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X-ray

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Hand mit Ringen: print of Wilhelm Röntgen's first "medical" x-ray, of his wife's hand, taken on 22 December 1895 and presented to Professor Ludwig  Zehnder of the Physik Institut, University of Freiburg, on 1 January 1896
Hand mit Ringen: print of Wilhelm Röntgen's first "medical" x-ray, of his wife's hand, taken on 22 December 1895 and presented to Professor Ludwig Zehnder of the Physik Institut, University of Freiburg, on 1 January 1896[1][2]

An X-ray (or Röntgen ray) is a form of electromagnetic radiation with a wavelength in the range of 10 to 0.01 nanometers, corresponding to frequencies in the range 30 PHz to 30 EHz. X-rays are primarily used for diagnostic radiography and crystallography. X-rays are a form of ionizing radiation and as such can be dangerous. In many languages it is called Röntgen radiation after Wilhelm Conrad Röntgen, who discovered the X-Rays on November, the 8th, 1895 in Würzburg.

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[edit] Unit of measure and exposure

The rem is the traditional unit of dose equivalent. This describes the Energy delivered by γ or X-radiation (indirectly ionizing radiation) for humans. The SI counterpart is the sievert (Sv). One sievert is equal to 100 rem. Because the rem is a relatively large unit, typical equivalent dose is measured in millirem (mrem), or one thousandth of a rem.

The average person living in the United States is exposed to approximately 150 mrem annually from background sources alone.

Reported dosage due to dental X-rays seems to vary significantly. Depending on the source, a typical dental X-ray of a human results in an exposure of perhaps, 3[3], 40[4], 300[5], or as many as 900[6] mrems (30 to 9,000 μSv).

[edit] Physics

X-rays are a type of electromagnetic radiation with wavelengths of around 10-10 metres.

When medical X-rays are being produced, a thin metallic sheet is placed between the emitter and the target, effectively filtering out the lower energy (soft) X-rays. This is often placed close to the window of the X-ray tube. The resultant X-ray is said to be hard. Soft X-rays overlap the range of extreme ultraviolet. The frequency of hard X-rays is higher than that of soft X-rays, and the wavelength is shorter. Hard X-rays overlap the range of "long"-wavelength (lower energy) gamma rays, however the distinction between the two terms depends on the source of the radiation, not its wavelength; X-ray photons are generated by energetic electron processes, gamma rays by transitions within atomic nuclei.

X-ray K-series spectral line wavelengths (nm) for some common target materials.[7]
Target Kβ₁ Kβ₂ Kα₁ Kα₂
Fe 0.17566 0.17442 0.193604 0.193998
Ni 0.15001 0.14886 0.165791 0.166175
Cu 0.139222 0.138109 0.154056 0.154439
Zr 0.070173 0.068993 0.078593 0.079015
Mo 0.063229 0.062099 0.070930 0.071359

The basic production of X-rays is by accelerating electrons in order to collide with a metal target. (In medical applications, this is usually tungsten or a more crack resistant alloy of rhenium (5%) and tungsten (95%), but sometimes molybdenum for more specialised applications, such as when soft X-rays are needed as in mammography. In crystallography, a copper target is most common, with cobalt often being used when fluorescence from iron content in the sample might otherwise present a problem). Here the electrons suddenly decelerate upon colliding with the metal target and if enough energy is contained within the electron it is able to knock out an electron from the inner shell of the metal atom and as a result electrons from higher energy levels then fill up the vacancy and X-ray photons are emitted. This process is extremely inefficient (~0.1%) and thus to produce reasonable flux of X-rays plenty of energy has to be wasted into heat which has to be removed.

The spectral lines generated depends on the target (anode) element used and thus are called characteristic lines. Usually these are transitions from upper shells into K shell (called K lines), into L shell (called L lines) and so on. There is also a continuum Bremsstrahlung radiation given off by the electrons as they are scattered by the strong electric field near the high-Z (proton number) nuclei.

X-rays can detect cancer, cysts, and tumors. Due to their short wavelength, in medical applications X-rays act more like a particle than a wave. This is in contrast to their application in crystallography, where their wave-like nature is most important.

Nowadays, for many (non-medical) applications, X-ray production is achieved by synchrotrons (see synchrotron light).

To create a blood or artery X-ray, also called digital angiography, an initial image is taken of the anatomical region of interest. A second image is then taken of the same region after iodine (contrast) has been injected into the blood vessels within this area. These two images are then digitally subtracted leaving an image of only the iodine contrast outlining the blood vessels. Lastly, the results are printed on film. The doctor (Radiologist) or surgeon then compares the image obtained to normal anatomical images to determine if there is any damage or blockage of the vessel.

To take an X-ray of the bones, short X-ray pulses are shot through a body with photographic film behind. The bones absorb the most photons by the photoelectric process, because they are more electron dense. The x-rays that do not get absorbed turn the photographic film from white to black, leaving a white shadow of bones on the film.

[edit] Detectors

[edit] Photographic plate

The detection of X-rays is based on various methods. The most commonly known methods are a photographic plate, X-ray film in a cassette, and rare earth screens.

A photographic plate or film is used in hospitals to produce images of the internal organs and bones of a patient. They are also used in industrial radiography processes. Since photographic plates are sensitive to X-rays, they provide a convenient and easy means of recording the image. X-ray film is usually provided as pre-loaded paper cartridges with the film inside a light proof paper envelope. An additional paper coated in a thin layer of lead is often included in contact with the photographic film. The lead reflects the x-rays back through the photo film thus more or less doubling the sensitivity of the assembly. Thus the photographic film has to be used the right way round, and is marked as such. The emulsion is frequently coated on both sides of the film or plate in order to increase the sensitivity further.

The part of the patient to be X-rayed is placed between the X-ray source and the photographic receptor to produce what is a shadow of all the internal structure of that particular part of the body being X-rayed. The X-rays are blocked by dense tissues such as bone and pass through soft tissues. Those areas where the X-rays strike the photographic receptor turn black when it is developed. So where the X-rays pass through "soft" parts of the body such as organs, muscle, and skin, the plate or film turns black. Contrast compounds containing barium or iodine, which are radiopaque, can be injected in the artery of a particular organ, or given intravenously. The contrast compounds essentially block the X-rays and hence the circulation of the organ can be more readily seen. Many years ago thorium was used as a contrast medium (Thorotrast) — this caused many people to be injured or even die from the effects of thorium radiation.

Photographic plates are losing favour in many X-ray facilities because of the necessity to have processing facilities readily to hand, and because the photographic plates themselves, plus the processing chemicals are relatively expensive consumables.

[edit] Photostimulable phosphors (PSPs)

An increasingly common method of detecting X-rays is the use of Photostimulable Luminescence (PSL), pioneered by Fuji in the 1980s. In modern hospitals a PSP plate is used in place of the photographic plate. After the plate is X-rayed, excited electrons in the phosphor material remain 'trapped' in 'colour centres' in the crystal lattice until stimulated by a laser beam passed over the plate surface. The light given off during laser stimulation is collected by a photomultiplier tube and the resulting signal is converted into a digital image by computer technology, which gives this process its common name, computed radiography (also referred to as digital radiography). The PSP plate can be used over and over again, and existing x-ray equipment requires no modification to use them.

[edit] Geiger counter

Initially, most common detection methods were based on the ionization of gases, as in the Geiger-Müller counter: a sealed volume, usually a cylinder, with a mica, polymer or thin metal window contains a gas, and a wire, and a high voltage is applied between the cylinder (cathode) and the wire (anode). When an X-ray photon enters the cylinder, it ionizes the gas and forms ions and electrons. Electrons accelerate toward the anode, in the process causing further ionization along their trajectory. This process, known as an avalanche, is detected as a sudden flow of current, called a "count" or "event".

Ultimately, the electrons form a virtual cathode around the anode wire drastically reducing the electric field in the outer portions of the tube. This halts the collisional ionizations and limits further growth of avalanches. As a result, all "counts" on a Geiger counter are the same size and it can give no indication as to the particle energy of the radiation, unlike the proportional counter. The intensity of the radiation is measurable by the Geiger counter as the counting-rate of the system.

In order to gain energy spectrum information a diffracting crystal may be used to first separate the different photons, the method is called wavelength dispersive X-ray spectroscopy (WDX or WDS). Position-sensitive detectors are often used in conjunction with dispersive elements. Other detection equipment may be used which are inherently energy-resolving, such as the aforementioned proportional counters. In either case, use of suitable pulse-processing (MCA) equipment allows digital spectra to be created for later analysis.

For many applications, counters are not sealed but are constantly fed with purified gas (thus reducing problems of contamination or gas aging). These are called "flow counter".

[edit] Scintillators

Some materials such as sodium iodide (NaI) can "convert" an X-ray photon to a visible photon; an electronic detector can be built by adding a photomultiplier. These detectors are called "scintillators", filmscreens or "scintillation counters". The main advantage of using these is that an adequate image can be obtained while subjecting the patient to a much lower dose of X-rays.

[edit] Image intensification

X-ray during Cholecystectomy
X-ray during Cholecystectomy

X-rays are also used in "real-time" procedures such as angiography or contrast studies of the hollow organs (e.g. barium enema of the small or large intestine) using fluoroscopy acquired using an X-ray image intensifier. Angioplasty, medical interventions of the arterial system, rely heavily on X-ray-sensitive contrast to identify potentially treatable lesions.

[edit] Direct semiconductor detectors

Since the 1970s, new semiconductor detectors have been developed (silicon or germanium doped with lithium, Si(Li) or Ge(Li)). X-ray photons are converted to electron-hole pairs in the semiconductor and are collected to detect the X-rays. When the temperature is low enough (the detector is cooled by Peltier effect or even cooler liquid nitrogen), it is possible to directly determine the X-ray energy spectrum; this method is called energy dispersive X-ray spectroscopy (EDX or EDS); it is often used in small X-ray fluorescence spectrometers. These detectors are sometimes called "solid state detectors". Cadmium telluride (CdTe) and its alloy with zinc, cadmium zinc telluride detectors have an increased sensitivity, which allows lower doses of X-rays to be used.

Practical application in medical imaging didn't start taking place until the 1990s. Currently amorphous selenium is used in commercial large area flat panel X-ray detectors for mammography and chest radiography. Current research and development is focussed around pixel detectors, such as CERN's energy resolving Medipix detector.

Note: A standard semiconductor diode, such as a 1N4007, will produce a small amount of current when placed in an X-ray beam. A test device once used by Medical Imaging Service personnel was a small project box that contained several diodes of this type in series, which could be connected to an oscilloscope as a quick diagnostic.

Silicon drift detectors (SDDs), produced by conventional semiconductor fabrication, now provide a cost-effective and high resolving power radiation measurement. Unlike conventional X-ray detectors, such as Si(Li)s, they do not need to be cooled with liquid nitrogen.

[edit] Scintillator plus semiconductor detectors (indirect detection)

With the advent of large semiconductor array detectors it has become possible to design detector systems using a scintillator screen to convert from X-rays to visible light which is then converted to electrical signals in an array detector. Indirect Flat Panel Detectors (FPDs) are in widespread use today in medical, dental, veterinary and industrial applications. A common form of these detectors is based on amorphous silicon TFT/photodiode arrays.

The array technology is a variant on the amorphous silicon TFT arrays used in many flat panel displays, like the ones in computer laptops. The array consists of a sheet of glass covered with a thin layer of silicon that is in an amorphous or disordered state. At a microscopic scale, the silicon has been imprinted with millions of transistors arranged in a highly ordered array, like the grid on a sheet of graph paper. Each of these thin film transistors (TFTs) are attached to a light-absorbing photodiode making up an individual pixel (picture element). Photons striking the photodiode are converted into two carriers of electrical charge, called electron-hole pairs. Since the number of charge carriers produced will vary with the intensity of incoming light photons, an electrical pattern is created that can be swiftly converted to a voltage and then a digital signal, which is interpreted by a computer to produce a digital image. Although silicon has outstanding electronic properties, it is not a particularly good absorber of X-ray photons. For this reason, X-rays first impinge upon scintillators made from eg. gadolinium oxysulfide or cesium iodide. The scintillator absorbs the X-rays and converts them into visible light photons that then pass onto the photodiode array.

[edit] Visibility to the human eye

While generally considered invisible to the human eye, in special circumstances X-rays can be visible. Brandes, in an experiment a short time after Röntgen's landmark 1895 paper, reported after dark adaptation and placing his eye close to an X-ray tube, seeing a faint "blue-gray" glow which seemed to originate within the eye itself.[1] Upon hearing this, Röntgen reviewed his record books and found he too had seen the effect. When placing an X-ray tube on the opposite side of a wooden door Röntgen had noted the same blue glow, seeming to emanate from the eye itself, but thought his observations to be spurious because he only saw the effect when he used one type of tube. Later he realized that the tube which had created the effect was the only one powerful enough to make the glow plainly visible and the experiment was thereafter readily repeatable. The knowledge that X-rays are actually faintly visible to the dark-adapted naked eye has largely been forgotten today; this is probably due to the desire not to repeat what would now be seen as a recklessly dangerous and potentially harmful experiment with ionizing radiation. It is not known what exact mechanism in the eye produces the visibility: it could be due to conventional detection (excitation of rhodopsin molecules in the retina), direct excitation of retinal nerve cells, or secondary detection via, for instance, X-ray induction of phosphorescence in the eyeball with conventional retinal detection of the secondarily produced visible light.

If the intensity of an X-ray beam is high enough, the ionization of the air will make the beam visible with a white glow.

[edit] Medical uses

X-Ray Image of the Paranasal Sinuses, Lateral Projection
X-Ray Image of the Paranasal Sinuses, Lateral Projection

Since Röntgen's discovery that X-rays can identify bony structures, X-rays have been developed for their use in medical imaging. Radiology is a specialized field of medicine. Radiographers employ radiography and other techniques for diagnostic imaging. Indeed, this is probably the most common use of X-ray technology.

X-rays are especially useful in the detection of pathology of the skeletal system, but are also useful for detecting some disease processes in soft tissue. Some notable examples are the very common chest X-ray, which can be used to identify lung diseases such as pneumonia, lung cancer or pulmonary edema, and the abdominal X-ray, which can detect ileus (blockage of the intestine), free air (from visceral perforations) and free fluid (in ascites). In some cases, the use of X-rays is debatable, such as gallstones (which are rarely radiopaque) or kidney stones (which are often visible, but not always). Also, traditional plain X-rays pose very little use in the imaging of soft tissues such as the brain or muscle. Imaging alternatives for soft tissues are computed axial tomography (CAT or CT scanning), magnetic resonance imaging (MRI) or ultrasound. Since 2005, X-rays are listed as a carcinogen by the U.S. government.[8]

Radiotherapy, a curative medical intervention, now used almost exclusively for cancer, employs higher energies of radiation.

The efficiency of X-ray tubes is less than 2%. Most of the energy is used to heat up the anode.

[edit] Other uses

Each dot, called a reflection, in this diffraction pattern forms from the constructive interference of scattered X-rays passing through a crystal. The data can be used to determine the crystalline structure.
Each dot, called a reflection, in this diffraction pattern forms from the constructive interference of scattered X-rays passing through a crystal. The data can be used to determine the crystalline structure.

Other notable uses of X-rays include

[edit] History

Among the important early researchers in X-rays were Professor Ivan Pulyui, Sir William Crookes, Johann Wilhelm Hittorf, Eugen Goldstein, Heinrich Hertz, Philipp Lenard, Hermann von Helmholtz, Nikola Tesla, Thomas Edison, Charles Glover Barkla, Max von Laue, and Wilhelm Conrad Röntgen.

[edit] William Morgan

Welsh actuary, physicist and fellow of the Royal Society, William Morgan (1750 - 1833) was the first to record an experiment which produced X-rays,[9] presenting in 1785 his paper Electrical Experiments Made in Order to Ascertain the Non-Conducting Power of a Perfect Vacuum.[10][11] The experiment involved creating a potential difference in a vacuum and slowly reducing the completeness of the vacuum by introducing mercury vapour into it. From the paper: "according to the length of time during which the mercury was boiled, the 'electric' light turned violet, then purple, then a beautiful green...and then the light became invisible". This progression was the result of the wavelength of the radiation caused by the electric current decreasing beyond the visible range and into X-ray wavelengths.

[edit] Johann Hittorf

Physicist Johann Hittorf (1824 - 1914) observed tubes with energy rays extending from a negative electrode. These rays produced a fluorescence when they hit the glass walls of the tubes. In 1876 the effect was named "cathode rays" by Eugen Goldstein, and today are known to be streams of electrons. Later, English physicist William Crookes investigated the effects of electric currents in gases at low pressure, and constructed what is called the Crookes tube. It is a glass cylinder mostly (but not completely) evacuated, containing electrodes for discharges of a high voltage electric current. He found, when he placed unexposed photographic plates near the tube, that some of them were flawed by shadows, though he did not investigate this effect. Crookes also noted that his cathode rays caused the glass walls of his tube to glow a dull blue colour. Crookes failed to realise that it wasn't actually the cathode rays that caused the blue glow, but the low level x-rays produced when the cathode rays struck the glass.

[edit] Ivan Pulyui

As a result of experiments into what he called cold light Ivan Pulyui is reputed to have developed an X-ray emitting device as early as 1881. He reputedly first demonstrated an X-ray photograph of a 13-year-old boy's broken arm and an X-ray photograph of his daughter's hand with a pin lying under it. The device became known as the Pulyui lamp and was mass-produced for a period. Reputedly, Pulyui personally presented one to Wilhelm Conrad Röntgen who went on to be credited as the major developer of the technology. Pulyui published his results in a scientific paper, Luminous Electrical Matter and the Fourth State of Matter in the Notes of the Austrian Imperial Academy of Sciences (1880-1883), but expressed his ideas in an obscure manner using obsolete terminology. Pulyui did gain some recognition when the work was translated and published as a book by the Royal Society in the UK.

[edit] Nikola Tesla

In April 1887, Nikola Tesla began to investigate X-rays using high voltages and tubes of his own design, as well as Crookes tubes. From his technical publications, it is indicated that he invented and developed a special single-electrode X-ray tube [12] [13], which differed from other X-ray tubes in having no target electrode. The principle behind Tesla's device is nowadays called the Bremsstrahlung process, in which a high-energy secondary X-ray emission is produced when charged particles (such as electrons) pass through matter. By 1892, Tesla performed several such experiments, but he did not categorize the emissions as what were later called X-rays. Tesla generalized the phenomenon as radiant energy of "invisible" kinds.[14] [15] Tesla stated the facts of his methods concerning various experiments in his 1897 X-ray lecture [16] before the New York Academy of Sciences. Also in this lecture, Tesla stated the method of construction and safe operation of X-ray equipment. His X-ray experimentation by vacuum high field emissions also led him to alert the scientific community to the biological hazards associated with X-ray exposure.[17]

[edit] Fernando Sanford

X-rays were first generated and detected by Fernando Sanford (1854-1948), the foundation Professor of Physics at Stanford University, in 1891. From 1886 to 1888 he had studied in the Hermann Helmholtz laboratory in Berlin, where he became familiar with the cathode rays generated in vacuum tubes when a voltage was applied across separate electrodes, as previously studied by Heinrich Hertz and Philipp Lenard. His letter of January 6, 1893 (describing his discovery as "electric photography") to The Physical Review was duly published and an article entitled Without Lens or Light, Photographs Taken With Plate and Object in Darkness appeared in the San Francisco Examiner. [18]

[edit] Heinrich Hertz

In 1892, Heinrich Hertz began experimenting and demonstrated that cathode rays could penetrate very thin metal foil (such as aluminium). Philipp Lenard, a student of Heinrich Hertz, further researched this effect. He developed a version of the cathode tube and studied the penetration by X-rays of various materials. Philipp Lenard, though, did not realize that he was producing X-rays. Hermann von Helmholtz formulated mathematical equations for X-rays. He postulated a dispersion theory before Röntgen made his discovery and announcement. It was formed on the basis of the electromagnetic theory of light (Wiedmann's Annalen, Vol. XLVIII). However, he did not work with actual X-rays.

[edit] Wilhelm Röntgen

On November 8, 1895, Wilhelm Conrad Röntgen, a German physics professor, began observing and further documenting X-rays while experimenting with vacuum tubes. Röntgen, on December 28, 1895, wrote a preliminary report "On a new kind of ray: A preliminary communication". He submitted it to the Würzburg's Physical-Medical Society journal. This was the first formal and public recognition of the categorization of X-rays. Röntgen referred to the radiation as "X", to indicate that it was an unknown type of radiation. The name stuck, although (over Röntgen's great objections), many of his colleagues suggested calling them Röntgen rays. They are still referred to as such in many languages. Röntgen received the first Nobel Prize in Physics for his discovery.

Röntgen was working on a primitive cathode ray generator that was projected through a glass partially evacuated tube. Suddenly he noticed a faint green light against the wall. The odd thing he had noticed, was that the light from the cathode ray generator was traveling through a bunch of the materials in its way (paper, wood, and books). He then started to put various objects in front of the generator, and as he was doing this, he noticed that the outline of the bones from his hand were displayed on the wall. Röntgen said he did not know what to think and kept experimenting. Two months after his initial discovery, he published his paper translated "On a New Kind of Radiation" and gave a demonstration in 1896.

Rontgen discovered its medical use when he saw a picture of his wife's hand on a photographic plate formed due to X-rays. His wife's hand's photograph was the first ever photograph of a human body part using X-rays.

[edit] Thomas Edison

Diagram of a water cooled X-ray tube. (simplified/outdated)
Diagram of a water cooled X-ray tube. (simplified/outdated)

In 1895, Thomas Edison investigated materials' ability to fluoresce when exposed to X-rays, and found that calcium tungstate was the most effective substance. Around March 1896, the fluoroscope he developed became the standard for medical X-ray examinations. Nevertheless, Edison dropped X-ray research around 1903 after the death of Clarence Madison Dally, one of his glassblowers. Dally had a habit of testing X-ray tubes on his hands, and acquired a cancer in them so tenacious that both arms were amputated in a futile attempt to save his life. "At the 1901 Pan-American Exposition in Buffalo, New York, an assassin shot President William McKinley twice at close range with a .32 caliber revolver." The first bullet was removed but the second remained lodged somewhere in his stomach. McKinley survived for some time and requested that Thomas Edison "rush an X-ray machine to Buffalo to find the stray bullet. It arrived but wasn't used . . . McKinley died of septic shock due to bacterial infection." [19]

[edit] The 20th century and beyond

Prior to the 20th century and for a short while after, x-rays were generated in cold cathode tubes. These tubes had to contain a small quantity of gas (invariably air) as a current will not flow in such a tube if they are fully evacuated. One of the problems with early x-ray tubes is that the generated x-rays caused the glass to absorb the gas and consequently the efficiency quickly falls off. Larger and more frequently used tubes were provided with a means of restoring the air. This often took the form of small side tube which contained a small piece of mica - a substance that traps comparatively large quantities of air within its structure. A small electrical heater heats the mica and causes it to release a small amount of air restoring the tube's efficiency. However the mica itself has a limited life and the restore process was consequently difficult to control.

In 1904, Fleming invented the thermionic diode valve (tube). This used a heated cathode which permitted current to flow in a vacuum. The principle was quickly applied to x-ray tubes, and hard vacuum heated cathode x-ray tubes completely solved the problem of efficiency reduction.

Two years later, physicist Charles Barkla discovered that X-rays could be scattered by gases, and that each element had a characteristic X-ray. He won the 1917 Nobel Prize in Physics for this discovery. Max von Laue, Paul Knipping and Walter Friedrich observed for the first time the diffraction of X-rays by crystals in 1912. This discovery, along with the early works of Paul Peter Ewald, William Henry Bragg and William Lawrence Bragg gave birth to the field of X-ray crystallography. The Coolidge tube was invented the following year by William D. Coolidge which permitted continuous production of X-rays; this type of tube is still in use today.

ROSAT image of X-ray fluorescence of, and occultation of the X-ray background by, the Moon.
ROSAT image of X-ray fluorescence of, and occultation of the X-ray background by, the Moon.

The use of X-rays for medical purposes (to develop into the field of radiation therapy) was pioneered by Major John Hall-Edwards in Birmingham, England. In 1908, he had to have his left arm amputated owing to the spread of X-ray dermatitis[2].

The X-ray microscope were invented in the 1950s. The Chandra X-ray Observatory launched on July 23, 1999, has been allowing the exploration of the very violent processes in the universe which produce X-rays. Unlike visible light, which is a relatively stable view of the universe, the X-ray universe is unstable, it features stars being torn apart by black holes, galactic collisions, and novas, neutron stars that build up layers of plasma that then explode into space.

An X-ray laser device was proposed as part of the Reagan administration's Strategic Defense Initiative in the 1980s, but the first and only test of the device (a sort of laser "blaster", or death ray, powered by a thermonuclear explosion) gave inconclusive results. For technical and political reasons, the overall project (including the X-ray laser) was de-funded (though was later revived by the second Bush administration as National Missile Defense using different technologies).

[edit] See also

[edit] References

  1. ^ Kevles, Bettyann Holtzmann (1996). Naked to the Bone Medical Imaging in the Twentieth Century. Camden, NJ: Rutgers University Press, pp19-22. ISBN 0813523583.
  2. ^ Sample, Sharron (2007-03-27). X-rays. The electromagnetic spectrum. NASA. Retrieved on 2007-12-03.
  3. ^ http://www.doctorspiller.com/Dental%20_X-Rays.htm and http://www.dentalgentlecare.com/x-ray_safety.htm
  4. ^ http://hss.energy.gov/NuclearSafety/NSEA/fire/trainingdocs/radem3.pdf
  5. ^ http://www.hawkhill.com/114s.html
  6. ^ http://www.solarstorms.org/SWChapter8.html and http://www.powerattunements.com/x-ray.html
  7. ^ in David R. Lide: CRC Handbook of Chemistry and Physics 75th edition. CRC Press, 10-227. ISBN 0-8493-0475-X.
  8. ^ http://ntp.niehs.nih.gov/ntp/roc/toc11.html
  9. ^ Anderson, J.G., "William Morgan and x-rays", Transactions of the Faculty of Actuaries 17: pp219-221, <http://www.actuaries.org.uk/files/pdf/library/TFA-017/0219-0221.pdf>
  10. ^ Electrical Experiments Made in Order to Ascertain the Non-Conducting Power of a Perfect Vacuum (PDF). Philosophical Transactions of the Royal Society of London, Vol. 75, 1785. JSTOR.
  11. ^ William Morgan: Bridgend Hall of Fame (HTML). Bridgend County Borough Council.
  12. ^ Morton, William James, and Edwin W. Hammer, American Technical Book Co., 1896. Page 68.
  13. ^ U.S. Patent 514,170 , Incandescent Electric Light, and U.S. Patent 454,622 , System of Electric Lighting.
  14. ^ Cheney, Margaret, "Tesla: Man Out of Time ". Simon and Schuster, 2001. Page 77.
  15. ^ Thomas Commerford Martin (ed.), "The Inventions, Researches and Writings of Nikola Tesla". Page 252 "When it forms a drop, it will emit visible and invisible waves. [...]". (ed., this material originally appeared in an article by Nikola Tesla in The Electrical Engineer of 1894.)
  16. ^ Nikola Tesla, "The stream of Lenard and Roentgen and novel apparatus for their production", Apr. 6, 1897.
  17. ^ Cheney, Margaret, Robert Uth, and Jim Glenn, "Tesla, master of lightning". Barnes & Noble Publishing, 1999. Page 76. ISBN 0760710058
  18. ^ Wyman, Thomas (Spring 2005). "Fernando Sanford and the Discovery of X-rays". "Imprint", from the Associates of the Stanford University Libraries: pp. 5-15.
  19. ^ National Library of Medicine. "Could X-rays Have Saved President William McKinley?" Visible Proofs: Forensic Views of the Body. http://www.nlm.nih.gov/visibleproofs/galleries/cases/mckinley.html
  • NASA Goddard Space Flight centre introduction to X-rays.
  • Way Out There in the Blue: Reagan, Star Wars and the End of the Cold War, Frances Fitzgerald, Simon & Schuster (2001). ISBN 0-7432-0023-3

    Cosmic ray

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    Cosmic rays are energetic particles originating from space that impinge on Earth's atmosphere. Almost 90% of all the incoming cosmic ray particles are protons, about 9% are helium nuclei (alpha particles) and about 1% are electrons. The term "ray" is a misnomer, as cosmic particles arrive individually, not in the form of a ray or beam of particles.

    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.[1]

    The energy spectrum for cosmic rays
    The energy spectrum for cosmic rays

    Contents

    [hide]

    [edit] Cosmic ray sources

    Most cosmic rays originate from extrasolar sources within our own galaxy such as rotating neutron stars, supernovae, and black holes. However, the fact that some cosmic rays have extremely high energies provides evidence that at least some must be of extra-galactic origin (e.g. radio galaxies and quasars); the local galactic magnetic field would not be able to contain particles with such a high energy. The origin of cosmic rays with energies up to 1014 eV can be accounted for in terms of shock-wave acceleration in supernova shells. The origin of cosmic rays with energy greater than 1014 eV remained unknown until recently, when a large collaborative experiment at the Pierre Auger Observatory appears to have answered this question. In preliminary results announced in November, 2007 they showed a strong correlation between their 27 most energetic events and active galactic nuclei [AGN], showing a less than 1/100 chance that those highest energy protons did not originate from those AGN.

    Observations have shown that cosmic rays with an energy above 10 GeV (10 x 109 eV) approach the Earth’s surface isotropically (equally from all directions); it has been hypothesised that this is not due to an even distribution of cosmic ray sources, but instead is due to galactic magnetic fields causing cosmic rays to travel in spiral paths. This limits cosmic ray’s usefulness in positional astronomy as they carry no information of their direction of origin. At energies below 10 GeV there is a directional dependence, due to the interaction of the charged component of the cosmic rays with the Earth's magnetic field.

    [edit] Solar cosmic rays

    Solar cosmic rays or solar energetic particles (SEP) are cosmic rays that originate from the Sun. The average composition is similar to that of the Sun itself. There exists no clear and sharp boundary between the phase spaces of the solar wind and SEP plasma particle populations[2].

    The name solar cosmic ray itself is a misnomer because the term cosmic implies that the rays are from the cosmos and not the solar system, but it has stuck. The misnomer arose because there is continuity in the energy spectra, i.e., the flux of particles as a function of their energy, because the low-energy solar cosmic rays fade more or less smoothly into the galactic ones as one looks at increasingly higher energies.[citation needed] Until the mid-1960s the energy distributions were generally averaged over long time intervals, which also obscured the difference. Later, it was found that the solar cosmic rays vary widely in their intensity and spectrum, increasing in strength after some solar events such as solar flares. Further, an increase in the intensity of solar cosmic rays is followed by a decrease in all other cosmic rays, called the Forbush decrease after their discoverer, the physicist Scott Forbush. These decreases are due to the solar wind with its entrained magnetic field sweeping some of the galactic cosmic rays outwards, away from the Sun and Earth. The overall or average rate of Forbush decreases tends to follow the 11-year sunspot cycle, but individual events are tied to events on the Sun, as explained above.

    There are further differences between cosmic rays of solar and galactic origin, mainly in that the galactic cosmic rays show an enhancement of heavy elements such as calcium, iron and gallium, as well as of cosmically rare light elements such as lithium and beryllium. The latter result from the cosmic ray spallation (fragmentation) of heavy nuclei due to collisions in transit from the distant sources to the solar system.[citation needed]

    [edit] Galactic cosmic rays

    See Galactic cosmic ray.

    [edit] Extragalactic cosmic rays

    See Extragalactic cosmic ray.

    [edit] Ultra-high-energy cosmic rays

    See Ultra-high-energy cosmic ray.

    [edit] Anomalous cosmic rays

    Anomalous cosmic rays (ACRs) are cosmic rays with unexpectedly low energies. They are thought to be created near the edge of our solar system, in the heliosheath, the border region between the heliosphere and the interstellar medium. When electrically neutral atoms are able to enter the heliosheath (being unaffected by its magnetic fields) subsequently become ionized, they are thought to be accelerated into low-energy cosmic rays by the solar wind's termination shock which marks the inner edge of the heliosheath. It is also possible that high energy galactic cosmic rays which hit the shock front of the solar wind near the heliopause might be decelerated, resulting in their transformation into lower-energy anomalous cosmic rays.

    The Voyager 1 space probe crossed the termination shock on December 16, 2004, according to papers published in the journal Science.[3] Readings showed particle acceleration, but not of the kind that generates ACRs. It is unclear at this stage (September 2005) if this is typical of the termination shock (requiring a major rethink of the origin of ACRs), or a localised feature of that part of the termination shock that Voyager 1 passed through. Voyager 2 is expected to cross the termination shock during or after 2008, which will provide more data.

    [edit] 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 about 1% are electrons. The remaining fraction is made up of the other heavier nuclei which are abundant end products of star’s 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), into lithium, beryllium and boron. It is found that the energy spectra of Li, Be and B falls off somewhat steeper than that 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 Sc, Ti, V and Mn elements in cosmic rays, which are produced by collisions of Fe and Ni nuclei with interstellar matter; see Environmental radioactivity#Naturals.

    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.[4]

    [edit] 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. Solar wind is expanding magnetized plasma generated by the sun, which has the effect of decelerating the incoming particles as well as partially 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 over its regular eleven-year cycle. Hence the level of modulation varies in autocorrelation with solar activity. Also the Earth's magnetic field deflects some of the cosmic rays, which is confirmed by the fact that the intensity of cosmic radiation is dependent on latitude, longitude and azimuth. 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 termed 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 can be understood by the fact that charged particle tend to move in the direction of field lines and not across them. This is the reason the Aurorae 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 heliospause (the boundary marking the end of the heliosphere) is called the heliosheath. This region acts as a barrier to cosmic rays and it decreases their intensities at lower energies by about 90% indicating that it is not only the Earth's magnetic field that protect us from cosmic ray bombardment. For more on this topic and how the barrier effects occur the agile reader is referred to Mabedle Donald Ngobeni and Marius Potgieter (2007), and Mabedle Donald Ngobeni (2006).

    From modelling 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 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.

    [edit] Detection

    The Moon's cosmic ray shadow, as seen in secondary muons detected 700m below ground, at the Soudan 2 detector
    The Moon's cosmic ray shadow, as seen in secondary muons detected 700m below ground, at the Soudan 2 detector

    The nuclei 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.

    [edit] 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 P. Buford Price et al., 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 slowly 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.

    [edit] 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.

    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); one common collision is:

    p + \mathrm{O}^{16} \rightarrow n + \pi

    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 aboveground nuclear weapons testing in the early 1950s. This is an important fact used in radiocarbon dating which is used in archaeology.

    In addition to the maintance of a steady-state abundance of C-14 in earth's atmosphere, due to an equilibrium between the production rate from cosmic ray bombardment, and the decay rate due to C-14's radioactive beta decay, it has recently been theorized that the highest energy cosmic rays might also produce micro black holes [MBHs] as a component of the relativistic shower striking earth.

    Searches for such MBHs via a possible Hawking radiation emission signal might be taken at the Pierre Auger cosmic ray observatory. Additionally, searches for Hawking Radiation are planned for possible MBHs that might be created at the Large Hadron Collider [LHC].

    [edit] Unusual Cosmic Rays

    In 1975, a team of researchers headed by P. Buford Price at U.C. Berkeley announced the discovery[5] of a cosmic ray track in a particle detector slung under a high-altitude balloon that was significantly different from all others ever measured. Using the particle track-etch method pioneered by Price, et al., they discovered the track of a particle that had passed through 32 sheets of 1/4 mil Lexan plastic without any measureable change in ionization. Yet, the Cerenkov detector admitted only of particles less than 2/3 c [the speed of light in the clear plastic]. The charge was measured as being 137, the same as predicted by Paul Dirac who first predicted the theoretical existence of magnetic monopoles and the particle track was preliminarily identified as having been caused by a magnetic monopole by the lead researcher, Walter L. Wagner.[6]

    [edit] Research and experiments

    There are a number of cosmic ray research initiatives. These include, but are not limited to:

  • CHICOS
  • PAMELA
  • Alpha Magnetic Spectrometer
  • MARIACHI
  • Pierre Auger Observatory
  • Spaceship Earth

[edit] History

After the discovery of radioactivity by Henri Becquerel in 1896, it was generally believed that atmospheric electricity (ionization of the air) was caused only by radiation from radioactive elements in the ground or the radioactive gases (isotopes of radon) they produce. Measurements of ionization rates at increasing heights above the ground during the decade from 1900 to 1910 showed a decrease that could be explained as due to absorption of the ionizing radiation by the intervening air. Then, in 1912, Victor Hess carried three Wulf electrometers (a device to measure the rate of ion production inside a hermetically sealed container) to an altitude of 5300 meters in a free balloon flight. He found the ionization rate increased approximately fourfold over the rate at ground level. He concluded "The results of my observation are best explained by the assumption that a radiation of very great penetrating power enters our atmosphere from above." In 1913-14, Werner Kolhörster confirmed Victor Hess' results by measuring the increased ionization rate at an altitude of 9 km. Hess received the Nobel Prize in Physics in 1936 for his discovery of what came to be called "cosmic rays".

For many years it was generally believed that cosmic rays were high-energy photons (gamma rays) with some secondary electrons produced by Compton scattering of the gamma rays. Then, during the decade from 1927 to 1937 a wide variety of experimental investigations demonstrated that the primary cosmic rays are mostly positively charged particles, and the secondary radiation observed at ground level is composed primarily of a "soft component" of electrons and photons and a "hard component" of penetrating particles, muons. The muon was initially believed to be the unstable particle predicted by Hideki Yukawa in 1935 in his theory of the nuclear force. Experiments proved that the muon decays with a mean life of 2.2 microseconds into an electron and two neutrinos, but that it does not interact strongly with nuclei, so it could not be the Yukawa particle. The mystery was solved by the discovery in 1947 of the pion, which is produced directly in high-energy nuclear interactions. It decays into a muon and one neutrino with a mean life of 0.0026 microseconds. The pion→muon→electron decay sequence was observed directly in a microscopic examination of particle tracks in a special kind of photographic plate called a nuclear emulsion that had been exposed to cosmic rays at a high-altitude mountain station. In 1948, observations with nuclear emulsions carried by balloons to near the top of the atmosphere by Gottlieb and Van Allen showed that the primary cosmic particles are mostly protons with some helium nuclei (alpha particles) and a small fraction heavier nuclei.

In 1934 Bruno Rossi reported an observation of near-simultaneous discharges of two Geiger counters widely separated in a horizontal plane during a test of equipment he was using in a measurement of the so-called east-west effect. In his report on the experiment, Rossi wrote "...it seems that once in a while the recording equipment is struck by very extensive showers of particles, which causes coincidences between the counters, even placed at large distances from one another. Unfortunately, he did not have the time to study this phenomenon more closely." In 1937 Pierre Auger, unaware of Rossi's earlier report, detected the same phenomenon and investigated it in some detail. He concluded that extensive particle showers are generated by high-energy primary cosmic-ray particles that interact with air nuclei high in the atmosphere, initiating a cascade of secondary interactions that ultimately yield a shower of electrons, photons, and muons that reach ground level.

Homi J. Bhabha derived an expression for the probability of scattering positrons by electrons, a process now known as Bhabha scattering. His classic paper, jointly with Warren Heitler, published in 1937 described how primary cosmic rays from space interact with the upper atmosphere to produce particles observed at the ground level. Bhabha and Heitler explained the cosmic ray shower formation by the cascade production of gamma rays and positive and negative electron pairs. In 1938 Bhabha concluded that observations of the properties of such particles would lead to the straightforward experimental verification of Albert Einstein's theory of relativity.

Measurements of the energy and arrival directions of the ultra-high-energy primary cosmic rays by the techniques of "density sampling" and "fast timing" of extensive air showers were first carried out in 1954 by members of the Rossi Cosmic Ray Group at the Massachusetts Institute of Technology. The experiment employed eleven scintillation detectors arranged within a circle 460 meters in diameter on the grounds of the Agassiz Station of the Harvard College Observatory. From that work, and from many other experiments carried out all over the world, the energy spectrum of the primary cosmic rays is now known to extend beyond 1020 eV (past the GZK cutoff, beyond which very few cosmic rays should be observed). A huge air shower experiment called the Auger Project is currently operated at a site on the pampas of Argentina by an international consortium of physicists. Their aim is to explore the properties and arrival directions of the very highest energy primary cosmic rays. The results are expected to have important implications for particle physics and cosmology. In November, 2007 preliminary results were announced showing direction of origination of the 27 highest energy events were strongly correlated with the locations of active galactic nuclei [AGN], where bare protons are believed accelerated by strong magnetic fields associated with the large black holes at the AGN centers to energies of 1E20 eV and higher.

Three varieties of neutrino are produced when the unstable particles produced in cosmic ray showers decay. Since neutrinos interact only weakly with matter most of them simply pass through the Earth and exit the other side. They very occasionally interact, however, and these atmospheric neutrinos have been detected by several deep underground experiments. The Super-Kamiokande in Japan provided the first convincing evidence for neutrino oscillation in which one flavour of neutrino changes into another. The evidence was found in a difference in the ratio of electron neutrinos to muon neutrinos depending on the distance they have traveled through the air and earth.

[edit] Effects

[edit] Role in ambient radiation

Cosmic rays constitute a fraction of the annual radiation exposure of human beings on earth. For example, the average radiation exposure in Australia is 0.3 mSv due to cosmic rays, out of a total of 2.3 mSv.[1]

[edit] Significance to space travel

Understanding the effects of cosmic rays on the body will be vital for assessing the risks of space travel. R.A. Mewaldt estimated humans unshielded in interplanetary space receive annually roughly 400 to 900 mSv (compared to 2.4 mSv on Earth) and that a 30 month Mars mission might expose astronauts to 460 mSv (at Solar Maximum) to 1140 mSv (at Solar Minimum).[7] These doses approach the 1 to 4 Sv career limits advised by the National Council on Radiation Protection and Measurements for Low Earth orbit activities.

High speed cosmic rays can damage DNA, increasing the risk of cancer, cataracts, neurological disorders, and non-cancer mortality risks.[8]

Due to the potential negative effects of astronaut exposure to cosmic rays, solar activity may play a role in future space travel via the Forbush decrease effect. Coronal mass ejections (CMEs) can temporarily lower the local cosmic ray levels, and radiation from CMEs is easier to shield against than cosmic rays.

[edit] Role in lightning

Cosmic rays have been implicated in the triggering of electrical breakdown in lightning. It has been proposed (see Gurevich and Zybin, Physics Today, May 2005, "Runaway Breakdown and the Mysteries of Lightning") that essentially all lightning is triggered through a relativistic process, "runaway breakdown", seeded by cosmic ray secondaries. Subsequent development of the lightning discharge then occurs through "conventional breakdown" mechanisms.

[edit] Role in climate change

Whether cosmic rays have any role in climate change is disputed. Different groups have made different arguments regarding the role of cosmic ray forcing in climate change.

Shaviv et al. have argued that galactic cosmic ray (GCR) climate signals on geological time scales are attributable to changing positions of the galactic spiral arms of the Milky Way, and that cosmic ray flux variability is the dominant climate driver over these time periods.[9][10] They also argue that GCR flux variability plays an important role in climate variability over shorter time scales, though the relative contribution of anthropogenic factors in relation to GCR flux presently is a matter of continued debate.[11] Because of uncertainty about which GCR energies are the most important drivers of cloud cover variation (if any), and because of the paucity of historical data on cosmic ray flux at various ranges of energies, controversies remain.[12]

Henrik Svensmark et al. have argued that solar variations modulate the cosmic ray signal seen at the earth and that this would affect cloud formation and hence climate. Cosmic rays have been experimentally determined to be able to produce ultra-small aerosol particles,[13] orders of magnitude smaller than cloud condensation nuclei (CCN). Whether this mechanism is relevant to the real atmosphere is unknown; in particular, the steps from this to modulation of cloud formation and thence climate have not been established. The analogy is with the Wilson cloud chamber, however acting on a global scale, where earth's atmosphere acts as the cloud chamber and the cosmic rays catalyze the production of CCN. But unlike a cloud chamber, where the air is carefully purified, the real atmosphere always has many CCN naturally. Various proposals have been made for the mechanism by which cosmic rays might affect clouds, including ion mediated nucleation, and indirect effects on current flow density in the global electric circuit (see Tinsley 2000, and F. Yu 1999). Claims have been made of identification of GCR climate signals in atmospheric parameters such as high latitude precipitation (Todd & Kniveton), and Svensmark's annual cloud cover variations, which were said to be correlated to GCR variation.

That Svensmark's work can be extrapolated to suggest any meaningful connection with global warming is disputed:[14]

At the time we pointed out that while the experiments were potentially of interest, they are a long way from actually demonstrating an influence of cosmic rays on the real world climate, and in no way justify the hyperbole that Svensmark and colleagues put into their press releases and more 'popular' pieces. Even if the evidence for solar forcing were legitimate, any bizarre calculus that takes evidence for solar forcing of climate as evidence against greenhouse gases for current climate change is simply wrong. Whether cosmic rays are correlated with climate or not, they have been regularly measured by the neutron monitor at Climax Station (Colorado) since 1953 and show no long term trend. No trend = no explanation for current changes.[15]

See-also Global warming#Solar variation.

[edit] Cosmic rays and fiction

Because of the metaphysical connotations of the word "cosmic", the very name of these particles enables their misinterpretation by the public, giving them an aura of mysterious powers. Were they merely referred to as "high-speed protons and atomic nuclei" this might not be so.

In fiction, cosmic rays have been used as a catchall, mostly in comics (notably the Marvel Comics group the Fantastic Four), as a source for mutation and therefore the powers gained by being bombarded with them.

Also, in the book Atlas Shrugged by author Ayn Rand, Dr. Robert Stadler's research of cosmic rays is said to have contributed to Project X: a weapon of mass destruction.

[edit] Notes

  1. ^ Luis Anchordoqui, Thomas Paul, Stephen Reucroft, John Swain. Ultrahigh Energy Cosmic Rays: The state of the art before the Auger Observatory. (2002) arxiv:hep-ph/0206072
  2. ^ Solar wind and solar energetic particles: origins and effects
  3. ^ Science, 23 September 2005, Vol 309, Issue 5743
  4. ^ Lal, Devendra; A.J.T. Jullb, David Pollardc and Loic Vacher (2005-06-15). "Evidence for large century time-scale changes in solar activity in the past 32 Kyr, based on in-situ cosmogenic 14C in ice at Summit, Greenland". Earth and Planetary Science Letters 234 (3-4): 335-249.
  5. ^ "Evidence for the Detection of a Moving Magnetic Monopole", Physical. Review. Letters,. Vol. 35. (1975)
  6. ^ Time magazine, August 25, 1975, "Bring it Back Alive"
  7. ^ The Cosmic Ray Radiation Dose in Interplanetary Space – Present Day and Worst-Case Evaluations R.A. Mewaldt et al, page 103, 29th International Cosmic Ray Conference Pune (2005) 00, 101-104
  8. ^ NASA Facts: Understanding Space Radiation
  9. ^ sciencebits.com/CosmicRaysClimate
  10. ^ sciencebits.com/ice-ages
  11. ^ sciencebits.com/CO2orSolar
  12. ^ sciencebits.com/ClimateDebate
  13. ^ Henrik Svensmark, Jens Olaf Pepke Pedersen, Nigel Marsh, Martin Enghoff and Ulrik Uggerhøj, "Experimental Evidence for the role of Ions in Particle Nucleation under Atmospheric Conditions", Proceedings of the Royal Society A, (Early Online Publishing), 2006.
  14. ^ RealClimate: Taking Cosmic Rays for a spin retrieved 22-Feb-2007
  15. ^ RealClimate: Nigel Calder in the Times, retrieved 22-Feb-2007

[edit] References

  • C. D. Anderson and S. H. Neddermeyer, Cloud Chamber Observations of Cosmic Rays at 4300 Meters Elevation and Near Sea-Level, Phys. Rev 50, 263,(1936).
  • M. Boezio et al, Measurement of the flux of atmospheric muons with the CAPRICE94 apparatus, Phys. Rev. D 62, 032007, (2000).
  • R. Clay and B. Dawson, Cosmic Bullets, Allen & Unwin, 1997. ISBN 1864482044
  • T. K. Gaisser, Cosmic Rays and Particle Physics, Cambridge University Press, 1990. ISBN 0521326672
  • P. K. F. Grieder, Cosmic Rays at Earth: Researcher’s Reference Manual and Data Book, Elsevier, 2001. ISBN 0444507108
  • A. M. Hillas, Cosmic Rays, Pergamon Press, Oxford, 1972 ISBN 0080167241 - A good overview of the history and science of cosmic ray research including reprints of seminal papers by Hess, Anderson, Auger and others.
  • J. Kremer et al, Measurement of Ground-Level Muons at Two Geomagnetic Locations, Phys. Rev. Lett. 83, 4241, (1999).
  • S. H. Neddermeyer and C. D. Anderson, Note on the Nature of Cosmic-Ray Particles, Phys. Rev. 51, 844, (1937).
  • M. D. Ngobeni and M. S. Potgieter, Cosmic ray anisotropies in the outer heliosphere, Advances in Space Research, 2007.
  • M. D. Ngobeni, Aspects of the modulation of cosmic rays in the outer heliosphere, M.Sc Dissertation, Northwest University (Potchefstroom campus) South Africa 2006.
  • D. Perkins, Particle Astrophysics, Oxford University Press, 2003. - Very interesting and well written book. ISBN 0198509510
  • C. E. Rolfs and S. R. William, Cauldrons in the Cosmos, The University of Chicago Press, 1988. ISBN 0226724565
  • B. B. Rossi, Cosmic Rays, McGraw-Hill, New York, 1964.
  • Martin Walt, Introduction to Geomagnetically Trapped Radiation, 1994. ISBN 0521431433
  • J. F. Ziegler, The Background In Detectors Caused By Sea Level Cosmic Rays, Nuclear Instruments and Methods 191, 419, (1981).
  • TRACER Long Duration Balloon Project: the largest cosmic ray detector launched on balloons.
  • NOAA FTP: Lal, D., et al., 2005. Data on cosmic ray flux derived from C14 concentrations in the GISP2 Greenland ice core.

[edit] See also

Gamma ray burst

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The image above shows the optical afterglow of gamma ray burst GRB-990123 taken on January 23, 1999. The burst is seen as a bright dot denoted by a square on the left, with an enlarged cutout on the right. The object above it with the finger-like filaments is the originating galaxy. This galaxy seems to be distorted by a collision with another galaxy.
The image above shows the optical afterglow of gamma ray burst GRB-990123 taken on January 23, 1999. The burst is seen as a bright dot denoted by a square on the left, with an enlarged cutout on the right. The object above it with the finger-like filaments is the originating galaxy. This galaxy seems to be distorted by a collision with another galaxy.
Drawing of a massive star collapsing to form a black hole. Energy released as jets along the axis of rotation forms a gamma ray burst. Credit: Nicolle Rager Fuller/NSF
Drawing of a massive star collapsing to form a black hole. Energy released as jets along the axis of rotation forms a gamma ray burst. Credit: Nicolle Rager Fuller/NSF

Gamma-ray bursts (GRBs) are the most luminous events occurring in the universe since the Big Bang. They are flashes of gamma rays emanating from seemingly random places in deep space at random times. The duration of a gamma-ray burst is typically a few seconds, but can range from a few milliseconds to minutes, and the initial burst is usually followed by a longer-lived "afterglow" emitting at longer wavelengths (X-ray, ultraviolet, optical, infrared, and radio). Gamma-ray bursts are detected by orbiting satellites about two to three times per week, but their actual rate of occurrence is much higher because not all bursts are pointed at Earth.

Most observed GRBs appear to be collimated emission caused by the collapse of the core of a rapidly rotating, high-mass star into a black hole. A subclass of GRBs (the "short" bursts) appear to originate from a different process, the leading candidate being the collision of neutron stars orbiting in a binary system. All known GRBs originate from outside our own galaxy; though a related class of phenomena, SGR flares, are associated with Galactic magnetars. The sources of most GRBs are billions of light years away.

A nearby gamma ray burst could possibly cause mass extinctions on Earth.[1] Though the short duration of a gamma ray burst would limit the immediate damage to life, a nearby burst might alter atmospheric chemistry by reducing the ozone layer and generating acidic nitrogen oxides. These atmospheric changes could ultimately cause severe damage to the biosphere.

Contents

[hide]

[edit] Discovery and history

[edit] Vela and the discovery of GRBs

Cosmic gamma-ray bursts were discovered in the late 1960s by the US Vela nuclear test detection satellites. The Velas were built to detect gamma-radiation pulses emitted by nuclear weapon tests in space. The United States suspected that the USSR might attempt to conduct secret nuclear tests after signing the Nuclear Test Ban Treaty in 1963. Any discoveries of weapon tests has never been publicly declared and details of the Vela Incident, an as-yet unidentified flash of light over the South Atlantic on September 22, 1979, remain classified.

In a classic example of scientific serendipity, the satellites did detect flashes of radiation that looked nothing like a nuclear weapons signature, coming from seemingly random directions in deep space. These results were published in 1973,[2] prompting the scientific study of GRBs.

[edit] BATSE

The presence of GRBs was confirmed later by many space missions such as Apollo and the Soviet Venera probes. To explain these events, many speculative theories were advanced, most of which posited nearby Galactic sources. Little progress was made however until the 1991 launch of the Compton Gamma Ray Observatory and its Burst and Transient Source Explorer (BATSE) instrument, an extremely sensitive gamma-ray detector. This instrument provided crucial data indicating that gamma-ray bursts are isotropic[3] (not biased towards any particular direction in space, such as toward the galactic plane or the galactic center), and therefore ruling out nearly all galactic origins. BATSE data also showed that GRBs fall into two distinct categories: short-duration, hard-spectrum bursts ("short bursts"), and long-duration, soft-spectrum bursts ("long bursts").[4] Short bursts are typically less than two seconds in duration and are dominated by higher-energy photons; long bursts are typically more than two seconds in duration and dominated by lower-energy photons. The separation is not absolute and the populations overlap observationally, but the distinction suggests two different classes of progenitors.

[edit] BeppoSAX and the afterglow era

For decades after the discovery of GRBs astronomers could not find any counterpart or host to them, such as a star or galaxy, owing to poor resolution of their detectors. The best hope seemed to lie in finding a fainter, fading, longer wavelength emission after the burst itself, the "afterglow" of a GRB, as predicted by most models.[5]

In 1997 the Dutch/Italian satellite BeppoSAX detected a gamma-ray burst (GRB 970228)[6], and when the X-ray camera was pointed in direction from which the burst had originated it detected a fading X-ray emission. Ground-based telescopes later identified a fading optical counterpart as well.[7] The location of this event having been identified, once the GRB faded, deep imaging was able to identify a faint, very distant host galaxy in the GRB location, the first of many to come.[8] Within only a few weeks the long controversy about the distance scale ended: GRBs were extragalactic events originating inside faint galaxies[9] at enormous distance. By finally establishing the distance scale, characterizing the environments in which GRBs occur, and providing a new window on GRBs both observationally and theoretically, this discovery revolutionized the study of GRBs.[10]

[edit] Swift and GRBs today

As of 2007, a similar revolution in GRB astronomy is in progress, largely as a result of successful launch of NASA's Swift satellite in November 2004, which combines a sensitive gamma-ray detector with the ability to slew on-board X-ray and optical telescopes towards the direction of a new burst in less than a minute.[11] Swift's discoveries include the first observations of short burst afterglows and vast amount of data on the behavior of GRB afterglows at early stages during their evolution, even before the GRB's gamma-ray emission has stopped. The mission has also discovered huge X-ray flares appearing within minutes to days after the end of the GRB.

[edit] Distance scale and energetics

[edit] Galactic vs. extragalactic models

Prior to the launch of BATSE, the distance scale to GRBs was completely unknown. Theories for the location of these events ranged from the outer regions of our own solar system to the edges of the known universe. The discovery that bursts were isotropic—coming from completely random directions—narrowed down these possibilities greatly, and by the mid 1990s only two theories were considered generally viable: GRBs originate from a very large, diffuse halo (or "corona") around our own galaxy, or that they originate from distant galaxies far beyond our local group.

Supporters of the galactic model pointed to the class of well-known objects known as soft gamma repeaters (SGRs), highly magnetized galactic neutron stars known to periodically erupt in bright flares at gamma-ray and other wavelengths, and stated that there may be an unobserved population of similar objects at greater distances, producing GRBs.[12] Furthermore, the sheer brightness of a typical gamma-ray burst would impose enormous requirements on the energy released in such an event if it really occurred in a distant galaxy.

Supporters of the extragalactic model claimed that the galactic neutron-star hypothesis involved too many ad-hoc assumptions in order to reproduce the degree of isotropy reported by BATSE and that an extragalactic model was far more natural regardless of its problems.[13]

[edit] Extragalactic nature of GRBs

The discovery of afterglow emission associated with faraway galaxies definitively supported the extragalactic hypothesis. Not only are GRBs extragalactic events, but they are also observable to the limits of the visible universe; a typical GRB has a redshift of at least 1.0 (corresponding to a distance of 8 billion light-years), while the most distant known (GRB 050904) has a redshift of 6.29 (12.3 billion light years).[14] As observers are able to acquire spectra of only a fraction of bursts - usually the brightest ones - many GRBs may actually originate from even higher redshifts.

[edit] GRB Jets: collimated emission

Many GRBs have been observed to undergo a jet break in their light curve, during which the optical afterglow quickly changes from slowly fading to rapidly fading as the jet slows down.[15] Furthermore, features suggestive of significant asymmetry have been observed in at least one nearby type Ic supernova, which may have the same progenitor stars as GRBs and have been observed to accompany GRBs in some cases (see "Progenitors"). The jet opening angle (degree of beaming), however, varies greatly, from 2 degrees to more than 20 degrees. There is some evidence which suggests that the jet angles and apparent energy released are correlated in such a way that the true energy release of a (long) GRB is approximately constant—about 1044 J, or around 1/2000 of a solar mass.[16] This is comparable to the energy released in a bright type Ib/c supernova (sometimes termed a "hypernova"). Bright hypernovae do in fact appear to accompany some GRBs.[17]

The fact that GRBs are jetted also suggests that there are far more events occurring in the Universe than actually seen, even when factoring in the limited sensitivity of available detectors. Most jetted GRBs will "miss" the Earth and never be seen; only a small fraction happen to be pointed the right way to allow detection. Still, even with these considerations, the rate of GRBs is very small—about once per galaxy per 100,000 years.[18]

[edit] Short GRBs

The above arguments apply only to long-duration GRBs. Short GRBs, while also extragalactic, appear to come from a lower-redshift population and are less luminous than long GRBs.[19] They appear to be generally less beamed[20] or possibly not beamed,[21] intrinsically less energetic than their longer counterparts, and probably more frequent in the universe despite being observed rarely.

[edit] Progenitors: what makes GRBs explode?

The immense distances of most gamma-ray bursts has made pinning down the nature of the system the produces these explosions extremely difficult. The currently favored model for the origin of most observed gamma-ray bursts is the collapsar model[22], in which the core of an extremely massive, low-metallicity, rapidly-rotating star collapses into a black hole, and the infall of material from the star onto the black hole powers an extremely energetic jet that blasts outward through the stellar envelope. When the jet reaches the stellar surface, a gamma-ray burst is produced.

While the collapsar model has enjoyed a great deal of success, many other models exist that are still seriously considered. Winds from highly magnetized, newly-formed neutron stars (protomagnetars)[23], accretion-induced collapse of older neutron stars[24][25], and the mergers of binary neutron stars[26] have all been proposed as alternative models. The different models are not mutually exclusive, and it is possible that different bursts have different progenitors. For example, there is now good evidence that some short gamma-ray bursts (GRBs with a duration of less than about two seconds) occur in galaxies without massive stars[19], providing strong evidence that this subset of events are associated with a different progenitor population from longer bursts - for example, merging neutron stars.

[edit] Emission mechanisms

The means by which gamma-ray bursts convert energy into radiation remains poorly understood, and as of 2007 there is still no generally accepted model for how this process occurs.[27] A successful model of GRBs must explain not only the energy source, but also the physical process for generating an emission of gamma rays which matches the durations, light spectra, and other characteristics of observed GRBs.[28] The nature of the longer-wavelength (X-ray through radio) afterglow emission that follows gamma-ray bursts has been modeled much more successfully as synchrotron emission from a relativistic shock wave propagating through interstellar space[29], but this model has had difficulty explaining the observed features of some observed GRB afterglows (particularly at early times and in the X-ray band)[30], and may be incomplete, or in some cases even inaccurate.

[edit] Mass extinction on Earth

Research has been conducted to investigate the consequences of Earth being hit by a beam of gamma rays from a nearby (about 500 light years) gamma ray burst. This is motivated by the efforts to explain mass extinctions on Earth and estimate the probability of extraterrestrial life. A gamma ray burst at 6000 light years would result in mass extinction; a 1000 light year distant burst would be equivalent to a 100,000 megaton nuclear explosion -- like standing a couple miles from Hiroshima everywhere on earth. A burst 100 light years away would blow away the atmosphere, create tidal waves, and start to melt the surface of the earth. There is a one in a million chance that there could be a gamma ray burst as near as the earth's closest star, Alpha Centauri, in the lifetime of the earth. Such a burst, at 4.3 lightyears distant, would effectively incinerate the earth[31].

A consensus seems to have been arrived at the fact that damage by a gamma ray burst would be very limited because of its very short duration, and the fact that it would only cover half the Earth, the other half being in its shadow. A sufficiently close gamma ray burst would however, result in serious damage to the atmosphere, shutting down communications (due to electro-magnetic disturbances), perhaps instantly wiping out half the ozone layer, and causing nitrogen-oxygen recombination, thereby generating acidic nitrogen oxides. These effects could diffuse across to the other side of the Earth, severely diminish the global food supply, and result in long-term climate and atmospheric changes and a mass extinction, reducing the global population to perhaps 10% of what it can now support. However, the damage from a gamma ray burst would probably be significantly greater than a supernova at the same distance.

The idea that a nearby gamma-ray burst could significantly affect the Earth's atmosphere and potentially cause severe damage to the biosphere was introduced in 1995 by physicist Stephen Thorsett, then at Princeton University.[32] In 2005, Scientists at NASA and the University of Kansas released a more detailed study which suggested that the Ordovician-Silurian extinction events which occurred 450 million years ago could have been triggered by a gamma-ray burst. The scientists do not have direct evidence to suggest that such a burst resulted in the ancient extinction, rather the strength of their work is their atmospheric modeling, essentially a "what if" scenario. The scientists calculated that gamma-ray radiation from a relatively nearby star explosion, hitting the Earth for only ten seconds, could deplete up to half of the atmosphere's protective ozone layer, the recovery for which would take at least five years. With the ozone layer damaged, ultraviolet radiation from the Sun would kill much of the life on land and near the surface of oceans and lakes, disrupting the food chain. While gamma-ray bursts in our Milky Way galaxy are indeed rare, NASA scientists estimate that at least one nearby event has probably hit the Earth in the past billion years. Life on Earth is at least 3.5 billion years old. Dr. Bruce Lieberman, a paleontologist at the University of Kansas, originated the idea that a gamma-ray burst specifically could have caused the great Ordovician extinction. He said, "We do not know exactly when one came, but we're rather sure it did come - and left its mark. What's most surprising is that just a 10-second burst can cause years of devastating ozone damage."[33]

Comparative work in 2006 on galaxies in which GRBs have occurred suggests that metal-deficient galaxies are the most likely candidates. The likelihood of the metal-rich Milky Way galaxy hosting a GRB was estimated at less than 0.15%, significantly reducing the likelihood that a burst had caused mass extinction events on Earth.[34]

[edit] Notable GRBs

GRBs of significant historical or scientific importance include:

  • 670702: The first GRB ever detected.[35]
  • 970228: The first GRB with a successfully detected afterglow. The location of the afterglow was coincident with a very faint galaxy, providing strong evidence that GRBs are extragalactic.[36]
  • 970508: The first GRB with a measured redshift (distance), 0.835. This confirmed unambiguously that GRBs are extragalactic.[37]
  • 971214: In 1997, this was believed by some to be the most energetic event in the universe. This claim has since been discredited.[38][39]
  • 980425: The first GRB with an observed associated supernova (1998bw), providing strong evidence of the link between GRBs and supernovae. The GRB itself was very unusual for being extremely underluminous. Also the closest GRB to date.[40]
  • 990123: This GRB had the optically brightest afterglow measured to date, momentarily reaching or exceeding a magnitude of 8.9, which would be visible with an ordinary pair of binoculars, despite its distance of nearly 10 billion light years. This was also the first GRB for which optical emission was detected before the gamma-ray emission had ceased.[41]
  • 030329A: An extremely close (z=0.168),[42] and therefore extremely bright GRB, with an unambiguous supernova association.[43] GRB 030329 was so bright that its gamma radiation ionized the Earth's upper atmosphere.[44]
  • 050509B: The first short GRB with a host association. Provided evidence that (some) short GRBs, unlike long GRBs, occur in old galaxies and do not have accompanying supernovae.[45]
  • 050724: A thoroughly observed short gamma-ray burst with an afterglow suggesting the demise of a neutron star orbiting a black hole.[46]
  • 050904: The most distant GRB with a securely measured distance, at a redshift of 6.29 (13 billion light-years).[47]
  • 060218: A low-redshift GRB with an accompanying supernova.[48]
  • 060505: The first, well-observed, long duration GRB not accompanied by a bright supernova.[49]
  • 060614: Another recent gamma-ray burst not accompanied by an observable supernova.[50]

[edit] See also