A Losing Bet

The scientific vortex of the SNS is its target.

The scientific vortex of the SNS is its target—the focal point where high-energy proton pulses collide with atoms of mercury, creating showers of neutrons. Once focused into a beam, the neutrons enable researchers to study the structure and internal dynamics of materials in unprecedented detail.

The SNS target, unlike its predecessors at other neutron facilities, is filled with a self-cooling flow of liquid mercury that is pumped through the target at high speed. The first-of-its-kind device was designed to remain cool in the face of extraordinary quantities of energy deposited by the proton pulses.

"The target is a unique and amazing thing," Henderson says. "Because the SNS target is so unusual, there were a lot of early concerns about whether such a novel experiment would work at all."

Targets made of solid metals, like tungsten and tantalum, would normally have been used for a pulsed neutron source. As the design of the SNS evolved, scientists became convinced that the energy of the intended beam was going to be so high that a solid target would have trouble dissipating heat quickly enough.

Despite extreme conditions of operation, the SNS mercury target has proved to be remarkably durable and is still in use. SNS staff started a pool in early 2007, predicting when the target would need to be replaced. The entire staff lost.

The target was replaced during the August 2009 maintenance shutdown. Using a remotely controlled system, researchers will cut the original target into sections to look for signs of degradation.

Henderson says the new target will be essentially the same as the original. "We may spray a proton-activated, light-emitting coating on the surface of the target that is struck by the beam. This coating will enable us to see exactly where the beam strikes the target and how the beam is shaped at that precise point."

Otherwise the "guts" of the target are exactly the same. Why mess with success?

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