 The 169Yb isotope ( with a half @-@ life of 32 days ) , which is created along with the short @-@ lived 175Yb isotope ( half @-@ life 4 @.@ 2 days ) by neutron activation during the irradiation of ytterbium in nuclear reactors , has been used as a radiation source in portable X @-@ ray machines . Like X @-@ rays , the gamma rays emitted by the source pass through soft tissues of the body , but are blocked by bones and other dense materials . Thus , small 169Yb samples ( which emit gamma rays ) act like tiny X @-@ ray machines useful for radiography of small objects . Experiments show that radiographs taken with a 169Yb source are roughly equivalent to those taken with X @-@ rays having energies between 250 and 350 keV . 169Yb is also used in nuclear medicine . 
