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Transcript
Radioactivity
Ch 10
Radioactivity
• is the process in which an
unstable atomic nucleus
emits charged particles &
energy
• Any atom containing an
unstable nucleus is called a
radioactive isotope or
radioisotope
How is the Atom Unstable?
• The nuclear “glue” that holds
the nucleus together sometimes
isn’t strong enough.
• Over time the atom “drops”
some of it’s subatomic particles
…called radioactive decay!
39
19
New Way of Writing
Nuclides & Isotopes
K ; 4019 K
• Superscript is the mass
number
• Subscript is the atomic
number
14
12
• 6C ; 6 C
Isotopes
• have the same number of p+,
different number of no
• Another way to show an
isotope is to have the mass
number follow the name of the
element (Carbon-14 or C-14)
Types of Nuclear Radiation
1. Alpha
2.Beta
3. Gamma
Alpha Decay
• Alpha particle—a positively
charged particle made up of
two p+ & two no
• the least penetrating
• can be stopped by a sheet of
paper
Alpha Decay
• An alpha particle looks like a
helium atom (42He)
• mass reduces by 4
• atomic # reduces by 2
• Examples:
238
92 U 
209
84 Po 
Beta Decay
• A beta particle is an eemitted by an unstable
nucleus
• can be stopped by a thin
sheet of metal such as
aluminum
Beta Decay
• A beta particle is written 0-1 e
• mass remains the same & the
atomic # increases by one
• Examples:
214
82 Pb
218
84 Po
Gamma decay
• A gamma ray is a penetrating
ray of energy emitted by an
unstable nucleus
• Gamma rays are energy waves
that travel through space at the
speed of light
Gamma
decay
• atomic # and
mass remain
the same, but
the energy of nucleus decreases
• Gamma rays can be stopped by
several centimeters of lead or by
several meters of concrete
Thanks Cambridge Physics Outlet for amazing graphics!
STAR Questions
• U-238 loses 4 total subatomic particles, 2
being protons.
– What elements are formed?
• Thorium and Helium
– What type of decay has it undergone?
• Alpha decay
• What is the most penetrating decay?
• Gamma rays
Background radiation
• is nuclear radiation that
occurs naturally in the
environment (levels are low
enough to be safe)
– Radioisotopes in the air,
water, rocks, plants, &
animals all contribute
Background Radiation
– Cosmic rays (streams of
charged particles) from outer
space that collide with the
Earth’s atmosphere also
contribute
– When nuclear radiation
exceeds background levels,
cells in your body can mutate
• Devices
used to
detect
radiation
include
Geiger
counters &
film badges
Detecting
Radiation
RATES OF NUCLEAR DECAY
A half-life is the time required for one
half of a sample of radioactive
sample of a radioisotope to decay
– Unlike chemical reactions, nuclear
decay rates are constant
regardless of temperature,
pressure or surface area
• C-14 has a half life of 5730 years.
– What fraction of a sample will be remaining
after 1 half life?
• After 2 half lives?
• C-14 has a half life of 5730 years.
– If you have a sample of 50 grams. How much of the
sample will be remaining after 1 half life?
• After 2 half lives?
RATES OF NUCLEAR DECAY
Transmutation is the conversion of
atoms of one element to atoms
of another
Transuranium elements are
elements with atomic numbers
higher than 92 (Uranium)
A quark is a subatomic particle
theorized to be among the basic
units of matter
FISSION AND FUSION
Fission is the
splitting of an
atomic nucleus
into two smaller
parts
Fusion is a
process in which
the nuclei of two
atoms combine
to form a larger
nucleus
• A chain
reaction is a
chain of
fission
reactions
triggered by
neutrons
released
during the
fission of a
nucleus
Fission
About 20% of the
electricity in the US
comes from fission
reactions
Fission
• A tremendous amount of energy
is produced during a fission
reaction
Fission
• Advantages: is the
lack of air
pollution.
• Disadvantages:
include the risk of
exposure &
radioactive waste
Harris plant,
near Raleigh
Lake Harris
Not in my backyard…
Then sealed and transported by way of trucks and
trains for more permanent storage. Low-level
wastes will stay above ground until they become
“stable”.
If high-level, the wastes are stored deep underground, where
they wait for hundreds to thousands of years to become “stable”.
Fusion
• release huge
amounts of
energy
• occur in the
sun and stars
(plasma)
Fusion
• We do not use fusion reactions
for energy b/c of the extremely
high temperatures needed to
start the reaction & because
the plasma would need to be
contained.
THREE MILE ISLAND
• Three Mile Island power station is near Harrisburg,
Pennsylvania
• In 1979 at Three Mile Island nuclear power plant a
cooling malfunction caused part of the core to
melt in the # 2 reactor. The TMI-2 reactor was
destroyed.
• Some radioactive gas was released a couple of
days after the accident, but not enough to cause
any dose above background levels to local
residents.
• There were no injuries or adverse health effects
from the Three Mile Island accident.
CHERNOBYL
• The Chernobyl accident in 1986 was the result of a
flawed reactor design that was operated with
inadequately trained personnel & without proper regard
for safety.
• The resulting steam explosion & fire released at least
five percent of the radioactive reactor core into the
atmosphere and downwind.
• 28 people died within four months from radiation or
thermal burns, 19 have subsequently died, & there have
been around nine deaths from thyroid cancer apparently
due to the accident: total 56 fatalities as of 2004.
• An authoritative UN report in 2000 concluded that there
is no scientific evidence of any significant radiationrelated health effects to most people exposed. This was
confirmed in a very thorough 2005-06 study.
Examples
• Iodine-131 has a half-life of 8.07 days.
What fraction of a sample of iodine-131 is
left unchanged after 16.14 days?
Examples
• The radioactive isotope Ni-63 has a halflife of 100 yrs. How much of a 10g sample
remains after 300 yrs?
Examples
• How long will it take a sample of Po-194 to
decay to 1/8 of its original amount, if Po194 has a half life of 0.7 seconds?
Examples
• A sample of Cl-38 is observed to decay to 25%
of its original amount in 74.4 min. What is the
half-life of Cl-38?
• sorry…no time for Star cards this time :o)