A Gamma-ray Shower over Milagro

                Milagro is an unusual telescope which detects very high energy light from very unusual astronomical objects of interest.  The light is about one trillion times the energy of ordinary room light.  These high energy photons (1 Tera-electron Volt) are the upper end of the scale of radiation generally called gamma-rays.

   The illustration is meant to depict TeV gamma rays coming from some unspecified extra-galactic source and being detected by the telescope. Starting at the top of the picture, representing deep intergalactic space, some powerful mechanism generates the rays.  We don't know the exact mechanisms, but unfamiliar objects such as large black holes, swallowing nearby matter or one another, are suspected to be the cause. In the illustration, you see the gamma-rays radiating out from the point source in various directions.  Only a very small fraction would come toward the Earth, and even fewer would be caught by the telescope. Many of them will encounter starlight on their journey and be annihilated as they produce matter and antimatter (in the form of electrons and positrons, depicted by the red "balls").  Only one, in this picture, reaches the atmosphere of the Earth.

Up around 20 km high, well above the altitude of the jet plane, the gamma-ray interacts with the thin air and initiates a "shower" of lower energy particles.  At each stage in this cascade of electrons, positrons and lower energy photons, interaction with the air makes the shower grow in number of particles and transverse size.  Eventually the lower energy particles begin to get absorbed in the atmosphere and the shower "dies out"-in number of surviving particles, but not in physical size.  The illustration shows it at an instant of time; the leading edge of the shower is like a pancake only a few meters thick.  Notice that a gamma shower has a very strongly defined core of particles, unlike the more common showers caused by other particles impinging on the atmosphere.

We show a cutaway view of the opaque membrane covering the Milagro pond, which is a reservoir containing five million gallons of very pure water.  In the pond you can see two horizontal layers of photomultiplier tubes, which are electronic devices which can sense blue or ultraviolet light when even one photon of such falls upon them. The purpose of the water is to convert the particles of the pancake (which pass through the membrane as if it were almost transparent) into the type of light the tubes can "see".  How does the water do this?

  A particle with electric charge, moving at a speed close to the velocity of light in vacuum, finds itself moving faster than the speed of light in water, which is slower than in vacuum.  Nature doesn't like this situation and makes the particle slow down by generating blue and ultraviolet light, coming out in a cone around the direction of the particle-like a shock wave from a plane breaking the sonic barrier. The picture shows these "Cherenkov cones" from a few charged particles (red balls entering the water, producing violet balls representing the UV light).  If an energetic photon itself (green ball) enters the water, two mechanisms convert it into the Cherenkov light.  One (at left) is the same old electron-positron pair production mentioned before.  The other (at right) is when the photon bounces off a water molecule and knocks an electron out of an atom.  Eventually all the particles incident on the water convert to near-visible light.  The particles of the core of the shower act in concert and produce a large ring of UV light deep in the water, shown as a second snapshot in time.

 The telescope finds the direction of the original single TeV photon by recording the time at which each tube first sees the UV light; this time will depend on the angle at which the pancake comes into the water.  The energy of that initial photon is measured roughly by how many tubes get hit by UV light-that can vary from a few to many hundreds.  The signals are carried by underwater cables to be decoded and recorded by electronics remote from the pond.

 Milagro then can produce a "sky map" of where the point sources of TeV gamma rays are.  The telescope operates night and day, independent of weather or other factors, and sweeps out the entire Northern sky as the Earth turns.  It records showers at the rate of about 1500/second, most of which will be the common (and rather uninteresting) showers from cosmic rays, uniformly coming from all directions in the sky.  The problem is rather like looking for stars during twilight-the background is too bright.  But Milagro patiently keeps adding up the true TeV gammas from point sources until it sees the "stars" against the cosmic ray twilight.  For explosions of gamma rays from bursting sources (GRBs), there isn't much background and Milagro can see them in a matter of seconds.

  Milagro has been in operation only since December of 1999.  Previous limited prototypes established that the detector can do what it advertised, and saw early evidence for TeV radiation from a GRB. (see the New York Times science section, Oct. 26, 1999).

 This illustration was produced in cooperation with the UCSC Visualization Laboratory; we thank Michael Gross for his cooperation. Description  written by Michael.