GROUP SIZE: Large or Small
TIME: 60 minutes
TYPE OF ACTIVITY: Teacher/Student Demonstration
TEACHING STRATEGY: Expository Guided Discovery
CONCEPTS: Radiation Radioactivity Detection
SKILLS: Reading for Information Observation/Inference
Objectives: To relate the flashes sometimes seen in your eyes to cosmic radiation; to review and strengthen the concept of radioactivity.
Materials: The radioactivity detector from the previous lesson; a Geiger counter (optional).
Teacher Background Information:
An unexpected effect of cosmic rays on humans was first noted on the Apollo 11 mission to the Moon in 1969. Astronaut “Buzz” Aldrin reported that he kept seeing flashes of light while his eyes were closed. On later flights, astronauts counted these flashes and Bill Pogue (on Skylab in 1973) saw eight flashes per minute. T. F. Budinger, the Principal Investigator for Experiment MA-106, collected descriptions of these flashes and did some experiments with high-energy ions shot out of a cyclotron in his laboratory. He knew exactly what kind of particles were in the cyclotron beam and what energy they had as they entered the heads of the volunteers. If Budinger aimed the beam through the brain, the volunteer did not see a flash. It was only when the high-energy ions passed through the eye that flashes were seen.
The many different shapes and sizes of flashes reported by the astronauts in space and by the volunteers at the cyclotron are shown in the study sheet. These different shapes and sizes complicate the explanation of how the flashes are produced. The simplest idea is that a cosmic ray causes a flash when it passes through the eye’s “detector,” the retina. The retina is a region at the back of the eyeball where nerve endings are sensitive to the light focused there by the lens at the front of the eye. The cosmic ray ionizes a few atoms or molecules, which produces a signal in the optic nerve. This signal is interpreted by the brain as a “single star” or perhaps a “comma.” The “diffuse cloud” flashes seem to be something different. (See illustration)
The “long streak” might be caused by a cosmic ray that crosses along the surface of the retina. Double stars and double streaks are most probably caused by a cosmic ray that hits two parts of one eye or perhaps passes through both eyes (since no one can tell which eye sees any flash) The diffuse cloud could be caused by galactic cosmic rays with very high energy, high mass and high atomic number Z. Because they are moving so fast these “HZE particles” (for High-Z, High-Energy) might produce a flash of light like a shock wave as they enter the eyeball
The MA-106 Experiment on Apollo-Soyuz involved two astronauts. They were blindfolded 15 minutes before the experiment started so that their eyes would adapt to the darkness They would then record any flash and describe what they saw into a tape recorder. Cosmic ray detectors were mounted near the heads of the astronauts and the records of the responses of the astronauts were compared with the data from the detectors.
- Ask the students if they have ever experienced flashes of light in their eyes. Ask what they think these might be.
- Blindfold several volunteers and ask them to record any flashes they might see over a 30 minute period. Emphasize the importance of accuracy and not calling out flashes that don’t really occur.
- Set up some of the detectors from the last lesson near the head of the volunteers to see if they react to the strike, if any, called out. If you have a Geiger counter set this up near the head and have a student record any strikes the instrument reports.
- Compare the data from the instruments with that reported by the students.
The students may not report any findings during this short experiment. If they try to stay aware of random strikes during the next few days they may be able to describe what they experienced and report back to class. The astronauts experienced many more strikes near the North and South poles. Why do you suppose that was?
Have the students read about the experiments carried out to gather information on these flashes. what other radiation effects have been explored in relationship to astronauts? Do pilots who fly commercial jets at high altitudes experience the same amounts of radiation exposure as astronauts? Why or why not? Have the students explore high energy particles and their nature.