Transcript Radioactivity I

Radiation True or False?
1. You are being bombarded with radiation right now.
2. You are giving off radiation right now
3. Your breakfast was irradiated with deadly radiation
4. You can see radioactivity
5. You can feel radioactivity
True
True
True ??
False
False
6. Radioactive substances are only harmful if you touch them
7. If you are irradiated then you become radioactive
8. Radioactivity is not very useful
9. We completely understand what causes radioactivity
10. Do you know the three types of radioactivity. Write them
down..
False
False
False
False
?
DP 5.1: Distinguish between stable and radioactive
isotopes and describe the conditions under which the
nucleus is unstable.
Alpha radiation -  or 42He
Description:
2 neutrons, 2 protons (helium nuclei)
Electric Charge:
+2
Relative Atomic Mass:
4
Penetration power:
Stopped by paper or a few cm of air
Helium nuclei
?
Ionisation effect:
Strongly ionising
Effects of Magnetic/Electric Field:
Deflected towards the negative
Beta radiation -  or 0-1e
Description:
High energy electron
Electric Charge:
-1
Relative Atomic Mass:
1/1860
high energy electron
Penetration power:
Stopped by few mm of aluminium
Ionisation effect:
Weakly ionising
Effects of Magnetic/Electric Field:
Strongly deflected towards the positive
positron radiation - or 01e
Description:
High energy electron
Electric Charge:
+1
Relative Atomic Mass:
1/1860
high energy positron
Penetration power:
Stopped by few mm of aluminium
Ionisation effect:
Weakly ionising
Effects of Magnetic/Electric Field:
Strongly deflected towards the negative
Gamma radiation - 
Description:
High energy electromagnetic radiation
Electric Charge:
0
Relative Atomic Mass:
0
Penetration power:
Electromagnetic radiation
Reduced by several cms of lead or
several metres of concrete
Ionisation effect:
Very weakly ionising
Effects of Magnetic/Electric Field:
NO deflection
The penetration power of the three types
of radiation.



Thin mica
Skin or paper
stops ALPHA
Thin aluminium
stops BETA
Thick lead
reduces GAMMA
Which type of radiation is…..
1. The most penetrating?
Gamma
2. The least penetrating?
Alpha
3. Least dangerous outside the body?
Alpha
4. Most dangerous inside the body?
Alpha
5. High energy electrons?
Beta
6. Has a negative charge?
Beta
7. Is weakly ionising?
Beta
8. Has zero charge and zero mass?
Gamma
9. Only reduced in intensity by lead and concrete?
Gamma
So what exactly does cause this radiation?
First we need to look at the structure of the atom
Draw diagrams to represent:
How did you go?
6 protons
6 protons
6 protons
6 neutrons
7 neutrons
8 neutrons
6 electrons
6 electrons
6 electrons
What do we call these?
6 protons
6 protons
6 protons
6 neutrons
7 neutrons
8 neutrons
6 electrons
6 electrons
6 electrons
Isotopes
Atoms of the same element containing the same number
of protons but different numbers of neutrons in their
nuclei.
Because they have the same number of electrons there
is NO difference to their chemical behaviour.
Stable and unstable Isotopes
• Over 2000 different isotopes have been
discovered
so far.
• If the nucleus of the isotope spontaneously emits
radiation it is said to be unstable or radioactive.
• Unstable isotopes are called radioisotopes (or
radioactive isotopes).
• Radioactive isotopes are unstable. They emit
radiation as they spontaneously release energy.
This is called radioactive decay. .
.
Stable and unstable Isotopes
p= n
For light elements,
stable nuclei have a
proton:neutron ratio
close to 1:1.
For heavy elements
the stable nuclei have a
proton:neutron ratio
close to 1:1.5
Most nuclei out of these
ranges are unstable
Stable and unstable Isotopes
All nuclei with p >83 are
unstable.
alpha emission
• When a nucleus emits an alpha particle it loses
2 protons and 2 neutrons, the same as a helium nucleus.
• Gamma rays are often emitted along with alpha decay.
Radon, plutonium, polonium all have alpha emitting
isotopes.
• Example:
•
238 U
92
4 He
2
+
234
90Th
• Now try these…
•
220 Rn
86
4 He
2
+
216 Po
84
•
239 Pu
94
4
+
235
2He
92U
beta emission
• During beta decay, the nucleus emits an electron. But how?
• A neutron decomposes into a proton and an electron, as
follows…
1
1
0n
+1p
+ 0-1e
• Èxample:
60
• Now you try:
0 e +
• 146C
-1
•
3
1H
0
0
27Co
-1e +
14 N
7
3
2He
-1e +
60
28Ni
Beta decay is also
accompanied by gamma
ray emission. Gamma
rays are not emitted on
their own but accompany
alpha or beta decay
positron emission
• During positron, the nucleus emits a positron. But how?
• A proton decomposes into a neutron and an electron, as
follows…
1 P
1
1
on
+ 01e
• Èxample:
38
• Now you try:
0 e +
• 127N
1
•
48
23V
0
0
19K
1e +
1e +
12 C
6
48
22Ti
38
18Ar
During alpha decay, which of the following is true?
A.
Relative atomic mass increases by 2
B.
Relative atomic mass decreases by 2
C.
Relative atomic mass increases by 4
D.
Relative atomic mass decreases by 4 
During beta (-) decay, which of the following is true?
A.
Atomic number increases by 1 
B.
Atomic number decreases by 1
C.
Atomic number increases by 2
D.
Atomic number decreases by 2
Now you try…
• Pg93, Q1, 2, 3 + 5.
Half-life
• The time taken for half of the number of atoms in a
sample of radioisotope to decay is called its half-life.
Eg.
• The half-life of fluorine-20 is 11 seconds.
• The half-life of carbon-14 is 5.7x103 years.
• The half-life of uranium-238 is 4.5x109 years.
Radioactive half-life
What is the half life of Carbon-15?
Number of carbon-15 atoms
Carbon 15 / g
present (x 108)
Decay of carbon 15
10
8
6
4
2
0
0
1
2
3
4
5
6
Time / s
7
8
9
10
Radioactive half-life
The average time taken for half of the substance to decay is
called the radioactive half-life.
Carbon
15 / g
left undecayed
% Atom
Decay of carbon 15
100
10
80
8
60
6
40
4
20
2
0
0
0
1
2
3
4
5
6
Time / s
7
8
9
10
DP 5.5
Identify one use of a named radioisotope:
in industry
in medicine
DP 5.6
Describe the way in which the above named
industrial and medical isotopes are used and
explain their use in terms of their properties.
Thickness Control Mill
radioactive
IfAnot
enough
source is onisone
radioactivity
side of the
detected
then
material
the
rollersand a
detector on
compress
to the
other.the
make
material thinner.
If too much
radioactivity
is
This
method is
getting
through,
used
in the
then the material
manufacture
of
is too
thin and
lots
of sheet
the rollers open
materials:
up a bit paper,
to make
plastics,
the material
sheet
steel.
Hydraulic
thicker.
ram
Beta Source
detector
Electronic instructions to adjust rollers.
Leak detection in pipes
The radioactive isotope is injected into the pipe. Then the
outside of the pipe is checked with a Geiger-Muller detector,
to find areas of high radioactivity. These are the points
where the pipe is leaking. This is useful for underground
pipes that are hard to get near.
The isotope must
have a short half life
so the material does
not become a long
term problem.
The radioactive isotope must be a gamma emitter so that it can be
detected through the metal and the earth where the pipe leaks.
Alpha and beta rays would be blocked by the metal and the earth.
Cobalt-60 Sterilisation
Gamma rays are used to kill bacteria, mould and insects in ood.
Also used to kill bacteria on hospital equipment.
This is useful particularly on packaged food or on plastic items
which would be damaged by heat sterilisation.
It can affect the taste and the vitamin content, but it lengthens the
shelf life.
unsterilised
Gamma Source
sterilised
Sterilisation
Cobalt-60 is used as it is a gamma emitter – very penetrating.
It has a half life of 5.3 years so the machines can run cheaply without
regular maintenance.
You don’t need external power to produce the gamma rays as you do
with x-rays
Cobalt-60 is held in a chemically inert form in a sealed container.
When the cobalt-60 is exhausted it can easily be replaced.
unsterilised
Gamma Source
sterilised
Named radioisotopes - industry
• Industry - cobalt-60
• Cobalt-60 is used to sterilise food because when it decays by
beta decay it also releases gamma radiation.
• Gamma rays are used to kill bacteria, mould and insects in food.
They are also used to kill bacteria on hospital equipment,
dressings and bandages.
• This is useful particularly on packaged food or on plastic items
which would be damaged by heat sterilisation.
• There are arguments for using cobalt-60 to sterilise food are that
it prolongs freshness and so reduces wastage. Arguments
against say that it does not effectively kill all bacteria and
destroys vitamin content. It may also cause harmful products in
the food.
• Cobalt-60 is used because It has a half life of 5.3 years so the
machines can run cheaply without regular maintenance. It
produces low energy gamma rays which do not make the food
radioactive.
• You don’t need external power to produce the gamma rays as
you do with x-rays
Where does it come from?
•Co-60 is artificially generated.
Co-59 is bombarded with a neutron in a nuclear reactor
59Co
60Co
+ 01n → 27
27
Co-60 then decays by negative beta
60Co → 59Ni + 0B
42
43
-1
the Ni-59 them emits two gamma
59Ni → 59Ni + γ + γ
43
43
The gamma rays are used to kill bacteria / microbes
Use of Technetium 99m in
medicine
Tc-99m is bound to a carrier
molecule
Tc-99m
Technitium-99m is useful because it is a
transition metal and chemically binds easily
with non-bonding electron pairs on atoms (such
as N) in a range of biological molecules which
are specific to different organs.
It can also be bound to immune system proteins
which bind to cancer cells.
It also has a half life of 6 hours so it does not
expose the patient to radiation for any significant
length of time.
In all other respects the carrier molecule is
identical to normal biological equivalent
Tc-99m
The biological molecule is introduced into the
body
The Tc-99m is carried to the specific organ
(depends on the molecule)
Special gamma cameras then take pictures of the
gamma rays emitted by the technetium
Higher signals will be recorded where there is a build up of the
radioisotope either in a blood clot, tumour, or constrictions in blood
vessels.
The benefits: surgeons and doctors can make an
accurate diagnosis without performing surgery
•The problems:
Tissue damage – leading to sickness and ultimately
death for high exposure
Can cause cancer – leukemia/lung – up to 20 years after
treatment
Genetic damage causes deformities in offspring.
Where does it come from?
•Tc-99m is artificially generated but has a ½ life of only 6 hours
which means it is impractical to deliver it to a hospital.
Mo-98 is bombarded with a neutron in a nuclear reactor
98Mo + 1n → 99Mo
42
0
42
Mo-99 is delivered to hospitals where it decays ½ = 66 hrs
99Mo → 99mTc + 0B
-1
42
43
the Tc-99m is chemically extracted and used
99mTc → 99Tc + γ
43
43
The gamma ray is v. low energy and therefore very safe – weakly
ionising
PM S5 P6 – Use available evidence to analyse
benefits and problems with the use of radioactive
isotopes in identified industries and machines.
Benefits:
Problems:
•Non-invasive diagnostic
procedures
•Tissue damage for people
exposed
•Treatement of cancers
•Risk of cancer if exposed
•Sensitive monitoring of
inductrial processes
•Genetic damages to
people exposed
•Sterilisation
•Hard to dispose of some
isotopes (long half-life)
•Non-invasive examination
of pipes / aircraft etc.
More benefits and problems
• In either medicine or industry the problems associated
with radiation is its effect on living cells.
• Radiation changes the structure of enzymes so they
cannot acts as catalysts.
• The structure of membranes can be changed preventing
transport in and out of the cell.
• The structure of DNA molecules can be altered so that it
cannot function correctly.
• Sex cells can be altered so that changes can result in
defects in offspring.
• Other problems are that some isotopes are made in
nuclear reactors so there is the problem of waste products
to be dealt with. Also security issues concerned with
transport and storage of nuclear waste.
Radiotherapy
A carefully controlled beam of gamma rays can be
used to kill cancer cells. It must be directed
carefully to minimise the damage to normal cells.
However, some damage is unavoidable and this
can make the patient ill.
It is therefore a balancing act - getting the dose
high enough to kill the cancerous cells, but as low
as possible to minimise the harm to the patient.
Describe how transuranic elements are produced
• Transuranic elements have an atomic number
greater than 92 [uranium].
• These elements are synthesised in nuclear reactors
and particle accelerators (cyclotrons).
• Most are radioactive and only exist for short periods.
• In a nuclear reactor the nucleus to be used is
bombarded with neutrons produced by uranium decay.
Neptunium and americium are produced this way.
• In a cyclotron, target nuclei are bombarded with positive
particles such as protons or atoms of helium or carbon,
at great speed, until they fuse together on collision.
• Very high speeds are necessary because the positive
particles have to fuse with a positive nucleus and the
two experience repulsion when near each other. High
speeds help to overcome this barrier.
Describe how transuranic elements are produced
• Californium is produced in an accelerator in
the following way:
•
238
92U
+ 126C
246
98Cf
+ 4(10n)
• In both cases the new nucleus formed will be
unstable and starts to emit particles and/or
radiation in order to become stable.
Describe how transuranic elements are produced
• Neptunium is produced in a reactor:
238 U + 1 n
239 U
239 Np +
92
0
92
93
0
-1e
(Uranium-238 is not fissile [will not split on bombardment
with neutrons] so forms uranium-239 when hit with a
neutron). This rapidly decays to neptunium by beta decay.
• Neptunium rapidly decays to plutonium by beta decay
239 Np
239 Pu + 0 e
93
94
-1
(Plutonium can be used to make americium. It is used in
energy generation and nuclear weapons. It was also used
as a fuel in the long distance craft Voyager and Voyager II,
which went on missions to Jupiter, Uranus, Neptune and
Pluto. It has a half-life of almost 25,000 years).
Describe how transuranic elements are produced
• Americium is produced in a reactor:
239 Pu + 2(1 n)
241 Am + 0 e
94
0
95
-1
• Americium is used in most domestic fire alarms
• Americium has a half-life of 432 years.
• It decays by alpha decay to neptunium and also
releases low energy gamma rays, so very, very
small amounts are used in fire alarms
(0.2microgram)
• Both neptunium and americium are artificial
elements made in reactors
Describe how commercial radioisotopes are
produced
• There are about 20 widespread commercial radioisotopes in
use, produced by cyclotrons and nuclear reactors.
• An accelerator is a machine that allows positive particles
[protons, small nuclei] to be accelerated to high speed, fired
at nuclei of atoms with controlled energies in order to study
nuclear reaction or make radioisotopes. Accelerators
produce neutron-deficient isotopes like fluorine-18.
14
7N
+
4
2He
18
9F
• Fluorine-18 is used in the treatment of cancers.
• Cyclotrons are found in major hospitals in Sydney,
Melbourne and Brisbane to produce useful radioisotopes
with short half-lives
• Iodine-123 is also made in a cyclotron.
Describe how commercial radioisotopes are
produced
• A nuclear reactor is a device that allows a uranium
chain reaction to occur safely, releasing neutrons at a
slow and controlled rate. A target is bombarded with
neutrons to produce a radioactive species with extra
neutrons. Nuclear reactors produce neutron-rich
isotopes: iodine-131, strontium-90, cobalt-60.
• Example:
59 Co
27
+ 10n
60 Co
27
• Example: Technetium-99m is formed from molybdenum99, which is a fission product of uranium-235
99 Mo
99 Tc + -1 e
42
43
0
• Tc-99m is an important medical diagnostic isotope
Commercial radioisotopes – nuclear
reactors
• technetium-99m: is formed by the decay of
molybdenum-99, which is a fission product of uranium235 (when uranium-235 atoms are split when
bombarded by neutrons). Molybdenum has a half life of
66 hours so it can be made at Lucas Heights reactor
and transported to hospitals all over Australia.
Technetium-99m has a half life of 6hrs so is suitable to
use in medical diagnosis as it decays rapidly causing
minimal damage to the patient. It can be attached to
biological molecules [eg. incorporated into blood serum]
and used to detect blood clots, tumours, damaged heart
tissue.
Commercial radioisotopes – n/reactor
•Americium-241: recovered from nuclear waste, used
in smoke alarms
•Plutonium-238: fuel for space probes
•Strontium-90: produces beta particles. Used in
industry to monitor paper/card thickness. A long halflife means that it doesn’t need replacing too often
• Cobalt-60: produced when cobalt-59 is bombarded
with a neutron. cobalt-60 decays and emits beta
particles and gamma rays. Has medicinal and
industrial uses.