NAIAD: an advanced sodium iodide scintillation detector with improved sensitivity to WIMP dark matter

A novel bulk NaI detector, NAIAD, developed by the UK Dark Matter Collaboration (UKDMC), combines improved light collection from suspending an unencapsulated crystal viewed by photomultipliers (PMTs) in mineral oil with background reduction from a liquid scintillator Compton veto. Low background NaI(Tl) detectors, which use pulse shape analysis (PSA) to distinguish scintillation from background electron recoils from nuclear recoils, currently set best limits on WIMP dark matter. Such detectors are difficult to use with vetos, and their hygroscopic crystals are encapsulated and linked to PMTs via silica lightguides to shield the crystal from activity in the PMT glass, resulting in poor light collection and reducing PSA efficiency. Experimental results and Monte Carlo simulations for a prototype NAIAD detector using specular, diffuse and total internal reflection lightguides coupled to a 50/spl times/50 mm crystal are presented. The effect of PTFE wrapping in oil on crystal light output is reported. Results indicate a possible twofold improvement in light collection by NAIAD over current detectors. A close-coupled configuration yielded >13 phe/keV.