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Radioactivity
Revision
History
• Wilhelm Conrad Roentgen
– Discovered x Rays
• Antoine Henri Becquerel
– Noticed the effects of Uranium on photgraphic
plates
• Marie Curie
– Coined the term Radioactivity
• Pierre Curie
– Shared Nobel prize with Marie and Becquerel
Radioactivity
• All substance are made of atoms.
These have electrons (e) around the
outside,
and a nucleus in the middle.
• The nucleus consists of
protons (p) and neutrons (n)
Radioactivity
• In some types of atom, the nucleus is unstable,
and will decay into a more stable atom. This
radioactive decay is completely spontaneous.
• You can heat the substance up, or subject it to high
pressure or strong magnetic fields - in fact, do
whatever you like to it - and you won't affect the rate
of decay in the slightest.
Definition
• Radioactivity is the
spontaneous breaking up of
certain unstable nuclei,
accompanied by the emission
of radiation
Types
• When an unstable nucleus decays, there are
three ways that it can do so.
It may give out:– an alpha particle (we use the symbol α)
– a beta particle (symbol β)
– a gamma ray (symbol γ)
• Many radioactive substances emit α particles and β particles as
well as γ rays.
In fact, you won't find a pure g source; anything that gives off g
rays will also give off a and/or b too.
Measuring Radiation
• Instrument: Geiger
- Muller tube
α, β or γ radiation
Ionises a gas,
causes an electric
current which is
amplified and acts
as a detector
Inside a GM Tube
alpha particle (α)
• Alpha particles are made of 2
protons and 2 neutrons. (Helium
Nuclei)
• This means that they have a
charge of +2, and a mass of 4
(the mass is measured in "atomic mass units",
where each proton & neutron=1)
• α particles are relatively slow and
heavy.
• They have a low penetrating
power - you can stop them with
just a sheet of paper.
• Because they have a large charge,
α particles ionise other atoms
strongly.
α particle emission
+
+
beta particle (symbol β)
• Beta particles have a charge of -1,
and a mass of about 1/2000th of a
proton. This means that beta
particles are the same as an
electron.
• They are fast, and light.
• β particles have a medium
penetrating power - they are
stopped by a sheet of aluminium
or plastics such as perspex.
• Beta particles IONISE atoms that
they pass, but not as strongly as
Alpha particles do.
β particle emission
-
gamma ray (symbol γ)
• Gamma rays are waves, not particles.
This means that they have no mass and
no charge.
• γ rays have a high penetrating power - it
takes a thick sheet of metal such as lead,
or concrete to reduce them significantly.
• γ rays do not directly ionise other atoms,
although they may cause atoms to emit
other particles which will then cause
ionisation.
– We don't find pure gamma sources - γ rays are emitted
alongside alpha or beta particles.
– Strictly speaking, gamma emission isn't 'radioactive
decay' because it doesn't change the state of the
nucleus, it just carries away some energy.
You need to know
Type of Radiation
Alpha particle
Beta particle
Symbol
α
β
Mass (atomic mass
units)
4
1/2000
0
Charge
+2
-1
0
Speed
slow
fast
very fast (speed of light)
Ionising ability
high
medium
0
Penetrating power
low
medium
high
paper
aluminium
lead
Stopped by:
Gamma ray
γ
(can look different,
depends on the font)
Loss of an alpha particle
• Radium-226 Radon-222 + alpha particle
•
226 Ra222 Rn
88
86
+42He
• Mass decreases by 4 [226222]
• Atomic number decreases by 2 [8886]
• Formed when a neutron decays into a proton
and an electron
Beta loss
• Carbon-14 nitrogen-14 + beta
• 146C  147N + 0-1e
• Mass number stays the same
• Atomic number increases by 1
Chemical & Nuclear Reactions
• Alpha decay of Americum 241 and the beta decay
of Carbon 14 are Nuclear Reactions
– Involve changes in the nucleus
• Chemical reactions involve bond making and
breaking ....electron allocation is changed
– Formation of compounds
– Compounds breaking up
– Rearranging aa compound
•
Note: Chemical reactions cannot result in an element changing into another element
Uses of Radiation
• Smoke Detectors
• Smoke alarms contain a weak
source made of Americium-241.
• Alpha particles are emitted from
here, which ionise the air, so that
the air conducts electricity and a
small current flows.
• If smoke enters the alarm, this
absorbs the a particles, the
current reduces, and the alarm
sounds.
• Am-241 has a half-life of 460
years
Uses
• Thickness Control
• In paper mills, the
thickness of the paper
can be controlled by
measuring how much
beta radiation passes
through the paper to a
Geiger counter.
Uses
• Sterilising
• Even after it has been
packaged, gamma rays can
be used to kill bacteria,
mould and insects in food.
• This process prolongs the
shelf-life of the food, but
sometimes changes the
taste.
• Gamma rays are also used
to sterilise hospital
equipment, especially
plastic syringes that would
be damaged if heated.
Uses
• Radioactive Dating
• Animals and plants have a
known proportion of Carbon-14
(a radioisotope of Carbon) in
their tissues.
• When they die they stop taking
Carbon in, then the amount of
Carbon-14 goes down at a
known rate
(Carbon-14 has a half-life of 5700 years).
• The age of the ancient organic
materials can be found by
measuring the amount of
Carbon-14 that is left.
Uses
• Cancer Treatment
• Because Gamma rays can
kill living cells, they are
used to kill cancer cells
without having to resort
to difficult surgery. This is
called "Radiotherapy",
and works because cancer
cells can't repair
themselves when
damaged by gamma rays,
as healthy cells can.
Radioactive isotopes
• For a start, just because something is called an isotope doesn't
necessarily mean it's radioactive.
•
You can think of different isotopes of an atom being different
"versions" of that atom.
• Consider a carbon atom. It has 6 protons and 6 neutrons - we call it
"carbon-12" because it has an atomic mass of 12 (6 plus 6).
If we add a neutron, it's still a carbon atom, but it's a different
isotope of carbon.
• One useful isotope of carbon is "carbon-14", which has 6 protons
and 8 neutrons. This is the atom we look for when we're carbon
dating an object.
Radioisotopes and half-life
• Unstable radioactive isotopes are called radioisopoes
• C12 is stable
• C14 is unstable
• Stable Isotopes become less common as the number of
Neutrons in the atom increases...all isotopes of
elements with atomic numbers >83 are radioactive
Half Life
• Point a Geiger counter at a radioactive
substance for a period of time and the
reading on the metre decreases as you
watch. This is shown on the graph.
• The radioactivity from some
substances dies away very fast perhaps in a few microseconds. Others
take thousands of years before you'll
notice that the radioactivity had
decreased at all.
• In theory, every radioactive substance
should stay slightly radioactive for ever
- the graph should never actually fall to
zero. This means that we can't usefully
talk about the "life" of a radioactive
source.
Half Life
8000
• Instead, we use the 7000
idea of "half-life".
6000
This is the time it
5000
takes for the
radioactivity to fall 4000
3000
by half.
2000
Half life of 2 hrs
Reading = 8000 @ time 0
Half life of 2 hrs
Reading = 4000 @ time 2
Half life of 2 hrs
Reading = 4000 @ time 6
1000
0000
0 1 2 3 4 5 6 7 8 9 10 11 1 2
Background Radiation
• Mainly natural radioactivity, all around us.
• Vast majority of our annual dose comes from
radon gas (emits alpha particles) , food & drink,
the ground, and cosmic rays (which are gamma rays
coming in from space).
• Unless you are having radiotherapy, your dose
from medical sources is quite low.
• Many people don't realise that your radiation
dose from cosmic rays is increased considerably if
you fly a great deal.
Background Radiation
Source
Radon Gas
Rocks and Soil
Food and Drink
Cosmic Rays
Weapons test fallout
Medical Sources
Nuclear Waste
%
60
10
10
10
0.2
9
0.1
That’s it