Chapter 8Cosmic Rays8.1 Composition and energy distributionCosmic rays can be broadly defined as the massive particles, photons (γ rays, X-rays, ultra-violet and infrared radiation, ...), neutrinos, and exotics (WIMPS, axions,...) striking theearth. The primary cosmic rays are those entering the upper atmosphere, the cosmic raysof the interstellar medium. Secondary cosmic rays are those produced by the interactions ofthe primary rays in the atmosphere or in the earth. Also products of cosmic ray interactionsin the interstellar medium (e.g., spallation products from cosmic ray - cosmic ray collisions)are also labeled as secondary cosmic rays. Cosmic rays can be of either galactic (includingsolar) or extragalactic origin.If we confine ourselves to the particle constituents (protons, nuclei, leptons), their motionin the galaxy has been roughly randomized by the galactic magnetic field. (We will mentionsome exceptions to this below.) Thus they provide very little information about the direc-tion of the source. The peak of the distribution in energy is in the range of 100 MeV - 1GeV. The intensity of cosmic rays of energy 1 Gev/ nucleon or greater is about 1/cm2sec.The energy density corresponding to this is thus about 1 ev/cm3. This can be compared tothe energy density of stellar light of 0.3 eV/cm3.The chemical composition of (primary) cosmic rays is shown in the figure. This distribu-tion is approximately independent of energy, at least over the dominant energy range of 10MeV/nucleon through several GeV/nucleon. About 90% of nuclear cosmic rays are protons,about 9% He nuclei, and 1% heavier nuclei. The composition has been measured by instru-ments mounted on balloons, satellites, and spacecraft. The figure also shows the chemicaldistribution of the elements in our solar system, which differs from that of the cosmic raysin some remarkable ways. The most dramatic of these is an enormous enrichment in the1cosmic rays of the elements Li/Be/B. Note also that there is enrichment is even Z elementsrelative to odd Z, when normalized to solar system abundances. Finally the cosmic rays arerelatively enriched in the heaviest elements relative to H and He.Although not shown in the figure, many elements heavier than the iron group have beenmeasured with typical abundances of 10−5of iron. Much of this information was gainedfrom satellite and spacecraft measurements over the last decade. Some of the conclusions:1) Abundances of even Z elements with 30∼<Z∼<60 are in reasonable agreement with solarsystem abundances.2) In the region 62∼<Z∼<80, which includes the platinum-lead region, abundances are en-hanced relative to solar by about a factor of two. This suggests an enhancement in r-processelements, which dominate this mass region.There are obvious connections between other astrophysics we have discussed (e.g., if ther-process site is core-collapse supernovae, then one would expect enrichment in r-processnuclei as supernovae are also believed to be the primary acceleration mechanism for lowerenergy cosmic rays) and possible deviations from solar abundances in the cosmic rays.Galactic cosmic rays are fully ionized: the acceleration mechanisms fully strip the ions. Cos-mic rays also have an antimatter component, as measured in the Space Shuttle DiscoveryAMS (Alpha Magnetic Spectrometer) experiment. The AMS detected about 200 antipro-tons above 1 GeV, generally attributed to nuclear collisions of CR particles with interstellarmatter.The energy distribution of cosmic rays from about 1010eV to about 1015eV has a power-lawdistributionparticles/cm2secMeV/n ∝ E−swith s between -1.6 and -1.7. However there is a break or “knee” in the curve at about 1015210 CHAPTER 2. GALACTIC COSMIC RAYSTable 2.1: Relativ e and absolute CR abundance (E>2.5GeV/nuc, [?])particlegroupnucleuschargein tegral parti-cle intensitynumber of particles per 105protonsm−2s−1sr−1in CR in the Universeprotons 1 1300 10000 10000helium 2 94 720 1600L3-52 1510−4M6-96.7 5214H10-192 156VH 20-30 0.5 4 0.06SH > 30 10−410−37 · 10−5electrons 1 13 100 10000antiprotons 1 > 0.1 5 ???trons from atoms, leaving isolated nuclei and electrons.The abundance of primary CR is essentially different from the standardabundance of nuclei in the Universe (Table. 2.1). The difference is biggestfor light n uclei (L = Li, Be, B) which are mainly produced b y CR collisionswith interstellar matter in the Galaxy. The relative abundance of differentelements in cosmic rays is shown also in Fig. 2.1.Among normal matter nuclei, there are also some antimatter nuclei. Nu-merous balloon experiments devoted to search for an timatter in space tookplace since 1970s. They ha ve collected, in total, several hundred antipro-tons. A big astroparticle experiment AMS (Alpha Magnetic Spectrometer)was lunched onboard the Space Shuttle Discovery and flew during 10 days inJune 1998 (Fig. 2.2). It collected about 200 an tiprotons with energy above1 GeV. According to the standard theory, the antiprotons do not originatein the birth of the universe but were produced inside the Galaxy in nu-clear collisions of the CR particles with the interstellar matter. However,some ideas of possible extra-galactic origin of antiprotons have also beenpresented. Unfortunately, data collected so far do not allow to distinguish2.1. COMPOSITION 11Figure 2.1: Relative abundance of elements in cosmic rays and in the solarsystem.20. Cosmic rays 3Differential flux (m2 sr s MeV/nucleon)−1Kinetic energy (MeV/nucleon)HHeCFe10510610410310210 10710−510−610−710−810−910−410−310−20.1101Figure 20.1: Major components of the primary cosmic radiation (from Ref. 1).Most measurements are made at ground level or near the top of the atmosphere,but there are also measurements of muons and electrons from airplanes and balloons.Fig. 20.3 includes recent measurements of negative muons [3,13,14,15]. Since µ+(µ−)areproduced in association with νµ(νµ), the measurement of muons near the maximum ofthe intensity curve for the parent pions serves to calibrate the atmospheric νµbeam [16].June 14, 2000 10:39eV. The slope sharpens above this knee (see the figure), falling with s ranging from -2.0 to-2.2, eventually steeping to an exponent of above -2.7. The knee is generally is attributedto the fact that supernovae acceleration of cosmic rays is limited to about this energy. Thiswould argue that the cosmic rays above this energy either have a different origin, or