One of the more important classes of background in WIMP detectors are from nuclear recoils. These can be produced by neutrons scattering off of argon nuclei. Because neutrons are electrically neutral, we don’t see the neutron directly; instead we only see the signal produced by the recoiling nucleus, which may look identical to the signal we expect from a WIMP! Not all neutrons produce a “perfect” WIMP background — some of them scatter more than once in the detector to produce “multiple recoil” events, and others produce nuclear recoils with energies higher than we expect from WIMP interactions. Still, it is possible that an incoming neutron may only scatter once and produce a signal in the WIMP recoil energy range. This means that — especially in detectors like DarkSide, where events other than nuclear recoils can be very efficiently rejected — neutron-induced nuclear recoils are typically the limiting background.
In DarkSide, we reduce, identify, and measure the rate of neutron-induced backgrounds using active suppression systems. There are two main classes of background-producing neutrons: radiogenic neutrons, which are produced from nuclear processes in the detector components, and cosmogenic neutrons, which are produced by the interactions of cosmic ray muons in the detector and surrounding materials. These different classes of neutrons are detected using two different suppression systems.
Like all direct detection dark matter experiments, DarkSide minimizes radiogenic neutron production by taking pains to select (and develop) detector materials with very low levels of intrinsic radioactivity. Some radiogenic neutron production is unavoidable, however, so to further suppress this background, the DarkSide detector is deployed within a Liquid Scintillator Veto (LSV), which is a spherical tank filled with boron-loaded liquid scintillator. After a neutron scatters on an argon nucleus, it will leave the inner detector, and go into the LSV. Inside the LSV, the neutron will scatter off of hydrogen and carbon nuclei as it slows down, producing a very fast signal that we can see. Once the neutrons are slowed down, they are captured by the boron with a very high efficiency. This capture produces a second signal that can also be used to efficiently identify these neutrons. By using the coincidences between the neutron veto and the inner detector to reject neutron recoil backgrounds in the argon, DarkSide is able to suppress the rate of background events induced by radiogenic neutrons to less than 1%. In addition, by measuring the efficiency with which neutrons are detected by the veto and the rate of neutron scatters in the argon, we can better understand the neutron background. This gives us a very strong handle on what our neutron background actually looks like, so we can also better predict how many neutrons may get past our cuts.
Cosmogenic neutrons have higher energies than radiogenic neutrons, and can therefore penetrate much further through the detector. Locating the DarkSide experiment underground reduces the flux of cosmogenic neutrons by reducing the rate of cosmic ray muons. The neutron veto is also effective against cosmogenic neutrons, although their higher energy makes them more likely to penetrate the veto without leaving a signal. In order to more fully suppress cosmogenic neutron backgrounds, the DarkSide LSV is deployed within the Water Cerenkov Detector, an 11m diameter and 10m high cylindrical water tank. Photomultiplier tubes in the water tank detect Cerenkov light produced by muons (and other particles in the muon shower) traversing the water. The large size of the water tank allows us veto cosmogenic showers with high efficiency and, in combination with the neutron veto, makes cosmogenic neutrons a sub-dominant background in DarkSide.