near where he found the new component of erbium. Marignac suspected that ytterbia was a compound of a new element that he called "ytterbium". In 1907, the French chemist Georges Urbain separated Marignac's ytterbia into two components: neoytterbia and lutecia. Neoytterbia later became known as the element ytterbium, and lutecia became known as the element lutetium. The Austrian chemist Carl Auer von Welsbach independently isolated these elements from ytterbia at about the same time, but he called them aldebaranium and cassiopeium; the American chemist Charles James also independently isolated these elements at about the same time. Urbain and Welsbach accused each other of publishing results based on the other party. The Commission on Atomic Mass, consisting of Frank Wigglesworth Clarke, Wilhelm Ostwald, and Georges Urbain, which was then responsible for the attribution of new element names, settled the dispute in 1909 by granting priority to Urbain and adopting his names as official ones, based on the fact that the separation of lutetium from Marignac's ytterbium was first described by Urbain. 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. Applications Source of gamma rays 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. High-stability atomic clocks 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