Radiation astronomy/Scatterings

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The image shows the Orion nebula surrounded by a ring of dust. Credit: NASA/JPL-Caltech/T. Megeath(University of Toledo).

"A lithometeor is the general term for particles suspended in a dry atmosphere; these include dry haze, smoke, dust, and sand."[1]

"Dry haze is an accumulation of very fine dust or salt particles in the atmosphere; it does not block light, but instead causes light rays to scatter. Dry haze particles produce a bluish color when viewed against a dark background, but look yellowish when viewed against a lighter background. This light-scattering phenomenon (called Mie scattering) also causes the visual ranges within a uniformly dense layer of haze to vary depending on whether the observer is looking into the sun or away from it."[1]

Neutrons[edit | edit source]

"The fast neutrons from uranium fission can be moderated by elastic collision with graphite or heavy water in a nuclear reactor until their velocity is reduced to that of thermal energies (average 2200 meters/sec= 0.025 electron volts); such very slow neutrons are called thermal neutrons. Unlike fast neutrons, whose principal reaction with matter is one of scattering, thermal neutrons because of their very low energy and velocity are more likely to be captured than scattered by the atoms they encounter. Most of the reactions of biological elements with thermal neutrons are capture reactions. After an atom captures a neutron, it forms a new compound nucleus with an excess of energy. This new compound nucleus may then:

1.) emit a gamma ray immediately to form a stable isotope, e.g.,1

1
1
H
+ 1n → [2
1
H
] → 2
1
H
+ γ;

2.) emit a heavy particle immediately to form a stable isotope, e.g.,

10
5
B
+ 1n → [11
5
B
] → 7
3
Li
+ α;

3.) emit immediately a capture radiation to form a radioactive daughter which subsequently emits beta or gamma rays at a rate characteristic for the isotope formed, e.g.,

14
7
N
+ 1n → [15
7
N
] → 14
6
C
* + p (half life = 6,000 years) → 14
7
N
+ β-."[2]

Synchrotrons[edit | edit source]

Superluminal motion in quasar 3C279 is shown in a "movie" mosaic of five radio images made over seven years. Credit: NRAO/AUI.

"We now assume that the γ-rays are produced [from 3C 279] by relativistic electrons via Compton scattering of synchrotron photons (SSC). In any such model, the fact that the γ-ray luminosity, produced via Compton scattering, is higher than that emitted at lower frequencies (1014 - 1016 Hz), supposedly via the synchrotron process, implies a radiation energy density, Ur, higher than the magnetic energy density, UB. From the observed power ratio we derive that Ur must be one order of magnitude greater than UB, which may be a lower limit if Klein-Nishina effects reduce the efficiency of the self-Compton emission. This result is independent of the degree of beaming, which, for a homogeneous source, affects both the synchrotron and the self-Compton fluxes in the same way. This source is therefore the first observed case of the result of a Compton catastrophe (Hoyle, Burbidge, & Sargent 1966)."[3]

"Superluminal motion in quasar 3C279 is shown [at right] in a "movie" mosaic of five radio images made over seven years. The stationary core is the bright red spot to the left of each image. The observed location of the rightmost blue-green blob moved about 25 light years from 1991 to 1998, hence the changes appear to an observer to be faster than the speed of light or "superluminal". The motion is not really faster than light, the measured speed is due to light-travel-time effects for a source moving near the speed of light almost directly toward the observer. The blue-green blob is part of a jet pointing within 2 degrees to our line of sight, and moving at a true speed of 0.997 times the speed of light. These five images are part of a larger set of twenty-eight images made with the VLBA and other radio telescopes from 1991 to 1997 to study the detailed properties of this energetic quasar."[4] The images are in the K band, 1.2 cm, 22 GHz.[4]

Induced Compton scattering[edit | edit source]

Def. "the non-linear scattering of radiation off electrons" is called induced Compton scattering.[5]

"The effect of scattering is to move photons to lower frequencies."[5] "[T]he fact that the radio pulses [from a pulsar] are not suppressed by induced scattering suggests that the wind's Lorentz factor exceeds ~104.[5]

The Lorentz factor is defined as:[6]

where:

  • v is the relative velocity between inertial reference frames,
  • β is the ratio of v to the speed of light c.
  • τ is the proper time for an observer (measuring time intervals in the observer's own frame),
  • c is the speed of light.

As an example, "[t]he power into the Crab Nebula is apparently supplied by an outflow [wind] of ~1038 erg/s from the pulsar"[5] where there are "electrons (and positrons) in such a wind"[5]. These beta particles coming out of the pulsar are moving very close to light speed.

Skyglows[edit | edit source]

This time exposure photo of New York City at night shows skyglow, one form of light pollution. Credit: Charliebrown7034.
A satellite image of Earth at night. Credit: Data courtesy Marc Imhoff of NASA GSFC and Christopher Elvidge of NOAA NGDC. Image by Craig Mayhew and Robert Simmon, NASA GSFC.
A composite image of the Earth at night in 1994–95. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio.

Scientific definitions of light pollution include the following:

  • Degradation of photic habitat by artificial light.[7]
  • Alteration of natural light levels in the outdoor environment owing to artificial light sources.[8]
  • Light pollution is the alteration of light levels in the outdoor environment (from those present naturally) due to man-made sources of light. Indoor light pollution is such alteration of light levels in the indoor environment due to sources of light, which compromises human health.[9]
  • Light pollution is the introduction by humans, directly or indirectly, of artificial light into the environment.[10]

To precisely measure how bright the sky gets, night time satellite imagery of the earth is used as raw input for the number and intensity of light sources. These are put into a physical model[11] of scattering due to air molecules and aerosoles to calculate cumulative sky brightness. Maps that show the enhanced sky brightness have been prepared for the entire world.[12]

Some astronomers use narrow-band "nebula filters" which only allow specific wavelengths of light commonly seen in nebulae, or broad-band "light pollution filters" which are designed to reduce (but not eliminate) the effects of light pollution by filtering out spectral lines commonly emitted by sodium- and mercury-vapor lamps, thus enhancing contrast and improving the view of dim objects such as galaxies and nebulae.[13] Unfortunately these light pollution reduction (LPR) filters are not a cure for light pollution. LPR filters reduce the brightness of the object under study and this limits the use of higher magnifications. LPR filters work by blocking light of certain wavelengths, which alters the color of the object, often creating a pronounced green cast. Furthermore, LPR filters only work on certain object types (mainly emission nebulae) and are of little use on galaxies and stars. No filter can match the effectiveness of a dark sky for visual or photographic purposes. Due to their low surface brightness, the visibility of diffuse sky objects such as nebulae and galaxies is affected by light pollution more than are stars. Most such objects are rendered invisible in heavily light polluted skies around major cities. A simple method for estimating the darkness of a location is to look for the Milky Way, which from truly dark skies appears bright enough to cast a shadow.[14]

In addition to skyglow, light trespass can impact observations when artificial light directly enters the tube of the telescope and is reflected from non-optical surfaces until it eventually reaches the eyepiece. This direct form of light pollution causes a glow across the field of view which reduces contrast. Light trespass also makes it hard for a visual observer to become sufficiently dark adapted. The usual measures to reduce this glare, if reducing the light directly is not an option, include flocking the telescope tube and accessories to reduce reflection, and putting a light shield (also usable as a dew shield) on the telescope to reduce light entering from angles other than those near the target. Under these conditions, some astronomers prefer to observe under a black cloth to ensure maximum dark adaptation.

Imaging Compton Telescope[edit | edit source]

This is a schematic of the various experiments aboard the Gamma-ray Observatory. Credit: NASA/JPL.
The Imaging Compton Telescope (COMPTEL) utilizes the Compton Effect and two layers of gamma-ray detectors. Credit: NASA.

For cosmic gamma-ray events, the experiment required two nearly simultaneous interactions, in a set of front and rear scintillators. Gamma rays would Compton scatter in a forward detector module, where the interaction energy E1, given to the recoil electron was measured, while the Compton scattered photon would then be caught in one of a second layer of scintillators to the rear, where its total energy, E2, would be measured. From these two energies, E1 and E2, the Compton scattering angle, angle θ, can be determined, along with the total energy, E1 + E2, of the incident photon. The positions of the interactions, in both the front and rear scintillators, was also measured. The vector, V, connecting the two interaction points determined a direction to the sky, and the angle θ about this direction, defined a cone about V on which the source of the photon must lie, and a corresponding "event circle" on the sky.

"COMPTEL's upper layer of detectors are filled with a liquid scintillator which scatters an incoming gamma-ray photon according to the Compton Effect. This photon is then absorbed by NaI crystals in the lower detectors. The instrument records the time, location, and energy of the events in each layer of detectors which makes it possible to determine the direction and energy of the original gamma-ray photon and reconstruct an image and energy spectrum of the source."[15]

Hypotheses[edit | edit source]

  1. The use of satellites should provide ten times the information as sounding rockets or balloons.

A control group for a radiation satellite would contain

  1. a radiation astronomy telescope,
  2. a two-way communication system,
  3. a positional locator,
  4. an orientation propulsion system, and
  5. power supplies and energy sources for all components.

A control group for radiation astronomy satellites may include an ideal or rigorously stable orbit so that the satellite observes the radiation at or to a much higher resolution than an Earth-based ground-level observatory is capable of.

See also[edit | edit source]

References[edit | edit source]

  1. 1.0 1.1 Mark R. Mireles; Kirth L. Pederson; Charles H. Elford (February 21, 2007). Meteorologial Techniques. Offutt Air Force Base, Nebraska, USA: Air Force Weather Agency/DNT. http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA466107. Retrieved 2013-02-17. 
  2. Alan D. Conger; Norman H. Giles Jr. (1950). "The Cytogenetic Effect of Slow Neutrons". Genetics 35 (4): 397–419. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1209493/pdf/397.pdf. Retrieved 2017-11-26. 
  3. L. Maraschi; G. Ghisellini; A. Celotti (September 1992). "A jet model for the gamma-ray emitting blazar 3C 279". The Astrophysical Journal 397 (1): L5-9. doi:10.1086/186531. http://adsabs.harvard.edu/abs/1992ApJ...397L...5M. Retrieved 2014-01-10. 
  4. 4.0 4.1 Ann Wehrle (1998). Apparent Superluminal Motion in 3C279. West Virginia, USA: National Radio Astronomy Observatory. http://images.nrao.edu/387. Retrieved 2014-03-16. 
  5. 5.0 5.1 5.2 5.3 5.4 D. B. Wilson; M. J. Rees (October 1978). "Induced Compton scattering in pulsar winds". Monthly Notices of the Royal Astronomical Society 185 (10): 297-304. 
  6. Dynamics and Relativity, J.R. Forshaw, A.G. Smith, Wiley, 2009, ISBN 978 0 470 01460 8
  7. . PMID 3896840. 
  8. Cinzano, P.; Falchi, F.; Elvidge, C. D.; Baugh, K. E. (2000). "The artificial night sky brightness mapped from DMSP Operational Linescan System measurements". Monthly Notices of the Royal Astronomical Society 318 (3): 641. doi:10.1046/j.1365-8711.2000.03562.x. http://www.lightpollution.it/cinzano/download/mnras_paper.pdf. 
  9. Hollan, J: What is light pollution, and how do we quantify it?. Darksky2008 conference paper, Vienna, August 2008. Updated April 2009.
  10. Marín, C. and Orlando, G. (eds.): Starlight Reserves and World Heritage. Starlight Initiative, IAC and the UNESCO World Heritage Centre. Fuerteventura, Spain, June 2009.
  11. P. Cinzano; F. Falchi; C. D. Elvidge (2001). "The first world atlas of the artificial night sky brightness". Mon.Not.Roy.Astron.Soc. 328 (3): 689–707. doi:10.1046/j.1365-8711.2001.04882.x. http://debora.pd.astro.it/cinzano/download/0108052.pdf. 
  12. The World Atlas of the Artificial Night Sky Brightness. Lightpollution.it. Retrieved on 2011-12-03.
  13. www.astronexus.com, Use of light pollution filters in astronomy. Astronexus.com. Retrieved on 2011-12-03.
  14. NASA Astronomy Picture of the Day, 2010 August 23. Apod.nasa.gov. Retrieved on 2011-12-03.
  15. Neil Gehrels (August 1, 2005). The Imaging Compton Telescope (COMPTEL). Greenbelt, Maryland USA: NASA Goddard Space Flight Center. http://heasarc.gsfc.nasa.gov/docs/cgro/cgro/comptel.html. Retrieved 2013-04-05. 

External links[edit | edit source]

{{Radiation astronomy resources}}{{Repellor vehicle}}