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Transcript
Radioactivity!
Standard: Differentiate among alpha
and beta particles and gamma radiation
Definition of Radiation

“Radiation is an energy in the form of electro-magnetic
waves or particulate matter, traveling in the air.”
Radioactivity: Elements & Atoms
Atoms are composed of smaller
particles referred to as:
– Protons
– Neutrons
– Electrons
Basic Model of a Neutral Atom.



Electrons (-) orbiting nucleus of protons (+) and neutrons.
Same number of electrons as protons; net charge = 0.
Atomic number (number of protons) determines
element.
Mass number (protons + neutrons)
Radioactivity

If a nucleus is unstable for any reason, it will emit and
absorb particles. There are many types of radiation and
they are all pertinent to everyday life and health as well as
nuclear physical applications.
Ionization
Ionizing radiation is produced by unstable
atoms. Unstable atoms differ from stable
atoms because they have an excess of
energy or mass or both.
Unstable atoms are said to be radioactive. In
order to reach stability, these atoms give off,
or emit, the excess energy or mass. These
emissions are called radiation.
Types of Radiation

Radiation is classified into:
 Ionizing
radiation
 Non-ionizing radiation
Ionizing Versus Non-ionizing
Radiation
Ionizing Radiation
– Higher energy electromagnetic waves
(gamma) or heavy particles (beta and alpha).
– High enough energy to pull electron from orbit.
Non-ionizing Radiation
– Lower energy electromagnetic waves.
– Not enough energy to pull electron from orbit,
but can excite the electron.
Primary Types of Ionizing
Radiation
Alpha particles
Beta particles
Gamma rays (or photons)
X-Rays (or photons)
Neutrons
Radioactivity

Radioactivity is the breaking down of atomic nuclei by
releasing particles (primarily alpha and beta particles) or
electromagnetic radiation (primarily gamma rays).
Alpha Particles


•
Alpha particles are positively charges particles.
Alpha particles consists of two protons and two
neutrons.
An alpha particle is simply a helium nuclei (He) which is
ejected with high energy from an unstable nucleus

An alpha particle is simply a helium nuclei (He) which is
ejected with high energy from an unstable nucleus.

When an atom loses an alpha particle, the Z number
(atomic number) is lower by two, so move back two
spaces on the periodic table to find what the new
element is. The new element has an A number (atomic
mass number) that is four less than the original elements.
Example

Because alpha particles are large and heavy, paper or
clothing or even dead skin cells shield form their effects.
Beta Particles





Beta particles are electrons. They are negatively charged
(-1).
They are fast moving because they are actually electrons.
It is produce when a neutron in the nucleus breaks up
into a proton and an electron.
The proton remains inside the nucleus, increasing its
atomic number by one but the electron is given off.
Example – Carbon -14
They are written as an electron
(along with a proton) which is emitted from the nucleus as
a neutron decays.




The Z number (atomic number) actually adds one since
its total must be the same on both the left and the right
of the arrow and the electron on the right adds a negative
one.
The A (atomic mass)number is unchanged.
The Z number determines the element so look for it on
the Periodic Table.
Gamma Rays





Gamma rays are electromagnetic waves with extremely
short wavelengths.
They have no mass or charge so the Z and A numbers are
not affected.
They travel at the speed of light.
It takes a thick block of lead to stop them.
Radioactive atoms often emit gamma rays along with
either alpha or beta particles.
Examples
Alpha & Beta Decay
http://www.kentchemistry.com/links/Nuclear/AlphaBetaGamma.htm
Gamma Radiation
Penetration power of radiation
Positron


A particle with the mass of an electron but a positive
charge.
Positron decay is like a mirror image of beta decay.
These points present a simplified view of what
positron decay actually is:
1) Something inside the nucleus of an atom breaks
down, which causes a proton to become a neutron.
2) It emits a positron and a neutrino which go
zooming off into space.
3) The atomic number goes DOWN by one and mass
number remains unchanged.
Some points to be made about the equation:
1) The nuclide that decays is the one on the lefthand side of the equation.
2) The order of the nuclides on the right-hand side
can be in any order.
3) The way it is written above is the usual way.
4) The mass number and atomic number of the
neutrino are zero.
5) The neutrino symbol is the Greek letter "nu.
Reading Assignment



Read the article on Radioactivity
Complete the one-pager
Discuss
Name
Alpha
Beta
Gamma
Greek
letter

-

2p+, 2n0
electron
EM wave
2+
1-
0
wood, glass
thick
concrete or
lead
Symbol
Composition
Charge
Stopped
by
paper, skin,
clothing
SUMMARY of alpha, beta, gamma
Alpha particles
• An alpha particle is simply a helium nuclei (He) which is ejected with high energy from an
unstable nucleus
• This particle, which consists of two protons and two neutrons, has a net positive charge.
• Although emitted with high energy, alpha particles lose energy quickly as they pass through
matter of air and therefore, do not travel long distances.
• They can even be stopped by a piece of paper or the outer layers of human skin.
• These slow moving particles are generally the product of heavier elements
Example : 23892U ----> 42He + 23490Th.
Beta particles
• Beta particles are identical to electrons and thus have a charge of (-1).
• This type of decay process leaves the mass number of the nuclei unchanged.
• The element is transformed to a new element
• A beta particle is minute in comparison to that of an alpha particle and has about one hundred
times the penetrating ability. Where an alpha particle can be stopped by a piece of paper a beta
particle can pass right through. It takes aluminum foil or even wood to stop a beta particle.
• The electron that is released was not present before the decay occured, but was actually created in
the decay process itself.
Example : 3215P ----> 0-1e + 3216S
Gamma Rays
• As the name implies, these are not particles but high energy photons and can be
found on the electromagnetic spectrum
• They are very similar to x-rays but have a shorter wavelength and therefore more
energy
• The penetrating ability of gamma rays is much greater than that of alpha or beta
particles. They can only be stopped by several centimeters of lead or more than a
meter of concrete. In fact, gamma rays can pass right through the human body.
Gamma rays often accompany other processes of decay such as alpha or beta.
process.
92U
238
---->
90Th
234
+ 200 + 42He
A ramification of alpha or beta particle production is that the newly formed nucleus
is left in a state of excess energy. A way for the nucleus to release this excess energy
is by emitting gamma rays. Since gamma rays have no mass, and are waves rather than
particles, the elements atomic number does not change after emission.
Standard: Explain the process of half-life as
related to radioactive decay.
Radioactive Decay
• During radioactive decay an unstable nucleus
spontaneously decomposes to form a different
nucleus, giving off radiation in the form of
atomic particles or high energy rays.
• This decay occurs at a constant, predictable
rate that is referred to as half-life.
• A stable nucleus will not undergo this kind of
decay and is thus non-radioactive.
Standard: Describe nuclear energy, its
practical application as an alternative
energy source, and
its potential problems.
How nuclear power works video:
http://www.neok12.com/php/watch.php?v=zX7e6c58536f7e4f45010763&t=Nuclear-Power
See AC Science Nuclear Energy lesson for potential nuclear problems.
Radioactivity Questions
1. A radioactive substance has a half-life of 10 years. What fraction of a sample of
the substance would be left after 30 years?
A. 1/2 B. 1/3
C. 1/8
D. 1/9
2. Which type of radiation, from an external source, will penetrate
deepest into the human body?
A alpha B gamma C ultraviolet
D x-ray
3. Which of the following is the LEAST likely reason for the popularity of fission as a
way of producing electricity?
A Spent uranium fuel is easier to dispose of than ashes from burned coal.
B Nuclear energy is sometimes less expensive than other energy sources.
C Uranium provides more energy than an equal amount of petroleum.
D Nuclear fission produces less air pollution than burning fossil fuels.
t (hr) Amount of
Gold-191
4. Gold-191 is a radioactive isotope that has a half-life of 12.4
Remaining
hours. If a lab starts with a 13.2-milligram sample of gold-191,
(mg)
how much will remain after 37.2 hours?
0
13.2
A 6.60 mg B 4.40 mg C 1.65 mg
D 0.825 mg
12.4
6.60
24.8
3.30
37.2
1.65