About three years ago I heard a discussion of target designs for the proposed European Spallation Source, Gabriel says. The Europeans were pushing for a mercury target. I was a little skeptical, but we gave it a serious look. The more I looked, the more logical it seemed for a high-powered target. Mercury became attractive largely because the SNS is going to be such a big gun that will likely get even bigger.
Conventional targets are solid, usually tantalum or tungsten.
In the SNS, each pulse of protons will strike the target with 17 kilojoules of energy. In watts, that represents enough energy to illuminate 17 million homes for a microsecond. Those pulses will arrive 60 times a second. At those energies, a solid target would be subject to destructive thermal shock and radiation damage. If the SNS is eventually upgraded to four megawatts, as planned, there is a risk that a solid target would quickly be reduced to burnt toast.