GCSE P2 Chapter 6

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  • Created by: emma998
  • Created on: 12-03-14 16:49

Observing nuclear radiation

  • The basic structure of an atom is a small central nucelus containing protons and neutrons surrounded by shells of electrons
  • The atoms of radioactive elements have unstable nuclei
  • They become stable by emitting radiation in a process called radiocative decay
  • Three types of radiation emitted are: Alpha, Beta, Gamma
  • Radioactive decay is a random process which is not influenced by external conditions therefore it is impossible to predict
  • Background radiation is radiation that is around us all the time
  • This is radiation from radioactive substances in the environment, from space, from devices such as Xray machines
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The discovery of the nucleus

  • At one point scientists thought that an atom was made up of spheres with a positive charge with electrons stuck into them. This became known as the plum pudding model of the atom
  • However Rutherford, Geiger and Marsden devised an alpa-particle scattering test in which they fired alpha particles at a thin sheet of metal foil

There results showed:

 

  • Most of the alpha particles passed striaght through the foil and were not deflected suggesting that most of an atom is made up of empty space
  • Some of the alpha particles were deflected by small angles suggesting that the nucleus has a positive charge 
  • A few of the particles were deflected by very large angles suggesting that the nucleus of an atom has a large mass and a very large positive charge
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Nuclear reactions

Alpha decay - alpha particle - 4, 2

  • Alpha article consists of 2 neutrons and 2 protons
  • The alpha particle is emitted from the nucleus
  • Mass number goes down by 4
  • Atomic number goes down by 2
  • Radium emits an alpha paticle and becomes radon

Beta decay - beta particle - 0,-1

  • Neutron in the nucleus changes into a proton and an electron
  • The electron is instantly emitted
  • The proton stays in the nucleus
  • Mass number stays the same
  • Atomic number does up by 1
  • Carbon-14 emits a beta particle when it becomes nitrogen

Gamma radiation is an electromagnetic wave so it has no mass and no charge.

  • No change in the atomic or mass number
  • Emitted from the nucleus
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More about alpha, beta and gamma radiation

  • When nuclear radiation travels through a material it collides with atoms of material and knock electrons off creating ions. This is ionisation and can damage and kill living cells

Alpha particles

  • Strongly ionising - large so have lots of collisions with atoms
  • Weakly penetrating - lots of collisions therefore doesn't penetrate far into a material
  • Stopped by a few cm of air, human skin or a piece of paper + Deflected by magnetic and electric fields

Beta particles

  • Less ionising - slightly smaller and faster so have fewer collisions + 
  • Stronger penetrating - less collisions so penetrate further into a material
  • Stopped by a few metres of air or a thin sheet of aluminium foil 
  • Deflected by electric+magnetic fields opposite direction 2 alpha particles- opposite charge

Gamma rays

  • Weakly ionising - travels far into a material before colliding with an atom + Strongly penetrating 
  • Unlimited range in air, several m concrete or few cm of lead absorb most radiation
  • Not deflected by electric and magnetic fields because it has no charge
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Half-life

  • The radioactivity of a sample can be measured by measuring the count rate of the isotope
  • The radioactivity of a sample decreases over time
  • Half life is the time taken for the unstable nuclei in the sample to halve
  • The half life of a substance is different for each isotope
  • Some take a few minutes and others take millions of years
  • The half life is the same for any sample of a paticular isotope
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Radioactivity at work

Alpha sources can be used in smoke alarms

  • They are weakly penetrating so is not dangerous
  • They need a half life of several years

Beta sources can be used in thickness monitoring

  • Beta particles can be fired at materials such as paper and metal foil
  • Alpha particles could not be used because they would be stopped by air or thin material and gamma rays could also not be used because they would pass straight through it
  • They need a half life of many years so that the decrease in count rate is due to the changes in thicknes of the materials not by the random radioactive decay

Gamma and beta sources can be used as tracers in medicine

 

  • Radiation injected or swallowed by patient + allowed to move through patient's body
  • The movement of the radiation is monitored by a detector outside the patient
  • Sample needs half life of few hours so not to expose patient to unecessary radiation

Radioactive dating can be used to find the age of ancient material. Carbon dating can be used to find the age of wood and uranium dating can be used to find the age of igneous rock.

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