Naturally Occurring Radiation Sources:
The Activity of Granite


Radioactive sources are typically classified into two categories: artificial or man-made sources, and natural sources. Exposure from man-made sources results primarily from the use of radioactive isotopes and X-radiation in medicine and dentistry. Nuclear fallout from atomic bomb testing in the 1950's and 1960's also contributes to radiation in this category. Some other man-made sources include such miscellaneous items as televisions, glow-in-the-dark watches, and smoke detectors. Finally, nuclear power production also contributes very slightly to the artificial radiation we receive.

Natural sources of radiation can be found in the rocks in the earth's crust. Some of these rocks, such as granite, are commonly used in building materials and so it is not uncommon for cities with many large buildings to have high levels of background radiation. Other natural sources occur in many of the foods we eat. Bananas and certain salt-substitutes contain high levels of potassium, which has a radioactive isotope. Finally, cosmic rays are another source of natural radiation. They consist of high-speed charged particles that constantly bombard the earth. These particles interact with the upper atmosphere and shower the earth with radiation. Most of this radiation decays before reaching the ground, and so it poses a very minimal threat to us. In today's experiment, we will measure the activity of samples of granite.


  1. Radioactivity measuring devices
  2. Stopwatches
  3. Triple-beam balances
  4. Granite samples
  5. Metric rulers or vernier calipers

Granite is a type of rock that is common in the earth's crust and it often contains large amounts of potassium. Approximately 93 percent of naturally occurring potassium is a non-radioactive isotope, potassium-39 (K-39). The remainder of a potassium sample is radioactive potassium-40 (K-40). This isotope has a half-life of 1.28 billion years, and so it can be used to determine the age of very old rock samples. One of the ways in which K-40 decays is through the process of beta-plus decay, also known as positron emission. In this process, one proton in the nucleus of the potassium atom is converted into a neutron, a positron (b+), and a massless particle called a neutrino (n ). The particle that results from this decay is a stable argon-40 nucleus:

Here, the positron and the neutrino are emitted from the nucleus. The positron undergoes a further process called electron-positron annihilation, where it violently collides with an electron and both particles are destroyed with a release of energy.

The activity of a sample of potassium-containing rock (like granite) can be determined by counting the number of decays that occur in a given amount of time.



  2. This measurement must be made with the source several meters from the Geiger counter. Do not obtain your source until this measurement has been made. Record the number of background counts on your data sheet for a time of at least 10 minutes. Calculate and record the background rate in counts per minute on the data sheet.
    1. Obtain a granite sample and measure its mass. Record this reading on your data sheet.
    2. Place your sample on the stage to be placed under the counter.
    3. Slide the stage into the counter so that the granite is directly under the detector. Select the level that allows the granite to be closest to, but not touching the detector. Measure the distance between the granite and the detector and record this reading on your data sheet.
    4. Take a reading for at least 20 minutes, but for no longer than 30 minutes.
    5. Record the total number of counts and the duration of the measurement on your data sheet.
    6. Return the granite to your lab instructor.
  1. Would you expect background radiation levels to be higher in Latrobe, PA or in Denver, CO? Explain. (Give at least 2 reasons for your answer.)
  2. Why is it safe to live in a building made of granite if granite is a source of radiation?
  3. Calculate the activity of your granite piece. (Use the activity of the known source at a given distance (from last lab) in a ratio.)
  4. Compare your results with others in the class. Give several reasons for any differences in your calculated activities.

Naturally Occurring Radiation Sources:
The Activity of Granite Data Sheet

Total Counts
Duration (s)


Mass (kg)
Distance (m)
Total Counts
Net Counts
Duration (s)



Source of known activity: Ao = 9.76 x 10-7 Ci
Distance (m)
Duration (s)
Net Counts


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Copyright 1999, Thomas McNulty
Last updated 9 August 1999