Re: Cerenkov radiation [ cosmic ray origin ]

["Franta, Jaroslav" <frantaj@aecl.ca>]

A little bit of web searching turned up these interesting sites...

....The images in "Air shower Cherenkov light simulations" are worth a
look-see !

[ PS. obviously my previous reference is a bit out of date w.r.t. the topic
of gamma-ray air showers...]

Jaro
frantaj@aecl.ca

http://www.mpi-hd.mpg.de/hfm/CosmicRay/ChLight/Cherenkov.html
Atmospheric Cherenkov light
 
Charged particles moving through the atmosphere with a velocity larger than
the local speed of light (the vacuum speed of light divided by the
refractive index of the air) emit Cherenkov light. This light is emitted on
a narrow cone around the direction of the particle. The opening angle alpha
is a function of the density of the air and, thus, of the height of
emission. It is increasing downwards but is always less than about 1.3
degrees. From each part of the particle track the Cherenkov light arrives on
a ring on the ground. In an air shower, the initial particle interacts with
the air atoms, producing many new particles. Most of those particles will be
stopped or decay before they reach the ground. The Cherenkov light of all
those shower particles faster than the local speed of light overlaps on the
ground.
Note: different transliterations are in use for the name of the Russian
physicist, who (together with Vavilov) discovered the nature of this kind of
light emission in 1934: Cherenkov or Cerenkov. In the literature, the effect
is sometimes called the Vavilov-Cherenkov effect. 

The first to realize that air showers would produce enough Cherenkov light
to be detectable was Blackett in 1948 and the first to detect this light was
Jelley in the 1950s. Although there are also other ways to detect air
showers, the Cherenkov method has the lowest energy threshold now. In the
1950s and 60s Cherenkov light was used to study properties of air showers
induced by cosmic rays (protons and heavier atomic nuclei). Starting at
about 1970, Cherenkov telescopes (for example the Whipple Gamma-Ray
Telescope) were used to search for sources of TeV energy gamma rays.
Although first detections were reported in the 1970s, it was not before 1988
when the imaging technique provided a means to distinguish between
cosmic-ray induced air showers (coming from all directions) and gamma-ray
induced air showers. Only gamma rays, being responsible only for a tiny
fraction of all air showers, travel on straight lines through our Galaxy and
can be used to image sources of such high-energy particles. For detecting an
enhancement of gamma-ray showers from a source direction above the isotropic
background of cosmic rays, the rejection of cosmic-ray showers is of crucial
importance. 

There are basically three ways to distinguish between cosmic-ray and
gammay-ray induced air showers: different image shapes with imaging
telescopes, different distributions of the light arriving on the ground, and
different distributions of arrival times of the Cherenkov photons.
The best cosmic-ray rejection is now achieved with stereoscopic systems of
several Cherenkov telescopes, like the HEGRA system. Stereoscopic here means
that several telescopes view the same showers from different angles. 

To get an impression how the distribution of Cherenkov light of air showers
looks like on the ground and in potential telescopes used for imaging this
light, go to the Cherenkov light simulations. 

http://www.mpi-hd.mpg.de/hfm/CosmicRay/ChLight/ChLat.html
Air shower Cherenkov light simulations
The following images show several examples of the lateral distribution of
Cherenkov light of air showers. The area displayed covers 400*400 m2 with
the shower core at the centre. Click on any of these images to view a larger
version of it. On these larger images you can click on any point to see what
an ideal telescope of 100 m2 mirror area would see at that point. These
telescope images have a field-of-view of 10*10 degree2, centered at the
shower direction. 
Technical details:
Simulation with CORSIKA 4.50 without any atmospheric extinction. Note that
extinction would reduce in particular the rings seen in the lateral
distributions of the proton showers which are produced high in the
atmosphere. The bright spots, on the other hand, are from particles reaching
the ground (mostly muons) and would not be affected by extinction. All
showers are vertical. The observation is assumed to be at a height of 2200
m. For efficiency reasons, Cherenkov photons were calculated in bunches of
up to 10. In the lateral distributions white pixels correspond to 80 or more
photons per m2 in the wavelength range 300-450 nm. In the telescope images
white corresponds to 30 or more photons per 0.1*0.1 degree2 for a telescope
area of 100 m2. 
These pages were created by K. Bernlöhr at the MPIK in Heidelberg. 

http://egret.sao.arizona.edu/whipple_home.html
The Whipple collaboration, which pioneered the Imaging Atmospheric Cherenkov
Technique for the detection of very high energy (VHE) gamma rays, is based
at the Fred Lawrence Whipple Observatory in Southern Arizona, in the United
States. The primary emphasis of the collaboration's research effort is the
search for and study of celestial sources of gamma-rays in the energy range
of 100 GeV - 10 TeV.
The collaboration currently operates a 10 meter optical reflector, designed
for the capture of light emitted as energetic particles striking the upper
atmosphere. 
 

http://www-hfm.mpi-hd.mpg.de/CosmicRay/CosmicRaySites.html#cherenkov
Atmospheric Cherenkov experiments (see intro ) 
Telescopes and telescope systems:
CANGAROO at Woomera, Australia [Collaboration between Australia and Nippon
for a GAmma Ray Observatory in the Outback]. See also the Australian page
from Adelaide. (Other pages are available from Kobe and Tokyo Institute of
Technology.)
New telescope installed: CANGAROO II 
CAT  [Cherenkov Array at Thémis] (Thémis experiments see also here). 
CLUE [C(h)erenkov Light Ultraviolet Experiment] at the HEGRA site on La
Palma 
HEGRA Cherenkov Telescopes on La Palma, Canary Islands 
Narrabri, Australia: telescopes of the University of Durham 
PACT [Pachmarhi Array of C(h)erenkov telescopes] at the High Energy Gamma
Ray Observatory at Pachmarhi, India (page apparently removed) 
Telescope Array (for Cherenkov and fluorescence light) 
Whipple Gamma-Ray Telescope on Mt. Hopkins, Arizona 
New telescope projects:
H.E.S.S. [High Energy Stereoscopic System] (to be built in Namibia, see also
pages from Paris) 
MAGIC (a 17 m telescope to be built on La Palma) 
VERITAS [Very Energetic Radiation Imaging Telescope Array System] 
Solar power facilities as light collectors:
CELESTE [CErenkov Low Energy Sampling and Timing Experiment] at Thémis,
France 
STACEE [Solar Tower Air Cherenkov Experiment] at Sandia Labs, New Mexico 
Solar Two Observatory (planned at the Solar Two facility, California) 
Cherenkov counter arrays:
AIROBICC (non-imaging counters in the HEGRA array) 
BLANCA [Broad LAteral Non-imaging C(h)erenkov Array] (at CASA, see paper) 
TUNKA-13 (array of non-imaging counters near Lake Baikal) 
Other Cherenkov light detection concepts:
SPHERE (a balloon project looking for light reflected on snow) 
Atmospheric fluorescence experiments
Auger Project Fluorescence Group 
EUSO [Extreme Universe Space Observatory ]: a proposed space experiment. 
HiRes The High Resolution Fly's Eye Cosmic Ray Detector 
(see also HiRes home pages at Adelaide and Columbia University). 
OWL-Airwatch [Orbiting Wide-angle Light collectors] (an ambitious plan to
build a satellite for air shower detection).
See also pages in Palermo and Huntsville. 
Telescope Array (for Cherenkov and fluorescence light) 


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