{"id":84,"date":"2012-02-08T15:25:58","date_gmt":"2012-02-08T15:25:58","guid":{"rendered":"http:\/\/blackhole.lngs.infn.it\/darkside\/?page_id=84"},"modified":"2012-02-08T15:25:58","modified_gmt":"2012-02-08T15:25:58","slug":"neutron-veto","status":"publish","type":"page","link":"https:\/\/darkside.lngs.infn.it\/?page_id=84","title":{"rendered":"Neutron Veto"},"content":{"rendered":"

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 \u201cperfect\u201d WIMP background \u2014 some of them scatter more than once in the detector to produce \u201cmultiple recoil\u201d 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 \u2014 especially in detectors like DarkSide, where events other than nuclear recoils can be very efficiently rejected \u2014 neutron-induced nuclear recoils are typically the limiting background.<\/p>\n

In DarkSide, we reduce, identify, and measure the rate of neutron-induced\u00a0backgrounds using active suppression systems. There are two\u00a0main classes of background-producing neutrons: radiogenic neutrons, which are produced from nuclear processes\u00a0in the detector components, and\u00a0cosmogenic neutrons, which are produced by the interactions of cosmic\u00a0ray muons in the detector and surrounding materials. These different\u00a0classes of neutrons are\u00a0detected using two different suppression\u00a0systems.<\/p>\n

\"\"<\/a>
Construction of the spherical liquid scintillator veto within the Borexino CTF.<\/figcaption><\/figure>\n

Like all direct detection dark\u00a0matter experiments, DarkSide minimizes radiogenic neutron\u00a0production by taking pains to select (and develop) detector materials with very\u00a0low levels of intrinsic radioactivity. Some radiogenic neutron\u00a0production is unavoidable, however, so to further suppress this\u00a0background, the DarkSide detector is\u00a0deployed within a Liquid Scintillator Veto (LSV), which is a spherical tank filled with boron-loaded liquid\u00a0scintillator. 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\u00a0the inner detector to reject neutron recoil backgrounds in the\u00a0argon, DarkSide\u00a0is\u00a0able to suppress the rate of\u00a0background events induced by radiogenic neutrons to less than 1%. In addition, by measuring the efficiency with\u00a0which neutrons\u00a0are 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.<\/p>\n

\"\"<\/a>
Sketch showing the two phase argon detector, surrounded by the liquid spherical scintillator veto, immersed in CTF water tank.<\/figcaption><\/figure>\n

Cosmogenic neutrons have higher energies than radiogenic neutrons, and\u00a0can therefore penetrate much further through the detector. Locating the\u00a0DarkSide experiment underground reduces the flux of cosmogenic\u00a0neutrons by reducing the rate of cosmic ray muons. The neutron veto\u00a0is also\u00a0effective against cosmogenic neutrons, although their higher energy makes them more likely to penetrate the veto without\u00a0leaving a signal. In order to more fully suppress cosmogenic neutron\u00a0backgrounds, the DarkSide LSV is\u00a0deployed\u00a0within the Water Cerenkov Detector, an 11m diameter and 10m high cylindrical water tank. Photomultiplier tubes in the water tank detect Cerenkov light produced by\u00a0muons (and other particles\u00a0in the muon shower) traversing\u00a0the water. The large size of the water\u00a0tank allows\u00a0us veto cosmogenic showers with high efficiency and,\u00a0in combination with the neutron veto, makes\u00a0cosmogenic neutrons\u00a0a sub-dominant background in DarkSide.<\/p>\n","protected":false},"excerpt":{"rendered":"

One of the more important classes of background in WIMP detectors are from nuclear recoils. These can be produced by […]<\/p>\n","protected":false},"author":2,"featured_media":0,"parent":0,"menu_order":10,"comment_status":"closed","ping_status":"closed","template":"sidebar-page.php","meta":{"footnotes":""},"class_list":["post-84","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/darkside.lngs.infn.it\/index.php?rest_route=\/wp\/v2\/pages\/84","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/darkside.lngs.infn.it\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/darkside.lngs.infn.it\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/darkside.lngs.infn.it\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/darkside.lngs.infn.it\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=84"}],"version-history":[{"count":0,"href":"https:\/\/darkside.lngs.infn.it\/index.php?rest_route=\/wp\/v2\/pages\/84\/revisions"}],"wp:attachment":[{"href":"https:\/\/darkside.lngs.infn.it\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=84"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}