1. Do Now (3/17/14):
What are some words and images that come to mind when you hear the word “radioactivity”?
What is an isotope?
What makes an isotope different than its element?

2. Radioactivity
4/23/12 

3. Lesson Objectives
Describe nuclear reactions and perform balancing of nuclear reactions by solving problems.
Apply radioactivity equations by solving problems.

4. Nuclear reaction
A reaction in which the number of protons or neutrons in the nucleus of an atom changes.

5. Number of protons in the nucleus of the atom
Atomic number
Number of protons in the nucleus of the atom

6. Sum of protons and neutrons in the nucleus of the atom
Mass number
Sum of protons and neutrons in the nucleus of the atom

7. Alpha decay
Radioactive decay process in which the nucleus of an atom emits an alpha particle

8. Alpha Particle
Nucleus of a helium atom

9. Beta decay
Radioactive decay that occurs when a neutron is changed to a proton within the nucleus of an atom, and a beta particle and an antineutrino are emitted

10. Gamma decay
Radioactive process of decay that takes place when the nucleus of an atom emits a gamma ray.

11. http://library. thinkquest
y/radioactivity.html

12. Isotope
Atomic nuclei having the same number of protons but different number of neutrons

13. 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
Radioactive Isotope display

14. A Half-Life Is the Time Required for ½ the Atoms of a Substance to Undergo Radioactive Decay Applet Animation
T1/2 = time for half the sample             to disintegrate
Assume T1/2   =  5 years
Number of nuclei present at time t = 0:
N0     =
When t = 5 yrs,    N = 50
t = 10 yrs,   N = t = 20 yrs,   N = 125.
Calculate the half-life animation

15. Applet Animation

16. Half Life:
Half-life: time needed for half of remaining mass of element to decay

17. Example #1:
Fermium-253 has a half-life of 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?

18. Decay Rate
T1/2=half life
λ=decay rate

19. Example #2:
The half life of Zn-71 is 2.4 minute. If one had 100 g at the beginning, what is the decay rate of Zn-71?

20. Mass remaining
m=mass remaining
Original mass

21. Example #3:
The half life of Zn-71 is 2.4 minute. If one had 100 g at the beginning, how many grams would be left after 7.2 minutes elapsed?

22. Practice:
Use the rest of class to work on the paper: Radioactivity; problems: #2,5,6, and 7

23. Do Now (4/24/12):
Pd-100 has a half-life of 3.6 days. If one had 6.02x1023 atoms at the start, how many atoms would be present

24. Do Now (4/24/12):
U-238 has a half-life of 4.46x109 years. How much U-238 should be present in a sample 2.5 x years old, if 2 grams were present initially?

25. Using Logarithms

26. Using Logarithms

27. Using Logarithms
Solving for λ:

28. Using Logarithms
Solving for t:

29. The Uranium Decay Series
Decay series animation
The only radium that exists today is that which is created as a result of the decay of uranium.

30. Carbon-14 Production
Neutron enters nucleus and kicks out a proton.                     0n1  +  7N >  6C14  + 1p1

31. Carbon-14 Enters the Ecosystem

32. 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.

33. 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  

34. 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.

35. Radioactivity Equations
N(t)  =  population at time t N(0) =  population at time zero N0   =   N(0) l     =   decay constant
Example:    N0 = 1000
l = 2 x 10-3 years -1 When will N = 200?
N   = N0 e-lt                     (1)
e-lt = N /N0                      (2)
ln (e-lt) = ln (N /N0)         (3)
-l t = ln (N /N0)             (4)
  N(t) = N0 e-lt  
t = - [ln (N /N0)] / l              (5)
= - [ln (200/1000)] /2 x10-3      (6)
=  805 years

36. Half-Life Problem
The half-life of a radioactive substance is 10 hours.   What is the decay constant, l? N = N0 e-lt                                    (1)
0.50 N0 = N0 e-l10                       (2)
e-l10 = 0.50                                  (3)
ln(e-l10) = ln(0.50)                       (4)
-10 l  =                        (5)
l   = hrs-1              (6) 

37. 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 t             (8)
t = ln (0.20)                  (9)
t = 23 hours

38. Decay Constant and Half-Life
N = N0 e-lt                    (1) N0 =  N0 e-lT                  (2)
(T = half-life)
      e-lT =  0.50                       (3)  ln(e-lT) =  ln(0.50)                   (4)
-lT =                      (5)  
T =  0.693/l                   (6)
l  = 0.693/T                   (7)

39. Half-Life Example
38Sr90 (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? l  = 0.693/T1/2
= / 28.5
= years-1   
0.02 = e t
t = - ln(0.02) / years-1
=  161 years

40. Radioactivity Units
A  =  number of disintegrations          per second, activity A  = lN       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 Bq.

41. 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

42. Radiation Sickness Dose (rems) Effect 50-300 Sickness 400-500
Lethal   50%   (LD50)
Above 600
Lethal 100%   (LD100)

43. Calculate Rems from Rads (Relative Biological Effectiveness)
Radiation R
(rems/rad)
a-particles
 20
Neutrons
 10
Protons
b-particles
   1
g-rays
   1
X-rays
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 = J
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

44. Radon Poisoning
Uranium in earth's crust decays to radium, which decays to radon.
Radon is an odorless, tasteless, lighter-than-air gas which rises from the ground through cracks and fissures in the earth into homes. When breathed, the alpha- emitting 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.

45. Practice: Complete any four problems from the Radioactivity Worksheet
When you are finished, raise your hand so I can stamp it
Bring this paper to school with you this week!