Cosmic Rays

By Nancy K. Cox, Chair

Imagine particles traveling from far across the Galaxy and entering our Earth’s atmosphere, and even at times, through us.  These are cosmic rays.  There are many reasons why they are fascinating.  As this is a history of astronomy group, we will discuss

·        the history of the discovery of cosmic rays,

·        early researchers,

·        what cosmic rays are,

·        why they are so very interesting, and

·        the many uses of cosmic rays.

Did you know that cosmic rays were first discovered by balloon in 1912 by the Austrian physicist Victor Hess, who was measuring the intensity of the radiation with electroscopes.  He noted a rise in the radiation level as the balloon rose in altitude.  He wrote “The results of these observations seem best explained by a radiation of great penetrating power entering our atmosphere from above.”  This was the beginning of cosmic ray astronomy.  24 years later, Hess shared the Nobel Prize in Physics for this discovery.

What led to these initial investigations was the way early-1900s researchers such as J. J. Thomson (discoverer of the electron) puzzled over the minute leakages of electricity from bodies, no matter how insulated they were.

Did you know that cosmic rays were responsible for the first discoveries of numerous other elementary particles (beyond the electron, proton and neutron).  First the meson, and other particles that are produced by the cascading “air showers” that occur when a high energy cosmic ray encounters the Earth’s atmosphere.  For 20 years, until the construction of the large accelerators of the 1950s, new elementary particles could be found only through cosmic rays – and always, of course, by accident, such as in the case of the accidental discovery of the positron (the antimatter particle to the electron).

“Cosmic Rays” is a catch-all term that includes several kinds of particles, and one kind of photon.  They come from many different sources, from within our solar system and beyond.  The Earth is continually being bombarded by cosmic rays:  “A thin cosmic rain”.  I’ll tell of my personal experience with cosmic rays and the Hubble Space Telescope.

What Are Cosmic Rays?

They are high-energy charged particles:  about 85% high-speed protons (hydrogen nuclei), 14% alpha particles (helium nuclei), and 1% heavy nuclei.  They all stream toward Earth at relativistic velocities.

The Several Sources of Cosmic Rays

Solar:  Our Sun ejects low-energy (10⁷ – 10¹⁰ eV) cosmic rays during solar flares.  These and other particles from the solar wind (an only fairly-recently-confirmed phenomenon) interact with our magnetosphere (the Van Allen Belts) to create the aurora – which is another interesting Sun/Earth interaction.

Intermediate-energy cosmic rays (10¹⁰ – 10¹⁶ eV) come equally from all directions.  They originate within the Galaxy, most likely from supernova explosions.  The exact mechanisms for accelerating the nuclei are not known, and they also come from the rotating magnetic fields of pulsars, which are the neutron-star remnants left behind by supernovae.

The highest-energy cosmic rays can come from supernovae or even from other galaxies, having enough energy to escape their home galaxy.  They all travel in very convoluted paths, some related to the galactic magnetic fields, not in straight lines.  New instruments such as the Fly’s Eye are studying these Ultra-High-Energy cosmic rays.

Neutrinos are also cosmic ray particles.  Many thousands of them pass through us at all times.

Gamma rays (the highest-energy form of electromagnetic photons) are also included among cosmic rays, and there have been many recent satellite instruments to study them:  BATSE, Compton Gamma Ray Observatory, etc.  Gamma rays are now thought to come from quasars and other active galactic nuclei, during the formation of black holes.

The Many Uses for Cosmic Rays

·        To study the solar wind, solar flares, solar coronal mass ejections, and Earth’s magnetosphere and auroras.

·        Studying chemical abundances with measurements of the amounts of heavy nuclei.  These measurements of abundances have cosmological implications, for research on the Big Bang (amounts of the light elements deuterium, lithium, and beryllium produced in the Big Bang, and for heavy elements, which are produced by nucleosynthesis in stars, and in supernova explosions.  The cosmic ray abundances of all of these differ from solar system abundances.

·        Age-dating the solar system, using radioisotopes such as ²⁶Al (with a half-life of 3/4 of a million years) in meteorites and lunar rocks.  These isotopes are produced by both galactic and solar cosmic rays.

·        The creation of radiocarbon, the famous “carbon-14”, ¹⁴C, by cosmic rays as they encounter Earth’s atmosphere; its half-life is 5,730 years.  ¹⁴C and some other isotopes are only produced by cosmic rays.  This allows for carbon-dating of ancient skeletons and other once-living organisms, so useful in archaeology and palaeontology.

·        Cosmic rays, with their high energy, might cause a certain number of cancers, but the cause is very difficult to trace.

There are now over 4,000 papers a year with “Cosmic Rays” in their titles, and a biennial International Cosmic Ray Conference, and a new journal, Astroparticle Physics.

Here is a most fitting commentary on cosmic rays made by J. J. Thomson and G. P. Thomson in 1928, long before the current range of this exciting field was known, as they wrote in Conduction of Electricity Though Gases:

It would be one of the romances of science if these obscure and prosaic minute leakages of electricity from well-insulated bodies should be the means by which the most fundamental problems in the evolution of the cosmos had to be investigated.

References

The 2 main works:

·        A Thin Cosmic Rain – Particles from Outer Space, Michael W. Friedlander, Harvard University Press, 2000.  He was a noted cosmic ray researcher and just died recently.  This book is now out in paperback.  It’s a very good popular-level exploration of the many aspects of cosmic rays.  Well illustrated.

·        Cosmic Ray Astrophysics by Reinhard Schlickeiser, Springer Verlag, 2002.  Includes the dreaded mathematical formulae.

Other works:

·        From Atoms to Quarks by Emilio Segrè, W. H. Freeman, 1980.  Gives the discovery and early studies of cosmic rays in a popular history of physics.  Segrè won a Nobel Prize for the first observations of antiprotons, produced by the UC Berkeley Bevatron.

·        The Particle Explosion by F. Close et al, Oxford University Press, 1987.  One of the best popular histories of particle physics.

·        Cosmic Rays by Bruno Rossi, McGraw-Hill, 1964.  A classic by an early pioneer.

·        Cosmic Rays by Martin A. Pomerantz, Van Nostrand, 1971.  An eloquent survey, especially with solar/terrestrial aspects.

·        What Are Cosmic Rays? by Pierre Auger, translated by Maurice M. Shapiro, University of Chicago Press, 1945.

·        Cosmic Rays by A. W. Wolfendale, Philosophical Library, 1963.

·        Cosmic Rays Thus Far by Harvey Brace Lemon, W. W. Norton, 1936.

·        The Story of Cosmic Rays by W. F. G. Swann, Sky, 1955.

·        The Origin of Cosmic Rays by V. L. Ginzburg, Gordon and Breach, 1969.