7.2.1 Describe the phenomenon of natural radioactive decay.

These notes were typed in association with Physics for use with
the IB Diploma Programme by Michael Dickinson
For further reading and explanation see:
Physics, Tsokos (purple): Ch 6.2
Physics, Giancoli (mountain): Ch 30
 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
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
Z 2
X N  2  He  
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****
Beta Decay (β) – is an electron
 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 elctron 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
X 
Y N 1  
Z 1
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.
Gamma Radiation (γ)
 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
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.
ms become ionized by gaining or
 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
 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
 Stable nuclides – Z < 20
 Protons and neutrons are about equal
 Above 20 the nuclides have more neutrons than protons.
 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

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