The Discoverers of the Iridium Isotopes
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The Discoverers of the Iridium Isotopes
THE THIRTY-SIX KNOWN IRIDIUM ISOTOPES FOUND BETWEEN 1934 AND 2001
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):
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
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
The Discoverers of the Iridium Isotopes
Mass number | Half-life | Decay modes | Year of discovery[i] | Discoverers | Ref. | Notes |
---|---|---|---|---|---|---|
Notes to the Table | ||||||
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A | 164 mIr | Mahmud 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). | ||||
B | 166 mIr | Only 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). | ||||
C | 167 mIr | Only 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). | ||||
D | 168Ir | Only the alpha energy was measured. The half-life was determined by Page et al. in 1995 (26). | ||||
E | 169Ir | The half-life was normalised from 638+462−237 ms determined by Poli et al. in 1999 (28). | ||||
F | 169 mIr | The 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). | ||||
G | 170Ir | The half-life was selected by Baglin (30). | ||||
H | 170 mIr | The isomeric state assignment was by Page et al. (26). The half-life was selected by Baglin (30). | ||||
I | 171 mIr | The half-life was selected by Baglin (33) who also assigned the activity to be an isomeric state. | ||||
J | 172 mIr | The isomeric state assignment was by Schmidt-Ott et al. (34). | ||||
K | 173 mIr | The isomeric state assignment was by Schmidt-Ott et al. (34). | ||||
L | 174 mIr | The isomeric state assignment was by Schmidt-Ott et al. (34). | ||||
M | 179Ir | Half-lifes determined by Nadzhakov et al. (39) appear to be systematically in error but the discovery is otherwise accepted. | ||||
N | 186Ir | The isotope was actually discovered by Smith and Hollander in 1955 (45) but was wrongly assigned to 187Ir. | ||||
O | 192Ir | The isotope was first observed as a non-specific activity by Amaldi and Fermi in 1936 (12). | ||||
P | 192 m1Ir | The isotope was actually discovered by McMillan, Kamen and Ruben in 1937 (16) but was wrongly assigned to 194Ir. | ||||
Q | 192 m2Ir | Scharff-Goldhaber and McKeown only determined the half-life to be greater than five years. The accepted value was determined by Harbottle in 1969 (59). | ||||
R | 194Ir | The isotope was first observed as a non-specific activity by Fermi et al. in 1934 (9) and Amaldi et al. in 1935 (10). | ||||
S | 194 m2Ir | A 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). | ||||
T | 196Ir | The isotope was first observed by Butement and Poë in 1953 (68) but was wrongly assigned to 198Ir. | ||||
U | 196 mIr | Jansen 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. | ||||
V | 197Ir | The 5.8 min half-life isotope was assigned to the ground state by Petry et al. in 1978 (71). | ||||
W | 197 mIr | The 8.9 min half-life isotope was assigned to be the isomeric state by Petry et al. in 1978 (71). | ||||
X | 198Ir | Details of this isotope were first given in the open literature by Szaley and Uray in 1973 (75). | ||||
Y | 199Ir | Only the mass of the isotope was determined. The half-life and decay mode were estimated from nuclear systematics (24). | ||||
Decay Modes | ||||||
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α | Alpha 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. | |||||
β− | Beta 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). | |||||
EC | Orbital 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. | |||||
IT | Isomeric 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. | |||||
p | The 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”. | |||||
164m | 100 μs | p | 2000 | 1: Kettunen et al. | 22 | A |
2: Mahmud et al. | 23 | |||||
165m | 300 μs | p, α | 1995 | Davids et al. | 20, 21 | |
166 | 10.5 ms | α, p | 1995 | Davids et al. | 20, 21 | |
166m | 15.1 ms | α, p | 1981 | Hofmann et al. | 25 | B |
167 | 35.2 ms | α, p, EC + β+ | 1995 | Davids et al. | 20, 21 | |
167m | 30.0 ms | α, EC + β+?, p | 1981 | Hofmann et al. | 25 | C |
168 | 125 ms | α?, EC + β+? | 1978 | Cabot et al. | 27 | D |
168m | 161 ms | α | 1995 | Page et al. | 26 | |
169 | 780 ms | α, EC + β+? | 1999 | Poli et al. | 28 | E |
169m | 310 ms | α, EC + β+ | 1978 | 1: Cabot et al. | 27 | F |
2: Schrewe et al. | 29 | |||||
170 | 870 ms | EC + β+, α | 1995 | Page et al. | 26 | G |
170m | 440 ms | EC + β+, IT, α | 1977 | 1: Cabot et al. | 31 | H |
2: Schrewe et al. | 29 | |||||
171 | 3.2 s | α, EC + β+, p? | 2001 | Rowe et al. | 32 | |
171m | 1.40 s | α, EC + β+, p? | 1966 | Siivola | 19 | I |
172 | 4.4 s | EC + β+, α | 1991 | Schmidt-Ott et al. | 34, 35 | |
172m | 2.0 s | EC + β+, α | 1966 | Siivola | 19 | J |
173 | 9.0 s | EC + β+, α | 1991 | 1: Bouldjedri et al. | 36 | |
2: Schmidt-Ott et al. | 34, 35 | |||||
173m | 2.20 s | EC + β+, α | 1966 | Siivola | 19 | K |
174 | 9 s | EC + β+, α | 1991 | 1: Bouldjedri et al. | 36 | |
2. Schmidt-Ott et al. | 34, 35 | |||||
174m | 4.9 s | EC + β+, α | 1966 | Siivola | 19 | L |
175 | 9 s | EC + β+, α | 1966 | Siivola | 19 | |
176 | 8 s | EC + β+, α | 1966 | Siivola | 19 | |
177 | 30 s | EC + β+, α | 1966 | Siivola | 19 | |
178 | 12 s | EC + β+ | 1970 | Akhmadzhanov et al. | 37, 38 | |
179 | 1.32 min | EC + β+ | 1971 | Nadzhakov et al. | 39 | M |
180 | 1.5 min | EC + β+ | 1970 | 1: Akhmadzhanov et al. | 37, 38 | |
2: Nadzhakov et al. | 39 | |||||
181 | 4.90 min | EC + β+ | 1970 | 1: Akhmadzhanov et al. | 37, 38 | |
2: Nadzhakov et al. | 39 | |||||
182 | 15 min | EC + β+ | 1961 | Diamond et al. | 40 | |
183 | 58 min | EC + β+ | 1960 | 1: Lavrukhina, Malysheva and Khotin | 41 | |
2: Diamond et al. | 40 | |||||
184 | 3.09 h | EC + β+ | 1960 | 1: Baranov et al. | 42 | |
2: Diamond et al. | 40 | |||||
185 | 14.4 h | EC + β+ | 1958 | Diamond and Hollander | 43 | |
186 | 16.64 h | EC + β+ | 1957 | Scharff-Goldhaber et al. | 44 | N |
186m | 1.92 h | EC + β+, IT? | 1962 | Bonch-Osmolovskaya et al. | 46 | |
187 | 10.5 h | EC + β+ | 1958 | Diamond and Hollander | 43 | |
187m | 30.03 ms | IT | 1962 | Ramaev, Gritsyna and Korda | 47 | |
188 | 41.5 h | EC + β+ | 1950 | Chu | 48 | |
188m | 4.2 ms | IT, EC + β+ | 1970 | Goncharov et al. | 49 | |
189 | 13.2 d | EC | 1955 | Smith and Hollander | 45 | |
189m1 | 13.3 ms | IT | 1962 | Ramaev, Gritsyna and Korda | 47 | |
189m2 | 3.7 ms | IT | 1974 | 1: André et al. | 50 | |
2: Kemnitz et al. | 51 | |||||
190 | 11.78 d | EC + β+ | 1946 | Goodman and Pool | 52 | |
190m1 | 1.120 h | IT | 1964 | Harmatz and Handley | 53 | |
190m2 | 3.087 h | EC + β+, IT | 1950 | Chu | 48 | |
191 | Stable | – | 1935 | Dempster | 3 | |
191m | 4.94 s | IT | 1954 | 1: Butement and Poë | 54 | |
2: Mihelich, McKeown and Goldhaber | 55 | |||||
3: Naumann and Gerhart | 56 | |||||
192 | 78.831 d | β−, EC | 1937 | McMillan, Kamen and Ruben | 16 | O |
192m1 | 1.45 min | IT, β− | 1947 | Goldhaber, Muehlhouse and Turkel | 57 | P |
192m2 | 241 y | IT | 1959 | Scharff-Goldhaber and McKeown | 58 | Q |
193 | Stable | – | 1935 | Dempster | 3 | |
193m | 10.53 d | IT | 1956 | Boehm and Marmier | 60 | |
194 | 19.28 h | β− | 1937 | McMillan, Kamen and Ruben | 16 | R |
194m1 | 31.85 ms | IT | 1959 | Campbell and Fettweiss | 61 | |
194m2 | 171 d | β− | 1968 | Sunjar, Scharff-Goldhaber and McKeown | 62 | S |
195 | 2.5 h | β− | 1952 | Christian, Mitchell and Martin | 65 | |
195m | 3.8 h | β−, IT | 1967 | Hofstetter and Daly | 66 | |
196 | 52 s | β− | 1966 | Venach, Münzer and Hille | 67 | T |
196m | 1.40 h | β−, IT? | 1966 | Jansen and Pauw | 69 | U |
197 | 5.8 min | β− | 1952 | Christian, Mitchell and Martin | 65 | V |
197m | 8.9 min | β−, IT? | 1976 | Petry et al. | 72, 73 | W |
198 | 8 s | β− | 1972 | Schweden and Kaffrell | 74 | X |
199 | (20 s) | β− | 1992 | Zhao et al. | 76 | Y |
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.
Appendices
Appendix
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 …”
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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.