Download 06-Nuclear shorter

Modern Physics
By
Neil Bronks
Atoms
Mass Number Number of protons +
Neutrons.
12
6
C
Atomic Number Number of protons
In a neutral atom the number of electrons and protons are the
same. In Carbon it is……… 6
Gravitational - Only Positive
- Very long range
Electromagnetic –
Positive and negative
4 Forces of Nature
Strong Nuclear –
Holds nucleus together
- Very Short Range
Weak NuclearAssociated with beta
decay
Radiation
J’ai fais ça !
 Decay of nucleus by the emission of a
particle or a ray.
 Discovered by Henri Becquerel
 Units 1 Bq is one decay per second
 Natural happens without outside
bombardment
 Artificial happens due to bombardment
Alpha Particles 
U  Th He
238
92
234
90
Helium Nuclei
Positive Charge
Heavy so not very penetrating
Very Ionizing
14 N
7
+42He 178O + 11H
4
2
Beta Particle ß




Th Pa 
234
90
234
91
Fast electron from the nucleus
Negative charge
Moderately Penetrating
Moderately ionizing
14
14 O + 0 ß
N

7
8
-1
0
1
Gamma Ray 
 High energy e-m wave (A Photon)
 No charge - not deflected by field
 Very penetrating – Need lead to stop most
of them
 Not very ionizing
 Release energy after reaction
Penetrating Power



Paper
Al foil
Concrete
Particles in Fields
Charged particles move in a
circular path as the force is
always at right angles to the
direction of motionFleming's Left Hand Rule



Solid State Detector
This a P-N junction in reverse bias.
This creates a huge depletion layer.
-
+
P
N
A piece of radiation passes through the depletion
layer and creates enough carriers to carry one
pulse of current.
Experiments
All experiments the same stick a
DETECTOR in front of a source and
count the decays.
Move it away for distance and plot
Time for half life and plot
Put things in front for penetration
Half-Life
Atoms
Not
Decayed
Time
1
2
3
4
Calculations
=0.693/T½
=0.693/3s
-1
=0.231s
Activity Calculations
Rate of Decay =
 x number you started with
dN/dt = -
 xN
Start with 4000 particles and =0.231
Activity = 4000 x 0.231=924 Bq
Isotopes
Same
atomic
number
different
mass
number
Rutherford Scattering – alpha
particles fired at gold foil.
Nucleus
A small number of high energy alphas are Deflected
More pass unaffected - So the nucleus is very small
Some reflected completely back - Nucleus totally positive.
Cockcroft and Walton
Hydrogen
discharge tube
Proton
4000kv
Accelerated by
An electric
field
Alpha strikes the
screen
Producing a flash
that
Is seen with the
microscope
0kv
Alpha 
Alpha

Lithium
Target
Alpha
Alpha 
Nobel Prize for Physics
 Proton + Lithium  2xAlpha + Energy
 Proves Einstein’s Law E=mc2
 First Transmutation by artificial
Bombardment of an element
7
3
Li  p     Energy
1
1
4
2
4
2
Ernest Walton
Use Famous Formula
E=mc2
E=
3.96x10-30 Kg
x
(3x108 m/s)
E = 3.56x10-13 Joules
2
Fusion
 Fusion is the joining together of 2 light nuclei
to make one nucleus with release of energy.
 Caused by a super fast collision at high
temperature in a magnetic bottle.
2
1H
2
1H
Fission-The breaking apart of a heavy nucleus to form
smaller products with release of energy.
1. Subtract mass
in a.m.u.
2. Convert to kg
3. Use E=mc2
Uranium-235
Also produced 3 fast neutrons that can
cause another fission and so a chain
reaction
Moderators slow
down the neutrons
to the right speed
Control Rods stop the
reaction and prevent
meltdown
Hot water to
turbine
Fuel rods contain the
Uranium-235 (Enriched to
ensure chain reaction)
Nuclear Equation
U  n Kr  Ba 3 n  Energy
238
92
1
0
97
36
139
56
1
0
In the isotope U-238 the neutrons must be
slowed down by a moderator – Graphite
Only need one neutron for stable Chain
Reaction (Self sustaining) more than one
=Meltdown (China Syndrome)
Leptons
Fundamental particles
1/1846 of an a.m.u.
Does not feel the strong nuclear force
Matter – Electron , Muon, Tau, ……
Anti-matter – Positron, Anti-Tau
Anti-matter first suggested by Paul Dirac
Annihilation
Matter combining with anti-matter to
form energy in the form of e-m
radiation
e01+
e01e+10 + e-10
2hf (2)
2 photons conserve momentum ?
Annihilation
2 photons conserve
momentum
Matter combining with anti-matter to
form energy in the form of e-m
radiation
e10+
e01-
An electron and a positron collide to make energy.
All the mass of the electrons gets turned into
gamma waves
Matter turns
2
So Energy E=mc
Into energy
To find frequency of wave E = 2h.f
Use Planks Equation
E=hf
h= planks constant
f= frequency
3.01x10-10 Joules = (6.6x10-34 js)x(f)
f= 3.01x10-10 Joules / 6.6x10-34 js
= 4.56x1023 Hz
Pair Creation
A matter and anti-matter pair
being created by energy from
an e-m wave
e+
eAn electron and a positron are created from a gamma
ray. (We can also get a proton and an anti-proton)
We do the calculation in reverse
To find energy of wave E = h.f
As we get 2 electrons E = 2mc2
Annihilation and Production
p+
p-
p+
p-
+
0
-
New particles are produced from the KE of the colliding
protons They must conserve charge
If we carry in 4Gev (1.6x10-19 . 4x109= 4x10-10 J)
As Energy to make 3 Pions is E=mc2 =(3x 2.4842x10-28xcxc)
=6.7x10-11 J
Subtracting we find the KE after collision.
Quarks - Inside the Hadrons
6 Quarks
6 Anti-Quarks – Opposite Signs
UP
+2/3
DOWN
-1/3
STRANGE
-1/3
TOP
CHARMED
+2/3
BOTTOM
-1/3
+2/3
Baryon
Meson
Beta decay
 In β− decay, the weak force converts a neutron
into a proton while emitting an electron and an
antineutrino
n0 → p+ + e- + νe
This explains loss in energy and momentum.
Pauli proposed it’s existence in 1930 but was
not discovered until 1956 as it is so weakly
interacting with other particles.
Particle Zoo
Hadrons
Subject
to all
forces
Baryons
Mesons
3 Quarks
Quark +
Anti-quark
Proton
uud
Pion
ud
Leptons
Fundamental
particles
Do not feel
Strong Nuclear
Force