Last week a group of LBNE collaborators, myself included, met by phone to discuss issues related to the photomultiplier tubes (PMT) we would use in the large water Cherenkov detector option. Without getting into details, a PMT can be thought of as a single pixel in a camera, and we look at the brightness of light that each PMT sees to discern a signal. We are lucky to have collaborators who are also on the Super-K experiment in Japan which is very similar to what we are talking about building. One important difference is that Super-K uses much bigger PMTs. The Super-K PMTs have a diameter of 20 inches (50cm) and one candidate we are considering for LBNE are half that size, 10 inches in diameter. In general, the larger PMTs will see more light than a smaller one, but in our case, the smaller PMTs are actually more efficient, and so it is not so clear cut.
An important thing to know is how much better or worse the 10inch PMTs are compared to the 20 inch ones. We can try to answer that question by compiling all of the data we have on each PMT and comparing, but there are a lot of uncertainties in that comparison. During the discussion we decided that we should try to compare these PMTs in a real lab experiment with the same light source. Coincidentally, there was only one collaborating institution that had one of each tube … us! So, I got volunteered to do this test, and we need the answer ASAP.
We had to make a number of modifications to our testing lab to accommodate the larger PMT, but we were able to make it work and are in the process of taking data.