COSMIC RAYS:

[Introduction]

Cosmic rays are high energy charged particles, originating in outer space, that travel at nearly the speed of light and strike the Earth from all directions. Most cosmic rays are the nuclei of atoms, ranging from the lightest to the heaviest elements in the periodic table. Cosmic rays also include high energy electrons, positrons, and other subatomic particles.

Cosmic Ray Energies and Acceleration: The energy of cosmic rays is usually measured in units of MeV, for mega-electron volts, or GeV, for giga-electron volts. (One electron volt is the energy gained when an electron is accelerated through a potential difference of 1 volt).

(electron)

(proton)

Most galactic cosmic rays have energies between 100 MeV (corresponding to a velocity for protons of 43% of the speed of light) and 10 GeV (corresponding to 99.6% of the speed of light). The number of cosmic rays with energies beyond 1 GeV decreases by about a factor of 50 for every factor of 10 increase in energy. Over a wide energy range the number of particles per m² per steradian per second with energy greater than E (measured in GeV) is given approximately by N(>E) = k(E + 1)^-a, where k ~ 5000 per m² per steradian per second and a ~1.6. The highest energy cosmic rays measured to date have had more than 10²⁰ eV, equivalent to the kinetic energy of a baseball traveling at approximately 100 mph!

It is believed that most galactic cosmic rays derive their energy from supernova explosions, which occur approximately once every 50 years in our Galaxy. To maintain the observed intensity of cosmic rays over millions of years requires that a few percent of the more than 10⁵¹ ergs released in a typical supernova explosion be converted to cosmic rays. There is considerable evidence that cosmic rays are accelerated as the shock waves from these explosions travel through the surrounding interstellar gas. The energy contributed to the Galaxy by cosmic rays (about 1 eV per cm³) is about equal to that contained in galactic magnetic fields, and in the thermal energy of the gas that pervades the space between the stars.

They may produce secondary particles that penetrate the Earth's atmosphere and surface. Most primary cosmic rays (those that enter the atmosphere from deep space) are composed of familiar stable subatomic particles that normally occur on Earth, such as protons, atomic nuclei, or electrons. However, a very small fraction are stable particles of antimatter, such as positrons or antiprotons, and the precise nature of this remaining fraction is an area of active research.

About 89% of cosmic rays are simple protons or hydrogen nuclei, 10% are helium nuclei or alpha particles, and 1% are the nuclei of heavier elements. These nuclei constitute 99% of the cosmic rays. Solitary electrons (much like beta particles, although their ultimate source is unknown) constitute much of the remaining 1%.

Cosmic rays may broadly be divided into two categories: primary and secondary. The cosmic rays that originate from astrophysical sources are primary cosmic rays; these primary cosmic rays interact with interstellar matter creating secondary cosmic rays. The Sun also emits low energy cosmic rays associated with solar flares. Almost 90% of cosmic rays are protons, about 9% are helium nuclei (alpha particles) and nearly 1% are electrons. The ratio of hydrogen to helium nuclei (28%) is about the same as the primordial elemental abundance ratio of these elements (24%). The remaining fraction is made up of the other heavier nuclei that are nuclear synthesis end products, products of the Big Bang, primarily lithium, beryllium, and boron. These light nuclei appear in cosmic rays in much greater abundance (~1%) than in the solar atmosphere, where their abundance is about 10⁻⁹% that of helium.

The variety of particle energies reflects the wide variety of sources. The origins range from processes on the Sun (and presumably other stars as well), to as yet unknown physical mechanisms in the farthest reaches of the observable universe.

The obscure mechanism of cosmic ray production at galactic distances is partly a result of the fact that (unlike other radiations) magnetic fields in our galaxy and other galaxies bend cosmic ray direction severely, so that they arrive nearly randomly from all directions, hiding any clue of the direction of their initial sources. Cosmic rays can have energies of over 10²⁰ eV, far higher than the 10¹² to 10¹³ eV that terrestrial particle accelerators can produce.

Cosmic rays compose a part of natural background radiation on Earth, averaging about 10-15% of it. However, persons living at higher altitude can obtain several times more cosmic radiation than at sea level, and long distance airline crews can double their yearly ionizing radiation exposure due to this source. Since the intensity of cosmic rays is much larger outside the Earth's atmosphere and magnetic field, it is expected to have a major impact on the design of spacecraft that can safely transport humans in interplanetary space.

The flux of incoming cosmic rays at the upper atmosphere is dependent on the solar wind, the Earth's magnetic field, and the energy of the cosmic rays.

The magnitude of the energy of cosmic ray flux in interstellar space is very comparable to that of other deep space energies: cosmic ray energy density averages about one electron-volt per cubic centimeter of interstellar space, or ~1 eV/cm³, which is comparable to the energy density of visible starlight at 0.3 eV/cm³, the galactic magnetic field energy density (assumed 3 microgauss) which is ~0.25 eV/cm³, or the cosmic microwave background (CMB) radiation energy density at ~ 0.25 eV/cm³.^[6]

However, cosmic rays, unlike the other energy components above, are composed of ionizing particles, and this is far more damaging to biological processes than simple energies suggest. As noted below, cosmic rays make up on average 10 to 15% of background ionizing radiation to humans on Earth, but this component can be several times larger for persons living at higher altitudes.

Air shower: When cosmic rays enter the Earth's atmosphere they collide with molecules, mainly oxygen and nitrogen, to produce a cascade of billions of lighter particles, a so-called air shower. All of the produced particles stay within about one degree of the primary particle's path. Typical particles produced in such collisions are charged mesons e.g. positive and negative pions and kaons. These subsequently decay into muons that are easily detected by many types of particle detectors.

Energies of cosmic ray (partcles):

E < 10 GeV : From solar flare

E > 1 TeV : Makes extensive air shower. Highest energy level that can be produced artificially.

E < 10¹⁵ eV : Below the knee. Made by pulsars, SNRs, and blackholes.

10¹⁵ eV < E < 10^18.5 eV : knee-ankle. Galactic SNR.

E > 10¹⁹ eV : Under investigation. From radio galaxy, AGN, GRB.

Detection of cosmic rays:

Vocano Ranch (US): Scintillator

Haverah Park (UK): Water Cerenkov

SUGAR (Australia): Scintillator

Fly's Eye (UK): Atmospheric fluorescent telescope

Yakutsk (Russia): Scintillator, atmospheric Cerenkov

AGASA (Japan): Akeno Giant Air Shower Array; scintillator, muon detector; 100 km2, 111 scintillation detectors and 27 muon detectors. Investigates UHECR. Operated by U Tokyo.

HiRes (US): High Resolution Fly's Eye. 10¹⁷-10¹⁸ eV cosmic rays have been detected. atmospheric fluorescent telescope

Auger-S Observatory (Arhentina): Atmospheric fluorescent telescope, water Cerenkov

TA (Telescope Array) (US): Utah. Finds the origin of super GZK particles

EUSO

Exper-iment	Method	Covered Area	Duty Factor	Effective Aperture	Energy Thres.	Energy Resol.	Angle Resol.	Cost	Start Year
Unit		km^2	%	km^2str	eV	%	Deg.	$M	-
Fly's Eye	FD	300	10	100	~10^17	~20	~2^o	0.5	1986
AGASA	SD	100	100	250	~3x10^18	~20	~2^o	1	1992
HiRes	FD	4,000	10	1,000	~3x10^18	~10	~0.5^o	5	1999
Pierre-Auger (South)	SD	3,000	100	7,000	~10^19	~10	~1^o	50	2005
	Hybrid	3,000	10	700	~3x10^18	~5	~0.4^o
(South + North)	SD	6,000	100	14,000	~10^19	~10	~1^o	100	2007
	Hybrid	6,000	10	1,400	~3x10^18	~5	~0.4^o
EUSO	FD	200,000	10	50,000	~5x10^19	~30	~2^o	~250	~2010
OWL	FD	2,000,000	10	500,000	~10^20	~30	~2^o	?	>2015

GZK (Greistein-Zatsepin-Kuzmin) limit: Obtained from the special relativity. Interaction between cosmic rays and cosmic background radiation prevents particles with E > 10¹⁹ eV cannot propagate a distance greater than 100 Mpc. Partiles with E > 10¹⁹ eV have been detected on the Earth. The magnetic field in our galaxy cannot confine particles of such high energy that they are thought to have come from outer galaxies. An unsolved problem.

UHECR (Ulta-High-Energy Cosmic Ray): Particles with E > 300 EeV have been detected on 15-Oct-1991. Believed to have come from a massive blackhole (active galactic nucleus). Probability = a few particles per centry. TA (US) program aims to detect UHECRs. 1.2 km2, 576 scintillation detectors and 3 fluorescent telescopes. Participating countries = Japan, US, Taiwan, China, Korea.

For further reading, see

http://www.srl.caltech.edu/personnel/dick/cos_encyc.html