Transcript What is radioactivity?

Trefoil Symbol – symbol for ionising
radiation
The Discovery of Radioactivity
Henri Becquerel (1896):
• Investigated the effect
of sunlight on crystals of
uranium salts
• By chance he left sample
resting on a
photographic plate wrapped
in black paper in a dark
drawer and accidentally
discovered it gave off some
sort of radiation
• Investigated the
radiation emitted concluded emission of
radiation was
spontaneous i.e. no heat,
light needed
Marie & Pierre Curie (1898):
• worked with a Uranium ore
(pitchblende) and purified it.
• after thousands of recrystallisations they isolated
two new elements from this
ore:
- polonium
- radium
• 1903 – Nobel Prize for
physics
• 1911 – Nobel prize for
chemistry
What is radioactivity?
Radioactivity is the spontaneous breaking up
of unstable nuclei with the emission of one
or more types of radiation.
Radiation consists of :
Alpha particles
α
Beta particles
ß
Gamma radiation
γ
Note:
For a nucleus to be stable it must contain a
suitable ratio of protons to neutrons. If this
ratio is not suitable the nucleus disintegrates
throwing out fragments in an effort to achieve
a stable ratio. This is radioactivity/radiation
Alpha Particles
What are alpha particles???
• 2 protons and 2 neutrons stuck together.
*He24 same as Helium nucleus
• may be represented as He24
Properties of alpha particles:
• Low penetrating power – stopped by few cm of
air or sheet of paper
• Positively charged
• Strongly Ionising – it knocks electrons of
matter it passes through producing positive
ions
• Slowest moving radiation
• Are deflected by magnetic and electric fields
Example
A example of a radioactive element that
emits alpha particles is americium-241
Use of americiun-241 – smoke detectors
No Smoke present
Alpha Source
Detector
Smoke present
Alpha Source
Detector
Alpha particles reaching the Alpha particles do not reach
the detector – alarm goes off
detector - no alarm
Beta Particles
What are beta particles???
• Electrons
• Formed when a neutron in an unstable nucleus
is changed into a proton and an electron.
• The electron is ejected from the nucleus
Properties of Beta Particles:
• More penetrating than alpha particles – can
penetrate up to about 5 mm of aluminium
• Negatively charged
• Less ionising than alpha particles
• Deflected by magnetic and electric fields
Example
An example of an element that emits beta
particles is carbon-14
- carbon-14 is used to date archaeological
artefacts (carbon dating)
Gamma Radiation
What is gamma radiation???
• High energy electromagnetic radiation –
radiant energy not a particle (similar to X-Rays)
• An unstable nucleus emits gamma radiation in
order to lose surplus energy
Properties of Gamma Rays:
• Not deflected in magnetic/electric fields
• Has no charge or mass – can be emitted from a
nucleus that has emitted an α or β particle
• Poor ionising ability
• It is fast moving similar to the speed of
light
• High penetrating ability – only stopped by thick
slab of lead
• Very harmful to human tissue – absorbed =
cancer
Example
An example of an element that gives off gamma
rays is cobalt – 60
- cobalt-60 used in cancer treatment
(cancerous cells more susceptible to
damage by ionising radiation than normal
cells)
- cobalt-60 used in food irradiation
Absorption of Radiation
Demonstration:
To detect the presence of α particles, β particles
and gamma radiation, and to investigate the
penetrating power of each of these three types of
radiation
Note:
To detect radiation we can
use a Geiger - Müller tube
connected to a ratemeter.
Distinction between Chemical Reactions
and Nuclear Reactions
• Chemical reactions involve transfer of electrons:
(a) Elements combine to form compounds
(b) Compounds break up to form elements
(c) Elements in compounds rearrange to
form new compounds
• Nuclear reactions involve the emission of
radioactive radiation from the nuclei of
unstable atoms:
- Nuclear reactions cause changes
in nucleus
- Nuclear reactions cause elements
to change into other elements
Nuclear Reactions
Nuclear Equations – Alpha Emissions:
When an alpha particle is emitted from a
radioactive element the new element remaining is
found by:
- subtracting 4 from the given atomic mass no.
- subtracting 2 from the given atomic no.
(i.e. Move back two places on the periodic
table to identify element)
Alpha Particles Emission:
When an atom looses an alpha particle it
changes into an atom of the element two
places before it in the Periodic Table. The
mass number of the parent element decreases
by four
Example:
If an atom of radium-226 looses an alpha
particle what is the new element formed???
226 Ra
88
?
+
4 He
2
+ energy
1) Complete calculations:
A: 226 – 4 = 222 - New atomic mass no.
Z: 88 – 2 = 86 - New atomic no.
2) Use periodic table to identify new element:
226 Ra
88
222 Rn
86
+
4 He
2
+ energy
Complete the following equations:
a)
214 Po
84
?
+
4 He
2
+ energy
b)
238
92U
?
+
4 He
2
+ energy
Nuclear Equations – Beta Emissions:
When a beta particle is emitted one of the
neutrons is changed into an electron and a
proton. (One neutron less and one more proton).
The proton remains in the nucleus. There is:
- no change in the atomic mass number (A)
- the atomic number (Z) increases by 1
Beta Particles Emission:
When an atom loses a beta particle, it
changes into an atom of the element one place
after it in the Periodic Table. The mass
number of the atom stays the same.
Example:
If an atom of carbon-14 looses a beta particle
what is the new element formed???
14 C
6
?
+
0 e
-1
+ energy
1) Complete calculations
A: no change – atomic mass number
Z: 6 + 1 = 7
- new atomic number
2) Use periodic table to identify element
14 C
6
14 N
7
+
0 e
-1
+ energy
Complete the following equations:
a)
214 Pb
82
?
+
?
b)
198 Au
79
?
+
?
+
energy
+ energy
Gamma Radiation:
The loss of gamma radiation does not give
rise to any new atoms – energy (hf) is
simply lost from the nucleus
Transmutation:
The changing of one element into
another is called transmutation
Half-Life
The half life of an element is the time taken
for half of the nuclei in any given sample to
decay.
Example:
Carbon 14
half life = 5,700 years
Cobalt 60
half life = 5 years
Polonium 234 half life = 0.15 milliseconds
Significance of long half-life as an argument
against nuclear power
Radioisotopes
• Unstable isotopes that are radioactive
Most elements of atomic number above 83 do
not have stable isotopes
Many useful isotopes are made artificially. In
nuclear reactors e.g. Cobalt-60
Uses of Radioisotopes
(Must know 3)
1. Medical Uses:
Cobalt 60 (gamma radiation) can be used
to treat cancerous cells as cancerous
cells are more susceptible to damage by
ionising radiation than normal cells.
Gamma radiation also used to sterilise
medical instruments
2. Archaeological:
Carbon-14 (carbon dating) /beta emitter
• Used to indicate age of carbon containing
objects(wood, fabric….)
• Determination
Living organism contains C-12 and C-14 in
same proportions as in the air
On dying, the ratio of C-12/C-14 changes
with time due to radioactive decay of C-14
The extent of the changes in the ratio allows
the age to be calculated
3. Americium 241 (discussed previously):
• Alpha particle emitter
• These ionise the air molecules and conduct
electric current between two terminals
smoke cling to the ionised molecules and
slows them down
• The current decreases and a transistor
switch activates the alarm
Background Radiation
Radiation can be
detected all around us.
We call this
background radiation.
There are two types of
sources:
- natural (first 4)
- artificial
*Note: Radon is a
radioactive gas that
escapes from rocks and
increases the risk of cancer
Source
Percentage
Radon gas 60%
Rocks+ soil 10%
Food+
drink
10%
Cosmic
rays
10%
Medical
9%
weapons
0.2%
Nuclear
waste
<0.1%