radioactivity ppt.PPT

© Boardworks Ltd 2001
KS4 Radioactivity

Radioactivity
Contents
What is Radioactivity?
Radioactivity in the environment.
Types of Radioactivity.
Half-Life.
Effects of Radioactivity.
Uses of Radioactivity.
Questions.

Radioactivity
What is it?
Radioactivity occurs as a result of changes in the
nucleus (the centre) of an atom.
Atoms which give out radioactivity are called radioactive
isotopes.
The nucleus of a radioactive isotope is unstable.
It can become stable by emitting small particles and
energy.
These particles and energy are called radioactivity.

Radioactivity
Atoms can achieve stability in two ways:
Alpha decay and beta decay.
ALPHA () DECAY
Alpha() decay
Uranium-238 Thorium-234
-particle
(2 neutrons
and 2 protons so
4 units of mass
lost from the
nucleus)
-ray
(an electromagnetic wave like light)

Beta() decay
-particle
(an electron)
Carbon-14
(Atomic number 6)
Nitrogen-14
(Atomic number 7)
Radioactivity
BETA Decay ()
One proton is GAINED so the element
changes to the next one in the Periodic table.

Radioactivity
How do we detect it?
Photographic film
Cloud chamber
Gold-leaf electroscope
Spark counter
Geiger-Muller Tube - this is the detector we
mostly use in the laboratory.

Mica window
Argon gas
Geiger-Muller Tube
counter
Collision &
ionisation
radiation
124
125

The Geiger Muller Tube
The detector is a metal tube filled with gas. The tube
has a thin wire down the middle and a voltage
between the wire and the casing.
When the radioactivity enters
the tube, it ionises the gas in
the tube. This produces a
pulse of current which is
amplified and passed to a
counter.

The Spark Detector
The spark detector consists of a metal grid and a metal strip. A
high voltage is applied between the grid and the strip. The
voltage is increased until electrical arcing (sparking) across the
gap just occurs.
When ionising radiation ( and  radiation, see later) is placed
close to the detector there is a marked increasing in the amount
of sparking.
High voltage
supply

Radioactivity and the environment -
background radiation
Natural background radiation comes from several
sources:
From the sun and space – this radioactivity is known as
cosmic rays.
From naturally occurring unstable isotopes in rocks,
building materials and food etc.
From human activity. For example, from nuclear waste
and explosions. This is a very small proportion of the
total background radiation.
granite

12%
12%
14%
10%
52%
radon and thoron gas
food
medical x-rays
rocks and building materals
cosmic rays
The Relative Proportions of Background
Radiation.

Types of Radioactivity
Alpha particles  - these are the big radioactive
particles and are heavy and slow.
Alpha particles are helium nuclei (2 protons and 2
neutrons). They are positively charged with a charge
of +2.
Beta particles  - these are fast moving
electrons. They are small and light and carry a
negative charge of –1.
Gamma rays - these are very high frequency
(and therefore high energy) electromagnetic waves.
Being electromagnetic waves they are
UNCHARGED.

Thin mica Thin aluminium
stops BETA
Thick lead
stops GAMMA
Skin or paper
stops ALPHA
How to block the three types of radiation.




Radioactivity
How can we block radiation?
 particles are stopped by paper or skin.
 particles are stopped by thin aluminium.
 rays are stopped by thick lead.

Penetrating and Ionising Power
 particles are big and slow moving so they don’t
penetrate very well and are stopped easily.
Because they are big and heavy they are strongly
ionising - they easily knock electrons off atoms.
 particles are quite small and quite fast so they
penetrate moderately. They are moderately ionising.
 rays penetrate a long way without being stopped.
However, they are only weakly ionising as they tend
to pass through atoms rather than knocking off
electrons.

The half-life of a radioactive isotope is the time
taken for half the atoms to decay.
How many half lives would lead to 1/16 of the
original atoms?
The graph on the next slide shows a decay curve for
a radioactive material.
After 1 half life, half the atoms have disintegrated and
only half remain. After another half-life, the activity
has halved again so only a quarter of the original
atoms survive.
Half Life

activity
time
Graph to show how the activity of a
radioactive source changes with time.
One half-life, the original activity
has halved.
Two half lives, the
activity is now at a
quarter of its original
level.
The original activity.
100
50
25

Effects of Radioactivity
Alpha beta and gamma cause ionisation in living
cells. This can damage or kill the cell.
Low doses of this radiation can cause damage or
create mutant cells.
Mutant cells can divide in an uncontrolled manner.
This is cancer.
Higher doses of radiation kills cells, which causes
radiation sickness.

Alpha particles do not pass through the skin.
However, inside the body they can damage cells in
the local area. Therefore, substances that emit
alpha particles are dangerous if inhaled or ingested.
Beta and gamma rays are dangerous when outside
the body as they can penetrate the skin and
damage the vital organs.
Effects of Radioactivity

Uses of Radioactivity
Sterilisation
Radiotherapy
Leak detection in pipes
Thickness/level control
Tracers in medicine
Dating rocks
Nuclear Power

Sterilisation
Gamma rays are used to kill bacteria, mould and insects in
food. This can be done even after the food has been
packaged. It can affect the taste, but supermarkets like it
because it lengthens the shelf life.
Gamma rays are also used to kill bacteria on hospital
equipment. It is particularly useful with plastic equipment
that would be damaged by heat sterilisation.
Gamma Source
unsterilised sterilised

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.

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 radioactive isotope must be a
gamma emitter so that it can be detected
through metal and earth. Alpha and beta
rays would be blocked.
The isotope must
have a short half
life so the material
does not become
a long term
problem.

Hydraulic
ram
detector
Thickness Control Mill
Electronic
instructions
to adjust rollers.
Beta Source

Thickness/level control - how it
works
A radioactive source is on one side of the material
and a detector on the other.
If too much radioactivity is getting through, then the
material is too thin and the rollers open up a bit to
make the material thicker.
If not enough radioactivity is detected then the
rollers compress to make the material thinner.
This method is used in the manufacture of lots of
sheet materials: plastics, paper, sheet steel.

Thickness/level control
Why is a beta source used in paper mills and a gamma
source in sheet steel ?
Why is it important for the source material to have a
long half-life?
click
click
The radiation must be able to pass through the
material. Beta radiation will pass through paper but
not steel.
A source with a long half-life will last a long time
before it needs to be replaced.

Tracers in Medicine
Certain radioactive isotopes at low concentrations
are injected into people (or swallowed). The location
of the isotope can then be observed in the body
using a detector. This way any blockages or
absorption rates can be checked.
Which type of radioactive source would you prefer in
your body?
What sort of half-life would be best?
Gamma so that it does not cause too much cell
damage.
Very short, a few hours so that the radioactivity decays
quickly.
click
click

Carbon Dating
All living things take in a little radioactive carbon-14
in photosynthesis, as well as the normal carbon-12.
When living things die, they stop taking in carbon-14
and the carbon-14 present at death slowly decays to
carbon-12 (half-life is 5600 years). The radioactivity
due to the decay of carbon-14 can be used to date
bones, wood, paper and cloth.

Example
A fresh bone gives a radioactive count of 170
counts per minute. Another ancient bone of the
same mass gives a count rate of 50 counts per
minute. The background count is 10 counts per
minute. How old is the bone?
Counts due to bones are 170 - 10 = 160 (fresh) and
50 - 10 =40 (ancient)
The count rate of the carbon-14 has fallen to one quarter of
its original value, i.e. 160/2 = 80, 80/2=40.
This is two half lives,
So the bone is 5600 x 2 =11200 years old.
click

Nuclear Power
When a nucleus decays it gives out heat energy - that’s
what keeps the centre of the earth hot!
In a nuclear power station, uranium-235 atoms decay and
give out energy and neutrons.
Each time a uranium atom splits it produces 3 neutrons
These go on to hit other uranium atoms, which causes
them to decay. A chain reaction is set up where more and
more energy is released. In a nuclear reactor the process
is carefully controlled so that neutrons are absorbed
harmlessly and the energy released is controlled.
In a nuclear bomb the reaction is not controlled, and the
bomb explodes!

Nuclear Power - fission
Fast neutron
from previous
decay
Kr
Ba
n
n
n
Uranium

Nuclear Power
Kr
Ba
n
n
n
n
Uranium

Questions
1. A certain radioisotope has a half-life of 3 minutes.
What fraction of the original atoms are still
unchanged after:
a) 3 minutes
b) 6 minutes and
c) 9 minutes?
a) after 3 minutes (1 half life) half of the original atoms
remain.
b) after 6 minutes (2 half lives) one quarter of the
original atoms remain (1/2 x 1/2).
c) after 9 minutes one eighth of the original atoms
remain (1/2 x1/2 x1/2).
click

2. What percentage of the original activity of a
radioactive substance remains after four half-lives
have passed?
Four half lives, so fraction of original activity is
1/2 x1/2 x 1/2 x 1/2 =1/16
% = 1/16 x 100 =6.25%.
click
3. A radioactive source has an activity of 1.5 MBq.
How many decays would occur in 1 hour?
(Note: 1Bq= 1 decay per second).
1500000 decays every second. Therefore in one hour
there will be:
60 x 60 x 1500000 decays
= 5.4 x 109 decays.

4. In a laboratory where the background count is 25
c.p.m. the uncorrected count rate from a radioisotope falls
from 960 c.p.m to 54 c.p.m over 1 hour 15 minutes. What
is the half-life of the isotope?
Starting count due to radioisotope is 960-25 =935
After 75 minutes, end count rate is 54 -25 =29
29/935 approx 1/32 which is 5 half lives
75 mins is 5 half lives so 1 half-life is 75/5 =15 minutes
5. The half-life of a radioisotope is 3.7 days. How long
would it take for the activity of a sample to fall to one
sixty-fourth of its original value?
1/64 is 6 half-lives (1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2)
6 x 3.7 = 22.2 days.
click

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