Post on 28-Dec-2015
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
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
QuestionShow the equation for Plutonium 239 (Pu) decaying by alpha emission to Uranium (atomic number 92).
Pu239
94U
235
92α
4
2+
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
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
QuestionShow the equation for Sodium 25 (Na), atomic number 11, decaying by beta emission to Magnesium (Mg).
Na25
11Mg
25
12 β-
0
-1+
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.
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
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)
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
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
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
half-life (t½)
Number of nuclei undecayed
timeA graph of the count rate against time will be the same shape
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
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
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
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
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
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
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
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
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
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
Experiment Dicium 25
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.
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
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:
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
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.
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?