COSMIC RAYS An Overview

Cosmic rays-a long story C.T.R Wilson discovered in 1900 the continuous atmospheric ionization. It was believed to be due to the natural radiation of 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.

Condensation tracks on ions Wilson suspected the tracks might be condensation on nuclei - ions that were the cause of the “residue” conductivity of the atmosphere.

The Wilson Cloud Chamber

Where did the ions come from? At the beginning of the 20th 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 solved on a balloon flight in 1912 from the University of Vienna.

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 increase as the balloon climbed.

Victor Hess and the Balloon Victor discovered that up to about 700 m the ionization rate decreased but then increased with altitude showing an outer space origin for ionization.

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.

Birth Cries of the Atoms 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.

Cosmic Ray Electroscope Electroscope of cosmic ray apparatus used by Millikan. Millikan and a fellow scientist, Compton, were locked in a debate about the nature of cosmic rays. Compton won, arguing that they were charge particles. Millikan believed they were uncharged.

The nature of the rays: Muons and Protons Seth Nedermeyer and Carl Anderson discover muons in cosmic rays. T.H. Johnson discovered that the ionization rate increased from east to west viewing angle indicating they were positively charged particles (protons). The increase occurs because the rays are deflected by the earths magnetic field, which changes in its strength with latitude.

Charged Particles! In 1929 a Russian scientists, D. Skobelzyn, discovered ghostly tracks made by cosmic rays in a cloud chamber. Also in 1929 Bothe and Kolhorster verified that the cloud chamber tracks were curved. Thus the cosmic radiation was charged particles.

The Caltech Cloud Chamber Milliken became President of Caltech and was instrumental in the building of a high magnetic field cloud chamber. Carl Anderson and Milliken made numerous photographs of both positive and negative particles tracks. The conclusion was that the positive particles must be protons.

Occhialini and Blackett Giuseppe Occhialini and Patrick Blackett devised a method of making cosmic rays take their own photographs. They observed in 1932 the formation of multiple particles, pair production. Occasionally they observed the production of particle showers using lead and copper plated places in the cloud chamber.

Extensive air showers Pierre Auger noticed that two detectors located several meters apart detected particles at the same time. He discovered EAS, showers of secondary nuclei produced by the interaction of the primary particle with air molecules. (1938)

EAS It is the secondary particles resulting from the interaction of the the primary particle that are detected by the detectors used in our detectors and others arrays.

An Extensive Air Shower Cosmic rays enter the earth’s upper atmosphere and interact with nuclei. Secondary particles result that also interact. The shower grows with time. Certain particles never reach the surface. Some particles, such as muons, do reach the surface and can be detected. It is these that we wish to detect.

Other tools: The emulsion plate The study of cosmic rays was greatly enhanced by the use of photographic emulsion plates. The plates were taken to numerous places, including the Pyrenees and left for extended periods of time. The results were images of complete pion decays including the discovery of the so called “strange particles”.

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.

Early Discoveries from CR The mass of the proton was determined to a 15% error (Anderson, Chadwick) The discovery of the antiparticle of the electron, the positron (Klemperer). The discovery of the mesotron, with mass between the electron and the proton The first evidence of meson decay (Williams and Roberts) The measurement of the decay (Rossi) The discovery of He nuclei and heavier elements in CR (Frier)

Present Cosmic Ray Studies Cosmic Ray studies continue in spite of the development of high energy particle accelerators. The energy of the highest energy cosmic rays still cannot be duplicated in accelerators. The field is still very active as indicated by the presentation of over 300 papers at the most recent international conference on cosmic rays.

What are cosmic rays? Primaries are particles with energies from 109 eV to 1021 eV. An eV is a unit of energy. A 40 W reading light uses about 1034 eV of energy in one hour. (from James Pinfoli, Pinfold@phys.ualberta.ca) Cosmic rays within the range of 1012 eV to 1015 eV have been determined to be: 50% protons 25% alpha particles 13% C, N, and O nuclei <1% electrons <0.1% gammas

The Energy Spectrum The highest energies! Existing models for the production of cosmic rays only work to 1015 eV. CR in excess of 1019 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)

The Oh My God Particle In 1991 at the Fly’s Eye CR observatory in Utah a primary particle of 3 x 1020 eV was recorded. This is the equivalent of 51 joules At present particle accelerators can reach energies of 1012 eV. The Fly Eye (from www.physics.adelaide.edu)

The AGASMA EVENT In Japan, in 1993, the worlds largest array recorded a large air shower believed to be the result of a primary particle measured at 1021 eV. These particles have energies six times higher than present theories allow. The mystery is, of course, what is the source of the high energy particles including these ultrahigh energy particles.

Where do they come from? Low energy rays (less than 10 GeV) come from the sun. Supernovae may be the source of particles up to 1015 eV. The sources for ultrahigh cosmic rays are probably, active galactic nuclei and gamma ray bursts. (www.phys.washington.edu)

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 1015 eV (image from:www.drjoshuadavidstone.com/ astro/supernova.jpg

The ultra high particles? Without going into great detail the problem with the source of the UHECR is that to achieve the high energies they must originate in a very large extragalactic field or from a process that doesn’t require such distance. Suggestions abound but there is not a agreement as to the origin. Maybe there isn’t a single source. One suggestions is that UHECRE’s originate from the decay of more primary particles resulting from the big bang.

A Summary Lower energy, < 1016 eV: Ultra High energy, > 1016 eV. Direct observation possible, 85% are protons. Most likely source are supernova shock wave acceleration. These are particles below the knee in the energy spectrum. Ultra High energy, > 1016 eV. Only indirect EAR shower information is available. Source of the particles with > 1016 eV is unknown.

High School Based Detectors Numerous detector arrays using high schools as sites for individual detectors have been built or are in the process of development. The projects range from arrays using hundreds of detectors covering thousands of km2 to small arrays involving only a few detectors in an area only a few hundred meters square.

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 1018 to 1021 eV using refurbished detectors from a neutrino experiment and 1 m2 scintillators Currently 51 sites are setup and working. Image from www.chicos.caltech.edu

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)

ALTA MAP

CROP (Cosmic Ray Observatory Project, University of Nebraska) A project to study EAS from particles > 1018 eV. Thirty operating schools covering 75000 sq miles is the goal of the project. Detectors are 1 m2 scintillators donated by the Chicago Air Shower Array. Image from Marion High School. Http://marian.creighton. edu

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/~wilkes

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 )

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.

Tentative Plans: 3-week quarknet workshop Tentative Plans: 3-week quarknet workshop. Summer, 2004 Julia Thompson Prof. of Physics, Univ. of Pittsburgh Adjunct Prof. of physics and/or in participating in a proposed project to put cosmic ray detectors in high schools, eventually perhaps linking them into a shared network. Teachers can register for 1-3 credit hours from UMSL for the workshop, and will receive a personal stipend of $300./week, Physics, Univ. of Missouri at St. Louis A 3-week summer workshop for physics teachers is expected at the University of Missouri at St. Louis (UMSL) in summer, 2004. through the quarknet program The workshop would be open to area high school physics teachers interested in expanding their knowledge of current modern and a $250. instructional materials stipend.