Cosmic Ray Collisions
(By Shirley Burris, Bayview Community School, Mahone Bay, Nova Scotia)
This activity models cosmic ray collision. Velcro balls represent the cosmic rays, and a Velcro target represents the point of collision. The purpose of the activity is to enable students to internalize an appreciation for the probability of collision of particles.
Students will use the Velcro balls to attempt to hit a target, in order to:
 Determine the probability that the tennis balls will hit
 The target
 One another at the target spot
 Record percentage of strikes of balls thrown.
Once the data is collected, students can compare their collisions with those that take place between cosmic rays and atoms in space. Cosmic rays travel around the Milky Way Galaxy in all directions, and they travel through a mostly uniform medium between the stars, in which there is about 1 hydrogen atom for every cubic centimeter. There is a 1/30 chance of a cosmic ray interacting with an atom in space before it reaches the earth.
Students should discuss the size of the balls, the size of the room, and distance thrown, with respect to the probability of collision. Compare to size of cosmic rays, and distance traveled, to internalize the concept of collision of cosmic rays. The activity gives a "jumping off" point for students to begin to think about the size of the universe, the size of the particles involved in cosmic ray collisions. Students will see the difficulty experienced in getting single tennis balls to hit the target, and the greater difficulty in orchestrating the connection of more than one at target point. They can then reason that if this level of difficulty is reached with objects the size of tennis balls in a small area such as the classroom, then the probability of objects the size of cosmic rays, over distances even as "close" as the Sun, will be very low.
Procedure:
Students will need 36 Velcro balls. (Tennis balls can be used "as is" or can be wrapped in Velcro strips.) A 3" x 5" strip of Velcro can be taped to a spot on the wall in the classroom. Students will place themselves at the opposite end of the classroom for purposes of throwing the balls. Care should be taken to clear the path of the tennis balls, and to instruct students to throw with reasonable care. (Alternatively, the target may be placed on the floor and the balls rolled toward that target. Or the activity can be done in a gymnasium.)
First they will be invited to throw 36 balls to the target across the room. Students will try this first with one ball from varying positions, and then with two, or three, requiring that all balls hit the same target at the same time.
Students will be invited to predict the probability of a "hit" in all instances; will discuss factors involved in the success or failure of hits to occur, and will discuss and record the rate of hits, and outcomes related to position and number.
Students will compare the ball size and distance thrown to the size of cosmic rays and distance traveled.
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