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![Page 1: Alpha decay Alpha particles consist of two protons plus two neutrons. They are emitted by some of the isotopes of the heaviest elements.](https://reader036.fdocuments.us/reader036/viewer/2022081420/56649e615503460f94b5cd5e/html5/thumbnails/1.jpg)
Alpha decay
Alpha particles consist of two protons plus two neutrons.
They are emitted by some of the isotopes of the heaviest elements.
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Example: The decay of Uranium 238
U238
92Th
234
90α
4
2+
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:He
4
2
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QuestionShow the equation for Plutonium 239 (Pu) decaying by alpha emission to Uranium (atomic number 92).
Pu239
94U
235
92α
4
2+
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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.
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Example: The decay of Carbon 14
C14
6N
14
7 β-
0
-1+
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:e
0
-1
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QuestionShow the equation for Sodium 25 (Na), atomic number 11, decaying by beta emission to Magnesium (Mg).
Na25
11Mg
25
12 β-
0
-1+
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Gamma decayGamma 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
Co60
27γ
0
0+Co
60
27
Cobalt 60 with excess ENERGY decays to
Cobalt 60 with less ENERGY plus gamma radiation.
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Do Now copy and completeChanging 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
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Complete the decay equations below:
Fe59
26Co
59
27 β-
0
-1+
Ra224
88Rn
220
86α
4
2+
N16
7O
16
8 β-
0
-1+
(a)
(c)
(b)
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Write equations showing how Lead 202 could decay into Gold. (This cannot happen in reality!)
Pb202
82Hg
198
80α
4
2+
Pt194
78Au
194
79β
-0
-1+
Element Sym Z
Platinum Pt 78
Gold Au 79
Mercury Hg 80
Thallium Tl 81
Lead Pb 82
Bismuth Bi 83
Hg198
80Pt
194
78α
4
2+
There are other correct solutions
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Choose appropriate words to fill in the gaps below:
When an unstable nucleus emits an alpha particle its atomic number falls by _______ and its mass number by ______.
Beta particles are emitted by nuclei with too many ________. In this case the atomic number increases by ______ while the ________ number remains unchanged.
Gamma rays consist of ______________ radiation that is emitted from a nucleus when it loses ________, often after undergoing alpha or beta decay.
electromagneticenergy masstwofour one
WORD SELECTION:
neutrons
electromagnetic
energy
mass
two four
one
neutrons
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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
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½ - life – copy please
• This is the time it takes for half the nuclei present in any given sample to decay
half-life (t½)
Number of nuclei undecayed
timeA graph of the count rate against time will be the same shape
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Different ½ - lives
• Different isotopes have different half-lives
• The ½-life could be a few milliseconds or 5000 million years!half life applet
half-life (t½)
Number of nuclei undecayed
time
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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?
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Examples
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ActivityThe 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
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Question 1At 10am in the morning a radioactive sample contains 80g of a radioactive isotope. If the isotope has a half-life 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
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Question 2Calculate 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
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Example 2 – The decay of source ZSource 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
12 noon
3 pm
6 pm
9 pm
midnight 500
1000
2000
4000
8000
16000
When will the activity have fallen to 125 Bq? 6 am
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Example 3 – The decay of isotope X
Isotope X decays to Isotope Y with a half-life of 2 hours.
At 2 pm there are 6400 nuclei of isotope X.
Time Nuclei of X
Nuclei of Y
2 pm
4 pm
6 pm
8 pm
10 pm
midnight 200
400
800
1600
3200
6400
6200
6000
5600
4800
3200
0
When will the nuclei of isotope X fallen to 25? 6 am
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Question 3A 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
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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 X
Nuclei of Y
9 am
9:15
9:30
9:45
10 am
10:15 40
80
160
320
640
1280
1240
1200
1120
960
640
0
How many nuclei of substance X will be left at 11 am? 5
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Question 5A 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
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Experiment Dicium 25
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Finding half-life from a graph
0
100
200
300
400
500
600
0 20 40 60 80 100 120
time (seconds)
num
ber
of n
ucle
i
half-life
The half-life in this example is about 30 seconds.
A more accurate value can be obtained be repeating this method for a other initial nuclei numbers and then taking an average.
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Question 6
0
100
200
300
400
500
600
700
800
900
0 10 20 30 40 50 60 70 80 90 100
time (seconds)
acti
vity
(B
q)
Estimate the half-life of the substance whose decay graph is shown opposite.
The half-life is approximately 20 seconds half-life
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Question 7The 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)
0 1 2 3 4 5 6 7 8
Mass (g) 650 493 373 283 214 163 123 93 71
The half-life should be about 2 hours:
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Choose appropriate words or numbers to fill in the gaps below:
The ________ of a radioactive substance is the average time taken for half of the _______of the substance to decay. It is also equal to the average time taken for the ________ of the substance to halve.
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 years time this should have fallen to ______ Bq. 11200 years later the activity should have fallen to ____ Bq.
The number of carbon 14 nuclei would have also decreased by ______ times.
eight half-life5600 425 activity1700
WORD & NUMBER SELECTION:
nuclei
eight
half-life
5600
425
activity
1700
nuclei
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Revision Simulations
Half-Life - S-Cool section on half-life and uses of radioactivity including an on-screen 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.
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Uses of radioactive isotopes
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Smoke detection
• Uses
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Thickness control
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Thickness control
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Used as Tracers
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Used as Tracers
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Killing microbes
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Killing microbes
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Checking welds
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Used as Tracers
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Carbon dating – write notes using the book page 265
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Summary sheet
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“Can you………?”
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Test!
Thursday
27th September 2012
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Can you answer the questions on pages 261 and
265?