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Professor Michael DuVernois goes to extreme heights and lengths in utilizing
balloon-borne and ground-based space physics experiments.
by Melissa Eblen
In order to understand the research of many IT faculty members, a good place to begin is with a tour of the lab where they work. For Professor Michael DuVernois, an appropriate lab tour might involve a trip to Antarctica, where his next balloon borne experiment will be launched, or Argentina, where he is part of a collaboration building a detector that covers an area the size of Rhode Island. Contrary to expectations, DuVernois is not interested in studying extreme climates or the geography of the Southern Hemisphere. Instead, it's the study of cosmic rays that propels him.
DuVernois' research interests have not always required him to travel to distant locations or construct highly complex systems. When he began his graduate career at the University of Chicago, he thought he wanted to be a theorist in general relativity. However, a research assistantship with Professor John Simpson, a pioneer in the study of cosmic rays, drew him into the realm of experimental physics. DuVernois worked on the Ulysses High Energy Telescope, with the goal of measuring the chemical composition of cosmic rays. Although the experience provided him with a strong background in cosmic ray physics and extensive data analysis work, the experiment itself had been built prior to DuVernois' arrival. Before long, DuVernois the theorist found he could not satisfy the experimentalist inside himself.
"This experiment had been built by NASA-approved people 15 years earlier," he recalls. "Most NASA experiments real humans can't touch." DeVernois wanted more hands-on involvement.
Pursuing his desire to do more experimental work, DuVernois took a postdoctoral position with Professor James Beatty at Penn State University because, according to DuVernois, Beatty was "a hard-core experimentalist." Having only limited experience developing hardware and designing electronics, DuVernois found he had a lot to learn about building cosmic ray experiments.
"Luckily the experience wasn't 'sink or swim.' It was more swim or here is a very small inflatable device. I found it a supportive environment in which to grow," recalls DuVernois.
DuVernois' career literally "took off" with his work on the High Energy Antimatter Telescope (HEAT) at Penn State. This balloon-borne cosmic ray experiment, which is launched to 120,000 feet, allowed for lots of hands-on work. Unlike the Ulysses space mission he had worked on earlier, HEAT was not "a gold-plated experiment that you didn't get to fiddle around with." Unlike space missions, HEAT could be retrieved and revised. During his four years at Penn State, DuVernois developed several generations of cosmic ray detectors made of devices that measure various properties of the cosmic rays.
According to DuVernois, "Cosmic ray experiments serve three groups of people: a small group of cosmic physicists; astrophysicists interested in supernovae, galaxies and magnetic fields; and particle physicists."
Cosmic rays - single particles that carry immense amounts of energy - can originate either within our galaxy (low and medium energy cosmic rays) or outside the galaxy (the highest energy cosmic rays). Astrophysicists believe that most of the galactic cosmic rays originate in supernovae, where dying stars eject matter at supersonic velocities. These supernova shock waves are thought to serve as the accelerator for the charged particles in low and medium energy cosmic rays. Exactly what accelerates the highest energy cosmic rays is still unknown.
Although astrophysicists are interested in the source of cosmic rays, particle physicists are most interested in the particle production that occurs when these cosmic rays slam into atoms in the atmosphere. Much of the energy that a cosmic ray holds is converted into matter at the point of collision, creating a number of subatomic particles. These particles then collide with other atmospheric atoms to create a cascade of collisions known as an air shower. The particles in the first steps of the cascade have so much energy that they travel faster than the speed of light in the atmosphere, which produces Cerenkov radiation. Muons and other particles are produced in the later steps of the cascade.
DuVernois' experiments address a variety of questions surrounding cosmic rays. HEAT, for example, which will be flown once more this spring before being retired, examines the production of electrons and positrons (positively charged electrons) by cosmic rays.
One would expect there to be about an equal number of electrons and positrons if they are produced in pairs, but this is not the case. Instead there is an excess of electrons. Early experiments showed a rise in the percentage of positrons produced in cosmic rays with energies above 10 GeV. This phenomenon was thought perhaps to be evidence of a light supersymmetric particle, called a neutralino (not to be confused with a neutrino), which decays into an electron-positron pair. HEAT found no support for this hypothesis. However, it studied a number of other interesting particle physics problems including tracking atmospheric neutrinos and studying antiprotons, which are a rare compo nent of cosmic rays.DuVernois' next balloon born experiment after HEAT will be the Cosmic Ray Energetics and Mass (CREAM) project, which will study the energy spectrum of the two types of cosmic rays.
The most energetic cosmic rays must come from outside the galaxy because there is no mechanism within the galaxy that could provide the required acceleration to achieve such high energies.
According to DuVernois, "These represent two totally different types of cosmic rays, and it is almost pure coincidence that when plotting the [energy spectrum of the two different types] they come together, because there is no reason they should."
CREAM will use new balloon technology to conduct an ultra-long duration balloon flight to study the regime where the spectrums of the different types of cosmic rays meet. The idea is to take advantage of the vortex winds around the South Pole to keep the balloon at roughly constant latitude for 100 or more days.
"The [scientific] advantage of balloon - borne experiments is that we observe the cosmic rays directly ... at the top of the atmosphere instead of observing the air showers," says DuVernois.
In addition, balloon-borne experiments can be carried out much more quickly and less expensively than traditional space-based experiments. CREAM is designed as a testbed for ACCESS, an upcoming instrument to be placed on board the International Space Station to study the interface region of the cosmic ray spectrum. DuVernois says the CREAM collaborators, with their faster timeline like to joke about getting the important results first.
"We hope to skim the 'cream' off of ACCESS science," he says.
DuVernois also works with the ground-based Pierre Auger Project, headed by Nobel laureate James Cronin, which studies the highest energy cosmic rays. Physicists initially predicted that there was an upper limit on the energy of cosmic rays.
The universe is filled with remnant radiation from the Big Bang, called the cosmic microwave background (CMB), and cosmic rays with extremely high energies will not be able to travel very far without crashing into CMB photons and losing their energy.
Nevertheless, in the mid-1960s cosmic rays with energies above the upper limit were detected. Experiments in the late 1980s and early 1990s saw more of these highest energy cosmic rays and led to the development of the Auger project. Scientists now believe that these cosmic rays must be coming from sources that are close to us and do not have time to collide with CMB photons. In addition, these cosmic rays are so energetic that they are not bent by magnetic fields, so scientists can trace them back to their sources. A study of these highest energy cosmic rays should allow researchers to learn something about the rays' astronomical source.
According to DuVernois, "Auger will allow us to study interesting stuff like the role of dark matter or particles on a [Grand Unification Theory] scale as well as meat-and-potato high energy physics, like calculating cross sections, and astrophysics."
The physical proportions of the Auger project are gigantic because the frequency of hits by the highest energy cosmic rays is extremely low - one incident event per square kilometer per century. To detect any significant amount of data, detector size must be increased dramatically. Auger consists of a pair of 3,000-square-kilometer detectors, one in Argentina and one to be built in Utah, which can map the entire sky. Each detector actually consists of 1,600 water tanks scattered over the 3,000 square kilometers. The tanks measure the Cerenkov radiation of particles passing through them. Each tank has an instrument, that serves as a calorimeter by looking at the nitrogen fluorescing in the sky from the air showers.
DuVernois is part of the team developing the central data acquisition system for the Auger project. Networking 1,600 individual detectors spread over such a large area is challenging. The Auger collaboration turned to the cellular communication system used by oil rigs in North America to help solve the networking problem.
"Cable is too expensive and problematic over such long distances," says DuVernois.
Each individual tank has its own Global Positioning System antenna and a cell-phone antenna. Each tank monitors what it is measuring, and if something appears unusual a trigger is set off. In response to the trigger, the local site will call the central data acquisition system and report its location and the nature of the event registered. The central system, which monitors messages from each local site, will call each individual water tank requesting more complete information if there appears to be some multi-site event.
"This is a federal system," explains DuVernois, "so each province [water tank] sends information to the central system, which in turn asks the local provinces to do certain things."
This type of experiment, which promises to vastly expand our knowledge of the highest energy particles in our universe, also redefines how experiments are conducted. The advanced networking system allows a variety of different projects to be carried out simultaneously using several different computers at the central location. By writing computer code with triggers tailored for a specific type of cosmic ray event, single computers looking for a particular signature can check inputs from the 1,600 water tanks. If something interesting appears, the computer programmed to monitor that event can collect data from all relevant tanks.
Once the detectors are built and networked, a variety of different investigations can be carried out by simply swapping in a computer that will search the relayed messages from individual water tanks for signs of a particular event.
"The net investment [to search for a particular event] is only one computer, and so you can test some rather outlandish ideas that would not normally merit building a detection system of their own," explains DuVernois.
In many experiments, data is collected, saved, and then analyzed at a later date with a specific goal in mind. The Auger project decides what the scientific goals will be, and then each computer selectively collects data relevant to its specific goal. This division of labor helps overcome the information overload brought about by a project of such magnitude. According to DuVernois, the raw data from a few hundred of these detectors would completely saturate the Internet.
With CREAM and the Pierre Auger Project getting underway, the coming years promise to be filled with interesting discoveries for DuVernois and others in the cosmic ray physics community. DuVernois, who just joined the University faculty this year, is currently busy setting up his laboratory on the fourth floor of the Tate Laboratory of Physics. Then, like other IT faculty members, he will also have a lab to show visitors - one thatıs a bit closer to home.
