovered X-rays, a momentous
  event that instantly revolutionized the field of
  physics and medicine.
• He determined that a glowing fluorescent screen
  on a nearby table was caused by invisible rays
  originating from the partially evacuated glass
  Hittorf-Crookes tube he was using to study
  cathode rays.
• These rays penetrated the opaque black paper
  wrapped around the tube.
• For this discovery, Roentgen received the Nobel
  prize for physics in 1901.

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Important Experiments

• Pierre Curie(1859-1906) Marie Curie(1867-1934)
  In 1895, Marie and Pierre Curie were married and
  worked together on their research.
• The term radioactivity was first coined by
  Marie.
• The Curies experimented with the chemical
  extraction of uranium from the ore. The
  conclusion was that the ore contained, in
  addition to uranium, new elements that were also
  radioactive.
• This led to their discoveries of the elements of
  polonium and radium.
• For their work the Curies were awarded the Nobel
  prize in physics and years later, Marie was
  awarded the Nobel prize in chemistry.

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Important Experiments

• Antoine Henri Becquerel( 1852-1908) Initially
  Becquerel believed that the suns energy was
  being absorbed by the uranium which then emitted
  X-rays.
• He found that uranium emitted radiation without
  an external source of energy such as the sun.
• Becquerel had discovered radioactivity, the
  spontaneous emission of radiation by a material.
  
• For this discovery he was awarded the 1903 Nobel
  prize for physics.

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Half-Life

• The time taken for the number of atoms in a
  sample of an element to decay by half
• Half-life is fixed no matter how big the
  sample, what the temperature or pressure is, it
  is always the same length of time.
• A sample of a radioisotope will never completely
  disappear
• its radioactivity always disappears by half,
  even in the tiniest amounts.

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Some Half-Life Examples

• This is an example of a half-life graph for
  Americium-242
• It can be used to find the half-life of a
  radioisotope
• The heaviest naturally-occurring radioisotope is
  Uranium, which has a half-life of 4.5 x 109
  years
• The more unstable a radioactive isotope is, the
  shorter its half-life

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Dangers of Radiation

• The main danger from radioactivity is the damage
  it does to the cells in your body.
• Most of this damage is due to ionisation when
  the radiation passes.
• If levels of radiation are high there can be
  damage due to heating effects as your body
  absorbs the energy from the radiation, rather
  like heating food in a microwave oven.
• This is particularly true of gamma rays.

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Dangers of Radiation

• Alpha Particles These are slow and have a short
  range in air. These are the most dangerous type
  of particle and can turn cells cancerous.
• Beta Particles These have a longer range than
  alpha particles, but ionise much less strongly.
  They have more penetrating power which means they
  can get through your skin and affect the cells
  inside you.
• Gamma Rays Gamma rays hardly ionise at all, so
  do not cause damage directly in this way. They
  are very difficult to stop and when they are
  absorbed by an atom they can gain quite a bit of
  energy, and may then emit other particles.

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Uses of Radioactivity - Alpha

• Smoke detectors
• The contain a small amount of Americium-241,
  which emits a radiation. This ionises the air so
  a current flows.
• When smoke enters the detector it absorbs the a
  radiation and the circuit is broken. An alarm
  sounds.

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Uses of Radioactivity - Beta

• Thickness testing
• A radioactive source emitting ß emissions, and a
  Geiger counter, are placed either side of the
  paper.
• The amount of ß radiation reaching the counter
  through the paper is measured
• If too little or too much radiation gets through,
  the machine is automatically adjusted to make the
  paper thinner or thicker

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Uses of Radioactivity - Gamma

• Tracers
• Medical purposes-used to follow the route of
  substances through the body e.g. To detect a
  blocked kidney
• Civil purposes-used to detect leaks in pipes by
  putting ? source into pipe measuring emissions
  using a Geiger counter

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Uses of Radioactivity - Gamma

• Radiotherapy
• Cancer cells are exposed to gamma rays which
  kills them off
• Makes the patient feel unwell
• Correct dose vital too much can kill healthy
  cells, too little wont prevent spread of cancer
• Sterilisation
• Civil use ? radiation used to kill bacteria on
  some foods. Prolongs shelf-life but may change
  the taste.
• Medical use sterilises medical equipment that
  would be damaged by heat e.g. Syringes
• Welding
• Gamma emissions passed through metal onto
  photographic film, to check for bubbles

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Uses of Radioactivity Half-life

• Carbon-14 Dating
• All living things contain a fixed proportion of
  radioisotope Carbon-14 (14C)
• When animals and plants die, the proportion of
  14C starts to fall, because decaying 14C is no
  longer being replaced by 14C being taken in e.g.
  food
• 14C also has a known half-life, of 5700 years
• Scientists can work out the age of ancient
  organic substances e.g.. bones, by comparing the
  amount of 14C left to the proportion in living
  organisms, and using half-life

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Credits and Bibliography

• Jess Jones What is Radioactivity, Types of
  Radioactivity, Half-Life, Uses of Radioactivity
• Laura McManus A Brief History of Radioactivity,
  Important Experiments, Atomic Structure, Dangers
  of Radioactivity
• Bibliography
• Chemical Ideas Salters Advanced Chemistry
• BBC Online GCSE Bitesize
• http//www.darvill.clara.net/nucrad/index.htm