It could be the largest structure ever to be built from plastic. Its footprint of 1,052 square meters will cover an area
about the size of a quarter of a football field. Its height will rise past the top of a five-story apartment building. And with 368,640 tubes of white PVC, the structure will have about as many components as some of the largest LEGO structures built in the world.
But this huge structure, to be constructed in Ash River, Minn., won’t serve as a plastic replica. It will be the skeleton of a fully functional particle detector. Wired with fiber optic cables and filled with 500 truckloads of mineral oil, the 15,000-ton NOvA detector will enable scientists to discover how the masses of the three types of neutrinos—the lightest, tiniest particles known to mankind—stack up.
Last week, the preparations for the assembly of this white PVC behemoth passed a pivotal test. In an assembly building at Fermilab, 40 miles west of Chicago, scientists, engineers and technicians from Fermilab, Argonne National Laboratory and the University of Minnesota successfully operated for the first time the NOvA pivoter, the hydraulic system developed by Fermilab to move and rotate huge, 200-ton plastic blocks for the assembly of the NOvA detector.
“This is a big deal,” said Fermilab physicist Pat Lukens, who manages the assembly of the detector. “Now the focus will shift to Ash River. We will assemble 500 truckloads of plastic modules.”
But this is no ordinary plastic. Argonne’s Rich Talaga and other NOvA collaborators spent many years finding the right ingredients to produce the strongest and most reflective PVC for the 16-m-long tubes that hold and support the weight of the mineral oil.
“Ordinary plastic tends to deform under pressure,” said Talaga, who worked closely with Fermilab’s Anna Pla-Dalmau. “Think of a plastic coat hanger. It changes shape when you put a sweater on it. We had to find a plastic that has to be strong for 20 years and doesn’t get weaker and rupture.”
For Extrutech Plastics in Manitowoc, Wisc., a company that makes PVC wall and ceiling panels and other plastic products, the purchase order for the NOvA tubes was the largest ever. The company has begun the production of the PVC panels, which look like 16 extra-long downspouts with a 4-by-6-cm cross section attached side-by-side. The panels, which must meet the tight specifications for the thickness and uniformity of the NOvA plastic, are shipped to a warehouse rented by the University of Minnesota. There, students and technicians outfit each tube with a fiber optic cable that will capture the faint light that a neutrino creates when it breaks up an atom in the mineral oil. Avalanche photodiodes attached to each fiber will record and amplify the signal, which is then digitized and transmitted to the central data acquisition system.
To make sure that no light gets lost, Talaga and his group used a special PVC formulation that includes large amounts of titanium-dioxide to create a strong plastic that is white and highly reflective.
“The oil doesn’t absorb much light,” said Talaga. “The light created by a neutrino interaction is either absorbed by the walls of the tubes or by the fiber optic cable inside each tube. By making the walls highly reflective, the light bounces back eight, nine or ten times without significant absorption and you see a stronger signal in the fiber.”
To transform the roughly 24,000 plastic panels into one giant particle detector, technicians will place 24 panels next to each other to make a layer of tubes, 16 meters by 16 meters square. After an application of special no-drip glue, the next layer will be placed on top, with the tubes lying perpendicularly to the layer below. Gluing and lifting of the 1,000-pound panels will be done with machines developed and tested at Argonne, where the first set of machines was used to build the test block used on the pivoter at Fermilab.
The Argonne group just finished the installation of the first gluing machine at Ash River. The full-size pivoter, six times as wide as the one tested at Fermilab, is under construction and will be ready for operation early next year. Bill Miller, of the University of Minnesota, who participated in the pivoter test at Fermilab, will lead the assembly of the detector in Ash River. He will supervise local staff, hired by the University of Minnesota for the task.
“We plan to assemble the first block in Ash River this spring,” said Lukens, who’s overseen the development of the NOvA assembly plans for three years. “It will take 18 months to assemble the entire detector.”
Scientists from 28 institutions are working on the NOvA experiment. When operational, the experiment will examine the world’s highest-intensity, longest-distance neutrino beam, generated at the Fermilab. Accelerators will produce a beam of muon neutrinos that will travel straight through the earth to the NOvA detector in northern Minnesota. During their split-second trip to Ash River, some of these neutrinos will turn into electron neutrinos and tau neutrinos. By measuring the composition of the neutrino beam with a small, 222-ton detector at Fermilab and a large detector in Ash River, scientists expect to discover the neutrino mass hierarchy, determining whether there are two light neutrinos and one heavy one, or two heavy ones and a light one.
Source : http://www.rdmag.com