Our Cosmic Ray scientists invite you to share in their discoveries and in their Search for Cosmic Rays.
In the first activity students simulated Hess' balloon ride and discovered that the particle detection rate increases with altitude. The results from Hess' actual balloon ride presented scientists with a new puzzle, what are these particles, where are they coming from are they moving in a particular direction, or are they just floating around in the air? Scientists were scrambling to learn everything they possibly could about this new background radiation. Activities 2 and 3 allow students to begin to understand the answers to these questions and recreate the early tangible curiosity that surrounded this discovery.
In Activity 2 students will determine if the particles are coming from above, below, or from one side or the other by using a circular array of scintillators. By observing which scintillator tubes detect the particle first, and then observing which scintillator tubes detect the particle next, students can get an idea which direction the particle must have been moving.
Remember that early scientists did not know what cosmic rays were
and did not have any idea that they were coming from outer space. Experiments
like Activity 2 were done to understand that the detected particles in
experiments like Hess' Balloon Ride were actually moving and not just floating
in air. Activity 2 shows one way that early scientists may have discovered and
shown that these particles were in fact moving particles.
Students are also given two timing scopes with which to measure the time that
the particles take to travel from one side of the detector to the other.
The times are given in nanoseconds. One nanosecond (ns) is equal to 10^-9
seconds; one billion nanoseconds per one second. These particles, which are
called Cosmic Rays, are moving at almost the speed of light. Experiments like this
contributed to the name Cosmic Ray. It was understood for the first time
that these particles were coming from outer space and hitting the Earth with
incredible speeds and energies.
The diagram to the left shows why motion and direction
can not be determined without more than one scintillator tube. In Activities
2 and 3, arrays of scintillator tubes are used and it is important to understand
why entire arrays are needed. Observing one scintillator tube
light up and then another, indicates particle movement in the direction
tube 1 to tube 2.
(In this activity, red and green markers are used to show which tubes have been
hit by particles. The red and green colors have no significance in Activity 2.)
If both rows are hit by a particle, the event is sent over to a histogram which looks at which tube on the bottom row was hit and subtracts its number (1-12) from the number of the tube which was hit on the top row. So, if tube number 5 is hit on the top row of tubes, and tube number 5 is hit on the bottom number of tubes, the mark on the histogram is placed above the tick mark labeled zero. If tube number 11 is hit on the top row of tubes, and tube number 8 is hit on the bottom number of tubes, the mark on the histogram is placed above the tick mark labeled -3.
Ask the students to design a lab setup that would allow them to determine the direction the particles are coming from. Ask them to draw and describe their lab setup. Share the drawings with the class. Discuss the pros and cons of each setup. Help students discover that they must have at least two scintillator tubes, to determine the direction of motion of the particle.
Between Activity 2 and 3 ask the students how they could modify their lab setup to specifically look at the most common angles with which the particles are hitting their detector.
As the students conduct the online lab, the teacher should act as a facilitator. Allow students as much freedom as possible. Ideally, they will discover the relationships on their own. It is best if students work in small groups, preferably two or three students per group. If this lab is to be done using a computer and projector in front of a class, ask questions and allow students to take turns running the computer. This will help to keep everyone involved.
The teacher must conduct the lab activity prior to the students. This way the teacher will be able to answer student questions as they arise. Require that students read the instructions prior to proceeding to the next page. This will help them to learn more from the lab.
Activities 2 and 3 should take about 1 classroom period, depending on technical problems and skill of students. It is worthwhile to discuss any possible technical problems with the technical specialist prior to beginning the lab. Such problems may include: whether or not all students have network access, the number of available computers with internet access, whether or not the local server is up and running on the day you wish to conduct the lab.
A student lab packet is provided which includes student instructions, graph paper, and the questions from the lab. Teachers may wish to use this for classes that need extra support, or for classes with time constraints, or merely to facilitate the ease of conducting the investigation.
The students should recognize that the mysterious particles Hess measured
on his balloon ride are coming from outer space at angles almost always
perpendicular to the ground. Conduct a class discussion about the results.
Why do they think the particles' directions are mostly perpendicular to
the ground? (The Earth's atmosphere filters out most of the particles.
As the angle of particle entry gets farther and farther from perpendicular,
a particle has to go through more and more atmosphere. Only the most energetic
particles are able to make it through all of the atmosphere at shallow
angles. Very few particles (cosmic rays) are detected at shallow angles,
so we can deduce that compared to the total number of cosmic rays that
we detect, the number of cosmic rays with very high energies is very small.)
Draw a picture of the earth with diagrams indicating the direction most
particles hit the earth. Ask the students if they think this varies as
you go around the planet? (No, it doesn't.) Or is it mostly perpendicular
in all places? (Yes.) Why? (Because it is the atmosphere that
causes the cosmic rays to come in at mostly perpendicular angles.)
What if you were above the atmosphere? Would the majority of cosmic rays
still be perpendicular to the ground? (Cosmic rays come uniformly from
all directions, this is one of the reasons why it is hard to understand
where they come from, there is no preferred direction in outer space.)