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
 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
A
Z
XN
A4
Z 2
X N  2  He  
4
2
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
A
 N = 77
X 
Z
N
Y N 1  
A
Z 1



 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
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
 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
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.
 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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