Unit 5 Atomic physics

Section 5.2 Radioactivity

Detection of radioactivity

• Many nuclei of elements in the periodic table are stable but some are unstable. Unstable nuclei undergo radioactive decay.
• When a radioactive nucleus decays it may emit one or more of the following: alpha particles, beta particles or gamma rays.
• The radiations alpha, beta and gamma have their own individual characteristics, as shown in the table below.

  	Alpha particle	Beta particle	Gamma ray
  Nature	2 protons and 2 neutrons (helium nucleus)	electron	electromagnetic radiation
  Symbol	α or 24 He	β or –10 e	γ
  Charge	positive	negative	uncharged
  Affected by magnetic and electric fields	yes	yes	no
  Penetrating power	weak – stopped by thin paper	moderate – stopped by a few mm of aluminium	strong – only stopped by many cm of lead or many m of concrete
  Relative ionising effect	strongest	medium	weakest
  Dangerous	yes	yes	yes

  Alpha particle

  Nature	2 protons and 2 neutrons (helium nucleus)
  Symbol	α or 24 He
  Charge	positive
  Affected by magnetic and electric fields	yes
  Penetrating power	weak – stopped by thin paper
  Relative ionising effect	strongest
  Dangerous	yes

  Beta particle

  Nature	electron
  Symbol	β or –10 e
  Charge	negative
  Affected by magnetic and electric fields	yes
  Penetrating power	moderate – stopped by a few mm of aluminium
  Relative ionising effect	medium
  Dangerous	yes

  Gamma ray

  Nature	electromagnetic radiation
  Symbol	γ
  Charge	uncharged
  Affected by magnetic and electric fields	no
  Penetrating power	strong – only stopped by many cm of lead or many m of concrete
  Relative ionising effect	weakest
  Dangerous	yes

• There is always radiation present all around us. This is known as background radiation. Background radiation comes from cosmic rays, rocks and atmospheric gases. A small amount (about 3%) of background radiation comes from human-made sources such as medical equipment, nuclear power stations and nuclear weapons testing.