Machine Design

Anodized Aluminum Helps Focus On The Elusive Neutrino

The prototype horn's entire structure and coatings are held to tolerances of ±0.01 in. Critical high-current surfaces are masked from anodizing because they must be silver plated. The inner conductor is coated with electroless nickel.

A proprietary anodizing process put a protective coating on a 650-lb prototype horn used for focusing neutrinos. The anodization prevents corrosion which would otherwise cause loss of subatomic particles called pions.

The proprietary anodizing process is called Metalast. Used by Universal Metal Finishing(UMF) Co., Chicago, it acts as a dielectric insulator and also resists degradation from radiation. This is important because the horns will eventually become radioactive during the course of the experiment in which they serve.

UMF selected a 1.8 to 2.3-milthick sulfuric acid hardcoat followed by a midtemperature nickel seal as the coating of choice. UMF says thinner coatings didn't perform well and were prone to pitting. Porosity made thicker oxide coatings poor performers as well.

The Metalast process uses a special pulsing technique during the anodizing process. A computer controls ramp-up, voltage, and electrical current to help avoid the tendency of anodized parts to burn. If the oxide-build and electrical pulse aren't in sync, the parts will be attacked by the acid in the bath.

Metalast is said to give a consistent coating with a tighter, denser pore structure that is more elastic. The elasticity helps the surface resist cracking and crazing. Metalast is also said to be less prone to "edge effects" — the oxide doesn't fracture at the part corners.

The anodized focusing horns will help resolve a mighty conundrum: If 90% of matter is not composed of the smallest particles already identified to date (quarks and leptons) then what is it? Researchers hope to prove that the unidentified matter consists of elementary particles called neutrinos. Neutrinos have no charge, travel at the speed of light, and can pass through anything. They also seem to have no mass. If they have no mass, then one must ask: How can they constitute the missing 90% of matter?

According to Dr. David Ayres, Fermilab, Chicago, the answer may come if the Main Injector Neutrino Oscillation Search (Minos) experiment can observe neutrino oscillation. Basic physics stipulates that if something oscillates then it must have mass. Theoretically, neutrinos could have mass, and Dr. Ayres and his colleagues hope Minos will help prove they do.

Key to the experiment are two anodized focusing horns. Researchers at Fermilab will first extract and bend protons from their accelerator's main injector onto a graphite target. At point of impact, elementary particles called pions will be produced, spraying in all directions from the graphite target. Here the first focusing horn will collect and direct the errant pions into a column or beam. Some distance away, the pions go through a second horn for further focusing refinement.

Once focused, they enter a 650-m long pipe where they decay into neutrinos. To observe if the neutrinos do in fact oscillate, scientists will send them on a 735-km journey through an aquifer — 120 ft below grade — from Batavia, Ill., to Soudan, Minn. Detectors at each end of the route will characterize the neutrinos and look for any sign of oscillation.

The success of the project relies on the alignment accuracy of the beam. Once the pions decay to neutrinos they can no longer be steered. So it's imperative for the pion beam to be precisely on target. The focusing horns control the direction of the beam via a magnetic field. They are made from fairly conductive 6061 T-6 aluminum. Any corrosion of the horns will cause pion loss and ultimately may result in failure of the experiment.

Combating corrosion on the horns is a major challenge. Horn inner conductors are subjected to 200-kA pulses every 5 msec to generate the magnetic field. Because there is some inherent resistance in the aluminum, the conductors tend to heat up during the pulse. Designers built a series of water spray nozzles to help cool the 17 kW of heat the pulses generate.

According to mechanical engineer Kris Anderson of Fermilab, the service life of the first focusing horn is around 10 million pulses or one year of operation.

Horn two, he says, sees less mechanical stress and thus has a nearly infinite life. The experiment is expected to last eight to 10 years.

Information for this article was provided by Universal Metal Finishing Co., Chicago, Ill., (800)-325-7858,

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