Bolivian Air Shower 1963


10-min silent movie, by Pier Oddone, Summer 1963. Description by Hale Bradt (Oct. 2017)


The MIT group established an extensive air shower (EAS) experiment near the summit of Mt. Chacaltaya, Bolivia, at altitude 5200 meters (17,000 ft) for the purpose of studying EAS of relatively small size (or energy), which are more numerous than those of higher energy. It measured EAS containing 10^5 to 10^7 electrons with overall energies ranging from 1 to 100 PeV (1 PeV = 10^15 electronvolts), well above energies attainable with proton accelerators then and even now. (Currently the LHC accelerates protons to 0.007 PeV.) The experiment was a collaboration between MIT, Japanese, and Bolivian scientists and was called the Bolivian Air Shower Joint Experiment (BASJE). The leaders were George Clark of MIT, Koichi Suga of the University of Tokyo, and Ismael Escobar of the Universidad Mayor de San Andreas, Bolivia. The high altitude laboratory was about a 1.5 hour (25 miles) drive from La Paz at 12,000 feet to the laboratory at 17,000 feet. Most scientists and technicians working there would reside in La Paz and travel to the lab in the lab’s van for the day.

The Science

EAS are cascades of particles initiated most often by an energetic single cosmic-ray proton entering the earth’s atmosphere. It collides with a nucleus of an air atom (oxygen or carbon) near the top of the atmosphere. The resultant cascade of secondary particles propagates down to the earth’s surface. The cascade, an EAS, consists of a flat pancake of particles and photons a few meter thick and several tens or hundreds of meters in extent, all traveling in the direction of the primary particle, which is normal to the pancake surface, at nearly the speed of light. It consists mostly of electrons, photons, and muons except near the core where nuclear active particles might be present. Measurement of the electron densities at several points in the cascade with an array of 20 one-meter diameter unshielded scintillators distributed over a circular area 140 meters in radius provided an estimate of the primary energy. The arrival times at several different points in the shower front, measured with five “fast-timing” detectors, yielded the arrival direction of the EAS—and hence that of the primary proton—to within a few degrees.

It was astronomy with protons, but futile because protons, by virtue of their electric charge, are deflected by galactic magnetic fields. The arrival directions at earth do not reflect the directions of their origins. The origins of these most energetic particles were of the greatest interest.

One purpose of BASJE was to search for EAS initiated by primary gamma rays because they travel through the galaxy without significant deflection. Such gamma rays were predicted to come from the Milky Way galaxy; they would be created by collisions of extremely high-energy cosmic ray protons with nuclei in the gases of the Milky Way. But they would be rare compared to EAS initiated by protons, perhaps only one in 10,000. Their signature would be a paucity of muons; the EAS would have few of the nuclear interactions that create muons. Accordingly, the BASJE featured 60 square meters of scintillators shielded with several feet of galena ore, a lead compound, on its top and sides. The bags of galena would prevent the electrons in an EAS from reaching the detectors, whereas the more massive muons could easily reach it. EAS with a paucity of muons would be candidates for having gamma-ray primaries.

There were other detectors also, including a large multi-plate cloud chamber that could record photographically nuclear interactions and the resulting shower of particles. It was located on top of the 60 square meters of shielded muon detectors.

The experiment did find an excess of very low-mu EAS, but could not exclude the possibility that fluctuations in the nuclear production of ordinary showers caused them. The clincher would be if the arrival directions of those low-mu EAS clustered showed asymmetries such as more from the Milky Way. Unfortunately this effect, if present, was not convincingly detected due to limited statistics. (More recent EAS experiments do indicate asymmetries in arrival directions.) The discovery of celestial gamma rays (at much lower energies) awaited a NASA/MIT satellite (OSO-3) experiment by William Kraushaar, George Clark, and Gordon Garmire, launched in 1967.

The movie

This 10-minute film/video illustrates my summer-1963 trip to La Paz and the Mt. Chacaltaya laboratory with my wife and an MIT physics major, Piermaria Oddone, who brought along a 16-mm movie camera to record our adventures. (Pier went on to graduate school at Princeton and, from 2005 to 2013 was the director of Fermi National Accelerator Laboratory.) The purpose of our trip was to install automated digital electronics to record the signals from the multitude of detectors for each detected EAS. The detector signals would be printed by an IBM typewriter and also recorded on punched paper tape that could later be read into a computer. Previously the data were recorded by photographing arrays of cathode ray tubes displaying the signals. They would then be hand measured! All of this equipment, primitive and “1963 modern” can be seen in the video.

The movie has no sound track and shows, in sequence:

  • plane flight from Boston and over Peruvian mountains.
  • scenes of La Paz
  • the road trip up to the Mt. Chacaltaya laboratory and exterior of lab
  • schematic of lab layout, entrance gate to lab, outdoor electron and fast timing detectors
  • view from the summit looking around and also down on the laboratory
  •  inside of the laboratory where Martin LaPointe and I are working
  • the bags of galena ore and the muon detectors (plastic scintillators with a large photomultiplier)
  •  an outdoor unshielded detector on a platform
  • the cloud chamber and a photo of a particle cascade recorded with it.
  • a boat trip on Lake Titicaca
  • a van trip to Las Yungas. We stayed at a former resort gone to ruin (in 1963) several thousands of feet lower than La Paz, where the jungle is lush and the roads to it are narrow and precipitous in the extreme. I drove most of it and still consider it one of the most dangerous episodes of my life.


The people in the movie are me—Hale Bradt—an assistant professor of physics at MIT (thin, tall with bald spot) and my wife, Dorothy (white blouse and dark skirt or pantaloons to right in stern of boat), Martin LaPointe, an MIT researcher in our group (shorter, stockier with decorated sweater and cigarette in lab scenes), and his wife (white blouse and decorated skirt in boat scene), an unidentified Argentinian woman scientist in the boat scene, Pier Oddone (young fellow walking toward camera at Lake Titicaca scene), Paul Barker, a U. Michigan scientist (climbing past bags of galena to show the multi-plate cloud chamber. Paul died in a mountaineering accident in 1967.), and Alfredo “Fred” Hendel, now deceased, an Austrian physicist (seen pushing me into the pool at Las Yungas.) Fred emigrated to Bolivia before WWII, lived and worked there many years, and was later a professor at U. Michigan. In retirement, he wrote two books, published privately, about his Bolivian and mountaineering experiences, Mountains in Bolivia and Revolutions in Bolivia. They are a set of absolutely fascinating adventures that include the establishment of the Mt. Chacaltaya laboratory.