Edexcel IGCSE Physics: Double Science 复习笔记:7.1.3 Types of Radiation

Edexcel IGCSE Physics: Double Science 复习笔记:7.1.3 Types of Radiation

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)

     

 

Worked Example

Which of the following statements is not true?

 

A    Isotopes can be unstable because they have too many or too few neutrons

 

B    The process of emitting particles or waves of energy from an unstable nucleus is called radioactive decay

 

C    Scientists can predict when a nucleus will decay

 

D    Radiation refers to the particles or waves emitted from a decaying nucleus

 

 

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

     

 

 

Exam Tip

The terms unstablerandom and decay have very particular meanings in this topic. Remember to use them correctly when answering questions!

Properties of Radiation

 

  • 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

     

penetration increase, IGCSE & GCSE Physics revision notes

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

 

ionising-the-atom 

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

 

 

Worked Example

A student has an unknown radioactive source. They are trying to work which type of radiation is being given off:

 

A    Alpha particles

B    Beta particles

C    Gamma rays

D    Neutrons

 

They measure the count-rate, using a Geiger-Muller tube, when the source is placed behind different material. Their results are shown in the table below:

4.2.2-WE-Absorption-table

Which type of radiation is being given off by the source?

 

 

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

转载自savemyexams

更多IGCSE课程
翰林国际教育资讯二维码