Project GRAND is a cosmic ray observatory located on the University of Notre Dame campus. The observatory features a grid of sixty-four proportional wire chamber (PWC) particle detectors positioned within a 10,000 m2 field. Project GRAND was designed and built by Notre Dame professor emeritus John Poirier and his students. The observatory operated mainly between 1989 and 2011. Project GRAND detected cosmic rays from the sun and extrasolar sources. Project GRAND was also able to discern the effect of atmospheric temperature and pressure on cosmic ray surface counts.
Concept and features Cosmic rays were discovered in 1912 by Austrian physicist Victor F. Hess for which he won the 1936 Nobel Prize in physics. Cosmic rays are particles, mostly protons, that are emitted by the sun and extrasolar sources. These particles impact earth's atmosphere to produce showers of particles ("extensive air showers" or EAS) that can be detected from the surface. In 1983, German physicists Wilhelm Stamm and Manfred Samorski were able to link cosmic rays to a source in space named Cygnus X-3 (the third brightest x-ray emitting object in the constellation Cygnus). Cygnus X-3 emits two major types of particles: protons and gamma ray photons. The gamma rays fall within two categories, "very high energy" (1012 eV) and "ultra high energy" (1015 eV). Of these categories, ultra high energy gamma rays can be observed using ground based cosmic ray detectors.
Professor emeritus John Poirier of the University of Notre Dame founded Project GRAND in the late 1980s. Poirier obtained his PhD in particle physics from Stanford University, and he later performed research at the Serpukhov accelerator in Russia and at Fermilab near Chicago. Poirier joined the Notre Dame Department of Physics faculty in 1964. Poirier later pursued the study of cosmic rays and their sources in space. He initially made plans to build a conventional optical detector that would be placed in a northern Indiana soybean field, but a leading expert in scintillation detector technology from Krakow, Poland then visiting the Notre Dame campus convinced Poirier to pursue a different approach, one that wouldn't be dependent upon weather conditions. Poirier employed Monte Carlo simulations to design a scintillator-based cosmic ray observatory that would detect extensive air showers produced by ultra high energy gamma rays and protons. The observatory, named Project GRAND, would be able to pinpoint the sources of cosmic ray particles to an angular resolution of 0.25° (an apparent angle of half the size of a full moon). Poirier presented this plan in a 1987 paper for the 20th International Cosmic Ray Conference held in Moscow. GRAND is an acronym for "Gamma Ray Astrophysics at Notre Dame". The observatory would be built with the assistance of the National Science Foundation (NSF) as well as funds from the University of Notre Dame and private individuals. Newspaper articles about Poirier and Project GRAND were published in the January 9 and November 19, 1989, editions of the South Bend Tribune.
In its heyday, Project GRAND used a set of 64 cosmic ray detecting installations that were located on a level field north of the Notre Dame main campus. Each of the 64 installations employed a set of eight vertically stacked proportional wire chambers (PWCs). (George Charpak won a 1992 Nobel Prize in physics for inventing the PWC.) At Project GRAND, each PWC chamber features 160 orthogonally positioned tungsten wires (two sets of 80 wires) sealed within an atmosphere consisting of argon and carbon dioxide (80% and 20% respectively). Cosmic ray "hits" were registered by the detection of ionized gases as voltage differences in the tungsten wires. The 64 installations, termed "huts", are plywood buildings arrayed in an 8 by 8 grid covering an area of 10,000 m2 (a 100 m by 100 m field). The grid is oriented directly north to south and, along with the stacked PWCs, allowed estimation of the angles of entry of cosmic ray induced particles. Extensive air showers display a cone of particles that impact earth's surface in an approximate 200 m diameter circular area. At Project GRAND, near simultaneous detections within multiple huts established the occurrences of an extensive air showers. A steel plate positioned above the bottom PWC in each hut was used to detect muons. Muons are heavy, short-lived cousins of the electron that are generated in earth's atmosphere by the impact of cosmic rays. The steel plate also allowed discrimination between air showers generated by gamma ray photons and those generated by protons, and the detection of muons enabled the differentiation of these sources of air showers. According to Poirier, in his proposal for the experiment, background protons would be reduced to zero by the detection of muons, and the detection of muons could then be linked to extended air showers produced by ultrahigh energy gamma rays. Each of the 64 huts was connected to a central trailer where data from the experiment was accumulated. The data trailer had been obtained from NASA surplus and may have been used as a quarantine facility for Mercury and Gemini mission astronauts.
Discoveries, educational opportunities The Project GRAND experiment operated between 1989 and 2011. As stated, the observatory received funds from the National Science Foundation. After construction, the main expense of the experiment was the purchase of the argon gas used in the PWCs.
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