1. COSMIC RAYS An Overview Dr. Darrel Smith Department of Physics Embry-Riddle Aeronautical University Prescott, AZ 86301

2. Cosmic rays-early beginnings  C.T.R Wilson discovered in 1900:  the Earth’s atmosphere was continually ionized.  It was believed to be due to the natural radiation from the Earth. In other words, from the ground up.  Wilson noticed the reappearance of drops of condensation in expanded dust free gas, the first cloud chamber.

3. The Wilson Cloud Chamber

4. Where did the ions come from?  At the beginning of the 20 th century, scientists were puzzled by the fact that more radiation existed in the environment than could be explained by natural background radiation.  The debate was resolved as a result of a balloon flight in 1912 from the University of Vienna.

5. Victor Hess  In 1912 a Victor Hess, a German scientist, took a radiation counter (a simple gold leaf electroscope) on a balloon flight.  He rose to 17,500 feet (without oxygen) and measured the amount of radiation as a function of altitude.

6. Victor Hess and the Balloon  Victor discovered that up to about 700 m the ionization rate decreased but then increased with altitude.  This showed that outer space was the source of the ionization.

7. Not from the Sun  During subsequent flights Hess determined that the ionizing radiation was not of solar origin since it was similar for day and night.  It was initially believed that the radiation consisted of gamma rays only.  But there was still a dispute as to whether the radiation was coming from above or from below.

8. Source of Cosmic Rays  In 1925 Robert Millikan of Caltech introduced the term “cosmic rays” after concluding that the particles came from above not below a cloud chamber.  He used elaborate electroscopes.

10. Extensive Air Showers  Cosmic rays enter the earth’s upper atmosphere and interact with nuclei.  Secondary particles result that also interact.  The shower grows with time.  Some particles never reach the surface.  Some particles, such as muons, do reach the surface and can be detected.

11. The Spark Chamber  In the 1960’s, spark chambers were common. When a charged particle ionizes gas between the plates, sparks fly along the track, marking the track of the particle.

12.  Primaries are particles with energies from 10 9 eV to 10 21 eV.  An eV is a unit of energy. A 40 W reading light uses about 10 34 eV of energy in one hour. ( from James Pinfoli, Pinfold@phys.ualberta.ca) Cosmic rays within the range of 10 12 eV to 10 15 eV have been determined to be:  50% protons  25% alpha particles  13% C, N, and O nuclei  <1% electrons  <0.1% gammas Composition of Cosmic Rays

13.  Existing models for the production of cosmic rays only work to 10 15 eV.  CR in excess of 10 19 eV are believed to come from sources relatively close to our Galaxy, but the sources are unknown. The highest energies! ( from,www.phys. washington.edu ) Cosmic Ray Energies

14.  Cosmic Ray studies continue in spite of the development of high energy particle accelerators ~ 10 12 eV.  The energy of the highest energy cosmic rays still cannot be duplicated in accelerators. Present Cosmic Ray Studies

15. Fermilab “Modern-Day Accelerator” E = 10 12 eV

16. Where do cosmic rays come from?  Low energy rays (less than 10 GeV) come from the sun.  Supernovae may be the source of particles up to 10 15 eV.  The sources for ultrahigh cosmic rays are probably, active galactic nuclei and gamma ray bursts. ( www.phys.washington.edu )

17. Supernovas  Nuclei receive energy from the shock wave of the supernova explosion.  The energy spectrum indicates that most of the supernova particles have less than 10 15 eV  (image from: www.drjoshuadavidstone.com/ astro/supernova.jpg

18. High School Based Detectors Numerous CR detector arrays have been built and are located at high schools. The projects range from  arrays using hundreds of detectors covering thousands of km 2 to  small arrays involving only a few detectors in an area only a few hundred meters square.

19. CHICOS ( California high school cosmic ray observatory)  Operated by Caltech, CHICOS is an active research array with a goal to study CR is the range of 10 18 to 10 21 eV using refurbished detectors from a neutrino experiment and 1 m 2 scintillators  Currently 51 sites are setup and working.  Image from www.chicos.caltech.edu www.chicos.caltech.edu

20. ALTA ( University of Alberta Large Time Coincidence Array)  The stated purpose of the ALTA project is to search for time correlations between EAS’s.  At present 16 high schools are involved.  The project is part of the Canadian learning standards with students receiving credit.  (image from www.physics.ubs.ca)

21. ALTA MAP

22. CROP ( Cosmic Ray Observatory Project, University of Nebraska)  A project to study EAS from particles > 10 18 eV.  Thirty operating schools covering 75,000 sq miles is the goal of the project.  Detectors are 1 m 2 scintillators donated by the Chicago Air Shower Array.  Image from Marion High School. Http://marian.creighton. edu Http://marian.creighton

23. SALTA (Snowmass Area Large-scale Time-coincidence Array)  A project to set up detectors in Colorado.  Linking high schools via Internet connecting to form a large array.  A modern hot-air balloon flight in 2001 reenacted Hess’s 1912 flight. Image from: http://faculty.washington.edu/~wi lkes

24. WALTA (Washington Large Area Time Array)  A project of the University of Washington.  As of late 2002 eighteen high schools around Seattle are participating. See image. ( from www.phys.washington.edu )

25. The Pitt/UMSL Projects A project of the University of Pitt and University of Mo at St. Louis. The project involves high school teachers building and using scintillator type detectors aimed at muon detection.