MONITORING MUONS FROM COSMIC RADIATON
James A. Petrait
Cosmic radiation from the solar system and beyond is constantly striking the upper layers of the atmosphere. Most of this primary radiation does not make it to the surface of the earth but it does produce a cascade of secondary particles.reaching the surface of the earth and below, The muon is an interesting particle which can be detected inside a classroom. A muon is about 206 times heavier than an electron and it has a lifetime of about 2 microseconds. In traveling the distance from the higher parts of the atmosphere, muons arrive at the surface of the earth and below after their lifetimes have expired. But because the muons are traveling near the speed of light, a time dilation effect allows them to complete their journey.
Observing and monitoring the muons can be an intriguing way to study the field of nuclear physics in high school science classes. Last year, I set up some lab demonstrations in observing the muons during my chemistry class. I also had one student do a project for the local science symposium in monitoring the muons over a period of several days. This year I plan to do some additional muon labs in my physics and chemistry classes and to have one or more students do some additional research on their own.
Geiger counters detect the muon flux which is around 5% to 10% of the total background radiation which is picked up. This is not an accurate way to find the muon flux rate and its variations. A better way is to place two geiger counters together and connect them with a coincidence counter (c-box). Only those muon particles that go through both geiger counters at the same time interval are counted with the help of the c-box.
Until the advent of a small geiger counter that uses a PC computer to count the particle clicks, it was difficult and expensive to do an experiment in trying to count the muons without the background count. Aware Electronics Corp. of Wilmington, DE has developed a geiger counter in a small, rectangular, plastic box that can easily be connected to a PC computer that records and stores the counts and can analyze the counts graphically. Two of these geiger counters can be placed together with a rubber band so that the geiger tubes in each one are directly parallel with each other. The phone line cords outputs from each counter are then connected to another small device called a coincidence-box (c-box) from Aware Electronics. Then a third phone line cord is connected to the computer by means of an adapter to the serial port. The c-box contains a circuit which passes the clicks from the muons particles that hit each of the 2 parallel geiger tubes. In effect, all background radiation is filtered out except for the muon particles which are only coming from above or from the side of the atmosphere. Although muons are energetic enough to penetrate the earth for a short distance, they are not able to pass through the entire earth so no muons are coming from below the earth.
Magnetic fields affect the arrival directions of the primary cosmic rays which in turn affect the arrival directions of muons which are also spread out from the creation of the particle cascade. The constant arrival of muons on the surface of the earth results in a muon flux which varies with the coordinates of the arrival location. This is a result of the magnetic fields and the muon flux variation and has been calculated by J.F.Ziegler (Terrestrial Cosmic Ray Intensities by J.F. Ziegler, IBM Journal of Research and Development, Volume 40, No.1) for various locations on the earth using New York as a base reference value of one. The Ziegler article was written as study of how cosmic radiation affects the failure rate of computer chips at different locations.
The actual muon count can be small using the lowest cost geiger counters from Aware Electronics. The number of counts is related to their statistical reliability. One way to increase the reliability without increasing the cost of the equipment is to take the muon counts over a period of one hour instead of one minute. On the average on one to two muons are recorded per minute with some minutes passing by with zero counts and others with 3 or 4 counts. If the muon counts are recorded by the hour then thecounts average between 60 to 120.
One of the interesting demonstrations to do during class would be to place the 2 geiger counter array at a straight up (90 deg.). That is the position for the maximum muon count rate. When turned sideways (0 deg.angle) which can be indicated as N/S, E/W etc.,the muon count dramatically drops to around 10 % or less of its straight up angle. This needs to be done over a longer time period as there may not be any count for 10 to 20 minutes.
For more ambitious class projects, the counts can be taken at various angles and compared. It would be difficult to point the array only towards the ground since it would also be picking up the muons from overhead. A tremendous amount of lead shielding would have to be used to eliminate the counts from overhead and to show that the count from the earth would be non-existent.
For her science symposium project, one of my students compared graphs of the counts made during various time periods of from 6 to 24 hours. This was regulated by the time available and the ability to keep the apparatus going. Using the graphing program set to the one hour interval in the computer, she was able to come up with some significant peaks and valleys of the graphs during the times observed. The sources of cosmic radiation which could affect the muon flux variation are the sun, our galaxy, and the rest of the universe. Variations in the radiation from the sun could affect the muon flux variation after large solar flares which tend to suppress the cosmic radiation reaching the earth while they are active. My student's project showed that there was significant variation in the muon count rate which was probably related to the solar radiation.
There are many other variations that can be made in the experimental designs. Besides those given above, some other possibilities are to observe and analyze the count variations over longer time periods, to relate the muon counts to peaks in solar activity, to adjust the angle of the geiger counters and try to pick up evidence of cosmic radiation from outside the solar system, and to measure the effects of what happens when the spacing between the two geiger counters is increased.
I find this research fascinating and I hope that it will continue to serve as a model for my students studies of real science that involves observing, measuring, monitoring and trying to solve the problems of the unknown universe.
Aware Electronics Geiger Counter
Inside of Aware Electronics Geiger Counter showing GM tube.
2 Aware geiger counters, coincidence detector, compass, level, protactor and index cards to create the angle. The 2 geiger counters are connected together with rubber bands. The angle is set to 20 degrees South.
2 geiger counters, coincidence detector, and level. The counters are pointing staright up towards the sky.
2 geiger counters, coincidence detector, and level. The counters are at a zero angle pointing to the horizon.
sample graph on the Aware Electronics program generated by the coincidence detector.
This article was published in the May, June 2003 issue of "Radio Astronomy", Journal of the Society of Amateur Radio Astronomers (SARA). For more detailed information consult my other muon and radiation articles that are posted on this website.
© 2003-16, James A. Petrait