A short description of MILAGRO:

Various energetic processes in the universe, such as the swallowing of matter by black holes, eject fast moving particles. MILAGRO is designed to detect high energy particles of light (photons), called "VHE gamma rays", and measure their arrival direction and energy. The arrival direction tells the observer where in the sky the particle came from, and the energy tells something about the physical mechanisms in the astrophysical source from which the particle was emitted.

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:

The MILAGRO Collaboration:

The MILAGRO experiment collaborators are from:

  • George Mason University
  • Los Alamos National Laboratory
  • Michigan State University
  • New York University
  • University of California, Irvine
  • University of California, Santa Cruz
  • University of Maryland, College Park
  • University of New Hampshire, Durham



The flux of cosmic ray protons and photons fall steeply with energy. At energies up to about 20 GeV the photon flux is high enough to have been well studied by relatively small area balloon and satellite detectors. Particles above about 100 TeV hitting the atmosphere generate air showers containing enough secondaries that penetrate the atmosphere to be studied by extensive air shower arrays. Individual sources in the in between range of 100 GeV to 100 TeV ( "very high energy" or VHE) have been studied by the air cerenkov technique, but Milagro was the first instrument capable of continuously monitoring the full overhead sky at these energies (Fig. 3).

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.

What is MILAGRO?

The MILAGRO (Multiple Institution Los Alamos Gamma Ray Observatory) consists of a man made pond lined on the bottom, covered on top and filled with water. Photomultiplier tubes (PMTs) - detectors which emit an electrical signal when hit by a photon - are tied with anchor lines to a weighted grid on the bottom of the pond and float up to fixed positions suspended in middle of the water. The anchor line lengths are chosen to form 2 planes of PMTs, one about 1.5 meters below the water surface and another 5 meters further down. A cable to each PMT connecting it to an electronics trailer just outside the pond is used to supply high voltage to the PMT and to relay the PMT signal to an electronic read out system. These signals are converted to digital form, analyzed by computer, and recorded to tape for later processing.

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.

How does MILAGRO work?

A VHE photon or proton entering the earth's atmosphere (the primary particle) interacts producing secondary particles. The secondaries interact producing more particles that in turn interact themselves. The result of this process is a large number of electrons and photons (and for proton primaries, some hadrons and muons and many neutrinos) known as an air shower (Fig. 4). When such a shower is incident on the MILAGRO detector, the charged particles emit cerenkov light in the water which is detected by the PMTs. Photons in the shower pair produce in the water producing additional charged particles which emit cerenkov light. The combination of sensitivity to particles over the full Milagro area, sensitivity to photons as well as charged particles, and location at high altitude account for Milagro's ability to detect air showers from VHE particles at energies well below those accessible to extensive air shower arrays.

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.

A brief history of MILAGRO

The MILAGRO detector is located in the Jemez Mountains near Los Alamos, New Mexico at an elevation of 8690 feet. The 80m x 60m x 8m pond was originally used for the Hot Dry Rock Experiment by Los Alamos and looked like this:

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:

The Milagro experiment was turned off in April 2008. Figure 31 shows the inside of the pond with the water level lowered to allow removal of top layer PMTs. Figure 32 shows some of the group involved in dismantling Milagro. They are not unhappy, because the PMTs are to be recyled for the new generation High Altitude Water Cerenkov detector, HAWC

Figures 31 and 32:

More Milagro reading

Go to for more Milagro information.

Milagro Refereed Publications:

"Milagro Limits and HAWC Sensitivity for the Rate-Density of Evaporating Primordial Black Holes," AstroPtl. Phys. 64, 4 (2015)

"Milagro Observations of Potential TeV Emitters", AstroPtl. Phys. 57-58, 16 (2014)

"The Study of TeV Variability and the Duty Cycle of Mrk 421 from 3 Yr of Observations with the Milagro Observatory", Ap.J. 782, 110 (2014)

"Spectrum and Morphology of the Two Brightest Milagro Sources in the Cygnus Region: MGRO J2019+37 and MGRO J2031+41", Ap.J. 753, 159 (2012)

"Constraints on the emission model of the `Naked-Eye Burst' GRB 080319", Ap.J. Lett. 753, L31 (2012)

"Observation and Spectral Measurements of the Crab Nebula with Milagro", Ap.J. 750, 63 (2012)

"The Large Scale Cosmic-Ray Anisotropy as Observed with Milagro", Ap.J. 698, 2121 (2009)

"Milagro Observations of Multi-TeV Emission from Galactic Sources in the Fermi Bright Source List" Ap.J. Lett. 700, L127 (2009); Erratum Ibid., 703, L185 (2009).

"A Measurement of the Spatial Distribution of Diffuse TeV Gamma Ray Emission from the Galactic Plane with Milagro", Ap.J. 688, p. 1078 (2008)

"Discovery of Localized Regions of Excess 10-TeV Cosmic Rays", Phys.Rev.Lett. 101, p. 221101 (2008)

"Milagro Constraints on Very High Energy Emission from Short Duration Gamma-Ray Bursts", Ap.J. 666, p. 361 (2007)

"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)

"Evidence for TeV Gamma-Ray Emission from a Region of the Galactic Plane", Phys. Rev. Lett. 95, 251103 (2005)

"Constraints on Very High Energy Gamma-Ray Emission from Gamma-Ray Bursts", Ap. J. 630, 996 (2005)

"Search for Very High-Energy Gamma Rays from WIMP Annihilations Near the Sun with the Milagro Detector", Phys. Rev. D70, 083516 (2004)

"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 TeV Gamma Rays from the Crab Nebula with Milagro using a New Background Rejection Technique", Ap. J. 595, 803 (2003)

"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)

"A Survey of the Northern Sky for TeV Point Sources", Ap. J. 558, 477 (2001)
"Milagrito, a TeV Air-Shower Array", Nucl. Inst. Meth. 449, 478 (2000)

"Evidence for TeV Emission from GRB 970417a", Ap. J. Lett. 533, L119 (2000)

"TeV Observations of Markarian 501 with the Milagrito Water Cerenkov Detector", Ap.J. Lett. 525, L25 (1999)


Most of the photos linked to above were taken by MILAGRO collaborators, including Jordan Goodman at the University of Maryland and Todd Haines at Los Alamos National Laboratory. The extensive air shower and energy range diagrams are courtesy of Jordan Goodman.

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.