Edexcel IGCSE Physics 复习笔记 7.1.3 Types of Radiation

Unstable Nuclei and Radiation

• Some atomic nuclei are unstable
• This is because of an imbalance in the forces within the nucleus
  • Forces exist between the particles in the nucleus

   

• Carbon-14 is an isotope of carbon which is unstable
  • It has two extra neutrons compared to stable carbon-12

   

Carbon-12 is stable, whereas carbon-14 is unstable. This is because carbon-14 has two extra neutrons

• Some isotopes are unstable because of their large size or because they have too many or too few neutrons

• Unstable nuclei can emit radiation to become more stable
• • Radiation can be in the form of a high energy particle or wave

   

Unstable nuclei decay by emitting high energy particles or waves

• As the radiation moves away from the nucleus, it takes some energy with it
  • This reduces the overall energy of the nucleus
  • This makes the nucleus more stable

   

• The process of emitting radiation is called radioactive decay
• Radioactive decay is a random process
  • This means it is not possible to know exactly when a particular nucleus will decay

   

• When an unstable nucleus decays it emits radiation, called nuclear radiation
• There are different types of radiation that can be emitted:
  • Alpha (α)
  • Beta (β^-)
  • Gamma (γ)

   

ANSWER:  C

• • Answer A is true. The number of neutrons in a nucleus determines the stability
  • Answer B is true. This is a suitable description of radioactive decay
  • Answer D is true. Radiation is about emissions. It is different to radioactive particles
  • Answer C is not true
  • Radioactive decay is a random process
  • It is not possible to predict precisely when a particular nucleus will decay

   

• The three different forms of nuclear radiation have different properties:

Alpha Particles

• The symbol for alpha is α
• An alpha particle is the same as a helium nucleus
  • This is because they consist of two neutrons and two protons

   

• Alpha particles have a charge of +2
  • This means they can be affected by an electric field

   

Beta Particles

• The symbol for beta is β^-
• Beta particles are fast-moving electrons
• They are produced in nuclei when a neutron changes into a proton and an electron
• Beta particles have a charge of -1
  • This means they can be affected by an electric field

   

Gamma Rays

• The symbol for gamma is γ
• Gamma rays are electromagnetic waves
• They have the highest energy of the different types of electromagnetic waves
• Gamma rays have no charge

Alpha particles, beta particles and gamma waves can be emitted from unstable nuclei

• The properties of Alpha, Beta and Gamma are given in this table, and then described in more detail below

Different Properties of Nuclear Radiation

• The trend down the table shows:
  • The range increases
  • Penetrating power increases
  • Ionisation decreases

   

Penetrating Power

• Alpha, beta and gamma have different properties
• They penetrate materials in different ways
  • This means they are stopped by different materials

   

Alpha, beta and gamma are different in how they penetrate materials. Alpha is the least penetrating, and gamma is the most penetrating

• Alpha is stopped by paper, whereas beta and gamma pass through it
• Beta is stopped by a few millimetres of aluminium
  • Gamma can pass through aluminium

   

• Gamma rays are only partially stopped by thick lead

Ionising Power

• All nuclear radiation is capable of ionising atoms that it hits
• When an atom is ionised, the number of electrons it has changes
• This gives it a non-zero charge

When radiation passes close to atoms it can knock out electrons, ionising the atom

• Alpha radiation is the most ionising form of nuclear radiation
  • This is because alpha particles have a charge of +2

   

• Gamma radiation is the least ionising form of nuclear radiation

ANSWER:  B

• • The answer is not A because the radiation passed through the paper almost unchanged
    • This means it is not alpha

     

  • The answer is not C or D because the aluminium decreased the count-rate significantly
    • This means it is not gamma (gamma penetrates aluminium)
    • This also means it is not neutrons (neutrons penetrate aluminium, however you do not need to know this for your GCSE)

     

  • Therefore, the source must be Beta particles

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