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Physics - Nuclear

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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.


Alpha Radiation

The Alpha particle is an electrically charged ( + ) particle emitted from the nucleus of some radioactive chemicals.   It contains 2 protons and 2 neutrons, and is the largest of the atomic particles emitted by radioactive chemicals.    Alpha Radiation is the least penetrating of all ionizing radiation and can be shielded by a few inches of air, pentrating power can be stopped by a piece of paper or the outer layer of skin.  Alpha radiation can cause ionization.

Beta radiation

Electrically charged ( - ) particles emitted from some radioactive chemicals.    It has the mass of an electron.   The Beta radiaton can be shielded by several inches of plastic, thin plywood and sheet metal.   Can penetrate up to 1/4 in. into the tissue  Beta particles can cause ionization.

Gamma ray short wave-length electromagnetic radiation

Gamma rays are released by some nuclear transformations ref above table.    It is similar to X-ray and will penetrate through the human body.    Both gamma and X-rays cause ionisation.  The effect of gamma rays can be reduced to permissible levels by shielding with lead, steel, or thick concrete.   Gamma radiation is strong enough to penetrate into the human body.   Gamma N X-rays cause ionization.


High energy neutrons can penetrate thick lead shields.   Neutrons can collide with atoms causing theme to eject electrons.   High density materials containing high levels of hydrogen atoms are necessary to stop neutron particles.  This radiation can penetrate through the human body

Measurement of Radiation /Dose

Note: The International Commission on Radiation Units and Measurements propose the use of the rad in favor of the gray (Gy), a unit 100 times larger.   Similarly, the rem is to be replaced by the sievert (Sv), again so that 100 rem = 1 Sv.  

Rad (rad) - Common Unit::

The rad represents a certain dose of energy absorbed by 1 gram of tissue.    It is a unit of concentration.   So if we could uniformly expose the entire body to radiation, the number of rads received would be the same whether we were speaking of a single cell, an organ or the entire body.

Rem (rem)- Common Unit::

Some forms of radiation are more efficient than others transferring their energy to the cell.    To have a level playing field, it is convenient to multiply the dose in rads by a quality factor (Q) for each type of radiation.    The resulting unit is the rem ("roentgen-equivalent man").    Thus, rem = rad x Q. X rays and gamma rays have a Q about 1, so the absorbed dose in rads is the same number in rems.   Neutrons have a Q of about 5 and alpha particles have a Q of about 20.   An absorbed dose of, say, 1 rad of these is equivalent to 5 rem and 20 rem respectively.

Becquerel (Bq)-SI unit:

The unit of radioactivity.   One Bq is 1 disintegration per second (dps).   One curie is 37 x 10 9 Bq.   Since the Bq represents such a small amount, you are likely to see a prefix used with Bq e.g. 37 GBq = 1 curie.

Curie:(Ci)- Common unit

A measure of the activity of the radioactive material.    (One Curie is equivalent of 3.7 x 10 10 disintegrations per second).

Roentgen (R)- Common Unit:

A special unit used for measuring exposure to radiation.   (2.58 x 10-4 coulomb per kilogram of air)

Gray (Gy)- Si Unit:

A quantity of energy imparted by ionizing radiation to a unit mass of matter.   A gray (abbreviated as Gy) is the amount of energy deposited in tissues; technically, 1 joule of energy per kilogram of tissue

Sievert (Sv)-SI Unit:

A unit of radiation dose that is used for radiation protection purposes. When an individual is exposed to mixed sources of radiation, the total biologically effective dose is calculated by multiplying the physical dose (expressed in units called gray) of each kind of radiation by a corresponding factor (called a quality factor or Q factor) specified for the type of radiation and its energy, after which these amounts are added together.   The factor for gamma rays is 1; therefore, 1 Sv = 1 Gy. The factor for the neutrons in atomic-bomb radiation is 10; therefore, 1 Sv = 0.1 Gy.

Coulomb/kilogram (C/kg):

Measure of exposure replacing the reontgen. 1 coulomb/kilogram (C/kg) = 3,880 roentgens

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 pCi1 nCi1 mCi 1 mCi1Ci
37 mBq37 Bq37 kBq37 MBq 37GBq
1 Bq1kBq1 MBq1 GBq 1TBq
27 pCi27 nCi27 mCi 27mCi 27 Ci
p = 10 -12n= 10 -9 m= 10 -6 m = 10 -3k = 10 3
M = 10 6G ) = 10 9    
Dose Quantities =Dose Units
Absorbed Dose(DTR)= Gray (Gy)
Equivalent Dose(HT)= Sievert (Sv)
Effective Dose(E)= Sievert (Sv)
Electrons/Photons All energies1
Neutrons < 10keV, > 20MeV , Protons > 2MeV5
Neutrons 10 - 100 keV, > 2-20MeV
Neutrons 100 keV, to 2 MeV
alpha Particles
Tissue wT
Skin, Bone ,surface 0.01
Bladder, Breast, Liver,Oeseophagus, thyroid,remainder0.05
Red bone marrow, colon,lung, stomach 0.12

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. Guidance for Radiation Accident Management....OAk Ridge Associated Universities ..Useful Notes on Radiation units etc
  8. Gamma Ray attenuation Properties of .......Detail document on radiation shielding.