These interactive lessons teach about Cosmic Rays by emphasizing the mystery that Cosmic Rays presented to early scientists. The scientific inquiries and investigations that Cosmic Rays prompted are interesting and important to understanding the way science works. Cosmic Rays are now being studied at research sites around the world. Much has been learned from early experiments and even more is being discovered with modern experiments, but many questions have yet to be answered.
Radioactive particles that come from space are called cosmic rays. These particles are moving at incredible speeds -- almost the speed of light. Most of the radiation measured far from radioactive sources (background radiation), are low-energy cosmic rays from the sun. (The Aurora Borealis and Aurora Australis are created primarily by Low-Energy Cosmic Ray particles getting caught in the magnetic field lines of the Earth.) The more scientifically fascinating cosmic rays are the Ultra-High Energy Cosmic Rays that the modern cosmic ray detectors have seen. These cosmic rays have so much energy that scientists are completely unsure of their origins.
Our Cosmic Ray scientists invite you to share in their discoveries and
in their Search for Cosmic Rays.
Cosmic Ray History
In the early 1900's Henri Becquerel discovered that some elements emit particles. Some of these particles moved slowly and had a positive charge, they were called alpha particles. Some moved faster and had a negative charge, they were called beta particles. Some moved very quickly and had a neutral charge, these were called gamma particles. These particles were named for the first 3 letters of the Greek alphabet. The discovery of radioactive elements was an important tool in uncovering the particles that make up the earth and the rest of the universe. Scientists were later able to discover that all matter is made of various ratios of beta particles, and the particles that make up alpha particles.
The early scientists working with radioactive elements used electroscopes, photosensitive plates and Geiger counters to detect these particles. Marie Curie discovered uranium is radioactive when a piece of pitchblende exposed the photographic plate she set it on. Victor Hess used an electroscope in the balloon experiment that is recreated in our first activity. In our activities we will be using Geiger counters instead of electoscopes. Invented by Hans Geiger, a scientist at Cambridge, the Geiger counter is able to detect and measure the rate of individual radioactive particles. Many students are familiar with Geiger counters.
When a Geiger counter is placed near a radioactive source, the rate of particles detected is very high. When the Geiger counter is moved very far from the source, the rates drop. However, the rates never drop to zero. In fact the particle rate a Geiger counter reads when it is far from any radioactive source is about one particle, each centimeter, each second. This perplexed scientists. Where are these particles coming from?
In the early 1900's Scientist Victor Francis Hess began to investigate these questions. He proposed that the particles were coming from the earth. Therefore, as you moved away from the earth the rate should decrease. To test this hypothesis he took an electroscope on a balloon ride, to compare the particle rate to the elevation. The first activity simulates Hess' historical balloon ride. Hess discovered that the rate actually increased! These particles did not appear to be coming from the earth! The results from Hess' balloon ride led to further questions, such as which direction are these particles coming from, and at what angles? Activities 2 and 3 allow students to begin to investigate these questions.
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.
While the students are conducting "Hess' Balloon Ride," the teacher should encourage the students to allow time for the rate to stabilize. Ask the students why they think the rate keeps changing? (The rate will statistically increase with altitude. This means that over time, the count rate at a certain altitude will approach a specific number, but since this is only an average rate, one should record data for enough time to allow the rates to stabilize. The longer you count (the "start counting" button), the more accurate your data and the smoother your data plot will be.)
This activity should take about 1 classroom period, depending on technical problems and student skills. 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, tables, 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.
Now that the students have had the chance to compare rates to altitude, conduct a class discussion about the results. Ask the students where they think the particles are coming from. (above the ground, outer space) Why do they think that the particle rates are increasing as elevation increases? (less atmosphere, more of the particles are able to get through) How could you test your hypothesis about where the particles are coming from? (Next activities)