Physics - Radioactivity

Background radiation comes from several sources. Some of these are naturally occurring and others are as a result of human activity.

       Radiation                      Percentage (%)

       Radon Gas                                                50

   Ground and buildings                                      14   

           Medical                                                     14

      Nuclear Power                                             0.3

      Cosmic Rays                                                10

    Food and Drink                                            11.5

         Other                                                       0.2

Atoms are made of protons, neutrons and electrons:

                                 Charge                          Mass

Protons                        +1                                   1

Neutrons                       0                                    1

Electrons                     -1                                1/1840

Each element has a different number of protons, for example Carbon has 6 protons and therefore has an atomic number of 6.

Not every atom of an element has the same number of neutrons and these different versions of the atom are referred to as isotopes. The number of protons and neutrons ina neucleus is the mass (neucleon) number.

Nuclei can be described using symbols

e.g 14 -------- Mass number which is the number of protons and neutrons. 

           c

       6 -------- Atomic number which is the number of protons.

Isotope    Symbol    Atomic Number    Mass Number    Number of neutrons

Carbon - 12           C                            6                                   12                                           6

Carbon - 14           C                            6                                   14                                           8

Uranium - 238       U                            92                                 238                                        146

Potassium - 41      K                           19                                   41                                          22

Radiation is produced when an unstable nucleus decays. This a random process which means that it is impossible to predict when a particular nucleus will decay.

Alpha Decay

Number on top or bottom must balance.

222                                 218                         4

       Ra        ------------              Rn         +           He     +      energy

 88                                   86                          2

Ra = Parent nucleus

Rn = Daughter nucleus

He = Alpha Particle

In Alpha Decay 2 protons and 2 electrons are emitted by the parent nucleus. These are ejected as an alpha particle to leave a daughter nucleus.

Beta Decay

14                               14                      0

       C   ----------------           N       +            e      +     energy

6                                  7                      -1

C = Parent nucleus

N = Daughter nucleus

e = Beta Particle

In Beta Decay a neutron in the parent nucleus decays to form a proton and electron (n ---- p + e). The proton stays in the nucleus increasing the atomic number by one and the electron is ejected as a fast moving beta particle.

Gamma Decay

Following alpha or beta decay the daughter nucleus can be left in an excited state with too much energy. The nucleus gets rid of its excess energy by emmiting an electromagnetic gamma ray (very strong radio wave).

Radiation will ionise molecules in living cells. This causes damage which may may cause cell mutation leading to cancer or in the case of high doses death of the cell.

If the source of radiation is outside the body the skin will stop alpha particles and therefore it will be the beta particles and gamma rays which will cause the most damage.

If the source of radiation is inside the body for example it has been eaten, alpha particles would pose the greatest risk due to their strong ionization effect.

Geiger-Muller Tube

The ions produced by radiation allow a small current to flow in the geiger-muller tube, this current is detected and converted into a count

Photographic Film

Radiographers, workers in nuclear power plants and air crew all wear a badge containing a small piece of photographic film wrapped in paper. Periodically the film is changed and sent to be developed. The darker the film the more radiation the wearer has been exposed to.

Radiation     Ionising Power     Penetrating power     Example of range in air

Alpha                   Strong                          Weak                                5-8 cm

Beta                    Medium                       Medium                          500-1000 cm

Gamma                Weak                          Strong                          Virtually infinite

Radiation stopped by                                          Description

           Paper                                       He Nucleus or 2 protons and 2 neutrons

    Thin Aluminium                                                     An Electron

   Thick lead sheet                                          An electromagnetic wave

Radiation is emitted when the nucleus of an atom decays.

The time taken for half of the total number of numbers in any radioactive sample to decay is known as the half life.

The amount of radiation produced by a source is known as its activity. The activity is measured in bequerels (Bq) which is the number of decays per second (1000 Bq = 1 KBq).

Each half life, the activity of the source halves.

Some isotopes for example Uranium - 235 have very long half lives (700 million years) others for example Radon - 222 have much shorter half lives.

Example

120g of living wood has an activity of 24 Bq due to the Carbon - 14 in it. A 120g sample of wood from an archaeological site is found to have an activity of 6 Bq. If the half life of Carbon - 14 is 5,600 years old, how old is the wood.

   24 Bq --------------------- 12 Bq --------------------- 6 Bq

------ = 5, 600 years

Age = 5600 + 5600 = 11200 years old 

Thickness Monitoring

Detects whether a material is too thick or too thin. The thicker the material the amount of radiation detected decreases. Beta is used for less dense materials such as paper and Gamma is used for more dense materials for example steel.

Non Destructive Testing

e.g Weld Inspection

Radiocarcon Dating

Tiny amounts of radioactive carbon - 14 pass in and out of all living things. When the living thing dies this carbon becomes trapped, and as timke passes the radiation emitted gets less and less. By measuring this radiation the age of a sample can be determined.

Medical Tracers

Small quantities of radioactive isotopes, usually gamma emmiters with a short  half life, can be iinjested, inhaled or injected into the human body to act as tracers. These tracers show up on a x-ray photograph and can help the diagnosis of health problems.

Radio Therapy

Ionising radiation upsets the life processes of cells. If such radiation is directed towards unwanted cells such as cancerous growths it will kill them.

When atoms of Uranium - 235 are bombarded with slow moving neutrons the nucleus splits into smaller nuclei (Barium - 144 and Krypton - 89) and 2 or 3 individual neutrons. These fission fragments have kinetic energy which can be harnessed as heat.

The 3 neutrons released in the process can go on to cause the fission of 3 further nuclei leading to a chain reaction.

A nuclear reactor consists of 4 main elements:

1) A steel and concrete pressure vessel.

2) Uranium - 235 fuel rods.

3) A graphite core or moderator, this slows down fast moving neutrons so that they are more likely to be absorbed by Uranium - 235 and cause fission.

4) Cadmium or Boron control rods, these absorb neutrons and can be moved in and out of the core. As they are withdrawn the fission process speeds up and more heat is produced.

A coolant is used to extract the heat from the reactor. This heat is then used to produce steam to drive steam turbines which turn the electrical generators.

The products formed by the fission process are radioactive and have very long half lives. Dealing with these products is difficult, currently they are buried deep underground.

In 1911, Ernest Rutherford divised an experiment which involved firing alpha particles at a thin sheet of gold foil. It was found that the vast majority of the alpha particles went straight through the gold foil, but that once in a while an alpha particle was deflected and that a small proportion (1 in 8,000) were repeled. This suggested that most of the atom was empty space surrounding a small positively charged core known as the nucleus.

The amount of deflection depends on:

The speed of the alpha particle, more speed = less deflection.

he charge on the nucleus, more charge = more deflection.

How close an alpha particle is to the nucleus, closer = more deflection.