Eu activation was found in the NBS decommissioned LINAC cave also.. Jim Tracy at NIST has more info. Also saw Zn-65 and Co-60, but the 13 y Eu-152 is the dominant long term residual nuclide. I assume it is a simple neutron capture by Eu-151 off of naturally occurring Eu in concrete. Others have seen this also. A little risk adds spice to life. slaback@MICF.NIST.gov