Alpha decay
Alpha particles consist of two protons plus two
neutrons.
They are emitted by some of the isotopes of the
heaviest elements.
Example: The decay of Uranium 238
238
92
U
234
90
Th +
4
2
α
Uranium 238 decays to Thorium 234 plus an alpha particle.
Notes:
1. The mass and atomic numbers must balance on each side
of the equation: (238 = 234 + 4 AND 92 = 90 +2)
2. The alpha particle can also be notated as:
4
2
He
Question
Show the equation for Plutonium 239 (Pu)
decaying by alpha emission to Uranium (atomic
number 92).
239
94
Pu
235
92
U
+
4
2
α
Beta decay
Beta particles consist of
high speed electrons.
They are emitted by
isotopes that have too many
neutrons.
One of these neutrons
decays into a proton and an
electron. The proton
remains in the nucleus but
the electron is emitted as
the beta particle.
Example: The decay of Carbon 14
14
6
C
14
7
N
+
0
-1
-
β
Carbon 14 decays to Nitrogen 14 plus a beta particle.
Notes:
1. The beta particle, being negatively charged, has an
effective atomic number of minus one.
2. The beta particle can also be notated as:
0
-1
e
Question
Show the equation for Sodium 25 (Na), atomic
number 11, decaying by beta emission to
Magnesium (Mg).
25
11
25
Na
12
Mg +
0
-1
-
β
Gamma decay
Gamma decay is the emission of electromagnetic radiation
from an unstable nucleus
Gamma radiation often occurs after a nucleus has emitted
an alpha or beta particle.
Example: Cobalt 60
60
27
Co
60
27
Co +
0
γ
0
Cobalt 60 with excess ENERGY decays to
Cobalt 60 with less ENERGY plus gamma radiation.
Do Now copy and complete
Changing elements
Both alpha and beta decay cause the an isotope to change
atomic number and therefore element. Alpha decay also
causes a change in mass number.
Decay type
Atomic number
Mass number
alpha
DOWN by 2
DOWN by 4
beta
UP by 1
NO CHANGE
gamma
NO CHANGE
NO CHANGE
Complete the decay equations below:
(a)
59
26
59
Fe
224
(b)
88
(c)
Ra
16
7
27
220
86
16
N
Co +
8
0
-1
Rn +
O +
0
-1
-
β
4
α
2
-
β
Write equations showing how Lead 202 could
decay into Gold. (This cannot happen in reality!)
Element Sym
Z
Platinum
Pt
78
Gold
Au
79
Mercury
Hg
80
202
198
4
Hg +
Pb
82
80
2
198
194
4
Hg
Pt
80
78
194
Thallium
Tl
81
Lead
Pb
82
194
Bismuth
Bi
83
78
Pt
α
2
0
Au
79
+
α
β
+
-
-1
There are other correct solutions
Choose appropriate words to fill in the gaps below:
When an unstable nucleus emits an alpha particle its atomic
two
four
number falls by _______
and its mass number by ______.
neutrons
Beta particles are emitted by nuclei with too many ________.
one
In this case the atomic number increases by ______
while the
mass
________
number remains unchanged.
electromagnetic radiation that is
Gamma rays consist of ______________
energy
emitted from a nucleus when it loses ________,
often after
undergoing alpha or beta decay.
WORD SELECTION:
four one energy
two neutrons mass electromagnetic
Today’s lesson
• Use the term half-life in simple
calculations, including the use of
information in tables or decay curves.
• Give and explain examples of practical
applications of isotopes.
• Title Half-life
½ - life – copy please
• This is the time it takes for half the nuclei
present in any given sample to decay
Number of
nuclei
undecayed
A graph of the count
rate against time will
be the same shape
time
half-life (t½)
Different ½ - lives
• Different isotopes have different half-lives
• The ½-life could be a few milliseconds or
5000 million years!half life applet
Number of
nuclei
undecayed
time
half-life (t½)
Examples
• A sample of a radioactive isotope of half
life 2 hours has a count rate of 30 000
counts per second. What will the count
rate be after 8 hours?
Examples
Activity
The activity of a radioactive
source is equal to the number
of decays per second.
Activity is measured
in bequerels (Bq)
1 becquerel
= 1 decay per second
Half life
Henri Becquerel
discovered
radioactivity in 1896
Question 1
At 10am in the morning a radioactive sample contains
80g of a radioactive isotope. If the isotope has a halflife of 20 minutes calculate the mass of the isotope
remaining at 11am.
10am to 11am = 60 minutes
= 3 x 20 minutes
= 3 half-lives
mass of isotope = ½ x ½ x ½ x 80g
mass at 11 am = 10g
Question 2
Calculate the half-life of the radioactive isotope in a
source if its mass decreases from 24g to 6g over a
period of 60 days.
24g x ½ = 12g
12g x ½ = 6g
therefore TWO half-lives occur in 60 days
half-life = 30 days
Example 2 – The decay of source Z
Source Z decays with a
half-life of three hours.
At 9 am the source has
an activity of 16000 Bq
The activity halves every
three hours.
Time
Activity
(Bq)
9 am
16000
12 noon
8000
3 pm
4000
6 pm
2000
9 pm
1000
midnight
500
When will the activity have fallen to 125 Bq?
6 am
Example 3 – The decay of isotope X
Isotope X decays to
Isotope Y with a halflife of 2 hours.
At 2 pm there are
6400 nuclei of
isotope X.
Time
Nuclei of Nuclei of
X
Y
2 pm
6400
0
4 pm
3200
3200
6 pm
1600
4800
8 pm
800
5600
10 pm
400
6000
midnight
200
6200
When will the nuclei of isotope X fallen to 25?
6 am
Question 3
A radioactive source has a half-life of 3 hours.
At 8 am it has an activity of 600 Bq.
What will be its activity at 2 pm?
at 8 am activity = 600 Bq
2 pm is 6 hours later
this is 2 half-lives later
therefore the activity will halve twice
that is: 600  300  150
activity at 2 pm = 150 Bq
Question 4 – The decay of substance P
Substance P decays
to substance Q with
a half-life of 15
minutes. At 9 am
there are 1280 nuclei
of substance P.
Complete the table.
Time
Nuclei of Nuclei of
X
Y
9 am
1280
0
9:15
640
640
9:30
320
960
9:45
160
1120
10 am
80
1200
10:15
40
1240
How many nuclei of substance X will be left at 11 am?
5
Question 5
A sample contains 8 billion nuclei of hydrogen 3
atoms. Hydrogen 3 has a half-life of 12 years. How
many nuclei should remain after a period 48 years?
48 years = 4 x 12 years
= FOUR half-lives
nuclei left = ½ x ½ x ½ x ½ x 8 billion
nuclei left = 500 million
Experiment
Dicium 25
Finding half-life from a graph
600
The half-life in this
example is about
30 seconds.
number of nuclei
500
400
300
200
100
half-life
0
0
20
40
60
80
time (seconds)
100
120
A more accurate
value can be
obtained be
repeating this
method for a other
initial nuclei
numbers and then
taking an average.
Question 6
The half-life is
approximately 20
seconds
900
800
700
activity (Bq)
Estimate the half-life of
the substance whose
decay graph is shown
opposite.
600
500
400
300
200
half-life
100
0
0
10
20
30
40
50
60
70
time (seconds)
80
90 100
Question 7
The mass of a radioactive substance over a 8 hour period
is shown in the table below.
Draw a graph of mass against time and use it to determine
the half-life of the substance.
Time
(hours)
Mass (g)
0
1
2
3
4
5
6
7
8
650
493
373
283
214
163
123
93
71
The half-life should be about 2 hours:
Choose appropriate words or numbers to fill in the gaps below:
half-life
The ________
of a radioactive substance is the average time
nuclei
taken for half of the _______of
the substance to decay. It is
activity
also equal to the average time taken for the ________
of the
substance to halve.
5600
The half-life of carbon 14 is about _______
years. If today a
sample of carbon 14 has an activity of 3400 Bq then in 5600
1700 Bq. 11200 years
years time this should have fallen to ______
later the activity should have fallen to ____
425 Bq.
The number of carbon 14 nuclei would have also decreased
eight times.
by ______
WORD & NUMBER SELECTION:
5600 nuclei eight half-life
425 1700 activity
Revision Simulations
Half-Life - S-Cool section on half-life and uses of radioactivity including an onscreen half-life calculation and an animation showing thickness control.
BBC AQA GCSE Bitesize Revision:
Detecting radiation
Natural sources of background radiation
Artificial radiation
Half life
Alpha Decay - PhET - Watch alpha particles escape from a Polonium
nucleus, causing radioactive alpha decay. See how random decay times
relate to the half life.
Uses of radioactive isotopes
Smoke detection
• Uses
Thickness control
Thickness control
Used as Tracers
Used as Tracers
Killing microbes
Killing microbes
Checking welds
Used as Tracers
Carbon dating – write notes
using the book page 265
Summary sheet
“Can you………?”
Test!
Thursday
27th September 2012
Can you answer the
questions on pages
261 and 265?