Reexamining the half-lives of 195Os and 195Ir
M. Birch, J. Flegenheimer, Z. Schaedig, B. Singh, M. Thoennessen
RReexamining the half-lives of
Os and Ir M. Birch, ∗ J. Flegenheimer, Z. Schaedig, B. Singh, and M. Thoennessen
3, 4 Department of Physics and Astronomy, McMaster University, Hamilton, Canada fl[email protected], Buenos Aires, Argentina National Superconducting Cyclotron Laboratory, Michigan State University, East Lansing, MI 48824, USA Department of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA
Currently the half-life of
Os is listed as unknown in most databases because the value of theonly available measurement had been reassigned. We argue that the original assignment is correctand re-evaluate the half-life of
Os to be 6.5(11) min, consistent with the original measurement.We also suggest to reassign the half-life of
Ir to 2.29(17) h.
Basic properties of neutron-rich nuclei along theN = 126 isotones are important for the astrophysical r-process (see for example [1]). However, below the doublymagic stable nucleus
Pb they are very difficult to pro-duce. While
Tl [2] and
Hg [3] have been known fora long time and the first half-life measurement of
Auwas reported in 1994 [4], even lighter isotones becameaccessible only recently. The discoveries of
Pt,
Ir,
Os as well as a few additional isotopes beyond N = 126were made possible by the development of improved sep-aration techniques at the FRS fragment separator at GSI[5–8].The half-life of one specific N = 119 nucleus,
Os,which one would expect to be known, is still contro-versial. While the Table of Isotopes lists a half-life of6.5 min [9], the majority of nuclear data bases and eval-uations [10–16] do not accept this value and quote onlyan approximate theoretical value of ∼ Ir is also not well established. Thecurrent ENSDF data evaluation [14] recommends a valueof 2.5(2) h which corresponds to an unweigthed mean oftwo measurements which do not agree with each otherwithin in the quoted uncertainties [18, 19].Rey and Baro first deduced a half-life of 6.5 min for
Os from the reaction
Pt(n, α ) and identified the iso-tope from the decay of the known daughter nucleus Ir[20–22]. Although recently two high-spin isomeric states,a short-lived state of 34 ns [23–25] and a long-lived stateof > Os ground state.The non-acceptance of the half-life measurement byRey and Baro is based on the apparent reassignmentof the
Ir daughter nucleus in a 1974 unpublished an-nual laboratory report by Colle et al. : “
Unfortunately,the then-existing assignment for
Ir has subsequentlybeen identified as Rb, arising from reactions inducedin target impurities. As a result, the present assignmentof
Os will not withstand careful scrutiny ” [27]. Thetimeline in this argument by Colle et al. is incorrect. Atthe time of the Rey and Baro discovery of
Os the ac-cepted half-life for
Ir was 140 min [28, 29]. A half-life ∗ [email protected] of 2.3 h was also reported in 1961 [30] from measure-ment of beta and gamma activity. Only one year later,in 1962, was this value replaced by Claflin et al. whodetermined a half-life of 4.2 h from the ( α ,p) reactionon a supposedly highly enriched Os target [31]. Thiswas the measurement that was subsequently questionedby Hoffstetter and Daly who demonstrated that the en-riched osmium target could have been contaminated byother elements and the observed half-life of 4.2 h actuallyresulted from either Br( α ,2n) or Br( α ,4n) reactionsand thus corresponded to Rb [18]. In addition, themeasurement by Rey and Baro could not have sufferedfrom the same contamination problem as the experimentby Claflin et al. because they did not use α -induced re-actions on enriched osmium targets but (n, α ) reactionson high-purity, natural platinum.Thus we believe that Rey and Baro indeed observed thedecay of Os. In order to extract the half-life of
OsRey and Baro included not only the growth and decayof the daughter
Ir but also contributions from
Os.Since the presently adopted half-lives for these isotopesdiffer from the values that Rey and Baro used in theirfit [14], we refitted their data as presented in Figure 2 ofReference [20]. The fit contained three components: thedecay of
Os, the growth and decay of
Ir, and thedecay of
Os.For the half-life of
Os the most recent value of29.830(18) h by Krane [32] was used. It should be men-tioned that this value differs from the currently acceptedvalue of 30.11(1) h [14, 33].As mentioned earlier, the currently adopted half-lifeof
Ir was deduced as the unweigthed average of twoindependent measurements: a 2.8(1) h half-life reportedby Hofstetter and Daly in 1968 [18] and a 2.3(2) half-life measured by Jansen, Pauw, and Toeset a few monthslater [19]. The first value was obtained from an analysisof the 99 keV γ -ray from the decay of the first excitedstate in Pt daughter, assigning this γ -ray only to theground state activity of Ir. However, Jansen et al. demonstrated that this state is also populated by thedecay of the 3.8(2) h isomeric state in
Ir [19, 34]. Thusthe value quoted by Hoffstetter and Daly is likely toohigh and should be discarded. Jansen et al. took thecontributions from both states into account and arrivedat the value of 2.3(2) h. This value was consistent with a r X i v : . [ nu c l - e x ] D ec A c t i v i t y -30-15015 0 1 2 3 4 R e s i dua l Time (h) Figure 1. (Color online) Decay curve and fit-residues for thedecay of
Os. The top panel shows our fit (solid red line) tothe data of Figure 2 in Rey and Baro’s work [20] (solid blackcircles) and the residuals of our fit are shown in the bottompanel. the first measurements of 140 min in the 1950’s [28, 29]which were known to Rey and Baro at the time of theirmeasurement of
Os.Present evaluations [10, 13–16] do not consider thatRey and Baro also independently measured the half-lifeof
Ir and it presented in the same papers reporting thediscovery of
Os [20, 22]. They deduced a half-life of2.2 h by chemically extracting iridium fractions followingthe decay of its parent
Os. This decay most probablypopulated only the 3/2 + ground state of Ir, ratherthan the 11/2 − isomer, since the ground state spin andparity of Os is expected to be 3/2 − [26]. An isomerof half-life > Os [26]with suggested spin-parity of 13/2 + is not expected tobe populated in the Pt(n, α ) reaction used by Rayand Baro [20].We digitized the data of Figure 2 of Ref. [22] displayingthe decay curve of Ir and deduced a value of 2.29(17) hfrom a least-squares fit. A similar analysis of Figure 3 of Ref. [22] gives a more precise half-life of 2.17(7) h,however, because of possible contamination from otherIr isotopes in this decay curve, we prefer the data fromFigure 2 of Ref. [22]. Hence, we recommend the value of2.29(17) h. We believe this represents the best and mostreliable half-life of the
Ir ground state and we haveused this value in the fit of
Os. This value agrees wellwith the result 2.3(2) h from [19], not with 2.8(1) h from[18].Therefore, there remain four free parameters for the fitof the
Os decay curve: the half-life of
Os, and theinitial amounts of
Os,
Ir, and
Os. These fourparameters were fitted by a least-squares method, wherethe minimum sum of squared residuals was determinedby differential evolution. The uncertainties in the fittedparameters were estimated by a Monte Carlo method inwhich many fits were performed on data sets generatedfrom sampling within the uncertainties of the data. Be-cause the original paper did not give uncertainties weassigned the statistical uncertainty given by √ N alongwith an uncertainty associated with the digitization ofthe plot. The sample standard deviations of the set offitted results from the simulated data sets were taken tobe the uncertainties in the best fit parameters. The re-sults from this procedure are shown in Figure 1. Thededuced half-life for Os is 6.5(11) min, in agreementwith 6.5 min value quoted in the original Rey and Baropapers. In addition, we conclude that the half-life of the
Ir ground state, based upon Rey and Baro’s work, beaccepted as 2.29(17) h in contrast to 2.5(2) h quoted inthe evaluated databases [14]. Furthermore, a new mea-surement of the
Os ground state half-life using state-of-the-art techniques is highly desirable.We would like to thank W. B. Walters for sending us acopy of the article “Decay of
Os and Search for
Os”in the 1974 University of Maryland Cyclotron LaboratoryProgress Report [27] and E. Browne for reading and eval-uating the original article “Un Nuevo Isotopo Del Osmio”by Rey and Baro [20] and also for useful comments onour manuscript. This work was in part supported by theNational Science Foundation under grant No. PHY11-02511 and by Office of Science, Office of Nuclear Physicsof the U.S. Department of Energy. [1] J. J. Cowan and F.-K. Thielemann, Physics Today (2004) Issue 10, p. 7[2] O. Hahn and L. Meitner, Phys. Z. (1908) 649[3] M. J. Nurmia, P. Kauranen, M. Karras, A. Siivola, A.Isola, G. Graeffe and A. Lyyjynen, Nature (1961)427[4] Ch. Wennemann, W.-D. Schmidt-Ott, T. Hild, K.Krumbholz, V. Kunze, F. Meissner, H. Keller, R. Kirch-ner, and E. Roeckl, Z. Phys. A 3¯47 (1994) 185[5] S. J. Steer, Zs. Podolyak, S. Pietri, M. Gorska, P. H. Re-gan, D. Rudolph, E. Werner-Malento, A. B. Garnswor-thy, R. Hoischen, J. Gerl, et al. , Phys. Rev. C (2008) 061302[6] H. Alvarez-Pol, J. Benlliure, E. Casarejos, L. Audouin,D. Cortina-Gil, T. Enqvist, B. Fernandez-Dominguez, A.R. Junghans, B. Jurado, P. Napolitani, J. Pereira, F.Rejmund, K.-H. Schmidt, O. Yordanov, Phys. Rev. C (2010) 041602[7] A. I. Morales, J. Benlliure, J. Agramunt, A. Algora,N. Alkhomashi, H. Alvarez-Pol, P. Boutachkov, A. M.Bruce, L. S. Caceres, E. Casarejos, et al. , Phys. Rev. C (2011) 011601[8] J. Kurcewicz, F. Farinon, H. Geissel, S. Pietri, C. Nocif-oro, A. Prochazka, H. Weick, J. S. Winfield, A. Estrade, P. R. P. Allegro, et al. , Phys. Lett. B (2012) 371[9] Table of Isotopes, seventh and eight editions (1978, 1996)and Table of Radioactive Isotopes (1986), John Wiley &Sons, Inc.[10] C. Zhou, Nucl. Data Sheets (1999) 645[11] J. W. Arblaster, Platinum Metals Rev. (2004) 173[12] R. Robinson and M. Thoennessen, At. Data Nucl. DataTables (2012) 911[13] G. Audi, F. G. Kondev, M. Wang, B. Pfeiffer, X. Sun, J.Blachot and M. MacCormick, Chinese Phys. C (1973) 101[18] K. J. Hofstetter and P. J. Daly, Nucl. Phys. A (1968)382[19] J. F. W. Jansen, H. Pauw, and C. J. Toeset, Nucl. Phys.A (1968) 321[20] P. Rey and G. B. Baro, Publ. Com. Nac. Ener. Atom.Ser. Quim. (1957), No. 10, 115[21] G. B. Baro and P. Rey, Z. Naturforsch. (1957) 520[22] P. Rey and G. B. Baro, Proc. Second United NationsInt. Conf. on the Peaceful uses of Atomic Energy, Geneva(1958), Volume 14, P/1570 (1958)[23] J. J. Valiente-Dobon, P. H. Regan, C. Wheldon, C. Y.Wu, N. Yoshinaga, K. Higashiyama, J. F. Smith, D. Cline, R. S. Chakrawarthy, R. Chapman, et al. , Phys.Rev. C (2004) 024316[24] M. Caamano, P. M. Walker, P. H. Regan, M. Pfutzner,Zs. Podolyak, J. Gerl, M. Hellstrom, P. Mayet, M. N.Mineva, A. Aprahamian, et al. , Eur. Phys. J. A (2005)201[25] S. J. Steer, Zs. Podolyak, S. Pietri, M. Gorska, H. Grawe,K. H. Maier, P. H. Regan, D. Rudolph, A. B. Garnswor-thy, R. Hoischen, et al. , Phys. Rev. C (2011) 044313[26] M. W. Reed, P. M. Walker, I. J. Cullen, Yu. A. Litvinov,D. Shubina, G. D. Dracoulis, K. Blaum, F. Bosch, C.Brandau, J. J. Carroll, et al. , Phys. Rev. C (2012)054321[27] R. Colle, P. Gallagher and W. B. Walters, Univ. Mary-land Cyclotron Lab., Progr. Rep. 1974, p. 119[28] D. Christian, R. F. Mitchell, and D. S. Martin Jr., Phys.Rev. (1952) 946[29] F. D. S. Butement and A. J. Poe, Phil. Mag. (1954)31[30] S. Homma, T. Kuroyanagi and H. Morinaga, J. Phys.Soc. (Japan) (1961) 841[31] A. B. Claflin, R. T. White and M. L. Pool, Nucl. Phys. (1962) 652[32] K. S. Krane, Phys. Rev. C (2012) 044319[33] M. S. Antony, D. Oster, and A. Hachem, J. Radioanal.Nucl. Chem. (1992) 303[34] J. F. W. Jansen, A. Faas, W. J. B. Winter, Z. Phys.261