Galactic Cosmic Rays (GCRS)

1. Galactic Cosmic Rays (GCRS) • Galactic cosmic rays (GCRs) come from outside the solar system but generally from within our Milky Way galaxy. • GCRs are atomic nuclei from which all of the surrounding electrons have been stripped away during their high-speed passage through the galaxy. • They have probably been accelerated within the last few million years, and have traveled many times across the galaxy, trapped by the galactic magnetic field. • GCRs have been accelerated to nearly the speed of light. As they travel through the very thin gas of interstellar space, some of the GCRs interact and emit gamma rays, which is how we know that they pass through the Milky Way and other galaxies. • The elemental makeup of GCRs has been studied in detail , and is very similar to the composition of the Earth and solar system. • but studies of the composition of the isotopes in GCRs may indicate the that the seed population for GCRs is neither the interstellar gas nor the shards of giant stars that went supernova. This is an area of current study. http://helios.gsfc.nasa.gov/gcr.html

2. Solar cosmic rays (SCR) • The solar cosmic rays (SCR) originate mostly from solar flares. • Composition is similar to galactic cosmic rays: mostly protons, about 10% of He and <1% heavier elements. • Solar cosmic rays were firstly discovered experimentally on 28 February 1942, as a sudden increase of Geiger counters counting rate associated with a large solar flare. • Since that time detectors, set up to monitor cosmic rays, have occasionally seen sudden increases in the intensity of the radiation associated with outbursts on the Sun, mostly with visible flares. • The cosmic ray intensity returns to normal within tens of minutes to hours, as the acceleration process ends and as accelerated ions disperse throughout interplanetary space. • The short increases of cosmic ray detectors count rate associated with solar particles arrival are called GLE - Ground Level Enchancement / Ground Level Events. http://www.oulu.fi/~spaceweb/textbook/scr.html

3. Primary and Secondary Cosmic Rays • There are two categories of cosmic rays: primary and secondary cosmic rays. • Real (or "primary") cosmic rays can generally be defined as all particles that come to earth from outer space. These primary cosmic rays generally do not make it through the earth's atmosphere, and constitute only a small fraction of what we can measure using a suitable set of particle detectors at the earth's surface. • we do measure particles at sea level in such detectors. What we measure, however, are mostly the remains from interactions of primary cosmic rays with the upper atmosphere. These remnants are also particles, referred to as "secondary" cosmic rays. Often, however, the specification "primary" or "secondary" is omitted. • secondary cosmic rays are neither "rays" nor "cosmic": they are particles rather than rays, and they come from the upper atmosphere rather than outer space. On the other hand, they are produced by real cosmic rays! http://www.oulu.fi/~spaceweb/textbook/scr.html

4. Unexpected observation in experiment Measuring the conductivity of gases. In spite of careful precautions, a significant residual conductivity remained. While by lead shielding, the residual conductivity reduces. Conclusion: external radiation of some form Cosmic Rays----A.W.Wolfendale

5. First hypothesis: these radiations from radioactive materials in the earth; • Then gas chambers flew in balloons to study the variation of conductivity with height; • Decreased with height from ground to about 700 meters, then increased steadily. • Hess put forward: the increase was due to an extremely penetrating radiation which was coming from outer space. • From absence of any significant difference in conductivity between day and night experiments, he deduced that the radiation was not of solar origin. This radiation soon came to known as the cosmic radiation. Cosmic Rays----A.W.Wolfendale

6. it was interpreted as γ-ray at first. • Then it was founded they were positively charged, not γ-ray. • According to the absorption properties of the cosmic radiation, it showed there were two main components: the hard (penetrating), the soft (easily absorbed); • Postulation: the mass of penetrating particles between that of electron and the proton.-----µ mesons.( two hundred electron masses); • future investigated that π-mesons could explain the decay of cosmic ray Cosmic Rays----A.W.Wolfendale

7. Then general features of cosmic ray were clear: • high energy protons from outside the earth’s atmosphere interact with the nuclei of the atmosphere to produce π-mesons and small numbers of other particles. Many of the π-mesons decay to form µ mesons ( muon) which survive down to ground level to from the penetrating component. Some µ mesons decayed to electrons which contribute to the soft component. Cosmic Rays----A.W.Wolfendale

8. The primary cosmic rays Interactions of the particles can be divided into two main processes: Physical interaction: scattering, disintegration on collision with galactic atoms; Magnetic interaction: deflection of cosmic ray trajectories by the magnetic fields Cosmic Rays----A.W.Wolfendale

9. Interactions of cosmic rays • Electromagnetic interactions: a fast charged particle passing through the medium atoms, energy transferred to the atom as a whole, excitation or ionization take place. • Excitation---an electron jumped to a larger radium and when it fell back (de-excitation), radiation is emitted. • Ionization: electron is removed completely from the atom • When energy gained by the electron is much higher, the interaction can be considered solely as being between particle and the electron. Cosmic Rays----A.W.Wolfendale

10. probability of collision between a particle and a free electron • Energy and momentum conservation for collision; • The probability increases with the thickness of the medium passed and with its density. Then using thickness expressed in units of mass per unit area, g/cm2 Cosmic Rays----A.W.Wolfendale

11. Interaction of photons (γ-ray )with matter • Three electromagnetic processes by which photons loses energy: photo electric effect, Compton scattering and pair production • E<100keV,photo electric effect: atom absorbs the photon, ejects electron.the number of photons reduced exponentially. Compton scattering: photon strikes an electron and rebounds with reduced energy; Pair production Energy converts to mass, a high energy photon becomes an electron pair. Cosmic Rays----A.W.Wolfendale

12. Nuclear interactions of cosmic rays: • Experiments of Blau and Wambacher in 1937: “stars” in photo graphic emulsions. • Then it is found that stars are mainly produced by neutrons and protons, nuclear disintegrations • π-mesons, then experiences more interactions. Cosmic Rays----A.W.Wolfendale

13. Energy of a few tens of Gev, Low energy particles can’t reach Sea leavel • Cosmic rays in the atmosphere and at sea level Cosmic Rays----A.W.Wolfendale Nuclear cascade schematic diagram

14. http://zebu.uoregon.edu/~js/glossary/cosmic_rays.html

15. Rossi(1952),total intensity of the various particles measured as a function of Altitude p----protons, corresponding to exponential proton absorption, falls down as a straight line in log-linear graph. υ---meson, rise rapidly to a maximum, by π meson decay. then falls off slowly by decaying into electrons. e----electron, same way as u meson. More higher peak due to π meson can lead to many electrons. Cosmic Rays----A.W.Wolfendale

16. http://www.ngdc.noaa.gov/stp/SOLAR/COSMIC_RAYS/cosmic.html