Introduction To Ultrahigh Energy Cosmic Ray Physics

Pierre Sokolsky

• Publisher: CRC Press
• Year: 2018
• ISBN:
  9780429979286
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

Cosmic ray physics has recently attracted a great deal of attention from the high energy physics community because of the discovery of new sources and the advent of new techniques. The result of a series of lectures prepared for graduate students and postdoctoral researchers, this book is a general introduction to experimental techniques and results in the field of ultrahigh energy cosmic rays. It succinctly summarizes the rapidly developing field, and provides modern results that include data from newer detectors. Combining experiment and theory, the text explores the results of a single, easy-to-understand experiment to tie together various issues involved in the physics of ultrahigh energy cosmic rays.

1

INTRODUCTION

This book originated as a set of lectures prepared for the Latin American School of Physics 1986, sponsored by the Universidad Nacional Autonoma de Mexico. The intent of the lectures was to introduce young high energy physicists (graduate students and postdoctoral researchers) to current issues of interest in ultrahigh energy cosmic rays. A number of chapters have been added to the original material and the scope has broadened considerably. The basic intent, however, remains the same.

The study of ultrahigh energy cosmic rays, long a backwater on the shoreline between high energy physics and astrophysics, has been recently revitalized. Part of this new energy comes from advances in experimental techniques which may lead to the solution of longstanding problems of cosmic ray physics, while part stems from new discoveries of structure in the cosmic ray spectrum and of new sources of ultrahigh energy γ-rays. Since much of the existing introductory literature is now outdated, and the resurgence of interest has brought many new people into the field (primarily from high energy physics), it seemed appropriate to write a new book on the subject. So this book is adressed to graduate students and researchers in astrophysics, high energy physics, cosmic rays, or astronomy who wish to get an understanding of the current issues and experimental techniques in the field of ultrahigh energy cosmic rays.

I have tried to keep the discussion largely phenomenological, tying the major physics questions to a description of the experimental techniques used to study them. Very little previous knowledge of astrophysics is assumed. The serious student is urged, however, to learn the needed background material on basic astrophysics and lower energy cosmic rays. This could be done by reading J. S. Longair’s admirable book High Energy Astrophysics, 1981. Another valuable text dealing with fundamentals (albeit outdated in some respects) is V. L. Ginzburg and S. I. Syrovatskii’s The Origin of Cosmic Rays, 1964. There are also a number of good review articles in the journal Physics Reports, dating back to the nineteen seventies. Since the study of cosmic rays is, in a sense, an application of the results of high energy physics to astrophysics, some familiarity with the physics of hadronic interactions is assumed.

A sincere attempt is made to strike a balance between the pedagogical approach and a systematic review of the field. Inevitably, the two directions often clash, in which case I have opted for pedagogy. I therefore make no claims for completeness or objectivity. In particular, the study of the hadronic content of extensive air showers, which is a complex and specialized field in its own right, gets short shrift, as does the study of TeV γ-ray astronomy. The interested reader is directed to recent review articles on these fields. The amount of space devoted to optical detection techniques (Cherenkov and fluorescence light) reflects my own interest and bias in addition to the fact that they are the newest and most promising approaches.

There is very little original material in this book. Instead, I have tried to bring together in one place information which new students of the field may have some difficulty in finding on their own. I have not attempted to give comprehensive references, but rather direct the reader to reviews and other secondary sources, or particularly illuminating original work. These can then serve as additional learning materials. Unfortunately, much that is published in this field is not in journal form, but rather exists in proceedings, particularly the proceedings of the widely-attended biennial International Cosmic Ray Conferences (these are always cited by the acronym “ICRC” along with their number, location, and year). Although these proceedings are not always easy to find, the serious student would do well to search them out.

Until recently, it has been the convention in cosmic ray physics to use electron volt units to describe energy over the entire twenty decades of interest. I have instead chosen to use TeV (10¹² eV), PeV (10¹⁵ eV), or EeV (10¹⁸ eV) units where possible, since many different experimental techniques fall naturally into the energy ranges described by this nomenclature.

I would like to thank all those who helped in this work including Bruce Dawson, whose readable Ph. D. thesis was a veritable treasure trove for obscure references; Vince Frederick, for help in unearthing many of the same; and my colleagues George Cassiday, Jerry Elbert, Brian Fick, Dave Liebing, and Gene Loh for stimulating discussions. David Liebing and Bruce Dawson also read the manuscript and made many helpful suggestions. Finally, I would like to thank Clicerio Avilez, and Tom O’Halloran for giving me the stimulus to publish the material.

2

SURVEY OF ULTRAHIGH ENERGY COSMIC RAYS

2.1 Introduction

This book considers questions relating to cosmic rays at the highest observable energies, i.e., energies> .1 EeV (10¹⁷ eV). Although there are many problems in the study of ultrahigh energy cosmic rays, there are two central questions. The first is a result of the fact that cosmic rays have been observed up to energies of 100 EeV, i.e., there is evidence of acceleration by sources either in our galaxy or other galaxies that produce particles with energies ranging over twenty orders of magnitude. The existence of cosmic rays with > .1 EeV energy raises the issue of what is the nature of the highly efficient acceleration mechanism necessary for their production. The first central question is thus that of origin, which in turn implies an understanding of the acceleration mechanisms involved. That subject is the main concern of this book. The second central question relates to the fact that the energy density of cosmic rays is a significant fraction of the energy density of the universe as a whole. As we shall see, it is of the same order as the energy density of starlight, galactic magnetic field, etc. It follows that cosmic rays must play an important role in the overall energy balance of the universe. This series of lectures will largely sidestep this “ecological” question, even though it is also of great interest.

The study of the sources of ultrahigh energy cosmic rays reduces to understanding the three different kinds of information that we have about them.

• The first kind of information is the nature of the particle and its energy spectrum as observed on the surface of the earth, i.e., the chemical composition of the cosmic rays and their energy distribution.

• The second is how the cosmic rays propagate through interstellar and intergalactic space to reach the earth. In their passage through intergalactic space we know that they traverse magnetic fields which affect their direction, and that they interact with dust and gas particles and optical photons. These interactions cause energy losses as well as the breakup (spallation) of any heavy nuclei which may be constituents of the cosmic rays. For the highest energies at which they have been detected, cosmic rays will interact with the 2.7°K blackbody radiation which permeates space and is presumably left over from the decoupling era in the early expansion of the universe. This interaction will modify both the observed charged particle spectrum and the composition of the primary cosmic ray spectrum.

• The third kind of information we can examine is the isotropy or lack thereof of cosmic rays with respect to potential sources within the galaxy or exterior to it.

As we shall see, answers to questions relating to chemical composition and spectra, propagation, and isotropy all affect each other. This is what makes the study of cosmic ray sources so difficult. What is needed at this time is experimental data that gives answers about the correlations of the three issues which have just been described.

2.2 Present Knowledge of Cosmic Rays

Before turning to these questions in greater detail, we will briefly survey what is known about cosmic rays and give order of magnitude numbers to orient the novice. The cosmic ray flux falls many orders of magnitude from the MeV range to the 100 EeV range. Experimental techniques that are used to detect this flux at different energies are therefore very different. At low energies (less than about .1 TeV to 1 TeV), the flux is high enough that direct measurements can be performed using spectrometers, calorimeters, and other similar techniques. Recently these direct measurements have been extended to about .1 PeV.

FIGURE 2.1 Relative abundances of solar and cosmic ray material.

2.2.1 Chemical Composition

The detailed relative abundance of solar and cosmic ray matter is shown in Figure 2.1. We can compare this composition with the average composition of stellar material in the solar system. Overall the compositions are quite similar; however, there are some differences in detail which are very important. Cosmic rays are overabundant in lithium, beryllium, and boron. The iron concentration agrees quite well with solar system composition, but there is an excess of elements just lighter than iron. There is also an underabundance of hydrogen and helium. One way to understand these detailed differences is to assume that cosmic rays have the same composition as solar matter at their origin. As they pass through interstellar space they interact with gas and dust particles, and the heavier nuclei spallate into lighter nuclei. Detailed models show that the abundance of lighter elements in cosmic rays vis a vis the composition of solar matter is in agreement with this kind of spallation due to propagation effects. The composition of cosmic rays at TeV to PeV energies is approximately 50% protons, 25% a-particles, 13% CNO and 13% Fe [1]. Electrons comprise ≤10⁻² and gammas on the order of 10⁻³ of the cosmic ray flux.

2.2.2 Spectrum

The spectrum of primary cosmic rays obeys an overall power law with a break and a change of the slope at around a PeV (see Fig. 2.2). The fact that a power law exists over many decades is important in restricting possible acceleration mechanisms, since the source of the cosmic rays must be such as to generate a power law spectrum. The flux of primary cosmic rays falls from approximately one particle/m²-sec-MeV at the lowest energies to one particle/km²-century at the highest energies. This magnitude of flux implies that the energy density in ultrahigh energy cosmic rays is very large. If the energy density that we observe on earth is similar to what exists in extragalactic space, a significant component of the total energy of the universe is in cosmic rays. The cosmic ray energy density integrated over all energies turns out to be approximately 1 eV/cm³. For comparison, starlight has an energy density of 0.6 eV/cm³ and the energy density of the galactic magnetic field is .2 eV/cm³. It is clear that cosmic rays form a major constituent of the interstellar medium. At the highest observed cosmic ray energy, the resultant energy density causes great problems in terms of source energy. If 100 EeV cosmic rays exist at the level at which they are believed to exist, their energy density corresponds to 10⁻⁸ eV/cm³. If we also assume that these cosmic rays fill the local supercluster of galaxies and have a lifetime of about 10⁸ years, then the sources of these cosmic rays in the supercluster must pump out approximately 5 × 10⁴¹ eV per second at 100 EeV to keep the flux constant[2]. This required energy input is comparable to the entire radio band energy output of the galaxies M87 or Cen A. It is clear that cosmic ray sources cannot follow a blackbody radiation spectrum! There must be nonthermal mechanisms for efficiently accelerating particles to these enormous energies.

FIGURE 2.2 Differential cosmic ray flux.

2.3 Candidate Sources

The presently accepted view is that ultrahigh energy cosmic rays are created and accelerated in active cosmic objects. These include: supernovae, pulsars, galactic nuclei, quasars, and radio galaxies. We will briefly describe the distance and energy scales involved. Our galaxy has a radius of approximately 10 kiloparsecs (kpc)[1 parsec is equal to 3.26 light-years] with the sun located approximately that distance from the galactic center. The thickness of the galactic disk is approximately 100 parsecs. Possible energetic sources of cosmic rays in the galaxy include: supernovae explosions, pulsars, and the galactic nucleus, which may contain a black hole. Our galaxy is a member of the local cluster of galaxies which has a scale of approximately two megaparsecs(Mpc). The local cluster is in turn a part of the local super cluster with a scale of 30 to 50 megaparsecs centered on Virgo, approximately 20 megaparsecs away. The local supercluster has a large number of highly energetic radio galaxies which can certainly produce the required energy output for low energy cosmic rays. Beyond the local supercluster are other superclusters extending to the edge of the visible universe. The edge of the universe is defined by the distance at which the velocity of recession of galaxies is equal to the speed of light. This corresponds to a distance of approximately 5,000 mega...

Table of contents

• Cover
• Title Page
• Copyright Page
• Table of Contents
• List of Figures
• List of Tables
• 1 Introduction
• 2 Survey of Ultrahigh Energy Cosmic Rays
• 3 The Extensive Air Shower
• 4 Experimental Techniques–Ground Arrays
• 5 Experimental Techniques: Cherenkov Light
• 6 Experimental Techniques: Air Fluorescence
• 7 The Cosmic Ray Spectrum
• 8 Anisotropy
• 9 Composition: Direct Methods
• 10 Composition: Muon and Hadron Detectors
• 11 Ultrahigh Energy Cosmic Ray Composition
• 12 The Greisen Cutoff
• 13 The Inelastic Total Cross Section
• 14 Point Sources of γ-Rays
• 15 Appendix: The Atmosphere
• Index