1. Radioactivity Types of particles: Alpha particles
Two protons + two neutrons
Same as helium-4 nucleus
+ 2 charge; deflected by a magnetic field, and attracted to negative charges

2. Alpha particles Largest particle of radioactivity Short range
Stopped by sheet of paper
Most damaging due to large mass
Alpha tracks in a cloud
chamber

3. Nuclear equations Mass must be conserved
Mass numbers and atomic numbers must have same sum on each side of equation
Result of alpha emission: mass number decreases by 4, atomic number decreases by 2
Note symbol for alpha particle – sometimes written 42a or just a

4. b Beta Particles Consist of free electrons Low mass, -1 charge
Medium range, medium penetrating power
Stopped by thick wood, thin sheet of lead b
Symbol is the Greek letter beta or 0-1e
Produced by a neutron, which turns into a proton

5. Nuclear equations
In beta decay a neutron turns into a proton and ejects an electron
Mass number does not change, and atomic number increases by 1
Example of transmutation

6. g Gamma Radiation Consists of high-energy photons
No rest mass, no charge
Not deflected by magnetic field
Long range, very penetrating
Accompanies many other types of decay g
Symbol is Greek letter gamma
Only product of IT – internal transition
Produces no change of mass or atomic numbers

7. Positron Emission Tomography
Other types of decay
Positron Emission
Positrons are the electron’s antiparticle
Same characteristics as electron, except for positive charge
Symbol: b+ or 01e
Positron Emission Tomography
(PET scan)

8. Positron emission
In positron emission a proton ejects a positron and becomes a neutron
Mass number does not change
Atomic number decreases by one

9. Electron Capture
If there are too many protons in a nucleus, it may capture an electron
A proton becomes a neutron
Symbol for an electron

10. Electron capture Mass number stays the same
Atomic number decreases by one
Same result as positron emission

12. Nuclear Stability
Nuclear particles (protons and neutrons) are called nucleons
Nucleons are held together by nuclear strong force (short range, very strong)
Neutrons are “glue” – necessary to hold the nucleus together
Without neutrons the nucleus would fly apart due to electrostatic repulsion

13. Nuclear Band of Stability

14. Nuclear Magic Numbers
Nuclei with certain numbers of protons or neutrons are especially stable
“Magic numbers” are 2, 8, 20, 28, 50, 82, and 126
When both neutrons and protons are magic numbers, the nucleus is specially stable: Pb

15. Nuclear Magic Numbers
Most stable nuclei have the same “magic number” of protons and neutrons: 42He, 168O, and 4020Ca
“Even-odd” rule: Nuclei with even numbers of protons and neutrons are more stable than odds:
Stable isotopes: 264
Both even: Both odd: 5

16. Stability and Decay Above the stability band: Too many neutrons
Beta decay reduces the neutron/proton ratio
Very large nuclei (Z>83) undergo alpha decay, which reduces the size of the nucleus

17. Stability and decay Below the band of stability: too many protons
Positron emission or electron capture
Protons are reduced, neutrons increased
11p n b
11p e n

18. Decay series

19. Induced Transmutation
Transmutation can be induced by allowing high-energy particles to strike atomic nuclei
42He + 147N  178O + 11p
23892U + 10n  23992U  23993Np + 0-1e
23993Np  23994Pu + 0-1e
10n + 147N  146C + 11H

20. Radioactive Decay Radioactive decay
Radioactive isotopes decay at predictable rates
Half Life: the time it takes for 1/2 of a sample to decay
Half of the remaining sample decays every half life period

21. Half Life Graph

22. Half Life Follows exponential decay
Moment of decay of any one particle is unpredictable
Example: Radon-222 decays with a half life of 3.8 days. Approximately how long will it take for 9.5 grams of a 10 gram sample to decay?

23. Half Life Problems
Solution: Divide sample mass in half until 0.5 grams or less is reached.
10/2 = 5 (one half life)
5/2 = 2.5 (two half lives)
2.5/2 = 1.25 (three half lives)
1.25/2 = (four half lives)
0.625/2 = (five half lives)

24. Half life Problems Four half lives = 4 HL x 3.8 days/HL = 15.2 days
Five half lives = 5 HL x 3.8 days/HL = 19 days
Therefore, 9.5 grams of a 10 gram sample will decay in somewhere between 15.2 and 19 days.

25. Half Life Problems
Example #2: Sally has a 15.0 g sample of phosphorus-32 (half life days). About how much will be left two months later (60 days)?
Find time in half-lives: 60 days/14.28 days/HL = 4.20 half lives.
Multiply the sample mass by (1/2)y, where y = number of half-lives (use xy key on calculator)

26. Nuclear Reactions and Energy
Half Life Problems
15.0g(1/2)4.20 = 15.0g(0.0544) = g remaining
Half life equation: Nt = N0(1/2)t/t1/2 or
Nt = N0e-lt where l is the decay constant
t = (t1/2/0.693)ln(N0/Nt)
Nuclear Reactions and Energy
Mass is not strictly conserved in nuclear reactions
Some mass is lost as energy

27. Nuclear Reactions and Energy
Mass to energy conversion is governed by DE = Dmc2, where c = the speed of light in a vacuum (3.0x108m/s)
Nuclear binding energy is the energy lost when the nucleus is formed.
Mass equivalent of the nuclear binding energy is the mass defect.
Protons and neutrons in the nucleus have less mass than separate nucleons

29. Calculating Binding Energy
Example:
Mass of 1 proton = amu
Mass of 1 neutron = amu
Mass of 1 electron = amu
If 1 amu = 1.66 x 10-24g, calculate the binding energy of an atom of helium-4 (mass amu)

30. Binding energy of helium-4
Mass of constituents
Protons: amu/p(2p) = amu
Neutrons: amu/n(2n) = amu
Electrons: amu/e(2e) = amu
Total: amu
amu(1.66x10-24g/amu) = 6.70x10-24g
Helium atom: amu(1.66x10-24g/amu) = 6.64x10-24g)

31. Binding energy of helium-4
Mass deficit = 6.70x10-24g x10-24g = 0.06x10-24g = 6x10-26g = 6x10-29kg
Binding energy: DE = Dmc2
DE = 6x10-29kg(3.00x108m/s)2 = 5x10-12J
Energy per gram: one gram of helium-4 would have 1g/(6.64x10-24) = 1.51x1023 atoms
1.51x1023 a/g(5 x 10-12J/a) = 8 x 1011J/g

32. Binding energy of helium-4
8 x 1011J/g(1 kW-hr/3600 J) = 2 x 108 kW-hr
Average household uses 10,656 kW-hr/yr
2 x 108 kW-hr/10,656 kW-hr/(house-yr) = 20,000
Binding energy in one gram of helium-4 could power 20,000 average households for one year
Alternatively, it could power one house for 20,000 years, or Al Gore’s mansion for 904 years.

33. Nuclear Fission
Some larger nuclei will split into two parts when struck by a neutron
The two smaller nuclei are more stable, so energy is released
The two smaller nuclei will have a higher binding energy per nucleon
Neutrons are also released, producing a chain reaction

34. Nuclear Fission

37. Nuclear chain reactions
Occur if the product of the reaction is necessary to start new reactions
10n U --> 23692U --> 9236Kr Ba + 310n
Critical mass - minimum mass necessary to sustain a chain reaction
Large enough critical mass will explode

38. Nuclear Power Plants
Nuclear fuel is usually a supercritical mass of U-235 enriched uranium
Reaction is promoted by a moderator - a material that slows neutrons down so they will cause fission - usually carbon or D2O
Nuclear reactor at Chernobyl

39. Nuclear Power Plants
Reaction is controlled by control rods (cadmium or boron), which absorb neutrons
Reaction generates heat, which makes steam to run a turbine
CROCUS, a small research
nuclear reactor

40. Geiger Counter Counts individual particles of radioactivity
Ionizing radiation enters the tube through a mica window
Ionization of gas in tube allows current to flow for an instant between high voltage cathode and anode