COSMIC RAYS

An Overview

Dr. Darrel SmithDepartment of PhysicsEmbry-Riddle Aeronautical UniversityPrescott, AZ 86301

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.

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 resolved as a result of a balloon flight in 1912 from the University of Vienna.

Victor HessIn 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.

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.

Not from the SunDuring 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.

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.

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.

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.

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 therange 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

Composition of Cosmic Rays

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)

Cosmic Ray Energies

Cosmic Ray studies continue in spite of the development of high energy particle accelerators ~ 1012 eV.

The energy of the highest energy cosmic rays still cannot be duplicated in accelerators.

Present Cosmic Ray Studies

Fermilab “Modern-Day Accelerator”

E = 1012 eV

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 1015 eV.

The sources for ultrahigh cosmic rays are probably, active galactic nuclei and gamma ray bursts.(www.phys.washington.edu)

SupernovasNuclei 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

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 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)

Fort McMurray

Hinton EDMONTON

ALTA DETECTOR MAP

Size of plannedAuger detector

ALTA MAP

CROP (Cosmic Ray Observatory Project, University of Nebraska)

A project to study EAS from particles > 1018 eV.

Thirty operating schools covering 75,000 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.