1. NUCLEAR DECAY

2. How Many Neutrons? The number of neutrons in a nucleus can vary
Range limited by the degree of instability created by
having too many neutrons
too few neutrons
Stable nuclei do not decay spontaneously
Unstable nuclei have a certain probability to decay

3. Nuclear Stability Facts
270 Stable nuclides
1700 radionuclides
Every element has at least one one radioisotope
For light elements (Z20), Z:N ratio is ~1
Z:N ratio increases toward 1.5 for heavy elements
For Z>83, all isotopes are radioactive

4. Nuclear Stability Facts
The greater the number of protons, the more neutrons are needed
“Magic numbers” of protons or neutrons which are unusually stable
2, 8, 20, 28, 50, 82, 126
Sn (Z=50) has 10 isotopes; In (Z=49)& Sb (Z=51) have only 2
Pb-208 has a double magic number (126n, 82p) & is very stable

5. Band of Nuclear Stability
Stable isotopes form a band of stability from H to U
Z:N ratios to either side of this band are too unstable & are not known

7. Nuclear Band of Stability

8. Radioactivity
The spontaneous decomposition of an unstable nucleus into a more stable nucleus by releasing fragments or energy.
Sometimes it releases both.

9. Electromagnetic Radiation
Electromagnetic radiation is a form of energy that can pass through empty space
The shorter the wavelength, the more energy it possesses
gamma rays are very energetic
radio waves are not ver energetic

10. TYPES OF RADIOACTIVE DECAY

11. Alpha Emission ZAX Z-2A-4Y + 24 92235U 90231Th + 24
Identity of the atom changes
Quick way for a large atom to lose a lot of nucleons (increases N:P ratio)

12. Beta Emission Ejection of a high speed electron from the nucleus
ZAX Z+1AY 
1940K Ca 
Decrease N:P ratio; identity of atom changes

13. Gamma Emission Emission of high energy electromagnetic radiation
Usually occurs after emission of a decay particle forms a metastable nucleus
Does not change the isotope or element

14. Radiation Energetics Alpha Particles
relatively heavy and doubly charged
lose energy quickly in matter
Beta Particles
much smaller and singly charged
interact more slowly with matter
Gamma Rays & X-rays
high energy
more lengthy interaction with matter

15. PENETRATING DISTANCES
alpha
beta
gamma

16. PENETRATING DISTANCES
paper
alpha
beta
gamma

17. PENETRATING DISTANCES
paper
alpha
beta
gamma
aluminum

18. PENETRATING DISTANCES
paper
alpha
beta
lead
gamma
aluminum

19. HAZARDS POSED BY DIFFERENT TYPES OF RADIATION
Alpha Emissions
easily shielded
considered hazardous if alpha emitting material is ingested or inhaled
Beta Emissions
shielded by thin layers of material
considered hazardous when beta emitter is ingested or inhaled
Gamma Emissions
need dense material for shielding
considered hazardous when external to the body

20. Radioactive Decay Rates
Relative stability of nuclei can be expressed in terms of the time required for half of the sample to decay
Examples: time for 1 g to decay to .5 g
Co yr
Cu h
U x 109 yr
U x 108 yr

21. HALF-LIFE
The level of radioactivity of an isotope is inversely proportional to its half-life.
The shorter the half-life, more unstable the nucleus
The half-life of a radionuclide is constant
Rate of disintegration is independent of temperature or the number of radioactive nuclei present

23. The grid below represents a quantity of C14. Each time you click,
one half-life goes by.
C14 – blue N14 - red
Half
lives
% C14
%N14
Ratio of
C14 to N14
100% 0%
no ratio
As we begin notice that no time
has gone by and that 100% of the
material is C14
Age = 0 half lives (5700 x 0 = 0 yrs)

24. The grid below represents a quantity of C14. Each time you click,
one half-life goes by.
C14 – blue N14 - red
Half
lives
% C14
%N14
Ratio of
C14 to N14
100% 0%
no ratio 1
50%
1:1
After 1 half-life (5700 years), 50% of
the C14 has decayed into N14. The ratio
of C14 to N14 is 1:1. There are equal
amounts of the 2 elements.
Age = 1 half lives (5700 x 1 = 5700 yrs)

25. The grid below represents a quantity of C14. Each time you click,
one half-life goes by.
C14 – blue N14 - red
Half
lives
% C14
%N14
Ratio of
C14 to N14
100% 0%
no ratio 1
50%
1:1 2
25%
75%
1:3
Now 2 half-lives have gone by for a total
of 11,400 years. Half of the C14 that was
present at the end of half-life #1 has now
decayed to N14. Notice the C:N ratio. It
will be useful later.
Age = 2 half lives (5700 x 2 = 11,400 yrs)

26. The grid below represents a quantity of C14. Each time you click,
one half-life goes by.
C14 – blue N14 - red
Half
lives
% C14
%N14
Ratio of
C14 to N14
100% 0%
no ratio 1
50%
1:1 2
25%
75%
1:3 3
12.5%
87.5%
1:7
After 3 half-lives (17,100 years) only
12.5% of the original C14 remains. For
each half-life period half of the material
present decays. And again, notice the
ratio, 1:7
Age = 3 half lives (5700 x 3 = 17,100 yrs)

27. How can we find the age of a sample without knowing how
C14 – blue N14 - red
How can we find the age of a
sample without knowing how
much C14 was in it to begin
with?
1) Send the sample to a lab which
will determine the C14 : N14
ratio.
2) Use the ratio to determine how
many half lives have gone by
since the sample formed.
Remember, 1:1 ratio = 1 half life
1:3 ratio = 2 half lives
1:7 ratio = 3 half lives
In the example above, the ratio is 1:3.
3) Look up the half life and multiply that
that value times the number of half lives determined by the ratio.
If the sample has a ratio of 1:3 that means it is 2 half lives old. If the half
life of C14 is 5,700 years then the sample is 2 x 5,700 or 11,400 years old.

28. C14 has a short half life and can only be used on organic material.
To date an ancient rock, use the uranium–lead method (U238 : Pb206).
Here is our sample. Remember we have no idea
how much U238 was in the rock originally but all
we need is the U:Pb ratio in the rock today. This
can be obtained by standard laboratory techniques.
As you can see the U:Pb ratio is 1:1. From what
we saw earlier a 1:1 ratio means that 1 half life
has passed.
Rock Sample
Now all we have to do is see what the half-life for U238 is. We can
find that information in half-life reference tables.
1 half-life = 4.5 x 109 years (4.5 billion), so the rock is 4.5 billion
years old.

29. Element X (Blue) decays into
Element Y (red)
The half life of element X is
2000 years.
How old is our sample?
See if this helps:
1 HL = 1:1 ratio
2 HL = 1:3
3 HL = 1:7
4 HL = 1:15
If you said that the sample was
8,000 years old, you understand
radioactive dating.

30. Element X (blue)
Element Y (red)
How old is our sample?
We know that the sample was
originally 100% element X. There are
three questions:
First: What is the X:Y ratio now?
Second: How many half-lives had to
go by to reach this ratio?
Third: How many years does this
number of half-lives represent?
1) There is 1 blue square and 15 red squares. Count them. This is a 1:15 ratio.
2) As seen in the list on the previous slide, 4 half-lives must go by in
order to reach a 1:15 ratio.
3) Since the half life of element X is 2,000 years, four half-lives
would be 4 x 2,000 or 8,000 years. This is the age of the sample.

31. The half-life of this element is 1 million years.
Question may involve
graphs like this one. The most
common questions are:
"What is the half-life of this
element?"
Just remember that at the end
of one half-life, 50% of the
element will remain. Find 50%
on the vertical axis, Follow the
blue line over to the red curve
and drop straight down to find
the answer:
The half-life of this element is 1 million years.

32. After 2 million years 25% of the original material will remain.
Another common question is:
"What percent of the material
originally present will remain
after 2 million years?"
Find 2 million years on the
bottom, horizontal axis. Then
follow the green line up to the
red curve. Go to the left and
find the answer.
After 2 million years 25% of the original material
will remain.

34. Trying To Reach Nuclear Stability
Some nuclides (particularly those Z>83) cannot attain a stable, nonradioactive nucleus by a single emission.
The product of such an emission is itself radioactive and will undergo a further decay process.
Heavy nuclei may undergo a whole decay series of nuclear disintegrations before reaching a nonradioactive product.

35. The Four Known Decay Series

36. Decay Series
A series of elements produced from the successive emission of alpha & beta particles

38. RADON GAS

39. Radon-222
Originates from U-238 which occurs naturally in most types of granite
Radon-222 has a half-life of days
It decays via alpha emissions
This isotope is a particular problem because it is a gas which can leave the surrounding rock and enter buildings with the atmospheric air