After Urbain's names were recognized, neoytterbium was reverted to ytterbium. The chemical and physical properties of ytterbium could not be determined with any precision until 1953, when the first nearly pure ytterbium metal was produced by using ion-exchange processes. The price of ytterbium was relatively stable between 1953 and 1998 at about US$1,000/kg. 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. Ytterbium clocks hold the record for stability with ticks stable to within less than two parts in 1 quintillion (). The clocks developed at the National Institute of Standards and Technology (NIST) rely on about 10,000 rare-earth atoms cooled to 10 microkelvin (10 millionths of a degree above absolute zero) and trapped in an optical lattice—a series of pancake-shaped wells made of laser light. Another laser that "ticks" 518 trillion times per second provokes a transition between two energy levels in the atoms. The large number of atoms is key to the clocks' high stability. Visible light waves oscillate faster than microwaves, and therefore optical clocks can be more precise than caesium atomic clocks. The Physikalisch-Technische Bundesanstalt is working on several such optical clocks. The model with one single ytterbium ion caught in an ion trap is highly accurate. The optical clock based on it is exact to 17 digits after the decimal point. A pair of experimental atomic clocks based on ytterbium atoms at the National Institute of Standards and Technology has set a record for stability. NIST physicists reported in the August 22, 2013 issue of Science Express that the ytterbium clocks' ticks are stable to within less than two parts in 1 quintillion (1 followed by 18 zeros), roughly 10 times better than the previous best published results for other atomic clocks.