K/Th/U in photomultiplier tubes and improved low-level NaI detectors

The study presented here is the first step in a program aimed at reducing significantly the background count rate of NaI scintillation units. We have investigated: (1) the residual background of a large well type NaI detector, i.e., when shielded with 10 cm of lead and operated deep underground, (2) low concentrations of primordial radioactivity in glass used for photomultiplier tubes (PMTs) and (3) the activity in whole tubes. The residual background of the NaI units is dominated by gamma radiation from potassium, thorium and uranium in the PMT, which severely limits their sensitivity. Activity in tubes made of new high purity glass was close to the detection level. The prospects of a new generation of low-level NaI detectors with these tubes are discussed.