Nuclaer Physics

i.e. 10n → 11p + 0-1e + νe  

• 146C → 147N + 0-1β + νe + energy

γ Radiation:

• less powerful ≈ 0.1MeV
• Can pass through many cm of lead
• No charge
• No mass
• Travels at the speed of light
• Given out in large quantities (other two aren’t)
• Gamma rays are actually electromagnetic radiation of high frequency, i.e. high energy light
• After decays and interactions, nuclei may have excess energy, therefore being unstable.  This is where they release a γ particle with the excess energy.  Often happens with the decay.
• E.g. 13153I → 13154Xe + β- + γ

ENERGY LEVELS:

• MeV (M=million) is lots of energy, most reactions involved only a few eV.  i.e. a millionth of the energy involved in ionizing radiation.
• 1eV is 1x10-19 Joules

RADIATION detection:

• can be found using different detectors:

• magnetic field, when charged particles go through, they bend.
• Geiger counter, ionization chamber

Note:

• There are natural isotopes but if humans create them, it is called artificial transmutation. Shooting neutrons at an isotope until it becomes unstable.

Half Life:

• No external properties have an effect on radioactive decay.
• Decay is a random process governed by the half-life of the isotope.
• The half life, T½, is used to state the time in which an isotope has a 50% CHANCE OF DECAY
• Half life is very accurate if there are many billions of atoms.  The bigger the atom, the more accurate the answer is.
• Two graphical methods:  either the number of radioisotopes left, or the number of decays of radioactive particles.

ACTIVITY

• Is measured in Becquerel (Bq) which is A MEASURE OF DECAYS PER SECOND.
• Therefore 100Bq means that 100 atoms decay every second.

• Some substances are chosen for their short half life (in the body, medical purposes), decay quickly and give out radiation but don’t last long.  Others are chosen for a long half life, not a lot of radioactivity but last a long time.

Radiation and YOU

• It is impossible to avoid radiation: cosmic radiation from space (γ radiation)
• Background radiation is not a risk
• High levels of radiation can cause permanent damage and even death
• Ionizing power of radiation:  break atoms apart, destroying body cells.
• How bad it is depends on the absorbed dose:
• Absorbed does = energy absorbed ÷ mass of tissue  
• Absorbed dose measured in JOULES/KILOGRAMS or GRAYS (Gy)
• Absorbed does – quick way to gauge danger but different types of radiation can be more dangerous.
• So better idea of the actual danger of radiation: dose equivalent is used.
• DOES EQUIVALENT IS MEASURED IN SIEVERTS (Sv)
• Dose equivalent = absorbed dose x quality factor

• 1Sv is a huge dose of radiation that could lead to severe radiation sickness
• most doses measured in milli-Sievert (mSv) or micro-Sievert (μSv)

DAMAGE:

• two different types of radiation damage:

1. Somatic (short term):  in your own life time.  When ordinary cells are damaged by radiation.  Sometimes it is immediate, sometimes it become apparent for many years (especially internal injuries).  Horrific tissue damage/cancer can be done.
2. Genetic (very long term):  kids/grandkids lifetime.  If reproductive organs are exposed, DNA and cells can be changed/mutated.  This will affect the child

• Amount of radiation/damage:

• In Australia we are exposed to 2000μSv per year.
• Chemistry is dependant on the electrons surrounding the atoms
• Ionization changes this (bond them or break them apart). Changes the way things function.
• Knock off electrons that form bonds between atoms. If DNA is damaged, cells can lose their function.
• Radiation can affect cell division rates, damaged cells reproduces too fast you can have cancer.
• Benefits can also come from radiation, help control and cure cancer.

Nuclear Fission

• Before 1932, particle collisions were hard to study as a positive needed to be shot at a positive, which was obviously very hard
• After 1932, Chadwick discovered the neutron and methods of firing neutrons were developed.
• Artificial transmutation is now possible (changing something artificially)
• E.g. 23992U → 23993Np + β
• New elements were found but THEY WERE UNSTABLE
• Sometimes these atoms became so unstable after this neutron catching that the whole atom splits.  This is FISSION and the atom that splits is the FISSILE ATOM.
• Weapons use fission as a chain reaction
• The two products made are called the “DAUGHTER PRODUCTS” or “FISSION FRAGMENTS”
• Fission means split in half, decay means that bits come off
• Daughter products are often β emitters, making them dangerous.  Energy released can be worked out using:
• E=mc2
• Right hand side has less mass, lost mass is ‘missing mass’.  To calculate the energy, used formula.  

The Unified Mass Unit and E=mc2

• Kilograms are too big for deal with atoms, therefore unified mass unit is used instead of kg for small scale discussions.
• 1u = 1.66x10-27 = 931.5MeV

• For working out masses:

• for working out energy:

• And REMEMBER IeV = 1.6x10-19 J (so 1 MeV = 1.6x10-13 J)

• Commonly used masses:

• me= 0.000549 u (electron)
• mp = 1.007277 u (proton)
• mn = 1.008665 u (neutron)
• mH = 1.007825 u (Hydrogen atom)

• The mass of Hydrogen does not equal the mass of an electron and proton separately.
• The difference is called the MASS DEFECT
• (energy and mass are the same ‘stuff’)
• Missing mass means that it has just changed due to conservation of mass / conservation of energy, it has turned into conservation of mass-energy
• There is energy that binds the electron into the Hydrogen.
• The difference in masses is because of the binding energy.
• E=mc2 can tell us how much.    (m in kg, produces answer in J)

Binding Energy

• Energy of ‘mass difference’ or ‘mass defect’ is the binding energy of an atom.  
• When protons and neutrons are combined, the energy holds them together,
• When they separate, energy is released.
• To compare different atoms we used the averaged binding energy per nucleon.  The larger the value, the more stable it is.

• The isotope of Iron, Fe-56, is at the top of the “mountain”, most stable.  
• All elements that are left of Fe-56 can be combined in nuclear fusion with the release of energy
• All elements that are right of Fe-56 can be split in nuclear fusion with the release of energy.

This chart shows:

• As atoms gain more protons, the number of neutrons that are needed for stability increases faster.  For smaller elements, the number of protons is usually similar to the number of neutrons but further up this is no longer the case.
• ALL atoms greater than Bismuth (83rd element) are radioactive.  And ALL atoms greater than Uranium have been man made and do not occur in nature.
• Of all the isotopes, majority are radioisotopes
• Line of stability is the line that curves away from the diagonal N=Z line.