The Discoverers of the Osmium Isotopes
The Discoverers of the Osmium Isotopes
THE THIRTY-FOUR KNOWN OSMIUM ISOTOPES FOUND BETWEEN 1931 AND 1989
This is the third in a series of reviews of circumstances surrounding the discoveries of the isotopes of the six platinum group elements; it concerns the discovery of the thirty-four isotopes of osmium. The first review on platinum isotopes was published in this Journal in October 2000, and the second review on iridium isotopes was published in October 2003 (1).
|Mass number||Isotopic abundance, %|
The discovery of the six major isotopes of osmium was reported by F. W. Aston (Figure 1) in 1931 (3) after being detected mass spectrographically at the Cavendish Laboratory, Cambridge University, England. The rare isotope, 184Os, was discovered by A. O. C. Nier (Figure 2) in 1937 (4) using a new type of high resolution mass spectrometer at Harvard University, in Cambridge, Massachusetts, U.S.A., where he was carrying out a redetermination of the isotopic abundances of osmium. Nier's abundance measurements became the definitive values for osmium for over half a century, only being superseded in 1990 by the measurements of Völkening, Walczyk and Heumann (5). These latter results were immediately incorporated into the 1991 atomic weight table (6).
Of the naturally occurring isotopes, Viola, Roche and Minor suggested in 1974 (7) that 186Os was radioactive with a half-life of 2.0 × 1015 years, and this remains the presently accepted value (8). Mattauch's Rule (9) states that if two adjacent elements have nuclides of the same mass then at least one of them must be radioactive. In the case of the naturally occurring pair 187Re–187Os, the 187Re is radioactive with a half-life of 4.12 × 1010 years (8). Since 187Re undergoes beta decay to 187Os, then in a mixture of rhenium-platinum element ores, the over-abundance of 187Os with respect to that expected for the primordial abundance (the actual measurements are 187Re/186Os and 187Os/186Os) leads to an estimate of the excess 187Os, and from this is obtained a direct assessment of the age of the ores. The half-life of 186Os is so long that it can be considered to be “stable” for these measurements.
The use of this cosmochronometer was first suggested by Clayton in 1963 (10).
Artificial Osmium Isotopes
Prior to 1940 there appears to have been mention of only one radioactive osmium isotope: by Kurchatov et al. in 1935 (11) with a half-life of 40 hours. This now seems likely to have been 193Os.
In 1940 Zingg (12) correctly identified both 191Os with a 10 day half-life (modern value 15.4 days) and 193Os with a 30 hour half-life (the presently accepted value is 30.11 hours). However, immediately afterwards Seaborg and Friedlander (13) switched these around and there then followed a bizarre period of seven years in which all of the property measurements on 191Os were ascribed to 193Os, and vice versa, until the mistake was finally corrected in the 1948 edition of the “Table of Isotopes” (14).
The most prolific period for the discovery of radioactive osmium isotopes was the 1970s with fourteen ground states and one isomeric state being identified. The most discovered by any one person or group was four by Cabot et al. in 1977 (15) and 1978 (16).
The lightest osmium isotope, 162Os, still only appears to be an alpha emitter with no evidence of proton decay, so the drip line for osmium has not been reached, in contrast to 167Ir for iridium. The most likely reason for this is that with an even atomic number, osmium nuclides are likely to be more tightly bound than those for iridium, which has an odd atomic number, so it is not surprising that proton decay has not yet been seen from osmium isotopes.
In Table II, the same criteria for discovery are used as in the prior reviews on platinum and iridium (1). Notes to Table II, Some of the terms used for this review, and the decay modes are given in Tables III, IV and V. The half-lifes given in the tables are from the revised NUBASE database (8).
|Mass number||Half-life||Decay modes||Year of discovery*||Discoverers||Ref.||Notes|
|162||1.87 ms||α||1989||Hofmann et al.||17|
|163||5.5 ms||α, EC + β+?||1981||Hofmann et al.||18, 19||A|
|164||21 ms||α, EC + β+||1981||Hofmann et al.||18, 19|
|165||71 ms||α, EC + β+||1978||Cabot et al.||16||B|
|166||216 ms||α, EC + β+||1977||Cabot et al.||15|
|167||810 ms||α, EC + β+||1977||Cabot et al.||15|
|168||2.06 s||EC + β+, α||1977||Cabot et al.||15|
|169||3.46 s||EC + β+, α||1972||Toth et al.||21|
|170||7.46 s||EC + β+, α||1972||Toth et al.||22|
|171||8.3 s||EC + β+, α||1972||Toth et al.||22|
|172||19.2 s||EC + β+, α||1970||Borgreen and Hyde||23|
|173||22.4 s||EC + β+, α||1970||Borgreen and Hyde||23|
|174||44 s||EC + β+, α||1970||Borgreen and Hyde||23|
|175||1.4 min||EC + β+||1972||Berlovich et al.||24|
|176||3.6 min||EC + β+||1970||1: Arlt et al.||25|
|2: de Boer et al.||26|
|177||3.0 min||EC + β+||1970||Arlt||25|
|178||5.0 min||EC + β+||1968||Belyaev et al.||27|
|179||6.5 min||EC + β+||1968||Belyaev et al.||27|
|180||21.5 min||EC + β+||1965||1: Belyaev et al.||28, 29||C|
|2: Hofstetter and Daly||30, 31|
|181||1.75 h||EC + β+||1966||Hofstetter and Daly||30||D|
|181m||2.7 min||EC + β+||1966||Hofstetter and Daly||30||E|
|183||13.0 h||EC + β+||1950||Stover||37|
|183m||9.9 h||EC + β+, IT||1957||Foster, Hilborn and Yaffe||32|
|185||93.6 d||EC||1946||Goodman and Pool||38|
|186||2.0 × 1015y||α||1931||Aston||3||F|
|189m||5.8 h||IT||1958||Scharff-Goldhaber et al.||43||I|
|190m||9.9 min||IT||1955||Aten et al.||44|
|191m||13.10 h||IT||1952||Swan and Hill||45|
|192m||5.9 s||IT, β−||1973||Pakkenen and Heikkinen||48||K|
|196||34.9 min||β−||1976||Katcoff et al.||58, 59|
|A||163Os||Alpha energy only. The half-life was determined by Page et al. in 1995 (20).|
|B||165Os||Alpha energy only. The half-life was determined by Hofmann et al. (18, 19).|
|C||180Os||In 1957 Foster, Holborn and Yaffe (32) assigned a 23 min half-life activity to 181Os but according to Hofstetter and Daly (30) this now appears to have more likely been 180Os. In 1965 Bedrosyan et al. (33) identified a 23 min half-life activity but did not assign a mass number.|
|D||181Os||A 2.7 h half-life activity described by Surkov et al. in 1960 (34) and apparently associated with 181Os was not observed by Hofstetter and Daly (30). Balyaev et al. (29) observed a 2.5 h half-life activity which appeared to confirm the observations of Surkov et al.|
|E||181mOs||Hofstetter and Daly's claim to have identified this isomeric state was only tentative but was confirmed by Goudsmit in 1967 (35). Shortly before the observations of Hofstetter and Daly, Aten and Kapteyn (36) also identified a 2.8 min half-life activity but gave no mass assignment.|
|F||186Os||The half-life was measured by Viola, Roche and Minor in 1974 (7).|
|G||187Os||Chu in 1950 (39) and Greenlees and Kuo in 1956 (40) observed activities with half-lifes of 35 h and 39 h, respectively, which they suggested could be an isomer of 187Os. However such an activity was not observed by either Newton (41) or Merz (42).|
|H||188Os||A 26 d half-life activity suggested by Greenlees and Kuo (40) as being an isomer of 188Os was not observed by Merz (42).|
|I||189mOs||A 6 h half-life activity observed by Chu in 1950 (39) and a 7.2 h half-life activity observed by Greenlees and Kuo in 1950 (40) were both likely to have been 189mOs.|
|J||192Os||Fremlin and Walters (46) suggested that the isotope, although described as being “stable”, could be radioactive with a half-life exceeding 2.3 × 1014 y. Tretyak and Zdesenko (47) reassessed the data and suggested a revised value of greater than 9.8 × 1012 y which indicates that the suggestion of radioactivity is inconclusive.|
|K||192mOs||A 6 s half-life activity assigned to 192Re by Blachot, Monnand and Moussa in 1965 (49) was reassigned to 192mOs by Pakkenen and Heikkinen (48). Hermann et al. (50) almost certainly discovered 192mOs in 1970 but could not decide as to whether it was 192Re or 192mOs.|
|L||193Os||A 40 h half-life activity described by Kurchatov et al. in 1935 (11) was assigned to 193Os by the “Table of Isotopes” (51).|
|M||195Os||In 1957 Baró and Rey (53) and Rey and Baró (54) identified a 6.5 min half-life activity which they assigned to 195Os, but in 1974 Colle et al. (55) showed that this was the rubidium isotope 81Rb, so 195Os remains undiscovered. Takahashi, Yamada and Kondoh (57) estimated the half-life to be about 9 min.|
Table IVSome of the Terms Used for this Review
Table VDecay Modes
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Cosmochronology is the measurement of the age of stellar bodies by studying rock samples, via measurement of the amount of a daughter nuclide formed in situ with the long lived radioactive parent nuclide. If the daughter nuclide is not radioactive, then if N2 is the number of daughter atoms and N10 the number of parent atoms, N2 = N10 e-λ1t, where λ1 is the decay constant of the parent nuclide (equal to loge2/half life) and t is the amount of time since the daughter started to be formed. This can indicate the age of the rock containing the nuclides.
If the daughter is also radioactive the formula becomes:
N2 = λ1/ λ2 - λ1 . N10 (e- λ1t - e- λ2t)
where λ2 is the decay constant of the daughter product.
In practice the situation is complicated, with the possible formation of the daughter nuclide at the same time as the parent in the same supernova shell, so both could be present in the ore body after condensation (in which case an additional term N20 . e- λ2t is added to the right hand side of the second equation, where N20 is the original number of daughter atoms).
Further nuclear reactions or weathering of rock samples so parent or daughter are removed, may also occur.
Positive Ray Analysis
In a discharge tube with a perforated disc cathode, luminous rays emerge in straight lines through the perforations away from the anode.
In 1895, J. Perrin showed that these rays had positive charge. W. Wein, in 1898, after studying the deflection of the rays in electric and magnetic fields, showed they consisted of electrically charged atoms or molecules of the same composition as the gases present in the discharge tube. In 1907, J. J. Thomson called them "positive rays". He realised that if the deflections of gases in the tubes were measured in electric and magnetic fields, and compared with standard gases such as hydrogen and oxygen, then their atomic masses could be determined.
However, Thomson did not realise that the apparatus was sensitive enough to distinguish individual isotopes. Hence, he thought that the observation of a second isotope of neon, of mass 22, was a hydride of neon, NeH2, even though such a substance cannot exist. After the First World War, Aston in England and A. J. Dempster in the U.S.A. improved the sophistication of the positive ray apparatus and designed and built the first mass spectrographs.
John W. Arblaster is Chief Chemist working in metallurgical analysis at Coleshill Laboratories, in the West Midlands of England. He is interested in the history of science and in the evaluation of the thermodynamic and crystallographic properties of the elements.