1. Nuclear Radiation

2. Some history
Insights associated with modern physics began with qualitative study of gaseous discharge and cathode ray tubes in the 1870s and 1880s.
Roentgen: x-rays in 1895
Thomson: electron ID in 1897
X-rays were discovered to ionize air
Up until then, only ions observed were in solution (e.g., Na+, Cl-)
Therefore, neutral atoms must possess minute electric charges as constituents
Atom is a complex structure
Electrical charges enter into its make-up

3. Anatomy of an atom Nucleus contains Protons Neutrons
Mass: x kg, sometimes abbreviated u
u = ‘atomic mass unit’, defined as x kg
Charge: 1.60 x C, sometimes abbreviated +1e
# of protons determines location on periodic table
Every atom of helium has two protons, every atom of oxygen has 8, etc.
Neutrons
Mass: x kg, u
Charge: 0 C
# of neutrons determines isotope and atomic weight

4. Anatomy of an atom Nucleus is orbited (kind of) by electrons
Charge: x C, frequently abbreviated -1e
Mass: 9.11 x kg, u
# of electrons typically matches number of protons
Electrically neutral atoms
When an atom loses an electron, it becomes a positively-charged ion
When an atom gains an electron, it becomes a negatively-charged ion

5. Nomenclature A = Z + N Mass number, A = number of protons, Z
+ number of neutrons, N
A = Z + N

6. Nomenclature
There is a range of neutron numbers that form stable nucleus.
Different number of neutrons  different isotopes
Written as Achemical symbol
20Ne has 10 protons (all neon atoms do) and 10 neutrons
22Ne has 10 protons and 12 neutrons

7. Think about…
Carbon is the sixth element in the periodic table. How many protons and neutrons in an isotope of 14C? Assuming it is electrically neutral, how many electrons?

8. Mass of nucleon
The nucleus of 4He (the most common isotope of helium) is made of two neutrons and two protons. If you add the mass of two neutrons and two protons, you will find that it is slightly larger than the mass of a helium atom. The difference is the total binding energy, calculated by E=mc2
Stable nuclei cluster around line of stability
No stable nuclei Z>83 (bismuth). Heavier elements, up to Z=92 (uranium) exist in nature but are radioactive
Unstable nuclei are in bands on both sides of line of stability
For lightest elements, N = Z
@ Z = 40, N/Z ~1.2
@ Z = 80, N/Z ~1.5

9. Nuclear binding energy
Some isotopes have more binding energy that others. More binding energy = more stable

10. Radioactivity
Marie and Pierre Curie discover that vial of radium maintains itself permanently above room temperature and that, when placed in a container, each gram of radium gives off 80 calories per hour.
Sufficient heat to melt its own weight of ice.
Where does energy come from?
Henri Becquerel discovers uranium can expose film and pass through objects.
Rutherford discovers uranium emits two types of rays.

11. Types of radiation
Eventually, three types of radiation were discovered
Alpha, 
Charge +2e
Mass: 4u
Beta, 
Charge: -1e
Mass: = electron
Gamma, 
Charge: 0
Mass: = photon

12. But… Both  and  radiation have mass Cloud chambers help illustrate
How can we account for continuous emission of material particles in the apparent absence of chemical change?
Cloud chambers help illustrate

13. Transmutation of elements
Elements change from one form to another and give off energy
In the form of moving mass
Electromagnetic waves

14. Nuclear stability
Stable nuclei cluster along ‘valley of stability’ No stable Z>83 Stable N/Z changes

15. Forces in the nucleus
Nucleus is made of positively-charged protons in very close proximity. Why don’t it fly apart? Something must be holding it together.
Strong nuclear force:
Attractive force between any two nucleons
Does not act on electrons
Very short range, i.e., ~radius of nucleus
In that range, stronger than electrostatic force
Extra neutrons = extra nuclear ‘glue’

16. Alpha Radiation
See Knight, Examples 30.2 and 30.3

17. Beta radiation - decay: + decay:
An unstable neutron decays into a proton and spits out an electron.
+ decay:
An unstable proton decays into a neutron and spits out a positron.

18. Gamma radiation
Atom in an excited state makes transition to lower-energy state by emitting a powerful photon.

19. Radiation vs. Radioactive
Energy associated with , , and  radiation is high enough to ionize nearby atoms and molecules
Bad for living things:
Ions drive chemical reactions
Potential damage to DNA
Mostly irrelevant to other objects
Irradiated objects do NOT become radioactive unless their nuclei changes, which they don’t.