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22.1 X-rays and radioactivity
Discovery of radioactivity
Do you know how
radioactivity is discovered?
 In 1895, Wilhelm Roentgen
discovered that rays are produced
when a heavy metal target is struck
by a beam of electrons.
 It was called X-rays.
 It could penetrate through solid
materials.
22.1 X-rays and radioactivity
 In 1896, Henri Becquerel
discovered radiation from
uranium salts that was
similar to X-rays.
 It could cause a photographic
plate wrapped inside the black
paper to blacken.
22.1 X-rays and radioactivity
Substances that emit this radiation are called
radioactive.
The phenomenon is known as radioactivity.
 The radiation is emitted when changes occur
within the nuclei of the atoms of the
radioactive substances.
 The radiation is called nuclear radiation.
22.1 X-rays and radioactivity
Types of nuclear radiation
 radiation  radiation
 radiation
Nature
helium nuclei
fast-moving
electrons
EM wave with
very short
wavelength
Speed
5% of
light speed
up to 90% of
light speed
light speed
Charge
+2e
e
neutral
largest
much smaller
than  particles
no mass
Mass
22.1 X-rays and radioactivity
 Wavelength of  radiation
 wavelength of X-ray
= 10-12 m
  radiation is emitted from the nuclei of atoms
while X-rays are not.
22.1 X-rays and radioactivity
Comparison of nuclear radiation
Different radiations have different:
 Ionizing power
 Range in air
 Penetrating power
 Deflection in electric field
 Deflection in magnetic field
22.1 X-rays and radioactivity
Ionizing power
 A gas is ionized when one or more electrons
are removed from some of its atoms.
 Ionization produces ion pairs which consist
of positive ions and free electrons.
electron
positive ion
22.1 X-rays and radioactivity
The ionizing power of nuclear radiation
– measure of the number of ion pairs produced
per unit length travelled by the radiation.
 Ionizing power of nuclear radiation:
 radiation >  radiation >  radiation
 An  particle can produce 100 times as many
ion pairs as a  particle for the same distance
travelled in air.
22.1 X-rays and radioactivity
Range in air
Energy is
required to
ionize air.
Nuclear
radiation loses
energy while
travelling in air.
It eventually
stops.
 Range = distance travelled by nuclear radiation
 The stronger the ionizing power of nuclear
radiation, the shorter the range in air.
22.1 X-rays and radioactivity
 Range of nuclear radiation:
 radiation <  radiation <  radiation
Radioactive
source
 radiation: strongest ionizing power 
shortest range in air
5 cm
 radiation: weaker
6m
ionizing power  longer
range in air
120 m
 radiation: lowest ionizing power 
longest range in air
22.1 X-rays and radioactivity
Penetrating power
  radiation has the lowest penetrating power.
Radioactive
source
paper
 radiation is
stopped by a few cm
of air or a sheet of
paper.
aluminium
(5 mm thick)
lead
(25 mm thick)
22.1 X-rays and radioactivity
Penetrating power
  radiation <  radiation
Radioactive
source  radiation
paper
 radiation can pass
aluminium
(5 mm thick)
through a sheet of paper but
is stopped by an aluminium
sheet 5 mm thick.
lead
(25 mm thick)
22.1 X-rays and radioactivity
Penetrating power
  radiation <  radiation <  radiation
Radioactive
source  radiation
 radiation
paper
 radiation’s strength
aluminium
onlythick)
(5 mm
reduces to half by
a
slab of lead 25 mm thick.
lead
(25 mm thick)
22.1 X-rays and radioactivity
Deflection of  radiation in electric field
Radioactive
source
negative ()
terminal
 radiation
(positively charged)
is attracted towards
the negative terminal.
positive (+)
terminal
22.1 X-rays and radioactivity
Deflection of  radiation in electric field
Radioactive
source
negative ()
terminal
 radiation
 radiation
positive (+)
terminal
(negatively charged) is
attracted towards the
positive terminal.
 Deflection of  radiation >  radiation
 particles are much lighter.

22.1 X-rays and radioactivity
Deflection of  radiation in electric field
Radioactive
source
negative ()
terminal
 radiation
 radiation is
undeflected,
because it is
neutral.
positive (+)
terminal
 radiation
22.1 X-rays and radioactivity
Deflection in magnetic field
magnetic field into paper
Radioactive
source
 radiation
 radiation
  and  radiation deflect in opposite directions.
 deflection of  radiation > deflection of 
radiation  particles are much lighter.

22.1 X-rays and radioactivity
Deflection in magnetic field
magnetic field into paper
Radioactive
source
 radiation is
undeflected,
because it is
neutral.
22.1 X-rays and radioactivity
Cloud chamber tracks
felt
lid
ring
Supersaturated
alcohol vapour
source
foam
dry
ice
 A diffusion cloud chamber is used to show a
visible track of nuclear radiation.
 Ions are produced along the path.
 Alcohol vapour condenses around ions to form droplets.
 The droplets appear as tracks in the light.
22.1 X-rays and radioactivity
Tracks produced in a cloud chamber
by  radiation
 Thick tracks
 Strong ionizing power
 Produce many ions for
alcohol vapour to
condense around.
 Straight tracks
 Heavy  particles are
not easily deflected by
collisions with air
molecules.
22.1 X-rays and radioactivity
Tracks produced in a cloud chamber
by  radiation
 Thinner tracks
 Weaker ionizing power
 Irregular tracks
 Light  particles are
easily deflected by
collisions with air
molecules.
22.1 X-rays and radioactivity
Tracks produced in a cloud chamber
by  radiation
 Scattered tracks
 Almost unobservable
tracks
 Produced by electrons
which have been
knocked out of gas
molecules by  radiation.
22.1 X-rays and radioactivity
Tracks produced when  radiation passes
through a cloud chamber filled with helium gas
 Right-angled fork track
 Produced when an 
particle collides with a
helium atom.
 The colliding particles
have the same mass.
  particles are helium
nuclei.
22.1 X-rays and radioactivity
That’s the end of Section 22.1
Check Point
Key Ideas
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Section 22.2
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