Download Radioactivity - Miami Beach Senior High School

Radioactivity
• Elements that emit particles and energy
from their nucleus are radioactive.
• Some large atoms are unstable and cannot
keep their nucleus together.
• A radioactive element emits one of these
forms of radiation: alpha ( ), beta ( )
and gamma ( ).
Alpha particles
• An alpha particle is a combination of two protons
and two neutrons (the nucleus of a helium atom).
• Alpha particles move quickly and have a lot of
kinetic energy. They can cause damage to the
surface of a material, especially living tissue. They
do not penetrate far because they are often trapped
by solid materials.
• Damage by alpha particles is limited, as they
travel through the air and combine with electrons
becoming harmless helium.
Beta particles
• Beta particles are negatively charged particles
emitted from the nucleus.
• Neutrons can change into protons and electrons
when unstable and emit an electron.
• They move faster than alpha particles, and so can
penetrate light materials, and can get quite deep
into the skin, where they can kill cells.
• Once stopped they become incorporated into the
material.
Gamma Rays
• Gamma radiation is an extremely energetic form
of electromagnetic radiation.
• Gamma radiation carries no electric charge and
has no mass.
• They can penetrate most materials, and can
damage molecules in our cells if they penetrate
our bodies.
• They cannot penetrate unusually dense material
like lead.
• If an atom changes its number of protons
(its atomic number), it becomes a different
atom.
• If U-238 emits an alpha particle – two
protons and two neutrons – its atomic mass
will decrease by 4, and it becomes Thorium,
Th-234. U-238 has an atomic number of 92,
Th-234 will have an atomic number of
_________.
Emitting beta particles
• A neutron can spontaneously transform into
a proton and electron in a nucleus with
more neutrons than protons.
• The electron is emitted form the nucleus.
• This is beta radiation.
• The element now increases its atomic
number by one, as it has an extra proton.
• Thorium-234 will become ______-234,
after gaining a proton
Show how Uranium-238 can
become lead by only alpha
emissions.
Why are elements radioactive?
• How is it possible that all positively charged
protons in the nucleus remain clumped together?
• This question led to the discovery of the strong
nuclear force, which acts between all nucleons.
• This force is very strong but acts only over very
short distances.
• The repulsive nuclear force acts over longer
distances.
• So what happens if the nucleus is large?
• A large nucleus is not as stable as a small nucleus.
• It emits high energy particles or gamma radiation.
• Neutrons act as a nuclear cement. Large nuclei
must have more neutrons than protons.
• A neutron by itself (not surrounded by protons) is
not stable and can spontaneously transform into a
proton and an electron. Adding more protons
increases the repulsive force.
Half-life
• The half-life of a radioactive element is the
time needed for it to decay to half of the
initial amount.
• Radium-226 has a half-life of 1620 years.
• If you start with 20g, then in 1620 years you
will have 10g.
• In how many years will there be 5g? In how
many will there be 2.5g?
Isotopic dating
• You can use radioactivity to find the age of
rocks and fossils.
• For fossils less than 50,000 years old, C-14
will work.
• For rocks millions of years old you can use
U-238
Carbon-14
• Carbon is a mixture of the two isotopes C-12 and
C-14.
• C-14 is radioactive and C-12 is stable.
• In the air the proportion of C-14 :C-12 is constant.
• Plants take in this mixture. Animals get their
carbon from plants.
• When an animal dies, C-14 starts to decay to N14. So now the proportion of C-14 to C-12
changes.
Dating a fossil
• If you want to date a fossil you can measure
the ratio of C-14 to C-12, compare it to the
ratio in the air, and figure out how long ago
the creature died.
Problems
• A sample of radioactive C-14 has decayed
to 12.5 % of its original amount. If the halflife of C-14 is 5730 years, how old is the
sample?
• A fossil has 10.0g of C-14. How long will it
take until it contains 0.625g?
• Element Z has a half-life of 1 week. Graph
the decay of 256g of Z over a 10 week
period.
Nuclear Fission
• Fission is another word for splitting. The
process of splitting a nucleus is called nuclear
fission.
• Uranium isotopes are normally used as the
fuel in nuclear reactors because uranium
atoms have relatively large nuclei that are
easy to split, especially when hit by neutrons.
• Fission occurs mainly in the rare isotope U235. Separating U-235 from U-238 is a
difficult task.
• When a uranium nucleus is hit by a neutron
the following happens:
• The nucleus splits into two smaller nuclei,
which are radioactive.
• More neutrons are released.
• The additional neutrons released may also hit
other uranium nuclei and cause them to split.
Even more neutrons are then released, which
in turn can split more uranium nuclei. This is
called a chain reaction. The chain reaction in
nuclear reactors is controlled to stop it going
too fast.
Nuclear Fusion
• Energy is produced as nuclei fuse.
• The mass of the fused nucleus is less than the
mass of each of the unfused nuclei.
• The difference in mass is converted to useful
energy.
• To fuse the nuclei must be traveling at very high
speeds.
• This can only occur at extremely high
temperatures (thermonuclear fusion)
• The hydrogen bomb is a thermonuclear bomb.