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Notes on Nuclear Physics

Nuclear Physics

Atom Nucleus

The classical atom comprises Protons, Neutrons, and Electrons.    The atom has a number of protons and neutrons bonded using nuclear forces in the central nucleus.   The atomic number of an element is the number of protons in the nucleus of its atoms.   The atomic number defines its chemical properties.

An element always has the same number of protons but can have a different number of neutrons.   The atomic weight of an element is the sum of the weight of the protons and the neutrons.   The different atomic weights of an element (resulting from having different numbers of neutrons ) are called the isotopes of the element.

The masses of atoms and molecules are measured with atomic mass units (u)
One atomic unit = 1 u = 1.660 x 10 -27 kg .... This is exactly the same as 1/12 the mass of a stable carbon atom.

The mass of a proton mp = 1.673 x 10 -27 kg = 1.007277 u
The mass of a neutron mn = 1.675 x 10 -27 kg = 1.008665 u

Binding Energy

The mass of an atom is always less than the sum of the masses of the neutrons, protons and electrons.   It has a mass defect..   The energy equivalent of the missing mass is called the binding energy the higher the binding energy the more stable the atom.    The binding energy is effectively the energy released when an atom is formed from its elemental particles.   The mass defect of a nucleus with Z atoms and N neutrons is calculated from its atomic mass m as follows

Dm = Z . mH + N . mn - m

mH = the mass of the Hydrogen atom = 1.07825 u

To calculate the energy in Mega-electronvolt Dm is multiplied by 931 MeV/u.

Nuclear Reactions

Nuclear reactions include nuclear fission and nuclear fusion.    Nuclei of intermediate size have the highest associated binding energy.    These are therefore more stable than materials having lighter and heavier nucleus.   To create atoms of a stable atom with a high binding energy levels from lighter atoms (Fusion) or heavier atoms (Fission) results in the release of energy.

Nuclear Fission

Nuclear fission occurs by making the nucleus unstable by causing the nucleus of a heavy atom to absorb an additional neutron.  Following the absorbtion of the neutron the excited nucleus splits into two almost equal parts.  The fission products include a range of elements of mass number 72 to 160.  . The fission reaction also results in the release of two or three neutrons and a significant quantity of energy.

The engineering of nuclear fission reactors is concerned with achieving the conditions to cause the creation of suitable neutrons and the conditions to enable them to be absorbed.   There is then the requirements to control the resulting released neutrons to enable them to be safely used to create further fissions in a sustainable manner.

Nuclear Fusion

In the fusion process the fusion of two nuclei to form a single heavier nuclei results in a more stable system with the release of energy.   At typical fusion reaction is

D + D -> He3 + n + 3.25 MeV

 D = Deuterium a hydrogen isotope with an additional neutron He3 = A helium isotope with only one neutron

To achieve fusion the high atomic repulsion forces must be overcome before the components can be brought sufficiently close to allow the reactions to take place.   Very high temperatures have to be achieved ( as in a Hydrogen Bomb).    The fusion process has been confirmed theoretically but it has not yet been possible to achieve the conditions such that there is a net gain in the energy obtained by the process.

Nuclear Decay

When nuclei are unstable they undergo radioactive decay in more stable nuclei.   Five types of decay are identified below with the resultant external effect.

 Type of Decay Gamma Decay Alpha Decay Beta Decay Electron Capture Positron Capture Decay Event Emmission of gamma ray reduces energy of nucleus Emmission of alpha particle reduces size of nucleus. Emmsion of electron by proton changes it to a neutron Capture of electron by proton changes it to a neutron Emmission of Positron by a proton changes it to a neutron Daughter Product Reduction in energy level Atomic Number change Atomic Number change Atomic Number change Atomic Number change Reason for instability Nucleus has excess energy Nucleus is too large Nucleus has too many neutrons relative to protons Nucleus has too many protons Nucleus has too many protons

A positron is a positive electron and an alpha particle is the nucleus of a helium atom.

Half Life

A nucleus subject to radiactive decay always has a definite probability of decay during any time interval.  The half-life of a radioactive isotope is the time required for half of any initial quantity to decay.
Typical Half lives

 Pure Radium...... 1620 Years Carbon 14...... 5730 Years Cobalt 60...... 5 Years Uranium 238...... 4.5 billion Years

Dose Uptake Assessment
 Dose Uptake Assesment 1 pCi 1 nCi 1 mCi 1 mCi 1Ci 37 mBq 37 Bq 37 kBq 37 MBq 37GBq 1 Bq 1kBq 1 MBq 1 GBq 1TBq 27 pCi 27 nCi 27 mCi 27mCi 27 Ci p = 10 -12 n= 10 -9 m= 10 -6 m = 10 -3 k = 10 3 M = 10 6 G ) = 10 9 Dose Quantities = Dose Units Absorbed Dose(DTR) = Gray (Gy) Equivalent Dose(HT) = Sievert (Sv) Effective Dose(E) = Sievert (Sv) Radiation wR Electrons/Photons All energies 1 Neutrons < 10keV, > 20MeV , Protons > 2MeV 5 Neutrons 10 - 100 keV, > 2-20MeV 10 Neutrons 100 keV, to 2 MeV alpha Particles 20 Tissue wT Skin, Bone ,surface 0.01 Bladder, Breast, Liver,Oeseophagus, thyroid,remainder 0.05 Red bone marrow, colon,lung, stomach 0.12 Gonads 0.2

Sites & Links For Nuclear Physics

1. Hyperphysics....A site with lots of scientific information in simple laymans language. ;...
2. ABC's of Nuclear Science ....Detailed notes on nuclear physics
3. Interactive Chart of Nuclides ....An interactive chart of nuclide.   What more could you want
4. GCSE Physics - Radioactivity ....Lots of very pretty and informative tutorials
5. Basics of Radioactivity ....University of Michigan student lessons- very informative
6. Ratical ....No Immediate Danger..Prognosis for a Radioactive Earth by Dr Rosalie Bertell
7. Ionizing Radiation ....Farmingdale State University ..Useful Notes
8. Guidance for Radiation Accident Management....OAk Ridge Associated Universities ..Useful Notes on Radiation units etc
9. Gamma Ray attenuation Properties of .......Detail document on radiation shielding.