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Radioactivity
• Particles emitted from nuclei as a result of
nuclear instability.
• Common types of radiation:
– Alpha
– Beta
– Gamma
Nucleons = anything in the nucleus
Half-lives
• Radioactive decay rates
• Half-life – time it takes for half of the particles
are emitted.
Alpha radioactivity
• Alpha particle is a nucleus of the
element helium = two protons
and two neutrons
• Range is less than a tenth of a
millimeter inside the body.
• Main hazard is when it is
ingested.
• Can have destructive power in
its short range.
Einstein
E = mc2
The difference in mass between the reactant
and products isn’t missing it was turned into
energy.
E = energy
M = mass
C = speed of light, 3x 10^8 m/s
• The energy is found (moslty) in the kinetic
energy of the alpha particle and daughter
nucleus moving away from each other.
Example 1: The iridium-168 isotope is known to go
through alpha decays. Write out a decay equation
that shows this process.
• Start by looking up iridium on your periodic table
so that you can find out its atomic number.
• Then write down the most basic decay reaction;
show what you started with (the iridium is your
• parent nucleus), and how it has decayed by
emitting an alpha particle and some other nuclei.
Masses
Proton = 1.007316 amu or 1.6727 x 10-24 g
Neutron = 1.008701 amu or 1.6750 x 10-24 g
Electron = 0.000549 amu or 9.110 x 10-28 g
1 amu = 1.66 x 10-27 kg
Beta particles
• A neutron turns into a proton and electron.
• The proton stays in the nucleus but the
electron is released.
• High energy electrons have greater range of
penetration than alpha particles, less than
gamma rays.
• Greatest hazard if ingested.
Beta
• Example 3: Write out the beta negative decay
reaction for calcium-46.
As with the alpha decay in Example 1, first find
your parent nucleus on the periodic table and
write out a basic decay reaction...
Beta negative decay
• A neutron is converted to a proton and an
electron.
• They found that the electron of a beta particle
is accompanied by a neutral particle called an
antineutrino
• Called antimatter (opposite spin as nuetrons)
Beta positive decay
• A proton turns into a neutron and emits a
positron.
• Exact same mass of electron but have a +1e
charge
• (antimatter version of electrons)
Half-life
• The nuclei of radioactive atoms are unstable.
They break down and change into a
completely different type of atom.
• Radioactive decay
• Ex. Carbon-14 decays to nitrogen-14 when it
emits beta radiation.
• Its not possible to predict when an individual
atom might decay .
• It is possible to measure how long it takes for
half the nuclei of a piece of radioactive
material to decay.
Half-life = the time it takes for the number of
nuclei of the isotope in a sample to halve.
Ex.
The half-life of carbon-14 is 5,715 years, but the
half-life of francium-223 is just 20 minutes.
Radioactive particles
• Alpha particles = (+) charged
• Beta particles = (-) charged
• Gamma rays = (o) nuetral, electromagnetic
waves (light waves)
Rutherford figured out the charged by placing
an electric and magnetic field around the
radioactive element to see where the particles
curved.
Gamma Decay
• Passes through anything but the densest of
matter.
• Happens because the nucleus has just been
through a lot, spitting out other subatomic
particles so on…
• Releases energy, not particles
• Example 5: The argon-40 that was produced in
Example 4 is in an excited state, so it releases
a burst of gamma radiation. Write the
equation for this.
The daughter nucleus may still be unstable and
will have to go through several decays in order
to be stable, this is called decay series.
Radiation risks
• Geiger counters calculate radiation.
• There's radiation everywhere that has been
here forever so it isn’t harmful.
• Large doses of radiation is harmful.
Rad: a rad is the older unit used to describe each
kilogram of tissue exposed absorbing 0.01 J of
energy.
• Gray (Gy): one Gray means each kilogram of
material absorbs 1 Joule of energy.
• So, 1 Gy = 100 rad
• Sievert (Sv): is a modified version of Grays,
because it takes into account the relative
Biological Effectiveness (RBE) of a particular kind
of radiation. The more dangerous a particular
kind of radiation is to a person, the more the
original Grays are multiplied to give Sieverts.
• In normal situations, a person can expect to
be exposed to about 0.5 mSv in a year. Any
exposure of about 6 Sv or higher will be fatal.
Radiation sickness
• Radiation can ionize cells.
• Radiation is knocking electrons off the cells,
interfering with cell division.
• Many people that survived the initial blast of
the bombs used at Hiroshima and Nagasaki
died from radiation sickness a few days later.
Genetic damage
• Damage to the actual DNA of cells.
• Can result in cancer, show up after several
years.
Alpha has a high ionization rate, but can not easily
penetrate matter. A layer of clothes or even
the top layer of skin (which is dead anyways) can
stop it. The alpha particles can only move
through the air about 5 cm before being stopped.
Alpha radiation is really only a danger if you
either breathe in or swallow the source of the alpha
radiation.
• Beta does not ionize as easily, but it can
penetrate matter more easily, traveling about
0.50 m
through the air and about 1 cm into a body. This
means that although beta radiation can be a bit
more of a risk, it is still most dangerous if the
source is ingested.
• Gamma can easily penetrate your body, since
it is EMR with a high frequency. Although it
doesn't ionize much, it causes the most damage
to a person. Even being near an unshielded
source of gamma radiation for a short period of
time is very dangerous!
• https://www.youtube.com/watch?v=FU6y1XI
ADdg
Half life
• Time it takes for half of the parent atoms to
transmutate into something lese (through
alpha or beta decays, or another process)
• Example 1: The half life of Carbon-14 is 5730
years.
Explain what you would expect to happen over
a long period of time. Imagine a sample of
carbon that originally had 100 of these carbon14 atoms. In reality we would need the sample
to have many more atoms, since statistics are
really only reliable for large numbers.
During the first few hundred years or so we would
notice that some of the carbon-14 atoms have
transmutated into some other element.
In fact, a lot of them have changed. Since we
started with a lot of the carbon-14 atoms, there is
the greatest chance of seeing quite a few change. It
would be like throwing 100 quarters into the air;
since there are so many, you've got a really good
chance of seeing a 50-50 split between heads and
tails when they hit the ground.
By the time 5730 years have passed, we would
expect to only have 50 carbon-14 atoms
remaining. Remember, the half life is the time it
takes for half of them to change. There are still
the same total number of atoms, just not as
many carbon-14 as we started with.
Some people think that if we wait another 5730
years, all of the carbon-14 will be gone... nope!
Remember, half life is the time it takes for half
the atoms to decay. So, after the next 5730 years
we would expect 25 carbon-14 to be left; that's
half of the 50 that we had after the first half life.
And so on, and so on... Eventually, after about
six or seven half lives have passed, the number
of carbon-14 atoms becomes so small that
probabilities fall apart and you basically have
the last few atoms decay whenever.
Half-life
• T1/2 the time required for half the amount of a
radioactive nuclide to decay.
• Less stable nuclides decay very quickly and
have shorter half-lives
Ex.
Decay series
• Series of radioactive nuclides produced by
successive radioactive decay until a stable
nuclide is reached.
• Parent nuclide – the heaviest nuclide of each
decay series.
• Daughter nuclide – nuclides produced by the
decay of the parent nuclides.
Half life equation
Example 2: Marie Curie had a 765g sample of
polonium-210 (half life = 138d) in a box. After 3.8
years of refining radium, she goes to the box to get
her polonium. Determine how much polonium-210
is in the box.
First, we need to get our units for time the same, so
figure out how many days there are in 3.8 years.
365 days = 1 year
1387 days
Now we can figure out how many half lives have
passed (the “n” value in the formula).
n = total length of time/half life
n = 1387 days/138 d half-life = 10.0507 half lives
Now we figure out how much polonium-210 is
remaining.
• By the time Madame Curie gets back to her
box, she’ll only find that 0.72g of polonium210 is remaining. There is still a total of 765g
of stuff in the box, but only 0.72g of it is
polonium-210. The other 764.28g of stuff
would be other elements that the polonium210 decayed into.
Example 3: You have 75g of lead-212. If it has a
half life of 10.6h, determine how long it will take
until only 9.3g remains. This question is tougher
than the previous example. If you write out the
formula, here’s what you get…
Now, if you’re good with logarithms in math, you
can go ahead and solve it. But in chem comm
you are not required use logs… we can do it an
easier way! Try the following...
1. Type 75 into your calculator and divide by 2.
You should get 37.5. So, after one half life you’ve
got 37.5g left.
2. Divide 37.5 by 2 to get 18.75… so after two
half lives you’ve got 18.75g.
3. Divide 18.75 by 2 to get 9.375. After three
half lives have passed you’ve got 9.375g left.
That’s pretty close to the 9.3g in the question,
so after just a little more than three half lives
you should have 9.3g left over.
Fission
• The process of causing a large nucleus (A > 120)
to split into multiple smaller nuclei, releasing
energy in the process.
• It can start when the large nuclei absorbs a
neutron, causing it to become unstable to the
point that it falls apart.
• This is the reaction that we use in nuclear power
plants and early nuclear weapons.
• Fission is relatively easy to do, but also leaves us
with lots of nuclear waste that must be stored for
thousands of years before it is safe.
Fusion
• The process of causing small nuclei to stick
together into a larger nucleus, in the process
releasing energy.
• This is the process that drives our sun, and all
other suns.
• We can do it under the right conditions in a lab,
but we end up putting in more energy than we
get out.
• The left over products of fusion are relatively
safe, which is why a lot of research is going into
developing fusion reactors.
Fission cont…
• The most typical fuel used in a fission reactor
is uranium-235.
• In 1939 four German scientists discovered that
uranium-235 would become very unstable if it
gained an extra neutron, forming uranium236.
• Uranium-236 is so unstable that a fraction of a
second later it will split to form two smaller
atoms, and in the process release energy.
• To keep this reaction going, do we need to keep
on adding neutrons?
• Well, we could, but it takes energy to isolate
neutrons and then throw them at the uranium235, so this isn’t the best idea.
• We do have an average of 2.5 neutrons thrown
off each reaction that is successful, so why not
just use those?
That’s exactly what we do!
• If exactly one neutron gives rise to another
reaction, the self sustaining reaction that results
is called critical. Each reaction leads to one
reaction afterwards. This is a “chain reaction”.
There are a few situations when we want this to
happen...
• in a nuclear bomb, since we want one reaction
we kick off to result in a cascade of
exponentially more and more reactions within
a split second
• when a nuclear power plant is first being
started up, until it reaches the number of
reactions that we can keep going at the same
time. Then it will be stepped down to just a
critical reaction.
There is also a situation when we do not want a
supercritical reaction, which is when a nuclear
power plant is going into a meltdown.
• This is what started to happen at the
Chernobyl Nuclear Reactor in Ukraine.
• http://en.wikipedia.org/wiki/Chernobyl_disast
er
If less than one neutron gives rise to more
reactions, the decreasing rate of reactions is called
subcritical.
• For example, lets say you have four reactions, but
the neutrons from only three of them feed later
reactions, and of those three only two continue,
then down to one… the reaction will eventually
die out.
• This is what happens when you shut down a
reactor.
Reactors use control rods to control the rate of the
reaction.
• Made from elements such as boron and
cadmium, control rods are very good at absorbing
neutrons.
• If a reaction is going supercritical, drop the
control rods further into the core to absorb extra
neutrons and the reaction slows.
• If the reaction is going subcritical, pull the control
rods out further, which lets more neutrons react
and get more reactions going again.