AQA A Level Physics复习笔记8.1.3 Alpha, Beta & Gamma Radiation

Alpha Particles

• Alpha (α) particles are high energy particles made up of 2 protons and 2 neutrons (the same as a helium nucleus)
• They are usually emitted from nuclei that are too large

Nuclear notation for an alpha particle (a helium nucleus)

• Alpha is the most ionising type of radiation
  • This is due to it having the highest charge of +2e
  • This means it produces the greatest number of ion pairs per mm in air
  • This also means it is able to do more damage to cells than the other types of radiation

   

• Alpha is the least penetrating type of radiation
  • This means it travels the shortest distance in air before being absorbed
  • Alpha particles have a range of around 3-7 cm in air

   

• Alpha can be stopped by a single piece of paper

Beta Particles

• Beta (β−) particles are high energy electrons emitted from the nucleus
• Beta (β+) particles are high energy positrons (antimatter of electrons) also emitted from the nucleus
  • β− particles are emitted by nuclei that have too many neutrons
  • β+ particles are emitted by nuclei that have too many protons

   

Nuclear notation for beta minus and beta plus particle

• Beta is a moderately ionising type of radiation
  • This is due to it having a charge of +1e
  • This means it is able to do some slight damage to cells (less than alpha but more than gamma)

   

• Beta is a moderately penetrating type of radiation
  • Beta particles have a range of around 20 cm - 3 m in air, depending on their energy

   

• Beta can be stopped by a few millimetres of aluminium foil

Gamma Radiation

• Gamma (γ) rays are high energy electromagnetic waves
• They are emitted by nuclei that need to lose some energy

Nuclear notation for gamma rays

• If these particles hit other atoms, they can knock out electrons, ionising the atom
• This can cause chemical changes in materials and can damage or kill living cells

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

• Gamma is the least ionising type of radiation
  • This is because it is an electromagnetic wave with no charge
  • This means it produces the least number of ion pairs per mm in air
  • It can still cause damage to cells, but not as much as alpha or beta radiation. This is why it is used for cancer radiotherapy

   

• Gamma is the most penetrating type of radiation
  • This means it travels the furthest distance in air before being absorbed
  • Gamma radiation has an infinite range and follows an inverse square law

   

• Gamma can be stopped by several metres of concrete or several centimetres of lead

Different types of radiation are stopped by different materials

Comparing Alpha, Beta & Gamma

• The properties of the different types of radiation are summarised in the table below:

Comparing Different Types of Radiation Table

ANSWER:     D

Smoke Detectors

• Smoke detectors contain a small amount of Americium-241, which is a weak alpha source
• Within the detector, alpha particles are emitted and cause the ionisation of nitrogen and oxygen molecules in the air
• These ionised molecules enable the air to conduct electricity and hence a small current can flow
• If smoke enters the alarm, it absorbs the alpha particles, hence reducing the current which causes the alarm to sound
• Am-241 has a half-life of 460 years, meaning over the course of a lifetime, the activity of the source will not decrease significantly and it will not have to be replaced

The operation of a smoke detector

Thickness Controls

• Beta radiation can be used to determine the thickness of aluminium foil, paper, plastic, and steel
• The thickness can be controlled by measuring how much beta radiation passes through the material to a Geiger counter
• Beta radiation must be used, because:
  • Alpha particles would be absorbed by all the materials
  • Gamma radiation would pass through undetected through the materials

   

• The Geiger counter controls the pressure of the rollers to maintain the correct thickness
• A source with a long half-life must be chosen so that it does not need to be replaced often

The pressure of the rollers can be adjusted to control the thickness of the aluminium foil depending on the amount of beta radiation detected

a) ANSWER: C

• • Alpha and low energy beta radiation would most likely be absorbed by the bag
  • Therefore, gamma radiation, or very high energy beta particles, would be needed to penetrate the bag
  • This would be best suited to Cobalt-60

b) ANSWER: D

• • Static electricity is an imbalance of electric charges on the surface of the polythene and is generally composed of negatively charged electrons
  • In order to get rid of the static charge, it will need to be neutralised
  • Beta-plus particles, or positrons, are the antimatter counterpart of the electron, and hence, are oppositely charged
  • When the positrons are directed at the surface of the polythene, the electrons will be attracted to them and become neutralised as the particles annihilate as they collide
  • Therefore, the beta-plus emitter, Fluorine-18, would be best suited to this job

   

c) ANSWER: B

• • Alpha particles would not be suitable for measuring the thickness of metal as they can be stopped by a thin sheet of paper
  • Gamma rays are the most penetrating of the radiations and hence would not be suitable where thickness monitoring is up to a few millimetres as they would all pass through
  • Beta particles are ideally suited as they have enough energy to pass through thin sheets of metal and any changes in thickness would be easily detected
  • Therefore, the beta-minus emitter Strontium-90 would be the most suitable isotope

d) ANSWER: A

• • Since smoke detectors are present inside homes and other buildings, they must pose no hazard to residents
  • This means the smoke detector must contain a very small amount of the radioactive material
  • Also, the radiation should not be too penetrating and should only be able to travel a few centimetres
  • Therefore, an alpha source should be selected – this means Americium-241 would be the most suitable isotope