Signal and noise
We can analyse the data in figure 4 to discriminate between nuclear recoils and electron recoils caused by background gamma-rays. Several hundred photons are detected for each event and their arrival times at the photomultiplier tubes follow a log-normal distribution. The position of the peak depends on the type of particle causing the recoil, while the width of the peak depends mainly on the number of photons in the event (which is determined by the event energy and the efficiency of the detector). Each detector is calibrated with neutron and gamma-ray sources before the experiment begins: a neutron signal (green line) is faster than a gamma one (blue) for the same recoil energy. Most of the data (red points) collected during a dark-matter search will fall on the gamma-ray curve. Statistical analysis is then used to determine the maximum level of neutron-like signals present. This improves the sensitivity of the detector from the blue line to the green line in figure 2. The higher light yields of the upgraded sodium-iodide systems mean that more photons are detected, which gives narrower distributions and hence better discrimination between the two types of signal.