The NOvA detector will comprise 368,640 PVC tubes that will be filled with mineral oil. A company in Wisconsin extrudes the tubes, which look like extra-long downspouts, in panels of 16. |
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.”
Engineers at Fermilab designed and tested a hydraulic system that will move and rotate the huge, 200-ton plastic blocks for the assembly of the NOvA detector. |
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.
More photos and information about NOvA
