Lesson 2 Types of radiation and decay..

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Transcript Lesson 2 Types of radiation and decay..

Electromagnetic spectrum
Mini-book – Due 17th September
• One type of EM wave for each page
• Each page should contain the wavelength
• Each page should contain uses and other
interesting information (including dangers)
• Attractive to look at (and funny?)-
Last year June 2012
• describe the structure of an atom in terms of
protons, neutrons and electrons and use
symbols to describe particular nuclei
• Rutherford’s alpha scattering experiment
• understand the terms atomic (proton) number,
mass (nucleon) number and isotope
Radioactivity
The atom
orbiting electrons
Nucleus (protons
and neutrons)
Nuclide notation
Atomic mass (mass number)
= number of protons and
neutrons
7
Li
3
Atomic number (proton number)
= number of protons
Isotopes
It is possible for the nuclei of the same element
to have different numbers of neutrons in the
nucleus (but it must have the same number of
protons)
7
6
3
3
Li
Li
Isotopes
For example, Lithium atoms occur in two forms,
Lithium-6 and Lithium-7
4 neutrons
3 neutrons
7
6
3
3
Li
Li
Relative atomic mass
On average, lithium atoms have a mass of 6.941
(relative to Carbon 12)
6.941
3
Li
Let’s try some questions.
Do Now!
1.Date &
Title –
Properties of Radiation
2. Do exam question 11 and
stick in your book
Unstable nuclei
Some nuclei are unstable, for example
Uranium 235
Hi! I’m uranium-235 and I’m
unstable. I really need to lose
some particles from my
nucleus to become more
stable.
Unstable nuclei
To become stable, an unstable nuclei
emits a particle
Weeeeeeeeeeeeee!
Unstable nuclei
We say the atom has decayed
Weeeeeeeeeeeeee!
Unstable nuclei
The decay of an unstable nucleus is random. We know it’s
going to happen, but we can’t say when! It cannot be
affected by temperature/pressure etc.
Weeeeeeeeeeeeee!
Today’s lesson
• 7.4 understand that alpha and beta particles and gamma
rays are ionising radiations emitted from unstable
nuclei in a random process
• 7.5 describe the nature of alpha and beta particles and
gamma rays and recall that they may be distinguished in
terms of penetrating power
• 7.6 Begin to describe the effects on the atomic and mass
numbers of a nucleus of the emission of each of the
three main types of radiation
You’ll learn
about some
really weird
ones in year 12!
3 types of particle
There are 3 (at least in
IGCSE!) types of
particles that can be
ejected from an
unstable nuclei.
Please fill in the table with
the information in the
Powerpoint
Ionisation – Please copy
Ionisation occurs when an
atom loses or gains one or
more electrons.
Lithium atom
(uncharged)
When an atom loses electrons
it becomes a positive ion.
When an atom gains electrons
it becomes a negative ion.
Lithium ion
(positively charged)
Alpha particles
α
Alpha particles
•
•
•
•
•
2 protons and 2 neutrons joined together
The same as the nucleus of a helium atom
Stopped by paper or a few cm of air
Highly ionising
Deflected by electric and strong magnetic
fields
2+
4
2
He
Alpha Decay
Atomic mass goes down by 4
235
231
92
90
U
Th +
Atomic number goes down by 2
α
Beta particles
β
Beta particles
•
•
•
•
Fast moving electrons from the nucleus
Stopped by about 3 mm of aluminium
Weakly ionising
Deflected by electric and magnetic fields
0
e
-1
Beta decay
• In the nucleus a neutron changes into an
electron (the beta particle which is ejected)
and a proton (which stays in the nucleus)
• During beta decay the mass number stays
the same but the proton number goes up
by 1.
231
Th
90
231
0
Pa + -1e
91
Gamma rays
Gamma rays
• High frequency electromagnetic radiation
• several cm of lead or thick concrete will
reduce the intensity.
• Very weakly ionising
• NOT affected by electric or magnetic fields
Gamma rays
Associated with alpha decay
235
231
92
90
U
Th +
α
Deflection by electric fields
-
-
-
Alpha and beta particles are
deflected in opposite
directions due to their
opposite charges.
Due to their much larger
mass alpha particles are
deflected far less than beta.
+ + +
Electric field produced by
positively and negatively
charged plates
Gamma rays are not
deflected because they are
not charged.
Deflection by magnetic fields
S
Magnetic south pole
placed behind the rays
Alpha and beta particles are
deflected in opposite
directions due to their
opposite charges.
Due to their much larger
mass alpha particles are
deflected far less than beta.
Gamma rays are not
deflected because they are
not charged.
Stick your table in your books
• please complete
information about
speed and mass from
the text book
The penetrating power of
alpha, beta and gamma radiation – Please copy
Paper or a few
cm of air stops
alpha particles
1cm or 1m of air
of aluminium
stops beta
particles
Several cm of lead or
1m of concrete
reduces the intensity
of gamma rays
Alpha, beta and gamma radiation
An alpha particle consists of two protons
and two neutrons.
It is strongly ionising.
A beta particle is a high speed electron.
It is produced when a neutron has decays into
an electron and proton.
It is moderately ionising.
Gamma rays are very high frequency
electromagnetic waves.
They are produced when an unstable nucleus
loses energy..
They are weakly ionising.
Let’s try some questions.
Do L15.1
ionizing
radiation and
question 2
page 253
Rayneritis!