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by Michelle Walter
How do you learn about something very small and nearly impossible to catch? Physicists studying neutrinos--elusive, subatomic particles that barely interact with anything--face just this problem. Currently a 5600 ton neutrino detector being assembled here in Minnesota.
What makes something so small worth all the effort? Understanding the neutrino may allow physicists to unlock the mysteries of the basic building blocks of the universe. Neutrinos, tiny neutral particles, are produced in nuclear reactions, such as those in stars and man-made nuclear processes. A million billion neutrinos pass through you every second, but they don't interact with you.
Billions of stars with billions of beta decays occurring every second produce most of the universe's neutrinos. These neutrinos shoot off in all directions with very high energy and little mass. Until recently, neutrinos were thought to have no mass. If they do--and if there are more than 100 neutrinos in every cubic centimeter of the universe--neutrinos may account for most of the mass in the universe.
The history of the neutrino problem
Wolfgang Pauli first proposed the neutrino in 1930. He noted that in beta decay (a neutron decaying into a proton and electron) the resultant energy doesn't add up to the initial energy. He figured that an additional particle might be produced in this reaction, but he didn't think it would ever be found. Enrico Fermi named this particle the neutrino and invented the theory of weak interactions to explain why it seldom interacts with matter. Although rare, the interactions of neutrinos with matter can be observed, and the first observation came in 1956 by Reines and Cowan.
Over the years scientists discovered three types, or "flavors", of neutrinos: electron, tau, and muon. Experiments determined the output and number of each neutrino flavor of neutrino in the universe, but the numbers didn't add up. For example, the sun produced less than half of electron neutrinos expected. In addition, the ratio of muon neutrinos to electron neutrinos resulting from the interaction of cosmic rays with atoms in the upper atmosphere was predicted to be two, but experiments showed this ratio was closer to one. From this, scientists predicted that the flavor of a neutrino could change as it traveled through space, oscillating from one type to another. But in order to change flavors, the neutrinos had to have mass. Physicists realized that precisely determining the mass of a neutrino as well as the quantity of neutrinos might help them figure out the mass of the universe and account for the "dark matter" thought to make up most of the cosmos.
Catching neutrinos
To better understand neutrino oscillations and to determine the neutrino masses, physicists must find a way to detect neutrinos--a difficult proposition considering how infrequently they interact with matter. Unlike many particles that easily react with matter, neutrinos can travel through the earth without being stopped. Therefore neutrino detectors are placed deep underground, so that other types of particles are blocked by the earth.
Current detectors employ two main methods for catching neutrino events. Both require a large mass so there is more chance of a traveling neutrino interacting with a particle.
Some detectors consist very large tanks of liquid placed deep underground. This liquid produces a noticeable discharge--a flash of light--when hit with a neutrino.
Others use a very dense material coupled with a detecting device. If the detectors contain enough liquid or solid material, a tiny fraction of the neutrinos will interact with it.
Planning the MINOS experiment
A promising new detector is currently being built for the Main Injector Neutrino Oscillation Search (MINOS) at the Soudan Underground Research Lab, a former iron mine in northern Minnesota.
The MINOS detector will be more effective than the lab's current dector, Soudan 2, which was originally constructed to search for nucleon decay. Like many other detectors, the Soudan 2 is examining muon- and electron neutrinos in the upper atmosphere. But scientists cannot control the composition of neutrinos coming from the sun or the universe in general.
The MINOS detector, however, will focus on a beam of neutrinos being shot at it from Fermilab in Chicago, some 735 km away. In this controlled environment, physicists will know the type of neutrinos shot toward the detector.
MINOS was designed and built in part by University faculty and students.
"Following the submission of several competing proposals and a lot of politics involving several independent reviews, the experiment proposed for Soudan was approved and the Department of Energy granted over $100 million to the experiment," says physics professor Keith Ruddick. "There are over 200 physicists from many institutions and more than 5 countries involved. The University's experimental high-energy group is the largest of the university groups."
Determining how to detect and record each neutrino interaction posed a significant challenge, says Ruddick. "Only about one trillionth of the neutrinos that come through the 5,600 ton detector will interact in it, producing a few thousand interactions per year. When they interact, muon neutrinos produce highly penetrating muons that are easily recognized, while if they have turned into electron or tau neutrinos, they produce electrons or taus that also have characteristic tracks in the detector."
The detector design alternates one inch-thick iron plates with a detecting layer. This layer, made of specially doped plastic developed after two years of University research, is called a scintillator.
When a neutrino hits a particle of matter in the iron plates, a charged secondary particle can fly off (a muon in the case of a muon neutrino). If this happens, the particle will travel into the scintillator and excite the particles in it that emit ultraviolet light. Normally the light would be absorbed very quickly, but the plastic is doped with a wavelength-shifter, or fluor (like fluorescence) that adsorbs the UV light and shifts it down to blue light that bounces around, aided by a white reflective coating that covers every piece of plastic.
The light then travels into a special fiber that shifts it from blue light to green. (This fiber only changes light of wavelengths less than that of green light; if you shined red or yellow of orange light on it, it would not emit any light because the wavelengths are too long.) About five percent of the green light is transmitted through the fiber into sixteen-pixel phototubes in a multiplexing box, or "mux box." The mux boxes are hooked up to a computer system that analyzes the data when a neutrino is detected.
The detector itself is an eight-meter wide octagon and 31 meters long. Each active layer of the scintillator is divided into eight modules. (The module layers must be cut into even smaller pieces because everything must fit in the 1880s-era elevator that that goes down the mine-shaft.)
"When complete this will be the largest amount of plastic scintillator ever assembled--around five football fields," says Ruddick.
Building a Behemoth
Half of the MINOS detector is beingbuilt at a temporary specially designed factory at the Minnesota Geological Survey building on University Avenue, about a half mile from campus. The other half is being constructed at the California Institute of Technology.
The first step in manufacturing one of the MINOS modules is to cut to size and bend the edges of an aluminum sheet. This serves as the base of a box to hold the plastic scintillators for one module. Then the 24-foot long plastic strips are cut to size and laid in the box. These strips are grooved on top for the fiber optics and have a white covering. The strips are glued together, covered in a plastic sheet, and the air is sucked out of them to allow the epoxy to set, leaving the plastic strips tightly glued to the aluminum.
After the glue has dried, the module is wheeled over to the threader, a cleverly-designed machine that runs on a track up and down the length of a module. The threader lays a green fiber in the groove of each plastic strip, adds glue, and then tapes the fiber down with aluminized mylar tape. The fibers at the end of the module are placed into light manifolds that guide them to an optical connector so they don't get shaken or damaged. These manifolds and many other plastic parts were designed by mechanical engineering professor Thomas Chase, who has worked closely with the physicists in the design of this project.
The third step in the production of the modules is to cut out of aluminum a top for the box. This "lid" is then glued on with epoxy, and then the box is further sealed using a technique designed by Chase. A hand-run device called a crimper runs up each edge of the aluminum and flattens and bends the edges neatly and consistently to facilitate handling and keep light out. Then the air is sucked out of the module again so the glue will dry. It takes 18 man-hours to produce one completed module.
Finally, every module is tested to make sure the fibers weren't stressed to the point of being unable to transmit light. Testing is done by a strong radioactive source, a small Cesium-137 gamma-source housed in a four-inch square lead container that walks up and down the module. Each wavelength-shifting fiber is connected to its own phototube for testing. This process maps the light output of the module. A module is rejected if it has more than two damaged fibers. The production goal was to have less than one percent rejection due to fiber damage. Of the more than 1,000 modules the University has made, only six have been rejected.
The Minnesota MINOS factory has been in production for more than a year and is ahead of schedule. They have made over 1,000 modules and are scheduled to finish production in January 2003. Also at this time, about 100 of the 489 layers of iron and scintillators have been installed in the mine.
Although researchers hoped to have MINOS fully assembled and collecting data by 2003, operation of the neutrino beam at Fermilab has been delayed until 2005. In the mean time, the detector will study atmospheric neutrinos and other products of the cosmic rays. If all goes according to plan, MINOS will soon propel University researchers to the forefront of neutrino research, aiding them in the search for more a better understanding of the building blocks of the universe.
FOR MORE INFORMATION:
www.hep.umn.edu/minos/images/minosminnesota.html
www-numi.fnal.gov
