Of the thirty-six isotopes of iridium known today, only two occur naturally with the following authorised isotopic abundances (2):

Although Arthur J. Dempster (3) is credited with the discovery of these two isotopes at the University of Chicago, Illinois, in late 1935, using a new type of mass spectrograph that he had developed; earlier that year Venkatesachar and Sibaiya (4) of the Department of Physics, Central College, Bangalore, India, had observed isotopic shifts in the hyperfine arc spectrum of iridium which they suggested were due to masses 191 and 193 in the approximate ratio of 1:2. At that time, this seemed to be incorrect as it resulted in an atomic weight for iridium of 192.4 which was much lower than the then accepted value of 193.1 (5). However, in 1936, Sampson and Bleakney (6) carried out a precision determination of the isotopic ratio using a mass spectrograph. This confirmed the above approximate ratio and eventually, in 1953, the atomic weight was lowered to 192.2 (7).

Artificial Iridium Isotopes

Almost immediately after the published discovery of artificial radioactivity by Curie and Joliot in 1934 (8), Fermi and colleagues of the Physics Laboratory, University of Rome, identified a 20 hour activity (activity is generally used to indicate the half-life of a non-specified isotope) after bombarding iridium with slow neutrons (9).

In 1935, the same group (10) refined the half- life to 19 hours, although Sosnowski (11) was unable to confirm this period but instead obtained activities of 50 minutes and three days for the half- lifes. In 1936, Amaldi and Fermi (12) also discovered an activity which they assigned to iridium, but its half-life was 60 days. In the same year Cork and Lawrence (13) bombarded platinum with deuterons (deuterium ions) and obtained activities with half-lifes of 28 minutes and 8.5 hours which they claimed were definitely associated with iridium following chemical identification. In 1937 Pool, Cork and Thornton (14) bombarded iridium with neutrons and obtained a 15 hour activity which was very similar to that obtained by Fermi and colleagues back in 1934.

Enrico Fermi 1901–1954 The physicist Enrico Fermi was born in Rome, Italy. In 1926 Fermi discovered the statistical laws governing the behaviour of particles of quantum spin one half, which are now known as fermions. A year later he became Professor of Theoretical Physics at the University of Rome where he evolved the theory of beta decay. In 1934 he set up the group which led to the discovery of numerous artificial radioactive isotopes obtained by bombarding elements with neutrons and for this he received the 1938 Nobel Prize in Physics. Immediately afterwards he moved to the United States, first to Columbia University, then to the University of Chicago to be Professor of Nuclear Studies. He was a leading member of the team that produced, on 2nd December 1942, the first controlled nuclear chain reaction. After the war he concentrated on high energy physics and cosmic rays. Element 100 is named fermium in his honour University of Chicago, courtesy of AIP Emilio Segrè Visual Archives View this figure

Philip John Woods Professor of Nuclear Physics at the University of Edinburgh. Philip Woods is a spokesman of a British-American collaboration that has performed experiments at the Argonne National Laboratory, Chicago, resulting in the discovery and measurement of a large number of proton-emitting isotopes. These include the four most unstable iridium isotopes from 164Ir to 167Ir. The object to the right of Philip Woods is the pioneering double-sided silicon strip detector (DSSD) used to identify iridium isotopes by their radioactive decays View this figure

However, although in 1935 Dempster (3) had identified the naturally occurring isotopes, and the various radioactive discoveries could probably be assigned to the missing ¹⁹²Ir, ¹⁹⁴Ir or ¹⁹⁵Ir, Livingston and Bethe (15), in a review in 1937, concluded that the situation was confused and that no firm mass assignments could be given at that time. However, in the same year McMillan, Kamen and Ruben of the Department of Physics and Chemistry at the University of California (16) confirmed the 19 hour activity of Fermi and colleagues (9, 10) and correctly assigned it to ¹⁹⁴Ir while they also confirmed the 60 day activity of Amaldi and Fermi (12) which they assigned to ¹⁹²Ir. Actually McMillan, Kamen and Ruben assigned the original identification of the 60 day activity to Fomin and Houtermans in 1936 (17), but these two appeared not to have produced this activity but simply mentioned its discovery by Amaldi and Fermi. However Fomin and Houtermans became credited with the first observation and this confusion was not resolved until 1951 (18). None of the other activities reported prior to 1938 have proved to be correct.

As with platinum, the most prolific decade for the discovery of iridium isotopes was the 1960s with Antti Siivola, who was then at the Lawrence Radiation Laboratory, Berkeley, California, producing and identifying seven new isotopes in 1966 (19). More recently there has been a concentration on the proton-rich isotopes, and in 1995 a British- American team, one of the leading members of which was Philip J. Woods, announced the production of the three proton-emitting isotopes ¹⁶⁵Ir, ¹⁶⁶Ir and ¹⁶⁷Ir and this research group was therefore the first to cross the proton drip line in iridium (20, 21). More recently two groups have independently discovered the even lighter proton-emitting isotope ¹⁶⁴Ir, the first at the Department of Physics, University of Jyväskylä, Finland (22), and the second by the British-American team mentioned above (23). The discovery of four particle-unstable isotopes (i.e. proton emitters) for one element is a record. Because the half-life of ¹⁶⁴Ir is likely to be less than 100 μs it is likely that there may be extreme difficulty in producing and identifying even lighter isotopes.

In the Table of the Discoverers of the Iridium Isotopes the date of discovery is a manuscript or conference date, or, if unavailable, then a publishing date. The half-lifes are mainly those selected in the NUBASE database (24) with new or revised values being referenced in the Notes to the Table.