MiniBooNE Ready To Move In

by Kurt Riesselmann

After investing $1.5 million and 12 months of construction work, the MiniBooNE collaboration is getting ready to take over the building that will host its neutrino experiment. The Whittaker Excavating company, which started work on this project in October 1999, is about to put the finishing touches on the building and a 40-foot-diameter tank located inside.

"We are on schedule, right at the point that we expected when we first started," said Fermilab physicist Peter Kasper, project manager for the MiniBooNE civil construction. "Whittaker will soon be done with its part of the project and we will have beneficial occupancy."

At that point, the MiniBooNE collaboration can start with the installation of various equipment needed to carry out their experiment.

The MiniBooNE collaboration, a group of 50 scientists from 12 institutions, will use the building and its tank to study the properties of neutrinos, ghost-like particles that account for perhaps as much as five percent of the total mass in our universe. As you read this sentence, billions of neutrinos traverse your body, completely undetected, rarely leaving a trace behind. Whether these particles have any mass at all will soon be clarified by the MiniBooNE experiment.

But before physicists are ready to detect any neutrino signals, another 12 months of work are needed to install mechanical and electrical systems, prepare the neutrino detector, set up the electronics and test all equipment. The civil construction is the foundation of the $16 million MiniBooNE experiment, but it ran into a problem with a coat of paint.

The 250,000-gallon tank inside the MiniBooNE building requires a special coat of reflective white paint that doesn't contain organic molecules, ring-shaped carbon compounds that emit light as they oscillate. The present paint contains a thin layer of organic non-white color on its surface.

"If it weren't for the paint problem we could have obtained beneficial occupancy at the beginning of November," said Kasper. "Now it looks as if it will be in the middle of November."

The design of the MiniBooNE experiment features 1,520 lightsensors, mounted on the inside of the tank. They are able to detect tiny amounts of light. The tank will be filled with mineral oil, a fluid with little intrinsic luminescence.

Starting in fall of 2001, an intense neutrino beam, created by Fermilab's accelerators, will cross the tank, causing approximately one neutrino-oil interaction every 20 seconds. The interactions create secondary particles that produce tiny flashes of light, bright enough to be registered by the lightsensors.

Additional light caused by ring-shaped carbon compounds would interfere with the dim neutrino-induced light flashes. Using a high-quality paint and keeping light-producing impurities out of the tank is a major requirement to conduct the MiniBooNE experiment.

But physicist Rex Tayloe and technician Andy Lathrop may have found a remedy that makes repainting the tank unnecessary. They learned that a powerwash with baking soda, usually used for cleaning graffiti from buildings, could remove the unwanted surface layer without damaging the perfectly white coat underneath. At present, physicists are working with Whittaker to find the best solution to the problem.

A schematic of MiniBooNE

The paint problem has caused the only construction delay. The project has swiftly moved along since breaking ground 12 months ago. Construction consisted of creating a cylindrical underground vault 50 feet in diameter and 45 feet deep, assembling a spherical tank inside the vault and building an electronics room above it. After completion, the whole construction was covered with an earth berm to reduce the number of cosmic particles entering the tank. A 24-foot-long entrance corridor provides access to the electronics room, the main tank and a smaller overflow tank located under the entrance tunnel.

"Right now people are working on the electrical system," explained Tayloe, who just started an Assistant Professorship at Indiana University. "The MiniBooNE experiment uses two independent power systems: one for the utilities, such as air conditioning, and one for the sensitive electronics."

With two systems, the electronics will be shielded from power surges and voltage variations as air conditioners and other equipment are switched on and off. Without these irregularities, the electronics will be more reliable in recording the faint electrical signals produced by the photosensors as light strikes their surfaces.

Tayloe is in charge of managing the installations inside the building. In Phase I, he and his colleagues need to prepare the interior so that scientists, led by a group from Princeton University, can start to mount the photosensors in January.

"Once beneficial occupancy is established, my job is to coordinate all the tasks involved with the installation of the detector components," explained Tayloe. "I have to determine and delegate who is doing what, when and where."

Talking to senior collaboration members, who have managed other physics projects, he thinks he drafted a realistic plan.

"Being conservative is always a good thing [when creating a schedule]," Tayloe pointed out. "The more details you can visualize, the better."

In May the installation of the photosensors, each eight inches in diameter, should be complete. Physicists will then connect them to the data acquisition system and test the electronics. To check the functionality of their experiment, the MiniBooNE collaboration will operate their experiment with an empty tank for about one month, studying background signals created by cosmic rays.

"For every signal caused by the Fermilab neutrino beam, there will be about 100,000 events from cosmic rays," said Bill Louis, physicist at Los Alamos National Laboratory and cospokesperson of the MiniBooNE collaboration. Fortunately, the neutrino beam is pulsed, and the majority of background events can be easily discarded as they occur when the beam is off.

To find out whether neutrinos have mass, the MiniBooNE experiment is looking for neutrino oscillations: Physicists will use Fermilab's Main Injector accelerator to create muon neutrinos and send them to the MiniBooNE building. The MiniBooNE detector will check whether some of the muon neutrinos, traveling a distance of 500 meters, have transformed into electron neutrinos, creating electrons as some of them interact with the mineral oil. If the results of an earlier experiment carried out at Los Alamos are confirmed, the MiniBooNE experiment should observe about 1000 transformations a year, clearly establishing neutrino oscillations.

MiniBooNE will test one scenario, either establishing or ruling out muon-neutrino to electron-neutrino oscillations beyond any doubt. Other experiments will test oscillation scenarios such as muon neutrinos to tau neutrinos.

"Presently, there exist three very different experimental results that hint at neutrino oscillations," said Louis. "MiniBooNE will be the first accelerator neutrino experiment to provide a definitive answer."