MILAGRO (the rectangle on the right in Fig. 1) consists of a large man-made pond filled with detectors (Fig. 2). When a VHE gamma ray enters the earth's atmosphere, it interacts producing new particles which in turn interact themselves producing even more particles. When the particles in this "shower" hit the pond, they emit light which is measured by the pond detectors. The time difference between different detectors being hit allows determination of the original particle's direction. The number of detectors hit and how much light they measure gives an indication of the original particle's energy.
Figures 1 and 2:
Figure 3:
MILAGRO has provided an all sky survey of the northern sky at VHE energies, studied time dependence of emission in known sources and discovered new ones. Since it is continuously operational, Milagro has been used to study the properties of gamma-ray bursts, setting limits on VHE emission from many bursts and possibly detecting one case of VHE photon emission. Since charged VHE cosmic rays are deflected by the solar magnetic field, measurement of shadowing of cosmic rays by the Sun allows study of this field and the search for anti-particles, which would be deflected in the opposite direction. Search for VHE photons from the Sun has also allowed setting a limit on exotic particles trapped in orbit around the Sun. Solar flares emitting particles of energy greater than about 5 GeV have been detected by an increase in the MILAGRO PMT rates. MILAGRO also allows searching for evaporating primordial black holes which would be identified by their unique photon emission characteristics.
Major parts of Milagro include the pond, liner, and cover; the PMTs and the anchor grid; a cover inflation system which is able to raise it, lower it, and keep it stable; a lightening protection system; the water purification and circulation system; the data acquisition electronics and computer system; The Environmental Monitoring System which keeps track of the experimental hardware and physical conditions; the calibration system; the off line reconstruction software; and the computer simulation software.
Figure 4:
Since the
shower front propagates near the speed of light, the time difference between
different PMTs getting hit can be used to determine the arrival angle of
the shower which gives the arrival direction of the primary. The amount of
Cerenkov light detected by the PMTs can be used to get a measure of the
primary particle energy.
Figure 5:
In order to use it as a cerenkov detector, the existing pond (Fig. 6) had to be cleaned (Fig. 7), contaminated water which couldn't be sprayed on national forest had to be evaporated (Fig. 8), and a new light tight cover was put on the pond (Fig. 9), after which it looked like Fig. 10. The pond cover was also coated with reflecting paint (Fig. 11) in the summer of 1996 (so that detectors and people wouldn't fry underneath the black cover (Fig. 12) which got very hot). For construction each day, the pond cover is inflated to allow entry of people and equipment. The cover is lowered at the end of the day, onto the bottom when construction is done in the empty pond, and onto the water when construction is done with water in the pond. The cover system is also designed to take up the slack in the cover when the pond is full of water during operation of the experiment. Because of the frequency of storms in the area (many of which come on quite suddenly and necessitate the difficult job of raising and lowering the cover quickly (Fig. 13), an improved cover control system (Fig. 14) and a lightening protection system (Fig. 15) were installed.
Figures 6 to 10:
Figures 11 to 15:
The support structure for the photmultiplier tubes (PMT) used to detect cerenkov light produced by shower particles was constructed underneath the cover in two stages. The pond bottom grid was installed in the summer of 1995 for the initial run of the experiment (Milagrisimo, Fig. 16) which, with 28 photomultiplier tubes (PMTs) spread over 600 square meters, took data from April to June of 1996. The full support structure (Fig. 17) including the slope was completed in the summer of 1996. Milagrito, an intermediate size detector with 228 PMTs on the pond bottom, collected data from February 1997 to April 1998. The grid was slightly repaired and reinforced (Fig. 18) in the summer of 1998.
An early, uncalibrated, timing distribution (Fig. 19) from Milagrito shows lines whose height is proportional to the shower arrival at each PMT. The primary direction is approximately perpendicular to the plane defined by the ends of these lines. The pulse height distribution (Fig. 20) can be used to get a measure of the primary particle energy.
Figures 16 to 20:
The full MILAGRO detector with 723 PMTs was installed in the summer and fall of 1998 by a dedicated group of hard working (Fig. 21) scientists. Shown below are how PMTs were lowered into the pond (Fig. 22), how the pond was filled (Fig. 23) with water, and how PMTs appeared in early filling stages (Figs. 24 and 25). In order to improve timing measurements, baffles (Fig. 26) were placed on all PMTs (Fig. 27) to cut out late arriving large angle light. Some PMT repair was done by boat and diving (Fig. 28) during the summer of 1999.
Figures 21 to 25:
Preliminary data recording with the full Milagro pond detector began in December of 1999. In the early sample event pictured (Fig. 29), the individual PMT scatter about the fit shower plane is shown.
Full interpretation of the measurements requires knowledge of the position of the air shower core (the center and most dense region of the particle distribution). For example, an energetic shower with a large number of particles but centered far from the pond can give a similar signal in the pond to a lower energy shower falling closer to the pond. An array of "outrigger detectors" surrounding the pond allow determination of the core position. The locations of these detectors, completed in 2004, is indicated by filled circles shown in Figure 30.
Figures 26 to 30:
Go to http://www.lanl.gov/milagro/ for more Milagro information.
"TeV Gamma-Ray Sources from a Survey of the Galactic Plane with Milagro", Ap. J. Lett. 664, L91 (2007)
"Discovery of TeV Gamma-Ray Emission from the Cygnus Region of the Galaxy", Ap. J. Lett. 658, L33 (2007)
"Constraints on Very High Energy Gamma-Ray Emission from Gamma-Ray Bursts", Ap. J. 630, 996 (2005)
"Tev Gamma-Ray Survey of the Northern Hemisphere Using the Milagro Observatory", Ap. J. 608, 680 (2004)
"Limits on Very High Energy Emission from Gamma-Ray Bursts with the Milagro Observatory", Ap. J. 604, L25 (2004)
"Observation of GeV Solar Energetic Particles from the 1997 November 6 Event Using Milagrito", Ap. J. 588, 557 (2003)
"The High-Energy Gamma-Ray Fluence and Energy Spectrum of GRB 970417a from Observations with Milagrito", Ap. J. 583, 824 (2003)
"Evidence for TeV Emission from GRB 970417a", Ap. J. Lett. 533, L119 (2000)
Milagro was made possible by the generous support of the National Science Foundation, the Department of Energy, Los Alamos National Laboratory, and the University of California.
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