LOW-BACKGROUND GERMANIUM SPECTROMETRY - THE BOTTOM LINE 3 YEARS LATER

Three years ago, state-of-the-art low-background germanium spectrometry was discussed, and speculations were advanced as to the origin of the remaining background. Some of those speculations have been shown to be incorrect. Contemporary lead shielding contains approximate to 100 Bg/kg of Pb-210. Our 450-year-old lead was shown to contain <100 mBq/kg. A high purity electroformed copper Marinelli shield was placed around the detector with no effect on the background, which implied that (he source is other than the 450-year-old shield. A new limit on the Pb-210 in this old lead shield is <9 mBq/kg. Electroformed copper components were found to contain Ra-226 and Th-228 contaminations at levels of 3500 and 110 mu Bq/kg, respectively. High purity H2SO4, recrystallized CuSO4, and a BaSO4 scavenge in the electroforming bath have reduced these contaminations to <25 and 9 mu Bq/kg, respectively. In copper cosmic ray induced nuclear reactions are now the dominant source of radioactivity. For example, Co-58 can be readily measured after only a 24-hour exposure at sea level. A new germanium spectrometer containing 2150 grams of 87.44% enriched Ge-76 has been fabricated to mitigate the effect of cosmogenic Ge-68 in the background. Current background spectra are presented, and potential sources identified.