Topic 7: Nuclear
WS / edited by JC
Becquerel, Marie and Pierre
Curie, Ernest Rutherford
 Rocks containing uranium that gave out some kind of
radiation were discovered and investigated.
 More than one substance was involved (Radium and
Polonium were discovered)
 More than one type of radiation was involved
 Alpha, Beta, Gamma
The properties of the electron were discovered (J.J.
Thompson). However there was no evidence for any of
the ideas about atom composition.
Geiger–Marsden experiment
1909
 The alpha particles were observed to occasionally scatter at
angles greater than 90°, which is physically impossible
unless they are scattering off something more massive than
themselves. This led Rutherford to deduce that the positive
charge in an atom is concentrated into a small compact
nucleus.
 The majority of the atom is
probably empty space as
most of alpha particles go
through the foil unchanged
Isotopes
 It was also discovered by JJ Thompson and others that
atoms of the same element could have different
masses.
 This meant that there had to be both a positive and a
neutral component in the nucleus.
This model is referred to as the:
“Simple Atomic Model”
Limitation of the simple model
 The main problem with this theory was that
accelerating charges are known to lose energy. If the
orbiting electrons were to lose energy they would spiral
into the nucleus.
 Also, this model does not explain the emission and
absorption spectrum.
 The model does not account for how the protons and
neutrons stay together in the nucleus.
Atomic energy levels
 Evidence for electron energy levels comes from
emission and absorption spectra.
 An energy level of 0 corresponds to the electron
escaping from the atom. Electrons attached to an atom
have negative energy levels.
Definitions
 Nuclide: The name given to a particular species of
atom (one whose nucleus contains a specified number
of protons and a specified number of neutrons)
 Isotope: Elements that contain the same number of
protons but a different number of neutrons.
 Nucleon: Protons and neutrons are collectively called
nucleons.
More Definitions
 A: Nucleon number – Number of nucleons (protons +
neutrons) in the nucleus
 Z: Proton number – also called atomic number, equal to
number of protons in the nucleus
 N: Neutron number – Number of neutrons in the
nucleus
 N=A–Z
Radioactive decay
 Radioactive decay is a random process and is not
affected by external influences. Some nuclei are more
stable than others. When an unstable nucleus
disintegrates to acquire a more stable state, radiations
are emitted.
 Nuclei also have energy levels!
Properties of alpha and beta
particles and gamma
Property
Alpha
Beta
Gamma
Effect on photographic film
Yes
Yes
Yes
Appropriate number of ions
produced in air
104 per mm travelled
102 per mm travelled
1 per mm travelled
Typical material needed to
absorb
10-2 mm aluminium, piece
of paper
A few mm aluminium
10cm lead
Penetration ability
Low
Medium
High
Typical path length in air
A few cm
Less than one m
Effectively infinite
Behaves like a negative
charge
Not deflected
About 108 m/s
Speed of light
Deflection by E and B fields Behaves like a positive
charge
Speed
About 107 m/s
What is ionizing?
 All three radiations are ionizing, which means that as
they go through a substance, collisions occur which
cause electrons to be removed from atoms. Atoms that
have lost or gained electrons are called ions.
 When ionisations occur in biologically important
molecules, such as DNA, mutations can occur.
Biological Effects
 At the molecular level, an ionisation could cause
damage directly to a biologically important molecule
such as DNA. This could cause it to cease functioning.
 Molecular damage can result in a disruption to the
functions that are taking place within the cells that
make up the organism. As well as potentially causing
the cell to die, this could just prevent cells from dividing
and multiplying.
 If malignant cells continue to grow then this is called
cancer.
Half-Life
 The time taken for the number (or mass) of radioactive
nuclei present to fall to half its value. This length of time
is constant at any point in time - showing that
radioactive decay is exponential.
Artificial (induced)
Transmutation
 Artificial transmutation is the process whereby a
nucleus is artificially made from another nucleus. It is
different from regular radioactivity in that the reaction is
not spontaneous; it is made to happen.
 When nitrogen gas was bombarded by alpha-particles
it was found that there were two products: oxygen gas
and positively charged particles.
Einstein Mass-Energy
Equivalence Relationship
 If an object increases in energy, then its mass also
increases. The relationship between mass and energy is
described by Einstein’s famous equation:
 E=mc2
 When energy is released, there is also a decrease in mass
of the products.
 In Einstein’s equation, 1kg of mass is equivalent to 9x1016J
of energy. Since mass and energy are equivalent it is
sometimes useful to work in units that avoid having to do
repeated multiplications by the speed of light.
 A new possible unit for mass is thus MeV c −2. If 1 MeV c−2
worth of mass is converted you get 1MeV worth of energy.
Binding Energy
 Mass defect: The difference between the mass of a
nucleus and the masses of its component nucleons.
 Binding energy: The amount of energy that is released
when a nucleus is assembled from its component
nucleons. It comes from a decrease in mass. The
binding energy would also be the energy that needs to
be added in order to separate a nucleus into its
individual nucleons.
 Mass defect x (Speed of light)2 = Binding Energy
Binding energy per nucleon
Binding energy per nucleon: is useful
measure of the stability of a nucleus
binding energy it is the total binding energy
divided by the total number of nucleons
Fission vs. Fusion
 Fission: Fission is the name given to the nuclear
reaction whereby large nuclei are induced to break up
into smaller nuclei and release energy in the process. It
is the reaction that is used in nuclear reactors and
atomic bombs. A typical single reaction might involve
bombarding a uranium nucleus with a neutron. This can
cause the uranium nucleus to break up into two smaller
nuclei.
 Fusion: Fusion is the name given to the nuclear
reaction whereby small nuclei are induced to join
together into larger nuclei and release energy in the
process. It is the reaction that fuels all stars.