Radioactive Decay
Predictions – Chapter 1, Activity 8
Radioactive Decay
• Some configurations of protons and neutrons in
the nucleus are not as stable as others.
• When the nucleus is not stable, it will
spontaneously transform into a more stable
configuration.
• For example, Carbon-14 is not stable and will
therefore spontaneously decay into Nitrogen-14.
• The process by which an atom spontaneously
changes into a more stable one is called
radioactive decay.
Radio-Isotopes
• Isotopes are configurations of atoms that have the
same number of protons, but different numbers of
neutrons.
• Radio-isotopes are unstable isotopes that go
through radio active decay.
Stable
12C
Unstable
14C
6
6
14N
7
+ e-
Types of Radioactive Decay
•
•
•
•
Alpha Decay
Beta Minus Decay
Beta Plus (Positron) Decay
Gamma Decay
Alpha Decay
Parent Atom
Daughter Atom
Alpha Decay occurs when two protons and two neutrons (or a
helium nucleus) are emitted from the nucleus of the parent
atom.
Beta Minus Decay
Parent Atom
Daughter Atom
Beta Minus Decay occurs when a neutron decays into a
proton and an electron, where the latter is emitted from the
nucleus.
Beta Plus Decay
Parent Atom
Daughter Atom
Beta Plus Decay occurs when a proton decays into a neutron
and a positive electron (positron), where the latter is emitted
from the nucleus.
Alpha particles can usually be stopped by a very thin barrier. Radioisotopes emitting alpha
particles are usually not hazardous outside the body, but they can cause damage if ingested.
Betas (streams of electrons) can pass through a hand, but are usually stopped by a modest
barrier such as a few millimeters of aluminum, or even a layer of clothing. As with alphas,
beta particles are more hazardous if inhaled or ingested.
Gammas can be very penetrating and can pass through thick barriers. Several feet of concrete
would be needed to stop some of the more energetic gammas. A natural gamma source found
in the environment (and in the human body) is 40K, an isotope of potassium.
Neutrons are also very penetrating. Some elements, like hydrogen, capture and scatter
neutrons. Water is commonly used as a neutron radiation shield.
Half-Life
• 1 half-life equals the amount of time required for ½ of the
original number of parent atoms to decay into daughter
atoms.
14C
14N + e6
7
Half-Life
Example:
•
You have 600 grams of iodine-133. The
half-life of iodine-133 is 21 hours.
a. How much of this sample would exist after 21 hours?
b. After 42 hours?
c. After 126 hours?
•
Remember: the amount of radioactive
substance diminishes by ½ for every halflife.
Example 1: (cont.)
•
Begin by determining the number of halflives by dividing the elapsed time by the
half-life.
a. 21 hours/21 hours = 1 half-life
b. 42 hours/21 hours = 2 half-lives
c. 126 hours/21 hours = 6 half-lives
Example 1: (cont.)
• Since the amount of radioactive material
will reduce by one-half with every passing
half-life, we can conclude:
Half-Life
Divider
Pattern
1
2
21
2
4
22
3
8
23
4
16
24
5
32
25
6
64
26
Divider = 2n where n
equals the number of
half-lifes.
Example 1: (cont.)
•
For every half-life, divide the original amount of
radioactive material by the divider
corresponding to the ½-life in question.
When n = 1, the divider equals 2 or 21.
m = 600 g/2 = 300 g
b. When n = 2, the divider equals 4 or 22.
m = 600 g/4 = 150 g
c. When n = 6, the divider equals 64 or 26.
m = 600 g/64 = 9.4 g
a.
Determining the Amount of
Radioactive Material Left
• To determine the amount of radioactive material
left in a sample after n number of ½ - lifes:
m = mo
2n
Where:
m = mass after n ½ - lifes
mo = initial mass
n = number of ½ - lifes
Applications of Radioactivity
• Dating materials:
• Carbon-14 (1/2-life = 5730 yrs) used for dating plants
and animals that were formerly alive.
• Uranium-238 (1/2-life = 4.47 billion years) used for
dating rocks.
• Potassium-40 (1/2-life = 1.3 billion years) used for
dating rocks.
• Medical diagnostics.
• Cancer Treatment
• Smoke detectors.
Carbon Dating
• Carbon-14 is created by cosmic radiation from the sun
acting on the atmosphere.
• CO2 makes up a small portion of our atmosphere that is
consumed by plants through photosynthesis.
• While all animals and plants are living, Carbon-14 is being
constantly replaced.
• When a living organism dies, the replenishment of carbon14 no longer exists and the decay process takes over.
• Every 5,730 years, the amount of Carbon-14 will decrease
by ½ into Nitrogen-14.
CARBON DATING
The rate at which 14C decays is absolutely constant. Given any set of 14C
atoms, half of them will decay in 5730 years. Since this rate is slow relative
to the movement of carbon through food chains (from plants to animals to
bacteria) all carbon in biomass at earth's surface contains atmospheric levels
of 14C. However, as soon as any carbon drops out of the cycle of biological
processes - for example, through burial in mud or soil - the abundance of 14C
begins to decline. After 5730 years only half remains. After another 5730
years only a quarter remains. This process, which continues until no 14C
remains, is the basis of carbon dating
NUCLEAR MEDICINE
Cobalt- 60 Cancer Treatment
•
•
•
•
Gamma ray emitter
Half life 5.23 years
Radiotherapy – treatment of cancer with radiation
Cancerous cells more susceptible to damage by
ionising radiation than normal cells
• Ionising radiation is directed on to the tumour
from different directions
• Tumour dose is high but normal tissue receives a
much lower, less harmful dose
Cobalt - 60
• Gamma rays kill
micro-organisms in
food
• May also involve
inhibiting sprouting,
controlling ripening
and pasteurising foods
• Fears of genetic
modification
Americium-241
• Emitted α-particles ionise
the air molecules, conduct
an electric current
between two terminals
• Smoke clings to ionised
air molecules and slows
them down
• Current decreases and a
transistor switch activates
the alarm
• This type of alarm not
used these days
NUCLEAR ENERGY
FISSION
Splitting the Uranium Atom:
Uranium is the principle element used in nuclear reactors and in certain types of
atomic bombs. The specific isotope used is 235U. When a stray neutron strikes a
235U nucleus, it is at first absorbed into it. This creates 236U. 236U is unstable and
this causes the atom to fission
Light atoms tend to combine and release energy as they do so.
Heavy atoms tend to split and release energy as they do so. Uranium and Plutonium are
particularly useful in this regard, and are the basis of nuclear fission.
Note there are three
neutrons released
for every incident
neutron. This is the
basis of a chain
reaction.
http://www.youtube.com/watch?v=JxzPN-vdP_0&feature=related
http://www.youtube.com/watch?v=XHitaEy-Xtg
Light atoms tend to combine and release energy as they do so.
Heavy atoms tend to split and release energy as they do so. Uranium and Plutonium are
particularly useful in this regard, and are the basis of nuclear fission.
Heavy nuclei break into
lighter nuclei and energy is
released.
Light nuclei fuse into heavy
nuclei and energy is
released.
CHAIN REACTION
NUCLEAR-ENERGY
POWER PLANTS
In a nuclear reactor, however, the last thing you
(and the rest of the world) want is all your
atoms splitting at once. But the reactor core
needs to be slightly supercritical so that plant
operators can raise and lower the temperature of
the reactor. The control rods give the operators a
way to absorb free neutrons so operators can
maintain the reactor at a critical level.To turn
nuclear fission into electrical energy, the first
step for nuclear power plant operators is to be
able to control the energy given off by the
enriched uranium and allow it to heat water into
steam