Limits on the release of Rb isotopes from a zeolite based 83mKr calibration source for the XENON project

The isomer Kr-83m with its half-life of 1.83 h is an ideal calibration source for a liquid noble gas dark matter experiment like the XENON project. However, the risk of contamination of the detector with traces of the much longer lived mother isotop Rb-83 (T1=2=86:2 d) has to be ruled out. In this work the release of Rb-83 atoms from a 1.8 MBq Rb-83 source embedded in zeolite beads has been investigated. To do so, a cryogenic trap has been connected to the source for about 10 days, after which it was removed and probed for the strongest(83)Rb g-rays with an ultra-sensitive Germanium detector. No signal has been found. The corresponding upper limit on the released Rb-83 activity means that the investigated type of source can be used in the XENON project and similar low-background experiments as Kr-83m generator without a significant risk of contaminating the detector. The measurements also allow to set upper limits on the possible release of the isotopes Rb-84 and Rb-86, traces of which were created alongside the production of Rb-83 at the Rez cyclotron.