Name ____________________________ Date ____________
Topic 7. 2 Radioactive Decay
7.2.1 Describe the phenomenon of natural radioactive decay.
 Certain isotope’s nuclides are said to be unstable – that is, the strong nuclear force does not bind the nucleus
together indefinitely and it eventually breaks apart.
 Radioactive decay – the process of these unstable nuclides breaking apart.
 These unstable nuclides are said to be radioactive.
 Natural radioactive decay (natural transmutation) – the break down and change of radioactive elements to
change into a different element.
 Three types of decay
 Alpha decay
 Beta decay
 Gamma Radiation
 During alpha and beta decay, the radioactive nuclide changes into one of a different atomic number. This is
therefore a different element. This change is called natural transmutation.
 Alpha Decay (α)
 An unstable nuclide emits an alpha particle to try and become more stable.
 Alpha particle – (2protons and 2neutrons)
 Nucleus of a helium atom
 Alpha decay generally occurs in very large nuclides. (lead)
 Larger nucleus means more repulsive Coulomb force acting between the protons, spans the entire
nucleus
 Strong force only acts between neighboring nucleons.
 If coulomb force becomes too big then the strong force is no longer able to hold nucleus together.
 Let’s look at radium – 222 (atomic number 88)
 Ra -222 – Parent nucleus
 Z = 88
 N = 134
 α – alpha particle
 Z=2
 N=2
 Rn – Daughter Nucleus
 Z = 86
 N = 134
 Beta Decay (β) – is an electron
 An unstable nuclide emits and beta particle to try and become more stable.
 Emits an electron from the nucleus.
 Electrons have a mass number of 0 and atomic number -1.
 Symbol - ****Draw on board****
 A neutron in the nucleus changes into a proton, and electron, β, and an almost undetectable particle
called an antineutrino. v (with a line)
 The electron and the antineutrino are emitted from the nucleus at very high speed.
 Since the number of nucleons remains the same, the mass number does not change.
 The number of protons and therefore the atomic number Z, increases by 1
 Let’s look at Iodine – 131 (atomic number 53)
 I – 131 – Parent nucleus
 Z = 53
 N = 78
 Xe – 131 – Daughter Nucleus
 Z = 54
 N = 77
 Gamma Radiation (γ)
 After alpha or beta emission, the daughter nucleus is left in an “excited state”. The protons and neutrons
reorganize themselves in an attempt to become more stable.
 As they do this they lose energy. This energy is emitted from the nucleus as a pulse of gamma radiation
 This energy emission is a little like the energy emitted from an atom as an electron drops to a lower
energy state.
 However, the energy levels oa a nucleus are much wider than that.
 Where an atom emits radiation in the order of a few eV, the nucleus emits radiation in the range of a few
keV, or a few MeV
7.2.2 Describe the properties of alpha (α) and beta (β) particles and gamma (γ) radiation.
 Alpha (α)
 Helium nucleus
 +2 charge
 Mass: 4u(7350 x me
 Strong ionizing ablility
 Stopped by a sheet of thick paper or card or by the skin.
 Travels a few cm in air
 Effect in an electric field ***See board***
 Effect in an magnetic field ***See board***
 Beta (β)
 High speed electron
 -1 charge
 Mass: 1/1800u
 Weak ionizing ability
 Stopped by a few mm of aluminum or other metals. Travels up to a meter in air.
 Effect in an electric field ***See board***
 Effect in an magnetic field ***See board***
 Gamma (γ)
 Electromagnetic Wave
 0
 Mass: 0u
 Very weak ionizing ability
 Never completely stopped although
reduced by thick concrete or lead.
 Effect in an electric field ***See
board***
 Effect in an magnetic field ***See
board***
 Decay Series
 Parent nuclide don’t always decay
into a stable daughter nucleus. The
daughter nucleus then needs become
the parent and decay again,
producing another daughter.
 This will continue until there is a
stable nuclide.
7.2.3 Describe the ionizing properties of alpha (α) and beta (β) particles and gamma (γ) radiation.
7.2.4 – Outline the biological effects of ionizing radiation.
 Ion is a charged atom. Atoms become ionized by gaining or losing electrons.
 Ionizing radiations – radiation that has the ability to strip away electrons.
 Can damage or destroy cells.
 Can alter the chemical information in cells.
 Can cause mutations and/or cancer.
 The ionization effects of radiation can be used to detect and identify the different forms of radiation.
7.2.5 Explain why some nuclei are stable while others are unstable.
 Two interactions inside the nucleus.
 Coulomb, electrostatic force and Strong Nuclear Force.
 Very fine balance must be maintained to stay stable.
 Neutrons help to increase the nucleus size and keep protons further apart to reduce the coulomb repulsive
forces.
 There has to be just the right ratio of neutrons to protons.
 If it’s not just right the strong force is reduced the nucleus becomes unstable
 This graph shows the relationship between neutrons and protons.
 Stable nuclides – Z < 20
 Protons and neutrons are about equal
 Above 20 the nuclides have more neutrons than protons.
 7.2.5 Explain why some nuclei are stable while others are unstable.
 Nuclides above the stability line have too many neutrons
 Beta minus (electron emission)
 Nuclides below the stability line have too few neutrons
 Beta plus (positron emission)
 Positron is same mass and size charge as electron only positive.
 Larger unstable nuclides decay by alpha emission.
 Above Z = 83
7.2.5 Explain why some nuclei are stable while others are unstable.
Detection of Radiation
 Gold Leaf Electroscope
 Source of radiation is brought close instrument
 Air around the cap is ionized.
 Gold leaf rises due to electrostatic induction.
 GLE only detects alpha radiation since alpha is highly
ionizing.
 Ionization Chamber
 Rutherford used
 Can detect all three forms of ionizing radiation
 Chamber is made of a metal box and a central wire electrode.
 Radioactive source is place in a chamber.
 As the air is ionized, to becomes electrically conductive
 A small current flows which is detected w/ a sensitive
ammeter.
 Geiger–Muller (GM) tube
 Used instead of Ionization chamber
 Works on same principle as the ionization chamber.
 Difference is the source does not have to be physically placed
inside the chamber.
 Measured in Becquerels, Bq
 If radiation enters the tube it ionizes the gas and causes a
high voltage spark between the central wire and the outer
metal body.
 An electric current flows.
 Counts the number of sparks which indicates the total number of particles to enter the tube.
Half – Life
7.2.6 State that radioactive decay is a random and spontaneous process and that the rate of decay decreases
exponentially with time.
7.2.7 Define the term radioactive half-life.
 If you were to toss a coin, then on average half the time the coin would turn up heads and the other half it
would turn up tails.
 However each toss is completely random and independent of other tosses.
 Tossing the coin is random of each toss but predictable over an extended series of tosses.
 Half-life – Random and spontaneous process – food for thought.
 Stochasticity (9:00 – 18:00) http://www.radiolab.org/2009/jun/15/
 Are we coins? http://www.radiolab.org/blogs/radiolab-blog/2009/jun/29/are-we-coins/
 This is similar with radioactive decay. It is a random process with a predictable outcome.
 Any nuclide, over time, half the unstable nuclei will decay in attempt to become more stable.
 Exponential decay
 Can be graphed/plotted
IB Definition
- Half–life is the time taken for half of the undecayed nuclei, remaining in a given sample, to decay.
- Applied to using a Geiger counter - half-life is the time taken for the activity of a given sample to fall to
half of its original value.
Number of undecayed
nuclei N (x10^6)
7.2.8 Determine the half – life of a nuclide from a decay curve.
 Lets use the isotope Iodine–131 – beta emission.
 The time taken for the
number of undecayed
2000
nuclei to drop from
1200x106 down to
600x106 is 8 days
1500
 The time taken for the
number of undecayed
nuclei to drop from
1000
6
400x10 down to
200x106 is 8 days as
500
well.
 It does NOT depend
on the INITIAL
0
amount of the
quantity.
0
10
20
Time t/days
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7.2.9 Solve radioactive decay problems involving integral numbers of half–lives .
Example problem 1
 A sample of Radon – 220 was measured to have an activity of 1440Bq. The same sample was measured 4
minutes 20 seconds later to have an activity of 45Bq. Calculate the half-life of Radom-220.
 Answer: 52seconds
 Example problem 2
 Nuclide X is radioactive, decaying with a half-life of 6.0 minutes to a daughter nuclide Y, which is stable. A
sample originally consists entirely of nuclide X.
 Construct a graph showing the number of undecayed x nuclei in the sample as a function of time. The initial
number is shown at point A.
 On the same axes construct another graph showing the number of daughter (Y) nuclei as a function of time.
Label this graph Y.
 Determine the ratio of Y and X nuclei after 18 minutes.
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