Transcript Today`s Objectives - RanelaghALevelPhysics

Strong Force & Radioactivity
Particle Physics 2
Homework Reminder!
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Ernest Rutherford
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What important
experiment did he direct
in the early 20th Century?
What did Rutherford
conclude from his
analysis of the
observations?
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Today’s Objectives
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The strong nuclear force:
-its role in keeping the nucleus stable;
- short-range attraction to about 3 fm, veryshort range repulsion below about 0.5 fm
If time:Equations for alpha decay and beta - decay
including the neutrino.
Size of the nucleus...
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If the atom were the size of the school canteen,
the nucleus would be the size of a pea dropped
in the middle.
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Really small compared to the atom!!
For fun?...(don’t worry about the
equations)
1 Q1Q2
F
2
40 r
Gm1m2
F 
2
r
Repulsive force between two
protons.
Gravitational Attraction.
Q1,Q2=e
G is gravitational constant.
m are proton masses
R is distance between the
two protons (10-15m).
Why doesn’t it fly apart?
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The repulsive force between the two charges is
much larger than the gravitational force.
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So why is it stuck together?
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We need another force – the STRONG force.
The Strong Force Graph
Important to Note
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It’s a weird force!
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At very short ranges, below 0.5 femtometres
(0.5 × 10-15 m) the strong nuclear force is
repulsive.
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It is attractive up to its maximum range of 3 fm
(3 × 10-15 m).
Strong Force
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What would happen if the strong force wasn’t
repulsive at short distances?
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It is about 200 N between two protons – very
strong compared to other forces.
Radioactive Decay
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Some isotopes are stable but others are not.
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Those which are not release radiation and
change into a more stable isotope (normally a
different element).
How much do you know?
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Complete the table (in pencil) that describes the
properties of the three common radiations:Radiation
Particle
Range in
air
Stopped By
Answers
Radiation
Particle
Range in air Stopped By
Alpha
Helium
nucleus (P)
Few mm (P)
Paper (P)
Beta
High speed
electron (P)
Few cm (P)
Aluminium
sheet (P)
Gamma
Energetic
photon (P)
Infinite (P)
Several cm
lead (P)
Alpha radiation
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Mostly comes from heavy nuclides with proton
numbers greater than 82, but smaller nuclides
with too few neutrons can also be alpha
emitters.
The general decay equation is summarised
below.
The term Q stands for the energy.
Can be written with He as α and without the Q.
Notes on α-decay
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The alpha particle is a helium nucleus (NOT
atom).
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Energy is released in the decay. The energy is
kinetic, with the majority going to the alpha
particle and a little going to the decayed nucleus.
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The velocity of the alpha particle is much greater
than that of the nucleus.
Example Decay
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The nucleon number goes down by 4, the
proton number by 2.
A typical alpha decay is:
Is this equation balanced? Explain your answer
Beta radiation
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Neutron rich nuclei tend to decay by beta
minus (b-) emission.
The beta particle is a high-speed electron
ejected from the nucleus, NOT the electron
clouds.
It is formed by the decay of neutrons, which are
slightly more energetic than a proton.
Isolated protons are stable; isolated neutrons last
about 10 minutes.
Equation for Beta decay.
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The neutron, having emitted an electron, is
converted to a proton, and this results in the
proton number of the nuclide going up by 1. A
new element is formed. The general equation is:
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Note the electron can be written as β, also
without the Q
Example Beta Decay
Notes
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The nucleon number remains the same ;
The proton number goes up by 1.
The beta particle is created at the instant of the
decay.
The antineutrino is very highly penetrating and
has a tiny mass. It is very hard to detect.
A precise amount of energy is released,
according to the nuclide.
That energy is shared among the nucleus, the
electron and the antineutrino.
Question
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What is the balanced nuclear equation for the
following decays?
(a) emission of a beta- particle from oxygen 19
(b) emission of an alpha particle from polonium
212
(c) emission of a beta + particle from cobalt 56
Proton numbers O – 8, F – 9, Fe – 26, Co –
27, Pb – 82, Po – 84
Answers
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(a)
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(b)
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(c)
Summary
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A graph of neutron number against proton
number shows that there are more neutrons in
larger nuclei
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This is needed to ensure stability of the nuclei.
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Natural decay occurs with alpha decay
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Or beta minus decay.