LAB EXERCISE #2: DETERMINATION OF THE EFFECT OF DISTANCE ON INTENSITY OF RADIATION

       Common sense dictates that if you increase the distance between yourself and a radioactive source, the intensity of the radiation to which you will be exposed will decrease. Mathematically this is best descibed as(select one):

Direct Proportionality.
Inverse(Indirect) Proportionality.

       In this experiment you will collect activity readings at various distances from the radioactive source. Using the data you will then be asked to establish a mathematical relationship to relate intensity(activity) of a radioisotope with distance.

Instrumental Procedures:

  1. Turn on the Scaler/Timer with G-M tube.
  2. Timer interval switch is preset for one minute readings.
  3. Record background activities as required (the data obtained in experiment 1 may be used).
  4. Using the mouse select and place the radioactive source in the initial position closest to the G-M tube. This corresponds to a distance of 1 cm from the source. *The distance can only be changed after the counter stops automatically.
  5. Obtain a one minute count of the activity at this distance.
  6. Advance the radioactive source to the second distance which corresponds to 2 cm from the source or essentially doubling the distance. Again obtain a one minute count at this distance.
  7. Successively place the source at the remaining levels that correspond to 3cm (triple distance), 4cm., and 5cm. Etc. At each level measure the activity. The readings must be made in a continuous series without interuption, and the counter and other conditions must not be disturbed throughout the readings. Each reading in counts per minute (cpm) should be recorded in your data table.
Go to Experiment

Treatment of Data

  1. Correct all recorded counts for background radiation.
  2. Plot a graph of activity readings(corrected for background) vs. distance of separation in your lab notebook.
  3. Based on your initial data and the graph establish if the relationship between activity and distance corresponds to a direct or inverse mathematical relationship.
  4. Write a trial mathematical proportionality that will relate activity and distance observed from your data.
  5. Using trial and error with your observed data see if you can establish a mathematical relationship between the activity and distance. [HINT: try squaring one side, square root, cube, cube root , etc.]