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Platinum Metals Rev., 2003, 47, (4), 167

The Discoverers of the Iridium Isotopes

THE THIRTY-SIX KNOWN IRIDIUM ISOTOPES FOUND BETWEEN 1934 AND 2001

  • J. W. Arblaster
  • Coleshill Laboratories,
  • Gorsey Lane, Coleshill, West Midlands B46 1JU, U.K.
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Article Synopsis

This paper is the second in a series of reviews of work performed that led up to the discoveries of the isotopes of the six platinum group elements. The first review, on platinum isotopes, was published in this Journal in October 2000 (1). Here, a brief history of the discovery of the thirty-six known isotopes of iridium in the sixty-seven years from the first discovery in 1934 to 2001 is considered in terms of the discoverers.

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

The Naturally Occurring Isotopes of Iridium

Mass numberIsotopic abundance, %
191Ir37.3
193Ir62.7

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

Enrico Fermi 1901–1954The 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 honourUniversity of Chicago, courtesy of AIP Emilio Segrè Visual Archives

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

Philip John WoodsProfessor 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

The Discoverers of the Iridium Isotopes

Mass numberHalf-lifeDecay modesYear of discovery[i]DiscoverersRef.Notes
Notes to the Table

A164 mIrMahmud et al. (23) considered that the nuclide observed was an isomeric state not the ground state. The half-life is a weighted average of 113+62−30 µs determined by Kettunen et al. (22) and 58+46−18 µs determined by Mahmud et al. (23).
B166 mIrOnly the alpha energy was measured. The half-life was determined by Page et al. in 1995 (26) while the isomeric state assignment was by Davids et al. (21).
C167 mIrOnly the alpha energy was measured. The half-life was determined by Page et al. in 1995 (26) while the isomeric state assignment was by Davids et al. (21).
D168IrOnly the alpha energy was measured. The half-life was determined by Page et al. in 1995 (26).
E169IrThe half-life was normalised from 638+462−237 ms determined by Poli et al. in 1999 (28).
F169 mIrThe isomeric state assignment was by Poli et al. (28). The half-life is a weighted average of 308 ± 22 ms determined by Page et al. (26) and 323 +90−60 ms by Poli et al. (28).
G170IrThe half-life was selected by Baglin (30).
H170 mIrThe isomeric state assignment was by Page et al. (26). The half-life was selected by Baglin (30).
I171 mIrThe half-life was selected by Baglin (33) who also assigned the activity to be an isomeric state.
J172 mIrThe isomeric state assignment was by Schmidt-Ott et al. (34).
K173 mIrThe isomeric state assignment was by Schmidt-Ott et al. (34).
L174 mIrThe isomeric state assignment was by Schmidt-Ott et al. (34).
M179IrHalf-lifes determined by Nadzhakov et al. (39) appear to be systematically in error but the discovery is otherwise accepted.
N186IrThe isotope was actually discovered by Smith and Hollander in 1955 (45) but was wrongly assigned to 187Ir.
O192IrThe isotope was first observed as a non-specific activity by Amaldi and Fermi in 1936 (12).
P192 m1IrThe isotope was actually discovered by McMillan, Kamen and Ruben in 1937 (16) but was wrongly assigned to 194Ir.
Q192 m2IrScharff-Goldhaber and McKeown only determined the half-life to be greater than five years. The accepted value was determined by Harbottle in 1969 (59).
R194IrThe isotope was first observed as a non-specific activity by Fermi et al. in 1934 (9) and Amaldi et al. in 1935 (10).
S194 m2IrA 47 s activity described as being an isomer of 194Ir by Hennies and Flammersfeld in 1959 (63) could not be found by Scharff-Goldhaber and McKeown (64).
T196IrThe isotope was first observed by Butement and Poë in 1953 (68) but was wrongly assigned to 198Ir.
U196 mIrJansen and Pauw (69) suggested that the 20 h activity originally assigned to 196 mIr by Bishop in 1964 (70) was actually a mixture of 196 mIr and 195Ir.
V197IrThe 5.8 min half-life isotope was assigned to the ground state by Petry et al. in 1978 (71).
W197 mIrThe 8.9 min half-life isotope was assigned to be the isomeric state by Petry et al. in 1978 (71).
X198IrDetails of this isotope were first given in the open literature by Szaley and Uray in 1973 (75).
Y199IrOnly the mass of the isotope was determined. The half-life and decay mode were estimated from nuclear systematics (24).
Decay Modes

αAlpha decay is the emittance of alpha particles which are 4He nuclei. Thus the atomic number of the daughter nuclide is lower by two and the mass number is lower by four.
βBeta or electron decay for neutron-rich nuclides is the emittance of an electron (and an anti-neutrino) as a neutron decays to a proton. The mass number of the daughter nucleus remains the same but the atomic number increases by one.
β+Beta or positron decay for neutron-deficient nuclides is the emittance of a positron (and a neutrino) as a proton decays to a neutron. The mass number of the daughter nucleus remains the same but the atomic number decreases by one. However, this decay mode cannot occur unless the decay energy exceeds 1.022 MeV (twice the electron mass in energy units). Positron decay is always associated with orbital electron capture (EC).
ECOrbital electron capture. The nucleus captures an extranuclear (orbital) electron which reacts with a proton to form a neutron and a neutrino, so that, as with positron decay, the mass number of the daughter nucleus remains the same but the atomic number decreases by one.
ITIsomeric transition, in which a high energy state of a nuclide (isomeric state or isomer) usually decays by cascade emission of γ (gamma) rays (the highest energy form of electromagnetic radiation) to lower energy levels until the ground state is reached. However, certain low level states may also decay independently to other nuclides.
pThe emittance of protons by highly neutron-deficient nuclides. As the neutron:proton ratio decreases a point is reached where there is insufficient binding energy for the last proton which is therefore unbound and is emitted. The point at which this occurs is known as the proton drip line and such nuclides are said to be “particle unstable”.
164m100 μsp20001: Kettunen et al.22A
2: Mahmud et al.23
165m300 μsp, α1995Davids et al.20, 21
16610.5 msα, p1995Davids et al.20, 21
166m15.1 msα, p1981Hofmann et al.25B
16735.2 msα, p, EC + β+1995Davids et al.20, 21
167m30.0 msα, EC + β+?, p1981Hofmann et al.25C
168125 msα?, EC + β+?1978Cabot et al.27D
168m161 msα1995Page et al.26
169780 msα, EC + β+?1999Poli et al.28E
169m310 msα, EC + β+19781: Cabot et al.27F
2: Schrewe et al.29
170870 msEC + β+, α1995Page et al.26G
170m440 msEC + β+, IT, α19771: Cabot et al.31H
2: Schrewe et al.29
1713.2 sα, EC + β+, p?2001Rowe et al.32
171m1.40 sα, EC + β+, p?1966Siivola19I
1724.4 sEC + β+, α1991Schmidt-Ott et al.34, 35
172m2.0 sEC + β+, α1966Siivola19J
1739.0 sEC + β+, α19911: Bouldjedri et al.36
2: Schmidt-Ott et al.34, 35
173m2.20 sEC + β+, α1966Siivola19K
1749 sEC + β+, α19911: Bouldjedri et al.36
2. Schmidt-Ott et al.34, 35
174m4.9 sEC + β+, α1966Siivola19L
1759 sEC + β+, α1966Siivola19
1768 sEC + β+, α1966Siivola19
17730 sEC + β+, α1966Siivola19
17812 sEC + β+1970Akhmadzhanov et al.37, 38
1791.32 minEC + β+1971Nadzhakov et al.39M
1801.5 minEC + β+19701: Akhmadzhanov et al.37, 38
2: Nadzhakov et al.39
1814.90 minEC + β+19701: Akhmadzhanov et al.37, 38
2: Nadzhakov et al.39
18215 minEC + β+1961Diamond et al.40
18358 minEC + β+19601: Lavrukhina, Malysheva and Khotin41
2: Diamond et al.40
1843.09 hEC + β+19601: Baranov et al.42
2: Diamond et al.40
18514.4 hEC + β+1958Diamond and Hollander43
18616.64 hEC + β+1957Scharff-Goldhaber et al.44N
186m1.92 hEC + β+, IT?1962Bonch-Osmolovskaya et al.46
18710.5 hEC + β+1958Diamond and Hollander43
187m30.03 msIT1962Ramaev, Gritsyna and Korda47
18841.5 hEC + β+1950Chu48
188m4.2 msIT, EC + β+1970Goncharov et al.49
18913.2 dEC1955Smith and Hollander45
189m113.3 msIT1962Ramaev, Gritsyna and Korda47
189m23.7 msIT19741: André et al.50
2: Kemnitz et al.51
19011.78 dEC + β+1946Goodman and Pool52
190m11.120 hIT1964Harmatz and Handley53
190m23.087 hEC + β+, IT1950Chu48
191Stable1935Dempster3
191m4.94 sIT19541: Butement and Poë54
2: Mihelich, McKeown and Goldhaber55
3: Naumann and Gerhart56
19278.831 dβ, EC1937McMillan, Kamen and Ruben16O
192m11.45 minIT, β1947Goldhaber, Muehlhouse and Turkel57P
192m2241 yIT1959Scharff-Goldhaber and McKeown58Q
193Stable1935Dempster3
193m10.53 dIT1956Boehm and Marmier60
19419.28 hβ1937McMillan, Kamen and Ruben16R
194m131.85 msIT1959Campbell and Fettweiss61
194m2171 dβ1968Sunjar, Scharff-Goldhaber and McKeown62S
1952.5 hβ1952Christian, Mitchell and Martin65
195m3.8 hβ, IT1967Hofstetter and Daly66
19652 sβ1966Venach, Münzer and Hille67T
196m1.40 hβ, IT?1966Jansen and Pauw69U
1975.8 minβ1952Christian, Mitchell and Martin65V
197m8.9 minβ, IT?1976Petry et al.72, 73W
1988 sβ1972Schweden and Kaffrell74X
199(20 s)β1992Zhao et al.76Y

[i] The year of discovery is taken as available manuscript and conference dates. Where these are not available then the year of discovery is the publishing date

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.

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Appendices

Appendix

Some of the Terms Used for this Review

Atomic numberthe number of protons in the nucleus
Mass numberthe combined number of protons and neutrons in the nucleus
Nuclide and isotopeA nuclide is an entity characterised by the number of protons and neutrons in the nucleus. For nuclides of the same element the number of protons remains the same but the number of neutrons may vary. Such nuclides are known collectively as the isotopes of the element. Although the term isotope implies plurality it is sometimes used loosely in place of nuclide.
Half-lifethe time taken for the activity of a radioactive nuclide to fall to half its previous value
Electron volt (eV)The energy acquired by any charged particle carrying a unit (electronic) charge when it falls through a potential of one volt, equivalent to 1.602 × 10−19 J. The more useful unit is the mega (million) electron volt, MeV.

Acknowledgement

Thanks to Mrs Linda Porter for typing the manuscript.

The Discoverers of the Platinum Isotopes

In the earlier review on platinum isotopes (1), on page 174, the β decay mode of 202Pt was inadvertently omitted.

On page 177, the second and third lines on the left hand column should be: “Table of Isotopes” and the fourth line should read “In the Table of the Discoverers of the Platinum Isotopes, the mass number of each isotope …”

The Author

John W. Arblaster is Chief Chemist working in metallurgical analysis at Coleshill Laboratories. He is interested in the history of science and in the evaluation of the thermodynamic and crystallographic properties of the elements.

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