Download Slide 1

Nuclear Physics 2
Radiation Properties
Wednesday, 24 May 2017
Leeds City College
1
Starter
Radiation
Alpha
Beta
Gamma
Particle
Range in air
Stopped by
Helium nucleus
Few mm
Paper, skin
High speed
electron
Less than 1 m
3 mm Aluminium
EM radiation
Infinite
2 cm lead
Strictly speaking the lead attenuates the gamma radiation so that it is little
above the background radiation.
Wednesday, 24 May 2017
Leeds City College
2
Equations for Decay
Alpha
A
Z
A–4
X
Z–2
4
Y +
2
+ Q
He
e.g.
228
224
90
88
Th
4
Ra +
2
He +
Q
Beta
1
1
0
1
n
A
A
X
Z+1
X
e.g.
29
13
Wednesday, 24 May 2017
Al
p
0
+
Y
+
29
14
0
-1
Si +
Leeds City College
+
e
-1
e
0
-1
0
ne
0
0
ne
+
e +
0
0
0
+
Q
+
Q
ne + Q
3
Production of Alpha Particles
They come from heavy elements
of mass greater than 106 atomic
mass units.
In classical physics, the strong
force balances the electromagnetic force, so the alpha
does not have the energy to get
out.
In quantum physics, there is a
small chance that the alpha can
get out by a process of quantum
tunnelling.
Energy well
Maximum energy
available
Escape route
by quantum
tunnelling
At this level we assume that all
the energy is kinetic. Alpha
particle energy is between 3 and
7 MeV
Wednesday, 24 May 2017
Leeds City College
4
Range of Alpha Particle
Ref: Helmut Paul
Wednesday, 24 May 2017
Leeds City College
5
Path of Alpha in a Magnetic Field
Use Fleming’s Left
Hand Rule to determine
the path.
Wednesday, 24 May 2017
Leeds City College
6
Beta Particles
In beta minus decay:
Joe Holdsworth
Wikimedia commons
Inductiveload
Wikimedia commons
Beta minus decay involves the change of a neutron to a proton via the
W- boson. It happens in smaller nuclei that have an excess of neutrons.
Wednesday, 24 May 2017
Leeds City College
7
• All energy is kinetic, as the
beta is a high speed electron
(no sub-structure).
• The energy is precise within an
isotope.
• There is a variable sharing
between the electron,
electron antineutrino, and the
nucleus itself.
• Momentum is conserved.
• We are only interested in the
maximum energy level.
Wednesday, 24 May 2017
Number of b
particles
Energy in Beta
Leeds City College
Max kinetic
energy = Q
Energy
8
Valley of Stability
The black line shows the stable
isotopes.
Unstable nuclides are radioactive.
If there is an excess of neutrons, beta
minus decay happens.
If there are too few neutrons, beta plus
decay happens.
The higher up the sides of the valley,
the less stable the nuclide.
Wednesday, 24 May 2017
Leeds City College
9
Alpha and Beta in a Magnetic Field
The magnetic field is
going into the board.
Which path does each
particle follow?
Alpha
Source
Gamma
Beta
Wednesday, 24 May 2017
Leeds City College
10
Force on the Charge
We know:
• Velocity = distance  time
• Current = charge  time
• F = BIl
The t terms cancel out to give:
F =B I l
I = Q
t
F  BQv
l = vt
Wednesday, 24 May 2017
Leeds City College
11
Motion in a magnetic field
B field going into the screen
The force at 90 o acts as a
centripetal force, keeping
the electron in a circular
path.
Force always acts at 90 o
Wednesday, 24 May 2017
Leeds City College
12
Bear Trap
Remember that the direction of the
electrons’ movement is in the opposite
direction to the conventional current. So if
the electrons are going from left to right, the
conventional current is going from right to
left.
When using Fleming’s Left Hand Rule, the
current is conventional.
Wednesday, 24 May 2017
Leeds City College
13
Equation
For centripetal force:
mv2
F
r
The v on the left cancels to get rid
of the v2 term on the right:
mv
Bq 
r
For a charge in a magnetic field:
F  Bqv
Rearranging:
BQr
v
m
We equate these to give:
mv2
Bqv 
r
Wednesday, 24 May 2017
Leeds City College
14
Valley of Stability
Wednesday, 24 May 2017
Leeds City College
15