Cosmic Rays

    The energetic subatomic particles that pervade the universe are known as "cosmic rays". The history of the discovery of these rays is fascinating, and reveals much about the scientific method of inquiry. This natural form of radiation is of great importance to biological evolution (contributing to the genetic mutation rate) and the resultant diversity of life on the surface of the planet earth. Our knowledge of biological history and the recent geological record has been greatly facilitated by ¹⁴C dating. This unstable form of carbon is produced in the earth's atmosphere by cosmic rays.

    The cosmic rays incident upon the earth are intercepted by the atomic nuclei high in the atmosphere. The "blanket" of air surrounding the earth acts as a very thick (equivalent to about 3 meters of concrete) radiation shield. The level of radiation is therefore much greater at high elevations. The top of the atmosphere (about 20 km above the earth's surface) is where the "primary" cosmic rays interact, forming unstable forms of matter (like ¹⁴C), and generating some "secondary" radiation that can penetrate to the earth's surface.

    The "primary" cosmic rays have energies mostly below about 10⁹eV (about the mass of the proton). The number of high energy cosmic rays for each energy is displayed in the figure below, where one can see that the number decreases steeply as the energy increases above 10⁹eV .

    The cosmic rays below about 10¹⁰eV are mostly of solar origin, are primarily protons, and are created in solar flares and other processes in the solar system. The more energetic particles at energies up to about 10¹⁶eV are due to acceleration in the shock waves of supernovae throughout the galaxy. These occur when massive stars explode, about 3 times per century in our galaxy, and produce copious quantities of energetic charged particles. The origin of "primary" cosmic rays above 10¹⁶eV is not understood at this time is is a very active frontier of modern astrophysics. These particles have more than a million times more energy than those we can study in our accelerator laboratories and represent the highest energy particles ever detected and studied.

    Collisions of cosmic rays with atoms in the upper atmosphere

1. produce debris like ¹⁴C, and
2. generate energetic new particles called p mesons.

The p mesons are important in that they are responsible for the secondary particles observed at the earth's surface and generate the dramatic air "showers" from the higher energy cosmic rays. These p mesons come in two types: charged p and neutral p. The charged p⁺ and p^- can collide with other nuclei (generating additional debris and more p mesons) or, with comparable probability, decay to a muon (m) and a neutrino (n). While neutrinos are of great interest in themselves, they are very penetrating and do not generally contribute to significant cosmic ray effects in the atmosphere.

    The muons are charged particles that slow down in the atmosphere, losing about 200 MeV per kilometer of atmosphere. Note that for the 20 kilometers of atmosphere one would expect that only muons of several GeV (10⁹eV) can make it to the earth's surface. These muons of several GeV are the main component of the observed (secondary) cosmics radiation at sea level. As can be inferred from the above energy spectrum, they arise from the interactions of 10¹⁰eV primary cosmic rays in the upper atmospere. As a result, there are about 200 muons per second incident for each square meter of area at the earth's surface. These represent the dominant radiation exposure for biological systems at sea level, and presumably contribute significantly to the mutation rate that drives biological evolution.
    The higher energy primary cosmic rays (over 10¹²eV) generate extended air "showers" of secondary particles in the atmosphere. Remarkably, these can produce signals in separated (by 100's of meters) particle detectors at the earth's surface. There are, of course, muons near the center of the shower from the charged p decays. Outside the central "core" of the shower, the particle flux is dominated by electrons and positrons from the gamma decays of neutral  p⁰ that generate copious quantities of these particles at lower energies. The various processes contributing to the production of  these showers is shown in the figure below.

    The extended air showers from really high energy "primary" cosmic rays can be studied in large arrays of detectors like the CHICOS array. While the individual (1 square meter) detectors observe the 200 Hz of incident muons, they only detect simultaneous ("coincident") events over a large area from the extended air showers of high energy "primaries".

    The study of these extended air showers affords us with an occasional glimpse of the very highest energy particles ever observed, and a unique view of extraordinary astrophysical processes. Recent experiments seem to indicate that there are more of these particles than expected, and that perhaps their arrival directions "cluster" as one might expect from "point" astrophysical sources. The CHICOS array will enable Southern California researchers to provide important new data relevant to these exciting forefront issues in physics and astrophysics.