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Main Nuclear Physics Natural Radioactivity LessonIV_1_1 Nuclear Physics

Natural Radioactivity - Lesson IV.1.1

Key Terms:

radioactivity | isotopes | alpha particles |beta particles |gamma rays |dosimetry |absorbed dose |dose equivalent |quality factor |becquerel | IL, NUM, CCT

Henri Becquerel accidentally discovered the radioactivity of uranium.

Pierre and Marie Curie studied uranium ores and discovered that pitchblende and chalcolite were radioactive due to the presence of polonium and radium.

For their research of radioactivity, the Curies shared the 1903 Noble Prize for physics with Henri Becquerel.

Marie Curie

Radioactive SymbolBUT WHAT IS RADIOACTIVITY?Radioactive Symbol


  • Is the spontaneous breakdown of an unstable nucleus.
  • Results in the emission of particles or electromagnetic radiation.
  • It is found naturally and in artificially produced sources.
  • Radioactivity cannot be detected by human senses.
  • All naturally occurring elements with atomic numbers greater than 83 are radioactive, as well as some isotopes of lighter elements. [Isotope Page]

As a result of the work of Rutherford, Soddy, Villard and others, three types of radiation were identified.

  1. Alpha particles Alpha
    • are positively charged particles emitted from alpha decay
    • these particles are helium nuclei Helium
    • are slightly deflected in an electric or magnetic field
    • are emitted at high speeds
    • have the lowest penetration power - up to 5 cm in air
    • can be stopped by a thin layer of paper or aluminum
    • results in the original nucleus changing - atomic mass decreases by four and atomic number decreases by two
  2. Beta particles Beta
    • are electrons that are emitted from beta decay electron
    • are deflected greatly in an electric or magnetic field
    • its direction of reflection is opposite to that of particles
    • they travel at various speeds, sometimes approaching the speed of light
    • medium penetration power - 10 m in air
    • can penetrate several centimeters of aluminum
    • results in the original nucleus changing - atomic mass remains the same and atomic number increases by one
  3. Gamma RaysGamma
    • are high energy electromagnetic radiation
    • highest penetration power - 2 km in air
    • can penetrate a minimum of 30 cm of lead
    • the composition of the original nucleus does not change when these rays are emitted

Sample Problem:
Use a periodic table to write the equation for the beta decay of neptunium - 239.


Check Point

As shown above, some radioactive nuclides have different properties. All radioactive nuclides have the following properties.

  • They undergo radioactive decay.
  • Their radiation affects the emulsion of photographic film, it ionizes surrounding air molecules, it makes some compounds fluoresce, and it has certain special biological effects.

Since radioactivity cannot be detected by human senses, special sensors are needed.

  • Photographic plate
  • Wilson cloud chamber
  • Electroscopes
  • Ionizing chambers
  • Geiger-Muller tube
  • Liquid and electronic bubble chambers
  • Spinthariscope
  • Solid state semiconductor devices
Research Research two of theses devices. For each, post a picture of the device and write a short paragraph on how it works.

Dosimetry is the measurement of radiation and the study of its effects on living organisms. Several units are used to measure radiation.

  1. Becquerel (Bq) - Measures the activity of a source. It is not an appropriate unit to used when studying the effect of radiation on a living organism.

    One becquerel is equal to one emission per second.
    1 Bq = 1 emission per second = 1 Curie (Ci)

  2. Absorbed dose - Describes the amount of energy deposited by a source per kilogram of exposed tissue. When a source gives 1 J of energy to 1 kg of tissue the absorbed dose is 1Gy. It depends on:
    1. the type of absorbing material exposed (bone, organs, flesh, etc.)
    2. number of particles per second hitting the organism
    3. the energy per particle

      Its SI unit is the gray (Gy) and a common non-SI unit is Rads.
      1 Gy = 1 J/kg = 100 Rads
      ? The radiation for the treatment of cancer generally involves an absorbed dose of 40 Gy.

  3. Quality factor - A number assigned to each type of radiation to describe its biological effects. It was defined by comparing its effects with those of a standard radiation of 200 keV X-rays. Quality factors are approximate since they depend on the energy of the radiation and the type of tissue being irradiated. An absorbed dose of 1 Gy of alpha particles can do 10 times the biological damage as the same dose of beta particles.
  4. Dose equivalent - Measures the biological damage produced on an organism. Its SI unit is the sievert (Sv). Dose equivalent (Sv) = absorbed dose (Gy) quality factor (Q).

REMEMBER: Radiation has different effects on different kinds of tissue and no exposure to radioactive emissions, for any period of time, should be considered "safe" to humans or other living organisms.


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