Properties of Radiation Notes

Edexcel IGCSE Physics: Revision notes

Key facts

  • Atomic number = protons; mass number = protons + neutrons; isotopes have the same protons but different neutrons.
  • Alpha, beta, gamma (and neutrons) are emitted randomly by unstable nuclei.
  • Alpha is the most ionising and least penetrating; gamma is the least ionising and most penetrating.
  • Alpha decay: mass number −4, atomic number −2. Beta decay: atomic number +1. Gamma: no change.
  • Nuclear equations must balance mass and atomic numbers.

Atoms and notation

The atomic number is the number of protons; the mass number is the number of protons plus neutrons.

An atom has a small, dense nucleus of protons and neutrons, surrounded by electrons. Protons are positive, electrons negative and neutrons neutral.

In 614C^{14}_{6}\text{C}, the top number is the mass number and the bottom is the atomic number. Isotopes have the same number of protons but different numbers of neutrons.

He2p 2n

Helium

2

Li3p 4n

Lithium

2,1

Helium and lithium atoms.
  • Atomic (proton) numbernumber of protons
  • Mass (nucleon) numberprotons + neutrons

Worked example

How many protons and neutrons are in 614C^{14}_{6}\text{C}?

How many neutrons are in 1123Na^{23}_{11}\text{Na}?

Types of radiation

Unstable nuclei randomly emit ionising radiation. The more strongly ionising a type, the less penetrating it is.

Emission is random: you cannot predict when a particular nucleus will decay. There are four main types of radiation.

Photographic film and Geiger–Müller tubes detect ionising radiation. Film darkens, and a GM tube clicks or counts.

  1. 1

    Alpha: strongly ionising

    stopped by paper or a few cm of air

  2. 2

    Beta: moderately ionising

    stopped by a few mm of aluminium

  3. 3

    Gamma: weakly ionising

    reduced by cm of lead or metres of concrete

  4. 4

    Neutron: not directly ionising

    stopped by thick concrete or water

The more strongly ionising a type, the less penetrating it is

Alpha

Nature:
2 protons + 2 neutrons (helium nucleus)
Penetration:
Stopped by paper or a few cm of air
Ionising ability:
Strongly ionising

Beta (β⁻)

Nature:
A fast-moving electron
Penetration:
Stopped by a few mm of aluminium
Ionising ability:
Moderately ionising

Gamma

Nature:
Electromagnetic radiation
Penetration:
Reduced by cm of lead or metres of concrete
Ionising ability:
Weakly ionising

Neutron

Nature:
A neutron from the nucleus
Penetration:
Stopped by thick concrete or water
Ionising ability:
Not directly ionising

Which radiation is stopped by a sheet of paper?

Testing penetrating power

Place absorbers between the source and a detector and see how the count rate changes, after subtracting background.

Use a source and a Geiger–Müller tube in line. Insert paper, then aluminium sheets, then lead, and record the count rate each time.

Background radiation must be measured and subtracted, or the readings overstate the source.

  1. 1

    Set up

    source and GM detector in line

  2. 2

    Add an absorber

    paper, aluminium or lead

  3. 3

    Record

    the count rate for each absorber

  4. 4

    Correct

    measure and subtract background radiation

Investigating penetrating power

The count rate falls to background when paper is added. The source emits

Nuclear equations

Each decay changes the mass and atomic numbers in a set way, and nuclear equations must balance on both sides.

In alpha decay, the nucleus loses 2 protons and 2 neutrons. In beta decay, a neutron becomes a proton and an electron is emitted. In gamma decay, the nucleus loses only energy.

The mass numbers must add to the same total on each side, and so must the atomic numbers.

050100150200Ra-226Rn-222He-4NuclideNumber
  • Mass number
  • Atomic number
Alpha decay of radium-226 balances: 222 + 4 = 226 and 86 + 2 = 88

Mass number

Alpha:
−4
Beta:
No change
Gamma:
No change
Neutron emission:
−1

Atomic number

Alpha:
−2
Beta:
+1
Gamma:
No change
Neutron emission:
No change

Worked example

Complete the alpha decay of radium-226: 88226Ra→??Rn+24He^{226}_{88}\text{Ra} \rightarrow {}^{?}_{?}\text{Rn} + {}^{4}_{2}\text{He}.

A nucleus undergoes beta decay. Its mass number

Try an exam question

A radioactive source emits alpha, beta and gamma radiation. Describe how a Geiger–Müller tube and absorbers can show that all three are present.

[4 marks]

That's the notes covered.

Carry on to the next subtopic.