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SHARE, Southern Hemisphere Air-shower Research and Exploration, is a program sponsored by
COSM at Hampton University in collaboration with
MARIACHI at Brookhaven National Laboratory. The purpose is
to bring small air-shower detection and research into the southern hemisphere using MARIACHI technology. To understand SHARE, we need to understand
Cosmic rays come in two main varieties. Primary cosmic rays are energetic particles from space which impact the earth or, more likely, the atmosphere of the earth. Particles from the sun are at the lower range of these energies, up to about 1014 eV; other particles, from the interstellar medium, come at higher energies. A very few have energies up to 1021 eV. By way of comparison, a proton acclerated in the Large Hadron Collider under construction at CERN will have an energy of 7 TeV, or about 1013 eV. When these primary particles encounter the atmosphere, they stimulate nuclear reactions which produce pions. The pions decay into muons, electrons, and neutrinos. Neutrinos are just about impossible to detect except at extremely low rates with very sophisticated detectors (see http://wwwlapp.in2p3.fr/neutrinos/anexp.html and http://cupp.oulu.fi/neutrino/). Electrons will interact easily with other particles in the atmosphere and few make it to the surface of the earth. Muons, however, have high momentum, interact but little, and, thanks to special relativity, often make it to the surface of the earth before they decay. Taken together, the particles in the atmosphere which result from the primary cosmic ray are called seondary cosmic rays. Air Showers When a very energetic (1015 eV or above) primary cosmic rays encounters the atmosphere, it creates a large shower of secondaries which rain down on a portion of the surface in a cone of energetic particles. This is an air-shower. Depending on the energy, an air shower stiking the earth can affect an area of diameter on the order of tens of meters to several kilometers. Pierre Auger and Hi-Res are two projects that have large, self-contained arrays of detectors to record and study air-showers. Another approach is taken by CROP and CHICOS, for example: individual detectors are placed in a variety of remote schools, universities, and other institutions to look for large-area signals. This is part of the MARIACHI approach. The Special Physics of Air Showers Air-showers may have some unusual consequences. One is that they may be a cause of lightning. Simply put, the potential difference between a thundercloud and the surface of the earth is not generally sufficient for a spark to jump from one to the other. It is hypothesized that air-showers may either add energy to electrons in the clouds in a process called "runaway breakdown" (Gurevich and Zybin, Physics Today, May 2005) or may create a conductive pathway between the clouds and the earth by ionization of the air. Ionization plays a key role in MARIACHI and SHARE. It is expected that an energetic air shower creates an ionization trail in the atmosphere, similar to -- but with different characteristics from -- the ionization trails of meteors. Like meteors, then, air-showers should create the conditions for sporadic reflections of VHF radio signals in the upper atmoshpere, allowing them to "skip" around the curvature of the earth and thus be detected beyond the line-of-sight. This is a well-known phenomenon for meteors; if it occurs with air showers as well, we can not only detect them in a different way but also learn more about how they ionize the air. But what is the cause of these rare, extreme energy (1019 - 1021 eV) air-showers? Are they simply protons or are they more exotic particles? How do they maintain their energy in the interstellar medium? Can we detect exotic particles, such as mini-black holes, that might shower, pass through the earth, and shower again up through the earth and out to the surface? MARIACHI and SHARE are designed to take small steps to answer these questions. Detection One of the sure-fire ways of detecting cosmic rays -- used in MARIACHI, CHICOS, CROP, and other collaborations -- is by means of scintillators. A scintillator is a piece of clear plastic or glass which is doped with molecules which will emit photons when charged particles pass near to them. If such a scintillator is wrapped with opaque material to keep it "light-tight" and coupled to a photomultiplier tube (PMT) to convert the photons to an electrical signal and amplify it to a readable level, it should only record passing muons and the like. Of course, there is a great deal of noise in the signal, much of it contributed by the PMT itself. If three or more such detectors are placed in a small array at least a few meters apart from each other and connected to a computer via a coincidence circuit, and air-shower can be detected when it occurs. This happens because the coincidence circuit only sends a signal to the computer if several of the detectors fired within the same small time interval, which most likely occurs only in a simultaneous shower of muons. In addition, MARIACHI and SHARE use signals bounced off the ionization trails from air-showers (passive radar). In combination, the scintillation detection method and passive radar will allow us to confirm the passive radar technique and to begin to study how air-showers affect VHF propagation. Look for more information on detection methods at http://www.ast.leeds.ac.uk/haverah/dets.shtml, http://www2.slac.stanford.edu/vvc/cosmicrays/default.htm, http://quarknet.fnal.gov/toolkits/ati/crdetectors.html, and on the MARIACHI site at http://www.cosmicray.bnl.gov/museum.html. Why the Southern Hemisphere? There are three science reasons to place MARIACHI-style detectors in the southern hemisphere: geometry, use of the earth as an absorber, and the search for lightning. The geometry argument is simple: more coverage in the southern hemisphere means that the global air-shower effort gets much closer to 4π2 solid angle coverage. There is growing evidence that the small extreme energy cosmic ray flux is isotropic, that is, comes from no preferred direction of source. However, to confirm this and to find if there are any excpetions, more study is needed. Use of the earth as absorber is useful for looking at rare exotic primary particles. A very high energy neutrino, a strange nucleus, or a mini-black hole may cause multiple showers as it passes through the atmosphere, the earth, and the atmosphere again. Coverage in both hemispheres increases the chance of detecting such an event. Lightning was already mentioned as a special case of air-shower physics. Lightning is most prevalent in the summer everywhere, meaning that when the lightning flux is low in the north it is high in the south. Thus, a presence in the southern hemisphere allows for year-round study. What's more, lighning effects are considered to be particularly strong in southern Africa, where SHARE plans to have an active presence. More To learn more about SHARE, contact Ken Cecire at ken.cecire@hamptonu.edu or view this presentation. |
SHARE Physics Resources:
Illustrations:  Click for greater detail. Coutesy Milagro collaboration website.   VHF skip due to air-showers.  A cosmic ray stimulates photon scintillation, causing a signal from the PMT.  A coincidence air-shower detector.  An exotic particle passes through the earth from one hemisphere to another.  Air-showers may be a cause of lightning. |
