Transcript Radioactivity

Radioactivity
Chapter 21
 Natural
occurring phenomena.
 In the nucleus of an atom there are
protons and neutrons.



Protons are positively charged so they
naturally want to repel each other.
A strong nuclear force keeps them inside.
Neutrons have no charge – so they are
neutral and help cushion the repulsive force
the protons feel.
• “the glue that holds the nucleus together”
Instability
 Once
the nucleus has 20 protons it needs
more neutrons than protons to keep the
nucleus together.
 And once you get to a certain number of
protons there is no number of neutrons
sufficient to keep the nucleus stable.
 Once the nucleus becomes unstable it
wants to break down or decay.
Band of Stability

The ratio between
neutrons and
protons determine
the stability of a
nucleus.
 If the number is too
large or too small the
nucleus is unstable.
 No atoms that have
an atomic numbers
larger than 83 and a
mass number larger
than 209 are stable.
Chemical Symbol

U = Uranium
 Mass number =
protons and neutrons
 Atomic number =
number of protons
3 Main Types of Radiation
Nuclear Equations
Alpha Decay
Alpha emission(greek symbol alpha - α )
1.
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•

emission of a nucleus with a mass number
of 4 and atomic number of 2.
can be stopped by a few sheets of paper.
Example:
Beta Decay
Beta emission (greek symbol β )
2.



emission of a high speed electron.
can be stopped by wood or heavy protecting
covering.
Example:
Gamma Decay
Gamma emission(greek symbol γ )
3.
•
•

emits a high energy non-particle radiation.
need several inches of lead to block
Gamma.
Example:
Other Types of Particles…
 Particles



in nuclear equations:
Proton
Positron
Neutron
Nuclear Bombardment
 When
atoms are bombarded by other
particles.
 Usually take place in a particle accelerator.
Overview of Radioactive
Emission
Radiation Exposure
 Radiation
is measure by the non SI unit R
(roentgen equivalent for man).
 Average dose received annually=120 R
 70% of radiation comes from natural
resources such as minerals and cosmic
rays. About 30% come from medical
procedures like x-rays.
Fission vs. Fusion

Fission reactions - a
very heavy nucleus,
spontaneously or
after absorbing
additional light
particles (usually
neutrons), splits into
two or sometimes
three pieces. (α
decay is not usually
called fission.)

Occurs only with a
critical mass.

Fusion reactions - two
light nuclei join to
form a heavier one,
with additional
particles (usually
protons or neutrons)
thrown off to conserve
momentum.
2H
+ 3H → 4He + n
Fission
Fusion
Uses of Radiation
 Nuclear
Powerplants
 Age Dating
 Radioactive Tracers
 Cancer Treatment
 Sterilizing
 Smoke Detectors
 Genetic Studies
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.
Radioactive Tracers



The most common tracer is called Technetium-99 and is
very safe because it only emits gamma rays and doesn't
cause much ionisation.
Radioisotopes can be used for medical purposes, such
as checking for a blocked kidney.
To do this a small amount of Iodine-123 is injected into
the patient, after 5 minutes 2 Geiger counters are placed
over the kidneys.
Also radioisotopes are used in industry, to detect leaking
pipes. To do this, a small amount is injected into the
pipe. It is then detected with a GM counter above
ground.
Cancer Treatment
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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.
It's vital to get the dose correct - too much and you'll
damage too many healthy cells, too little and you won't
stop the cancer from spreading in time.
Some cancers are easier to treat with radiotherapy than
others - it's not too difficult to aim gamma rays at a
breast tumor, but for lung cancer it's much harder to
avoid damaging healthy cells. Also, lungs are more
easily damaged by gamma rays, therefore other
treatments may be used.
Sterilizing
 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 sterilize
hospital equipment, especially plastic
syringes that would be damaged if heated.
Smoke Detectors
 Smoke
alarms contain a weak source
made of Americium-241.
Alpha particles are emitted from here,
which ionize 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.
Half-life
 The
half-life of a substance is the time it
takes for a radioactive sample to decay
into half.