WELCOME TO PERIOD 19: BETA DECAY AND
ITS APPLICATIONS
Homework #18 is due today.
Midterm 2: Weds, Mar 27, 7:45 – 8:55 pm
(Same room as your midterm 1 exam.)
Covers periods 10 – 19 and videos 3 & 4
Review: Tues, 3/26, 7:00 – 8:00 pm 2005 SM
Drop in: Weds, 3/27, 5:30 – 7:15 pm 2005 SM
PHYSICS 1104 – PERIOD 19
•How does a proton change into a neutron?
….a neutron into a proton?
•What are quarks?
•How can beta decay be used to date
materials?
Nucleus stability
Stable nuclei with more
than 20 protons and less
than 83 protons have
more neutrons than
protons.
Unstable nuclei beta
decay to change a
proton into a neutron
or a neutron into a
proton.
b+ decay
An unstable nuclei with too many protons changes one
proton into a neutron. 1
1
1
p  0n
Conservation of charge requires a positive antielectron.
1
1
p  01 n 
0
1
0
1
e
e
When an antielectron is emitted, a neutrino is also emitted n
1
1
p  01 n 
0
1
e   0o n
What does the spring represent? Energy!
1
1
p  01 n 
0
1
e  0o n  energy
b- decay
An unstable nuclei with too many neutrons changes one
neutron into a proton. 1
1
0
n  1p
Conservation of charge requires a negative
1
0
n  11 p 
0
1
0
electron.  1
e
e
When an electron is emitted, an antineutrino is emitted n
1
0
n  11 p 
0
1
e  00 n
Energy is also emitted.
1
0
n  11 p 
0
1
e  00 n  energy
Stable Fundamental Particles and Quarks
Neutrons and protons are made up of fundamental
particles called quarks.
The electron is a fundamental particle.
The Standard Model of Particle Physics attempts to
explain the fundamental particles and how they are
related to one another.
These relationships involve the strong nuclear, weak
nuclear, and electromagnetic forces.
Protons and
neutrons are
made of UP
and DOWN
quarks
http://en.wikipedia.org/wiki/File:Standard_Model_of_Elementary_Particles.svg
Quark electric charge and spin
Electric
Charge
+2/3
– 1/3
1st
generation
particle
u
up quark
d
down
quark
2nd generation
particle
3rd generation
particle
c
charm quark
t
top quark
s
b
strange quark bottom quark
Quarks, like many other particles, have a
rotation about their axes called intrinsic spin.
The intrinsic spin of quarks = ½
Combining quarks into nucleons
1)
Find a combination of quarks whose electric
charge equals the charge of the nucleon.
(up quark = + 2/3; down quark = - 1/3)
2)
For two quarks of the same type (2 up quarks or
2 down quarks), the spins must point in opposite
directions.
Note: Depending on how the quarks are combined,
the spin of the nucleon formed from the quarks will
point either up or down.
Intrinsic spin of particles
Almost all particles in physics have a rotation about
their axes called intrinsic spin. The intrinsic spin of
quarks and leptons = ½
When 3 quarks combine to form a proton or neutron,
the spins of the two quarks of the same type must
point in opposite directions.
Therefore, the total spin of a neutron or proton is
½ – ½ + ½ = ½.
Force binding quark trios into nucleons
The strong nuclear, weak nuclear, and
electromagnetic forces arise from the exchange of
carrier particles known as gauge bosons.
The gluon (g) is the gauge boson responsible for the
strong nuclear force that holds three quarks together
to form a neutron or a proton.
The gluon is
responsible
for the strong
nuclear force
that binds
quarks into
nucleons.
http://en.wikipedia.org/wiki/File:Standard_Model_of_Elementary_Particles.svg
The Higgs Boson
• On July 4, 2012, researchers using the Large Hadron
particle collider at CERN announced they had found the
Higgs boson.
• The Higgs boson plays a unique role in the Standard
Model by explaining why the other elementary particles,
except the photon and gluon, have mass.
• The Higgs boson has no intrinsic spin, and for that reason
it is classified as a boson.
• The Higgs boson is a very massive particle and decays
almost immediately when created.
• Therefore, a very high energy particle accelerator was
needed to observe the Higgs boson.
Source: http://en.wikipedia.org/wiki/Higgs_boson
Carbon isotopes
 Unstable Carbon-14
14
6
C can be used to date
archaeological sites.
Why is
14
6
C unstable?
Write a reaction showing how this isotope decays to
become more stable.
 Carbon-14 is produced when cosmic rays convert
stable nitrogen-14 147 N in the air into carbon-14.
Write a reaction showing how carbon-14 is produced
14
from 7 N
Carbon isotopes
Unstable Carbon-14 decays by emitting a b  particle
and an antineutrino:
14
6C

14
7N

0
1 e
 n
Carbon-14 is produced when cosmic rays convert
stable nitrogen-14 in the air into carbon-14.
A b  particle and a neutrino are emitted.
•
14
14
N

7
6C

0
1 e
 n
Half-life of radioactive sources
The half-life of a radioactive source is the time
required for half of the unstable nuclei to decay.
After one half-life, the material will be only half as
radioactive.
The number of the original nuclei remaining will be
only half what it was originally.
Radioactive decay simulation
1. Close switch to the
left to charge batteries.
9 volt
batteries
2. Close switch to the
right and start timer.
V
3. Record the voltage shown by the
multimeter every 15 seconds.
Finding Half-Life from a graph with background
1) Pick a data point on your graph and read the Y-axis
value (the voltage in our activity).
2) Subtract the background voltage.
3) Divide the result in half.
4) Add back in the background voltage. This gives ½
the original voltage, corrected for the background.
5) Find this voltage on your graph.
6) Read down to the X-axis from this point to find a
time in seconds.
7) The difference in seconds between this time and the
time of your original point is the half-life – the time it
took for ½ of the capacitor’s charge to be released.
Exponential growth and decay
N = B x 2t
exponential growth:
exponential decay:
N 
Bx2
t

1
Bx t
2
N = the amount of the quantity at a given time
t = the number of time periods elapsed
B = the initial amount of the quantity
Number of
Half Lives
0
1
2
3
4
5
Fraction of
Original
1
6
1

Number of
Half Lives
20
1
2

1
4

1
8

7
1
21
1
8
22
9
1
23
1
16

1
32

1
24
1
25
10
Fraction of
Original
1
64

1
128

1
256

1
512

1

1024
1
26
1
27
1
28
1
29
1
210
Example of exponential decay
A sample of radioactive material has a half life of 15
minutes. If there are 5.0 grams of the material at the
beginning of an experiment, how much will be left
after 1 hour has passed?
After 1 hour, four 15-minute half lives have passed.
N  Bx
 5.0 grams
1
2
4
1
2t

1
5.0 g
16

0.31 g
Radio carbon dating
Both stable Carbon-12 and unstable carbon-14
isotopes are present in the atmosphere.
Living organisms absorb both isotopes of carbon.
After an organism dies, it no longer absorbs any new
carbon-14, and the carbon-14 within it decays.
We can accurately estimate the time of an organism's
death, if we know
1) the ratio of carbon-12 to carbon-14 in the
atmosphere at the time the organism died and
2) the present ratio of carbon-12 to carbon-14 in the
fossil.
The half-life of carbon-14 is 5,730 years!
BEFORE THE NEXT CLASS…
Read textbook chapter 20.
Complete Homework Exercise 19.
Print out Activity Sheet 20.
Midterm 2: Weds, Mar 27, 7:45 – 8:55 pm
(Same room as your midterm 1 exam.)
Covers periods 10 – 19 and videos 3 & 4
Review: Tues, 3/26, 7:00 – 8:00 pm 2005 SM
Drop in: Weds, 3/27, 5:30 – 7:15 pm 2005 SM