CREATING A COSMIC RAY DETECTOR
Vicki A. Lewis, Berkeley Middle School
Douglas W. Higanbotham PhD, Thomas Jefferson National Particle Accelerator
ABSTRACT
Cosmic rays bombard the surface of the earth at a rate of 100 per square meter every second and cannot be seen or felt. A cosmic ray detector can be constructed to detect these particles, which are primarily muons. Designing and creating a working cosmic-ray spark chamber using excess equipment at Jefferson Lab and The College of William and Mary to use as a demonstration device for middle school student was the goal of the research. The completed spark chamber will allow skeptical observers to see and hear evidence of cosmic rays. A design was created taking into account the materials available at the lab and the college. The main components of the cosmic ray spark chamber include scintillation plastic, aluminum sheets which are alternately grounded or wired to a high voltage supply, photomultiplier tubes, a logic unit, a discriminator, a high-voltage supply, and a trigger. The main components will be housed in a gas-tight box, in which an inert gas will flow. As muons travel through the detector they will ionize the gas. A high-voltage discharge along the path of the ionized gas will create a spark and a pop. After assembly, the cosmic ray spark chamber should give us the same results of other similar models.
INTRODUCTION
Every second, every square meter of space is bombarded with hundreds of tiny particles from outer space. The nature of these particles, called cosmic rays, and creating a device in which to detect these particles is the thesis of this paper.
What is a cosmic ray?
A cosmic ray begins is a single proton – a hydrogen ion – which through a variety of mechanisms and sources collides with the earth’s upper atmosphere. From that initial reaction with the atmosphere, billions of smaller secondary particles are eventually created and rain down on earth in a cosmic ray shower. These secondary cosmic rays harmlessly penetrate everything in their path – air, water, rock, life forms. The charged particles are virtually unstoppable, effortlessly penetrating matter at about the speed of light. They cannot be seen or felt by ordinary means.
Where do they come from?
Cosmic rays have their origins in outer space and are classified according to what part of the celestial sky from which they originate. NASA classifies cosmic rays as galactic, anomalous, and solar (1). Galactic cosmic rays originate in our own Milky Way Galaxy, but outside our own solar system; solar cosmic rays are swept from the sun toward earth by the solar wind and anomalous cosmic rays originate in interstellar space as a result of super nova. A supernova occurs when the star uses up all of its nuclear energy and therefore no longer produces the pressure which throughout its lifetime had opposed its own masses’ forces of gravity. As a result, the heavy core implodes and releases a huge amount of energy and violently expels its envelop into space.
The single proton hitting the atmosphere starts a chain reaction of collisions after colliding with a single atom in the atmosphere, creating smaller particles – pions which decay into muons --which go on to collide with other atoms. Eventually, the proton – the primary particle -- is broken down into billions of pieces which rain down over an area of about 10 square miles (2). It is these small particles – muons – which we will detect with our spark chamber.
Why study cosmic rays?
*Some physicists have proposed the idea that periods of intense cosmic ray jets due to gamma ray bursts in outer space are responsible for some mass terrestrial extinction events in the past and the fast repopulation of new species afterwards due to biological mutations caused by ionizing radiation (3). In the future, early warnings of mass extinction events could possible by mapping and timing gamma ray bursts.
*Weather on earth is influenced by cosmic rays since they ionize atoms in the upper atmosphere. And without ionization, lightning and the resulting thunder would not occur (4).
*While earthbound humans have built-in protection from primary cosmic rays because of the earth’s atmosphere, astronauts in space do not. With President George W. Bush’s new vision for the future of space exploration which calls for a “Crew Exploration Vehicle” that will send humans to the moon as soon as 2015, with “the goal of living and working there for increasingly extended periods” (5), further research into the protection of and shielding from cosmic rays is paramount. The moon, which does not have its own atmosphere, is actually more inhospitable to humans, than Mars.
Why build a detector?
The detector we will be building will be similar to devices used for experimental purposes in the 50s and 60s. Today, our detector is considered obsolete for those purposes. However, as a demonstration device it will be very valuable, especially for middle school-age observers.
Teachers of particle physics face an uphill battle in asking students to believe that which they cannot see or that which is counter-intuitive and contrary to their students’ experiences. In a recent science teaching journal, a researcher found that after a well-planned lesson on Newton’s Laws of Motion, the students could recite the laws of motion, but they confessed to the researcher that didn’t believe them to be true. As a teacher of physics and chemistry in a middle school, I frequently encounter this predicament: the recitation of the “facts” by my students, who still maintain a large measure of skepticism.
Asking students to believe that every second, hundreds of charged particles are cursing through their bodies is a lot. The spark chamber will give students (and other visitors to Jefferson Lab) an opportunity to observe the physical reality of cosmic rays – or in other words, “seeing is believing.”
What components are needed to build a cosmic ray spark chamber?
The essential parts of a cosmic ray spark chamber include two sheets of scintillation plastic, two photomultiplier tubes, a logic unit, a discriminator, a source of high voltage, and a series of alternately grounded/high voltage aluminum plates, which are encased in a gas-tight container in which a mixture of helium and neon are piped.
What is the purpose of each component?
Scintillation plastic is a special type of material in which the atoms become excited as a cosmic ray passes through. The excited atoms will “kick off” a photon as they return to their stable state. Two sheets of scintillation plastic are needed in the detector: one on top and one on the bottom.
The photomultiplier tubes (PMT) are attached to the scintillation plastic with a light guide. The light guides help direct the photons in the scintillation plastic into each PMT. The PMT works on the principal of the photoelectric effect. As a photon comes in, it kicks off electrons in the photomultiplier does what its name implies. It multiplies the light which comes into it.
Each PMT is attached to the discriminator. When the discriminator receives signals from both the top and bottom scintillation/PMT combinations (denoting that a cosmic ray has passed through both) it sends out a digital signal.
The logic unit transforms simultaneous pulses into a third digital signal.
The logic is hooked up to a trigger. When the trigger receives the square signal, it will produce a high-voltage (5 mV).
A series of aluminum plates are stacked on top of each other about one centimeter apart with ceramic spacers. Every other plate is wired to the high voltage source. The other plates are grounded. The plates are enclosed in a gas-tight container.
A mixture of neon and helium is pumped into the container. The neon will create a colorful spark.
How do the components work together to create a spark?
A cosmic ray travels through the top scintillator, through the aluminum plates, through the bottom scintillator, and beyond. The rays ionize the gas in the chamber and excite the atoms in the scintillation plastic, which emit photons. The photons are guided into the photomultiplier tubes and -- due to the photoelectric effect -- kick off electrons from the semi-conductor inside the PMTs. A series of dynodes inside the tubes multiply the number of electrons. Signals from the PMTs are sent to the discriminator which produces digital signals. The logic transforms simultaneous pulses into a third digital signal, which triggers the high voltage supply to apply 5 kV of electricity to the hot aluminum plates. Because of the potential difference between the hot and grounded plates, the electricity discharges along the path of ions created by the cosmic rays, creating a spark and a pop.
Appendix A
Illustration by Vicki A. Lewis
Appendix B
Illustration by Vicki Lewis
REFERENCES
(1) http://helios.gsfc.nasa.gov/cosmic.html).
(2) http://www.auger.org
(3) http://prola.aps.org/pdf/PRL/v80/i26/p5813_1
(4) http://www2.slac.stanford.edu/vvc/cosmicrays/cratmos.html
(5) http://www.whitehouse.gov/news/releases/2004/01/20040114-3.html