by J. Allie Hajian and Maggie Masetti

This activity can be used to explain all three methods of X-ray detection: imaging, spectroscopy, and light curves. You will need two bags of m&m's (Tm) or Skittles (Tm), two boxes with a hole poked in each just large enough for individual candies to fit through, and either a meter square piece of paper with grid lines drawn on it, or a number of large egg cartons. You will need stopwatches, or watches with a second hand, for Part C.

Part A: Imaging

Put approximately 20 candies in one box, and approximately 10 in the other. This can be varied. Do not let your students know how many candies are in each box. Plug the hole in the box so the candies don't fall out. Have students hold the boxes approximately 1 ft. apart and 1 ft. above the grid or the egg cartons. These two boxes are now your X-ray sources, the candies are photons, and the grid or egg carton is your X-ray detector. Unplug the hole and have the students gently shake the boxes so the candies fall down onto the grid or into the egg carton. Have them draw the distribution of candies on paper. What does this distribution tell you about the number of candies in each box? If the candies are photons and the boxes are X-ray sources, what does the distribution tell you about the brightness of each source? If the students didn't know how many boxes of candies there originally were, would they be able to tell from the distribution of candies on the grid? If you hold the boxes close together and then shake the candies out, does it make it harder or easier to distinguish the number of X-ray sources you had? This can be a good lesson on resolution; sometimes X-ray sources that are really close together can appear to be one big source.

Part B: Spectroscopy

Repeat the exercise but this time have the students count the number of candies by color, i.e. the number of red, green, brown, and blue m&m's. Have them tally the candies over the whole grid, not just for each source. Have each color of candy represent a different energy. Depending on the age of your students, you can use real X-ray energies, i.e., have brown = 2-10 keV, red = 10-30 keV, blue = 30-100 keV, and green = 100-250 keV, or just make up energies. Have the students graph how many candies are at each energy. This is the spectrum for your sources. Next, pretend that your detector can see both how many photons are at each energy, and where they fall spatially. This means you can resolve both sources and you can make a spectrum for each one! Which source is more energetic?

Part C: Light Curves

Fill one of the boxes with candies. Plug the bottom. When you unplug it, time the candies draining out for six ten-second intervals. Have the students count how many candies fall out in each ten-second interval. (You can do this for longer than one minute). Then have the students graph how many "photons" there were over each interval; this is a light curve of intensity (how many candies) over time (60 seconds).