University of Minnesota
School of Physics & Astronomy

Spotlight

One Word: Plastics

Dan Cronin-Henessy
Dan Cronin-Hennessy
Wendy Tschampl
                                                       

School of Physics and Astronomy Professor Daniel Cronin-Hennessy is leading up machine development for the NOvA module factory. NOvA, or NuMI Off-axis electron-Neutrino Appearance experiment is a proposed 18 kiloton neutrino detector that will search for the yet unobserved oscillation of muon- neutrino to electron-neutrino. It will be located above ground near International Falls, Minnesota and will be made almost entirely of plastic.

"As far as we know it's the largest plastic structure ever built," Cronin-Hennessy said. The detector will be made up of approximately one-half million cells of PVC. When asked why the group choose plastic and not some other material commonly used in detectors such as steel, Cronin- Hennesy said that material was chosen because the particular flavor of neutrinos they are searching for, electron neutrinos, are easily absorbed by steel. "Also, given the surface location which exposes the detector to cosmic rays, we need the bulk of the detector to consist of an active medium, liquid scintillator, in this case." Therefore a thin, but strong material was needed. With the Minos detector, which is primarily housed in steel, most of the detector mass is dead. MINOS observes muons arising from muon neutrino interactions. High energy muons traverse steel relatively easily compared to electrons. NOνA which uses the same particle beam from Fermilab as Minos, will be searching for a different transition, from Muon to electron neutrino. "Primarily we need an accurate energy reading. The primary component, mineral oil doped with a scintillating compound provides this measurement."

According to Cronin-Hennessy, PVC was chosen as the structural medium because they need such a large amount of it. They needed a material that was relatively inexpensive and had a large industrial infrastructure behind it. Almost the entire detector, mineral oil, PVC, and fiber, is derived directly from oil. "The Department of Energy (DOE) came to us and asked for a projection of oil prices. The government agency with the relevant data turned out to be the Department of Energy."

Because the detector will use an existing beam, the one aimed at MINOS in the Soudan Underground Laboratory, the NOνA detector needs to be above ground further north. The beam enters the earth at an angle and comes out of the ground around International Falls. Because of the nature of electron neutrinos the background is more easily suprressed and the massive shielding required by MINOS is unnecessary with NOνA.

NOνa is scheduled to be begin taking useful data in February 2010 though the detector will not be entirely finished until July 2011. The modular nature of NOνa and the fact that the beam will be available immediately mean that it can be useful before it is finished. It is being designed to last for more than a ten year period.