4/22/14 
Do Now (4/22/14) (7 minutes):
 What are some words and images that come
to mind when you hear the word
“radioactivity”?
 Define:
•Atomic Number
•Mass Number
Pass in any Spring Break Bonuses
•Isotope
•Half Life
Atomic number
Number of protons
in the nucleus of the
atom
Mass number
Sum of protons and
neutrons in the
nucleus of the atom
Isotope
Atomic nuclei having
the same number of
protons but different
number of neutrons
Nuclear reaction
A reaction in which the
number of protons or
neutrons in the nucleus
of an atom changes.
Alpha decay
Radioactive decay
process in which the
nucleus of an atom
emits an alpha particle
Alpha Particle
Nucleus of a
helium atom
Alpha Particle
What is the mass
number of an
alpha particle?
4
Beta decay
Occurs when a neutron is
changed to a proton and a beta
particle and an antineutrino are
emitted
Gamma decay
Radioactive process of decay
that takes place when the
nucleus of an atom emits a
gamma ray.
Half Life
 What is the Half-Life of an element?
Half Life:
 Half-life: time needed for half of remaining mass of
element to decay
t  (# halflives)T1
2
Which of the following would decay the
slowest?
Example #1 (2 minutes):
Fermium-253 has a half-life of 0.334
seconds. A radioactive sample is
considered to be completely
decayed after 10 half-lives. How
much time will elapse for this
sample to be considered gone?
Decay Rate
 T1/2=half life
 λ=decay rate
0.693

T1
2
Example #2: (2 minutes)
The half life of Zn-71 is
2.4 minutes. If a mass of
Zn-71 had 100 g at the
beginning, what is the
decay rate of Zn-71?
Mass remaining
m  m0e
 m=mass remaining
 m0 =Original mass
 t
Example #3 (2 minutes):
The half life of Zn-71 is 2.4
minutes. If a mass of Zn-71
had 100 g at the beginning,
how many grams would be
left after 7.2 minutes elapsed?
All Elements Have
Radioactive Isotopes
 All elements have more than one isotope
 Some isotopes of all elements are radioactive
 Some half-lives are so short that the isotope is not found
naturally
Which of the following would likely not be
found in nature?
Practice:
Use the rest of class to
work on the paper:
Radioactivity
worksheet.
Do Now (4/24/14) (5 minutes):
U-238 has a half-life of 4.46x109
years. What percentage of a
sample of U-238 would remain
after 25 x10 9 years old?
1. Find the decay rate.
2. Find the percentage left or
m
m0
Fraction Left Over
m
 Fraction of Mass
m0
 Convert from Fraction to Percentage
Using Logarithms
m  m0e
 t
Using the equation
above, rearrange and
solve the equation for
time.
m
 t
e
m0
m
 t
e
m0
Using Logarithms
 m 
ln  t
m0 
Using Logarithms
 Solving for t:
m
ln  
m
0

t

Solve for Decay Rate (30 seconds)
 Rearrange this
equation and
solve for the
decay rate, λ.
m
ln  
m
0

t

Using Logarithms
 Solving for λ:
m
ln  
m
0 


t
Grades
Three weeks of school left.
Check your grades this weekend.
Practice Extension
If you are working all of class,
without having to be told to stay
on task, you may turn in the
practice tomorrow.
 You will not be given any warnings to
stay on task.
Tough Problems
 If you get stuck, write down your givens
and identify which equation you
should use.
 Pay attention to the difference between
a material decaying to 30% and a
material decaying by 30%.
Practice:
Worksheet is due at the end
of class.
Keys are on the board.
If you finish early there are
bonuses.
Do Now (4/28/14) (5 minutes):
 If Uranium-238 decays to Thorium-234,
what is the resulting change in mass
number?
 What is emitted from this decay?
Decay
 The change in mass number tells us what
particle was emitted as a result of the decay.
 We look at the pre-decay mass number, then
the post-decay mass number. The difference
tells us what was emitted.
Conservation of ????
This is an example of what
physics theory?
Conservation of Mass
Two Day Lab
We will be in the Lab on Monday
and Tuesday.
Two Labs and a Bonus
You will be working in assigned
groups, check the front of the Lab
End of Today
 You need to complete at least one of
the labs by the end of class today.
 This will be worth 4 points towards
your lab grade.
End of the Semester
These may be the last two lab
grades and one of the last quizzes is
Wednesday.
They are worth a large percentage
of your fourth-quarter grade.
Lab
 Meet here tomorrow.
 To the lab ROOM 128!!!
Do Now (4/29/14) (5 minutes):
 Write down three things that you will
put on your notecard.
 Pass in your Do Now!!!
Lab
 By the end of class today you should
completed both labs.
 The last 10 minutes of class we will
review a few concepts for the quiz.
Lab
 There is a bonus if you finish early.
 If you finish the bonus study for your
quiz and make your notecard.
 Head over to Room 128.
Decay series animation
The Uranium Decay Series
The only radium that exists today
is that which is created as a result
of the decay of uranium.
Carbon-14 Production
Neutron enters nucleus and kicks out a proton.
1
0n
+ 7N14 ---------> 6C14 + 1p1
Carbon-14 Enters the Ecosystem
Carbon Dating
 Since living organisms continually exchange carbon with the atmosphere in
the form of carbon dioxide, the ratio of C-14 to C-12 approaches that of the
atmosphere.
 From the known half-life of carbon-14 and the number of carbon atoms in a
gram of carbon, you can calculate the number of radioactive decays to be
about 15 decays per minute per gram of carbon in a living organism.
Measuring the Age of Organic
Matter
A German tourist in the
Italian Alps discovered
the remains of the
"Iceman" in the ice of a
glacier in 1991.
Calculating the Iceman's Age
The current activity per gram of
carbon half what it would be if
the Iceman were alive.
Since the half-life of carbon-14
is about 5700 years, the
Iceman's remains are about
5700 years old.
Radioactivity Equations
N(t) = population at time t
N(0) = population at time zero
N0 = N(0)

= decay constant
N(t) = N0 e-t
Example: N0 = 1000
 = 2 x 10-3 years -1
When will N = 200?
N = N0 e-t
e-t = N /N0
ln (e-t) = ln (N /N0)
- t = ln (N /N0)
(1)
(2)
(3)
(4)
t = - [ln (N /N0)] / 
(5)
= - [ln (200/1000)] /2 x10-3 (6)
= 805 years
Half-Life Problem
The half-life of a radioactive substance is
10 hours. What is the decay constant, ?
-------------------------------------------------------N = N0 e-t
(1)
0.50 N0 = N0 e-10
e-10 = 0.50
ln(e-10) = ln(0.50)
-10  = -0.693
 = 0.0693 hrs-1
(2)
(3)
(4)
(5)
(6)
Half-Life Problem
From the previous problem, how much time will it
take for the sample's activity to fall to only 20% of
what it was originally?
---------------------------------------------N = 0.20 N0
(7)
0.20 N0 = N0 e-0.0693 t
-0.0693 t = ln (0.20)
t = 23 hours
(8)
(9)
Decay Constant and Half-Life
N = N0 e-t
(1)
0.50 N0 = N0 e-T
(T = half-life)
e-T = 0.50
(3)
ln(e-T) = ln(0.50)
-T = -0.693
(4)
(5)
T = 0.693/
 = 0.693/T
(2)
(6)
(7)
Half-Life Example
38Sr
90
(strontium-90) has a half-life of 28.5 years.
How long will it take for 98% of a sample of
strontium-90 to disappear?
----------------------------------------------------------------- = 0.693/T1/2
= 0.693 / 28.5
= 0.0243 years-1
0.02 = e-0.0243 t
t = - ln(0.02) /0.0243 years-1
= 161 years
Radioactivity Units
A = number of disintegrations
per second, activity
A = N
One becquerel (Bq) is one
disintegration per second.
One curie is the number of
disintegrations per second (the
"activity") of one gram of radium, or
about 3.7 x 10 10 Bq.
Units of Absorbed Radiation
Rad: 10 milli-joules per kilogram
20 rads of X-rays doesn't do the
same damage to humans as 20
rads of alpha particles.
---------------------------------------------Rem: an absolute biological
damage unit
Radiation Sickness
Dose
(rems)
Effect
50-300
Sickness
400-500
Lethal 50% (LD50)
Above 600
Lethal 100% (LD100)
Calculate Rems from Rads
(Relative Biological Effectiveness)
Radiation
a-particles
R
(rems/rad)
20
Neutrons
10
Protons
10
b-particles
1
g-rays
1
X-rays
1
Example:
How many rads of protons
will kill a person?
-----------------------------600 rems is fatal
RBE for protons is 10
Number of rads = 600 / 10
= 60
Example:
One joule of energy per
kilogram is absorbed in the
form of neutrons.
Will this prove fatal?
-------------------------------1 rad is ten milli-joules
1 rad = 0.010 J
Radon Poisoning
Uranium in earth's crust decays to radium,
which decays to radon.
Radon is an odorless, tasteless, lighterthan-air gas which rises from the ground
through cracks and fissures in the earth
into homes. When breathed, the alphaemitting radon can cause cancer of the
lung.
Radon is the single greatest source of
radiations for humans, providing about
200 milli-rems per year per person.