Jumat, 04 Juni 2010

ComIcs

Cosmic ray

Cosmic rays are energetic particles originating from outer space that impinge on Earth's atmosphere. Almost 90% of all the incoming cosmic ray particles are simple protons, with nearly 10% being helium nuclei (alpha particles), and slightly under 1% are heavier elements, electrons (beta particles), or gamma ray photons.[1] The term ray is a misnomer, as cosmic particles arrive individually, not in the form of a ray or beam of particles. However, when they were first discovered, cosmic rays were thought to be rays. When their particle nature needs to be emphasized, "cosmic ray particle" is written.

The variety of particle energies reflects the wide variety of sources. The origins of these particles range from energetic processes on the Sun all the way to as yet unknown events in the farthest reaches of the visible universe. Cosmic rays can have energies of over 1020 eV, far higher than the 1012 to 1013 eV that man-made particle accelerators can produce. (See Ultra-high-energy cosmic rays for a description of the detection of a single particle with an energy of about 50 J, the same as a well-hit tennis ball at 42 m/s [about 94 mph].) There has been interest in investigating cosmic rays of even greater energies.[2]

Composition

Cosmic rays may broadly be divided into two categories, primary and secondary. The cosmic rays that arise in extrasolar astrophysical sources are primary cosmic rays; these primary cosmic rays can interact with interstellar matter to create secondary cosmic rays. The sun also emits low energy cosmic rays associated with solar flares. The exact composition of primary cosmic rays, outside the Earth's atmosphere, is dependent on which part of the energy spectrum is observed. However, in general, almost 90% of all the incoming cosmic rays are protons, about 9% are helium nuclei (alpha particles) and nearly 1% are electrons. The ratio of hydrogen to helium nuclei (28% helium by mass) is about the same as the primordial elemental abundance ratio of these elements (24% by mass He) in the universe.

The remaining fraction is made up of the other heavier nuclei which are abundant end products of stars' nuclear synthesis. Secondary cosmic rays consist of the other nuclei which are not abundant nuclear synthesis end products, or products of the Big Bang, primarily lithium, beryllium, and boron. These light nuclei appear in cosmic rays in much greater abundance (about 1:100 particles) than in solar atmospheres, where their abundance is about 10−7 that of helium.

This abundance difference is a result of the way secondary cosmic rays are formed. When the heavy nuclei components of primary cosmic rays, namely the carbon and oxygen nuclei, collide with interstellar matter, they break up into lighter nuclei (in a process termed cosmic ray spallation) - lithium, beryllium and boron. It is found that the energy spectra of Li, Be and B fall off somewhat more steeply than those of carbon or oxygen, indicating that less cosmic ray spallation occurs for the higher energy nuclei presumably due to their escape from the galactic magnetic field. Spallation is also responsible for the abundances of scandium, titanium, vanadium, and manganese ions in cosmic rays, which are produced by collisions of iron and nickel nuclei with interstellar matter. (See environmental radioactivity#Natural).

In the past, it was believed that the cosmic ray flux has remained fairly constant over time. Recent research has, however, produced evidence for 1.5 to 2-fold millennium-timescale changes in the cosmic ray flux in the past forty thousand years.[3]

Modulation

The flux (flow rate) of cosmic rays incident on the Earth’s upper atmosphere is modulated (varied) by two processes; the sun’s solar wind and the Earth's magnetic field. The Solar wind is expanding magnetized plasma generated by the sun, which has the effect of decelerating the incoming particles, as well as excluding some of the particles with energies below about 1 GeV. The amount of solar wind is not constant due to changes in solar activity, for instance over its regular eleven-year cycle. Hence the level of modulation varies in anticorrelation with solar activity. Also the Earth's magnetic field deflects some of the cosmic rays, giving rise to the observation that the intensity of cosmic radiation is dependent on latitude, longitude, and azimuth angle. The cosmic flux varies from eastern and western directions due to the polarity of the Earth's geomagnetic field and the positive charge dominance in primary cosmic rays. (This is called the "east-west effect"). The cosmic ray intensity at the Equator is lower than at the poles as the geomagnetic cutoff value is greatest at the equator. This is because charged particles tend to move in the direction of field lines and not across them, so that they are concentrated in the polar regions (where field lines are closest together). This is the reason the auroras occur at the poles, since the field lines curve down towards the Earth’s surface there. Finally, the longitude dependence arises from the fact that the geomagnetic dipole axis is not parallel to the Earth's rotation axis.

This modulation which describes the change in the interstellar intensities of cosmic rays as they propagate in the heliosphere is highly energy and spatial dependent, and it is described by the Parker's Transport Equation in the heliosphere. At large radial distances, far from the Sun (~94 AU), there exists the region where the solar wind undergoes a transition from supersonic to subsonic speeds called the "solar wind termination shock". The region between the termination shock and the heliopause (the boundary marking the end of the heliosphere) is called the heliosheath. This region acts as a barrier to cosmic rays, decreasing their intensity at lower energies by about 90%; thus it is not only the Earth's magnetic field that protects us from cosmic ray bombardment.

From a scientific modeling point of view, there is a challenge in determining the Local Interstellar Spectra (LIS) due to large adiabatic energy changes these particles experience owing to the diverging solar wind in the heliosphere. However, significant progress has been made in the field of cosmic ray studies with the development of an improved state-of-the-art 2D numerical model that includes the simulation of the solar wind termination shock, drifts and the heliosheath coupled with fresh descriptions of the diffusion tensor, see Langner et al. (2004). But challenges also exist because the structure of the solar wind and the turbulent magnetic field in the heliosheath is not well understood indicating the heliosheath as the region unknown beyond. With lack of knowledge of the diffusion coefficient perpendicular to the magnetic field our knowledge of the heliosphere and from the modelling point of view is far from complete. There exist promising theories like ab initio method approaches, but the drawback is that such theories produce poor compatibility with observations (Minnie, 2006) indicating their failure in describing the mechanisms influencing the cosmic rays in the heliosphere.

Detection
The Moon's cosmic ray shadow, as seen in secondary muons detected 700 m below ground, at the Soudan 2 detector.
The Moon as seen by the Compton Gamma Ray Observatory, in gamma rays of greater than 20 MeV. These are produced by cosmic ray bombardment of its surface. The Sun, which has no similar surface of high atomic number to act as target for cosmic rays, cannot be seen at all at these energies, which are too high to emerge from primary nuclear reactions, such as solar nuclear fusion.[4]

The nucleus that make up cosmic rays are able to travel from their distant sources to the Earth because of the low density of matter in space. Nuclei interact strongly with other matter, so when the cosmic rays approach Earth they begin to collide with the nuclei of atmospheric gases. These collisions, in a process known as a shower, result in the production of many pions and kaons, unstable mesons which quickly decay into muons. Because muons do not interact strongly with the atmosphere and because of the relativistic effect of time dilation many of these muons are able to reach the surface of the Earth. Muons are ionizing radiation, and may easily be detected by many types of particle detectors such as bubble chambers or scintillation detectors. If several muons are observed by separated detectors at the same instant it is clear that they must have been produced in the same shower event.

Cosmic rays impacting other (non-Earth) bodies in the solar system which are made of elements heavier than hydrogen and helium, can be detected indirectly by observing high energy gamma ray emissions from these bodies using a gamma-ray telescope (see image at right). When such gammas are of energy too high to result from radioactive decay processes (> about 10 MeV) they must be secondary to cosmic ray bombardment.

Detection by particle track-etch technique

Cosmic rays can also be detected directly when they pass through particle detectors flown aboard satellites or in high altitude balloons. In a pioneering technique developed by Robert Fleischer, P. Buford Price, and Robert M. Walker,[5] sheets of clear plastic such as 1/4 mil Lexan polycarbonate can be stacked together and exposed directly to cosmic rays in space or high altitude. When returned to the laboratory, the plastic sheets are "etched" [literally, slowly dissolved] in warm caustic sodium hydroxide solution, which removes the surface material at a slow, known rate. Wherever a bare cosmic ray nucleus passes through the detector, the nuclear charge causes chemical bond breaking in the plastic. The slower the particle, the more extensive is the bond-breaking along the path; and the higher the charge [the higher the Z], the more extensive is the bond-breaking along the path. The caustic sodium hydroxide dissolves at a faster rate along the path of the damage, but thereafter dissolves at the slower base-rate along the surface of the minute hole that was drilled. The net result is a conical shaped pit in the plastic; typically with two pits per sheet [one originating from each side of the plastic]. The etch pits can be measured under a high power microscope [typically 1600X oil-immersion], and the etch rate plotted as a function of the depth in the stack of plastic. At the top of the stack, the ionization damage is less due to the higher speed. As the speed decreases due to deceleration in the stack, the ionization damage increases along the path. This generates a unique curve for each atomic nucleus of Z from 1 to 92, allowing identification of both the charge and energy [speed] of the particle that traverses the stack. This technique has been used with great success for detecting not only cosmic rays, but fission product nuclei for neutron detectors.

Interaction with the Earth's atmosphere

When cosmic ray particles enter the Earth's atmosphere they collide with molecules, mainly oxygen and nitrogen, to produce a cascade of lighter particles, a so-called air shower. The general idea is shown in the figure which shows a cosmic ray shower produced by a high energy proton of cosmic ray origin striking an atmospheric molecule.
Atmospheric Collision.svg

This image is a simplified picture of an air shower: in reality, the number of particles created in an air shower event can reach in the billions, depending on the energy and chemical environment (i.e. atmospheric) of the primary particle. 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). Cosmic rays are also responsible for the continuous production of a number of unstable isotopes in the Earth’s atmosphere, such as carbon-14, via the reaction:

n + \mathrm{N}^{14} \rightarrow p + \mathrm{C}^{14}

Cosmic rays kept the level of carbon-14 in the atmosphere roughly constant (70 tons) for at least the past 100,000 years, until the beginning of above-ground nuclear weapons testing in the early 1950s. This is an important fact used in radiocarbon dating which is used in archaeology.

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