The phototubes in the MiniBooNE detector are completely submerged in mineral oil inside the sealed tank; since mid-run access or replacement are essentially impossible, great care was taken to ensure that the tubes would operate properly over the experiment's two year run.
|Preparing tubes for installation.|
After a tube has been cleaned, a photomultiplier "base" is attached; this
device distributes the high voltage to the dynodes inside the tube. The base and
connecting cable are then sealed inside an oil-impervious plastic, and the tube
is ready for dark-testing.
The photomultipliers were tested by placing them in a photographic darkroom
and bringing the tubes up to their operating voltage. (This determined the
"dark rate," essentially a measure of how electrically noisy or quiet the tube was.) After this test, the tube was exposed to a carefully monitored amount of
light, to test its response and amplification level (the tube's gain).
After the tube's electrical response characteristics were measured, it was taken to the detector site, and lowered into the tank. The tubes were mounted on the inside of the detector, and electrically connected via cables to the electronics above and outside the tank.