Nuclear structure
27.(c) distinguish between nucleon number (mass number) and proton
number (atomic number)..
27.(e) use the usual notation for the representation of nuclides.
27.(d) show an understanding that an element can exist in various isotopic
forms each with a different number of neutrons
27.(f) appreciate that nucleon number, proton number, and energy and mass
are all conserved in nuclear processes
27.(g) represent simple nuclear reactions by nuclear equations of the form
14
7
N  24He 178O 11H
27.(h) show an appreciation of the spontaneous and random nature of
nuclear decay.
21.(j) infer the random nature of radioactive decay from the fluctuations in
count rate.
An atom
• Describe the structure of an atom
Atomic notation
• There are about 256 nuclides in nature which are so stable
that they have never been observed to decay. They occur
among the 80 different elements which have one or more
stable nuclides.
• Each nuclide can be identified by characterized by it’s
number of protons, its number of neutrons, (and its
energy content)
• The accepted notation is A Z X
• A = mass or nucleon number
• Z = atomic number
• N = neutron number are related (A = Z + N).
Isotopes
• Isotopes have the same atomic no, (Z) but
different mass number (A)
• How could this be?
• Some common examples:
– H, D and T,
– U-235 and U-238,
– C-14 and C-12.
• Find out the name for nuclides having the same A
but different Z, and the same N but different Z .
Interactions
•
•
•
•
What is the charge on a proton?
What happens when lots of protons are in close proximity?
So how does a nucleus stay together?
There must be an attractive force acting on the nucleons
which is stronger than the Coulomb force
• What about gravity?
• The rest mass of a proton is 1.672621637(83)×10−27 kg
• Also remember Gravitational force is 1042 weaker than the
Coulomb force
Nuclear force
• The other force is the strong nuclear force
• How does it compare to the Coulomb force?
• Do you think this is an inverse square law force
(field)?
• No it must be stronger over shorter distances
than the Coulomb force but weaker over
distances greater than the diameter of a nucleus.
• The decrease is approximately as a negative
exponential power of distance
N-Z plot (Segrè plot).
Neutron
number
n
p + b- n
A
bb+
B
p
n + b+ + n
Proton number
Radioactive decay
• The Segre plot shows isotopes which are
unstable are far from the line.
• These isotopes will naturally decay to become
more stable (moving closer to the line)
• They can do this by emitting alpha or beta
particles.
241
95
4
Am  237
93 Np  2 He.
Equation for alpha decay
• Nuclear decay processes can be represented by
nuclear equations. The word equation implies that the
two sides of the equation must ‘balance’ in some way.
• Decay processes
• alpha source eg americium-241,
241
95
Am 
237
93
Np  He.
4
2
Beta radiation
•
•
•
•
•
Beta sources are strontium-90,
The underlying process is:
n –> p + e- + ν
Here, ν is an antineutrino.
You can translate n –> p + e- into the AZ
notation:
90
38
Sr  Np 
90
39
0
1
e.
Gamma radiation
• gamma sources eg cobalt-60
• The γ radiation comes from the radioactive daughter of the
beta decay.
• The daughter is formed in an ‘excited state’ and immediately
loses the energy by emitting a gamma ray.
• They are only emitted after an alpha or beta decay
• All such gamma rays have a well-defined energy.
• So a cobalt-60 source which is a pure gamma emitter must be
designed so that betas are not emitted. How?
• By encasing in metal which is thick enough to absorb the
betas but which still allows gammas to escape.
Nature of radiations
• Use pages 334&5 to complete the table.
• Close your book turn over the page and see
how much you can do from memory.
Spontaneous
• Which of the following will cause a nuclide to
decay.
– Applying huge pressures using high explosives
– Applying exremely high temperatures such as at
the centre of stars
– Accelerating it to speeds near to the speed of light
Answer
None of them the process is spontaneous
and random
• If I toss a coin 15 times and each time get a
head the next toss is more likely to be
– A head
– A tail
•
•
•
•
Answer
Still 50/50 the toss is random
Nuclear decay is also random
You cannot predict exactly when a nucleus will
decay
To do
•
•
•
•
Complete the nuclear equations worksheet
P4U p347 Q1,2,3,4,8
P4U p370 Q7
Reading
– Atomic structure p 328-331.
– Nature of radiations p334-335
– Alpha and beta decay equations p338-339
– Background p344-346