Low stationary noise is necessary, but not sufficient. We adopt the term here to mean all those types of noise or unexplained transient phenomena that mimic real events. Background must be measured by operating the detector as an observing instrument. If only stationary noise is present, the resulting event energy distribution function has a shape similar to a Boltzman distribution. Unexplained events outside this thermal-like distribution are either background or real events.
Background is potentially due to every non-equilibrium or transient mechanism that one can imagine.
It is not much remarked in the literature, but we know that high background has been a serious problem for every detector when it actually started to make observations. We addressed them all during our reconstruction of the antenna. The crucial questions were whether it could be reduced to a low enough level, and how much effort would that take. These measures were successful, as shown in Figure 3. Data are shown for half of 1991 and three months of 1993.