1. Dr. Bill Pezzaglia Nuclear Physics1
AstroPhysics Notes
Dr. Bill Pezzaglia Nuclear Physics
Updated: 2014Feb14
Rough draft

2. Tom Lehrer: Elements

3. Nuclear Physics A. Nuclear Structure B. Nuclear Decay3
Nuclear Physics
A. Nuclear Structure
B. Nuclear Decay
C. Nuclear Reactions

4. 4
A. Nuclear Structure
Parts of the Atom
Isotopes
Nuclide Table

5. 1. Parts of Atom 5
Electrons (negative charge) orbital diameter approximately m
Nucleus size m
Nucleus made of
Protons (+ charge)
Neutrons (neutral)

6. 2. Isotopes 6
Isotopes have same atomic number (number of protons)

7. 2b. Nomenclature 7
Z: Atomic Number Number of Protons Tells what is chemical “X”
N: Neutron Number Number of Neutrons
A: Mass Number Number of Nucleons A=Z+N
Don’t really need “Z”: You know Carbon 14 has 6 protons, because its carbon.

8. 2c. Atomic Mass 8 AMU: Atomic Mass Unit Carbon 12 is exactly 12 amu
Or 1 mole of C12 is 12 grams [1 mole=6.021023 atoms]
Naturally occurring carbon
98.9% C12 ( amu)
1.1% C13 ( amu)
Average:

9. 3. Nuclide Table 9 Isotopes: same Z C12, C13
G. Seaborg 1940
Atomic number is on vertical axis, Neutron number on the horizontal
Isotopes: same Z C12, C13
Isotones: same N C14, N15, O16
Isobars: same A C14, N14, O14

10. Nuclide Table (Small Z)10

11. Nuclide Table (BIG Z) 11 97 96 95 94 93 92 91

12. B. Nuclear Decay Activity Decay Law Modes (Alpha, Beta, Gamma) Dosage12
Activity
Decay Law
Modes (Alpha, Beta, Gamma)
Dosage

13. 1. Radioactivity 13 (a) Phenomena
1898 Term coined by Pierre & Marie Curie (radiation-active)
1896 Becquerel discovers radioactive emissions (“Becquerel Rays”) of uranium salts (using photographic plates)
(b) Units
Activity: decays per second (emissions per second)
new SI unit Bq=becquerels= decays per second
Old Unit: Curie: 1 Ci = 3.7×1010 Bq (activity of 1 gram of radium 226)
(c) Decay Constant
Activity is proportional to number of nuclei present “N”
Activity = N
Decay Constant “” is probability of decay per second.
Antoine Henri Becquerel ( ), 1903 Nobel Prize for discovery of radioactivity

14. 14
2. Decay Law
1902 Rutherford & Soddy realized that all radioactive decays obeyed the same exponential decay law
Half Life: time for half of sample to decay. It is related to decay constant :
This “emination law” showed radioactive decay was not deterministic, but statistical (indeterminant) in nature.

15. 3. Decay Modes 15
Rutherford (1897) clarifies that there are two types of “Becquerel Rays”, alpha (which he identifies as a Helium nucleus), and beta which is 100x more penetrating.
By emitting any of these, the element undergoes “transmutation” into another element.

16. 3. Decay Modes 16

17. 3a. Alpha Decay Example: Most alpha emitters are heavy nuclei17
3a. Alpha Decay
Alpha particle is a Helium Nucleus
Example:
Most alpha emitters are heavy nuclei

18. 18
3b. Beta Decay
Beta particle is actually an electron, identified in 1897 by Thomson.
Beta decay involves a “neutrino” (described by Enrico Fermi in 1930s)
Example: Neutron decays to proton (plus beta and neutrino) with 12 min halflife

19. 19
3b. More Beta Decay
Nuclei with neutron excess will change a neutron into a proton by beta decay, emitting an “anti-neutrino” and beta minus (aka an “electron”)
Example: Carbon 14 decay

20. 20
3b. Inverse Beta Decay
Nuclei with proton excess will change a proton into a neutron by inverse beta decay emitting a neutrino and beta plus (aka positron or anti-electron).
Hydrogen fusion in sun changes 2 protons into neutrons to make helium:

21. 21
3c. Gamma Decay
“Gamma Rays” discovered 1900 by Villard (later identified as high energy photons, which were what Becquerel originally saw)
For example: If a - (electron) combines with its “antimatter” particle, the + (positron), they will annihilate, creating two gamma rays

22. 22
C. Nuclear Reactions
Stability
Fission & Fusion
Efficiency

23. 1. Nuclear Stability 23
(a) Binding Energy: the energy required to remove one nucleon from the nucleus
The mass of an atom is LESS than the sum of its parts due to negative potential energy of nuclear force.
Mass Defect: m=(Zmp+Nmn-matom)
Binding Energy: BE=m( MeV/u)

24. 1b. Binding energy per nucleon24
Low Z: more nucleons means more nuclear force, hence more stable
High Z: nuclear force is short range, big nuclei unstable
Iron is most stable nuclei

25. 25
1c. Nuclear Force
Aka “strong force”. This is what holds the protons together in a nucleus
Nucleons attract each other
Force is short range, hence big nuclei are unstable

26. 2b Fusion 26
Combine two (or more) small nuclei to make a bigger, more stable, nuclei
Fusion of 4 Hydrogen to Helium is how sun produces energy
Fusion of 3 Helium to Carbon is how “red giants” create energy
All elements up to iron in the universe were made this way inside of stars (“nucleosynthesis”).

27. 2c Fission 27
Large (bigger than iron), unstable nucleus is split into two (or more) smaller, more stable nuclei
Fission can be induced by tossing a slow neutron at a nucleus.
During fission, often 2 or more neutrons are released, which can create more fissions (chain reaction)
Nuclear reactors generate power from fission of U235.

28. 28
3. Efficiency
The reaction that the sun uses to generate energy is to fuse (four) hydrogen into helium.
The mass of 4 Hydrogen’s (protons) is: 4( ) = amu
This is more than the Helium ( ), so there was a small amount of mass converted to energy m=( )= amu
Converted to a percentage: / = or 0.7% of the mass was converted to energy. This is called the efficiency.

29. References/Notes 29
Physics Today, Feb (1996) 21-26, “The Discovery of Radioactivity”