Lunar and Planetary XXXVI (2005) 1739.pdf

CONSTRAINTS ON THE ROLE OF -247 AS A POSSIBLE SOURCE OF FISSION IN THE EARLY . Stephen Edwards, Chris Ballentine and Jamie Gilmour, SEAES, University of Manchester, Manchester, UK

Introduction: Curium is the heaviest known ele- expected value. As a working hypothesis we there- ment with possessing a geologically fore adopt a value for 244Pu /247Cm of ~5 for the significant half life. 247Cm has an alpha half life of early solar system. 16 Ma, decaying to via 235U. 248 has a Another uncertainty is the difference in 247Cm half life of 340 kyr (=91.6%, SF=8.4%). and 244Pu xenon fission spectra. Examination of Xenon isotopes are important indicators of at- fission isotope yield in Hyde [9] indicate that much mospheric, crustal and planetary evolution. They of the change in fission yield with increasing atomic are derived in part from the initial nebular inven- occurs in the lighter peak. The mean fragment tory, fractionated by accretion and nebular hydro- mass in this peak steadily increases with increasing dynamic processes. The other major source is de- . However, the heavy fragment peak, cay of unstable isotopes, notably 129I ( decay) containing xenon isotopes shows far smaller and 244Pu and 238U (). As 247Cm changes. As a result it is likely that the 136Xe fission has a long alpha half life, it is possible that it was a yield from 247Cm should be similar to that from significant source of fission xenon in the early solar 244Pu, though the associated xenon isotopic signa- system. However, a review of the literature and the ture would be significantly different. web e.g.[1,2,3] failed to produce a fission branching Limitations on current experimental techniques ratio for 247Cm. suggest that under favourable conditions as little as Our aim is to seek abundance and spontaneous 10 percent non 244Pu fission xenon can be detected fission branching ratio constraints at which 247Cm in a sample of 244Pu fission xenon. We thus adopt a would be a significant xenon source. 10% contribution of 247Cm fission xenon in a suit- Abundance and Branching Ratio: 244Pu was able sample of early solar system material as a de- the most important recognized source of fission xe- tection limit. non in the early solar system. The 247Cm abun- Differences in geochemical behaviour might also dance/spontaneous half life constraints will there- contribute to enrichment or depletion of the Cm/Pu fore be normalized to 244Pu. 244Pu has an alpha half ratio. However, both elements have highly refrac- life of 80 Ma. Its spontaneous fission branching tory and, in early solar system high tempera- ratio of 0.12% thus corresponds to a half life of 67 ture inclusions, are likely to show similar levels of Ga. fractionation [10]. Fractionation might occur in Since 244Pu and 247Cm are both r-process ele- lower temperature, more hydrous systems. ments the initial solar 244Pu /247Cm ratio is deter- Finally the difference in relative alpha half life mined by the details of r-process production and the between 247Cm and 244Pu must be considered. The interval of free decay between synthesis in a super- alpha half life of 244Pu is approximately 5 times that nova and solar system formation. Goriely and Ar- of 247Cm. This results in additional factor of 5 for 136 136 nould [4] estimated r process heavy synthe- production of XePu relative to XeCm. sis. Their values give 244Pu /247Cm ratios of 0.64 to Using the above constraints on initial 244 247 136 136 49.3, with a mean of 5. Friedrich et al. [5] consid- Pu/ Cm and XeCm/ XePu the maximum spon- ered both and to esti- taneous fission half life of 247Cm at which curium mate a steady state 247Cm abundance relative to 235U derived fission xenon will be detectable is ~25 Ga. of 0.012 and assume that incorporation into the so- This corresponds to a minimum branching ratio of lar nebula occured within one 247Cm halflife, giving ~6.5x10-4. Since an experimental spontaneous fis- a 247Cm/235U ratio of 0.005. Longer assimilation sion value for 247Cm is not available, we now exam- times would lead to more pronounced 247Cm deple- inine the known fission half lives of adjacent heavy tion. . Since the initial 244Pu/238U ratio is estimated to We note that the log of half life of the longest be 0.004-0.008 [6,7] and 235U/238U = 0.23 this isotope at each even atomic mass exhibits a linear yields 244Pu/247Cm ~3.5-7. The maximum 247Cm dependence on atomic mass e.g.[3]. However, odd abundance has been experimentally constrained by atomic mass isotopes must be considered separately 235U/238U variations in meteoric material [8]. Adopt- due to additional stabilizing, specialization ener- ing their maximum 247Cm/238U ratio of 0.0015 gies. Fig. 1 shows a similar plot of log spontaneous to 244Pu/247Cm = 2.7-5.4, similar to the theoretically fission half life against atomic mass for the most stable odd numbered actinide isotopes. The rela- Lunar and Planetary Science XXXVI (2005) 1739.pdf

tionship is reasonably linear and allows an interpo- References: [1] The Berkeley Laboratory Isotopes Pro- lation of the spontaneous fission half life of 247Cm jects ie.lbl.gov /education/isotopes.htm [2] Seaborg, at around 26 Ga subject to a range of 420 Ma – G.T., Loveland, W. D. (1990) The elements beyond Ura- 3500 Ga. nium Wiley-Interscience [3] Seaborg, G.T., Loveland, The New Alternatively, we can examine the neigbouring W.D. (2000) in Hall, N. ed. Cam- bridge University Press [4] Arnould, S. and Goriely. S. isotopes. In this case the 3 adjacent isotopes 245Cm, 249 249 (2001) and Astrophysics 379, 1113-1122 [5] Cf and Fm are used, and the logs of their fis- Friedrich, J.M., Ott, U., Lugmair, G.W. (2004) revisiting sion half lives are averaged. This gives an estimated extraterrestrial U isotope ratios. Lunar and planetary half life of 57 Ga. science XXXV [6] Crozaz, G. et al. (1989) Planet Discussion: For curium to have a detectable Sci Lett. 93, 157-169 [7] Turner, G. et al, (2004) Science impact on early solar system fission xenon a maxi- 306, 89-91 [8] Chen, J. H. and Wasserburg, G. J. (1981) mum fission half life of the order of 25 Ga has been Earth Planet. Sci. Lett. 52, 1-15. [9] Hyde, E.K. (1964) estimated. The nuclear properties of the heavy elements vols 1-3 The 16 Ma half life of curium combined with the Dover [10] Boynton, W.V. (1978), Fractionation in the solar nebula Earth Planet. Sci. Lett. 40, 63-70 [11] Don- low frequency of supernovae means that the early nelly, J. et al., (2001) The abundances of actinides in solar system Pu/Cm ratio is constrained to lie within cosmic as clues to origin. Proceed- a wide range of possible values. It also limits the ings of The ICRC 1715 [12] Wallner, C. et al. (2003) 247 possibility of estimating the presolar Cm to produced and anthropogeic 244Pu in deep sea 244Pu ratio using present day Cm/Pu ratios from encrustations. New Astronomy Reviews xx heavy ions in cosmic rays [11], or in manganese Elsevier. nodules in which recent supernova 244Pu has been recorded [12]. Given these caveats, the 25 Ga half life limit for detectable 247Cm fission xenon overlaps with ap- proximately 50 percent of the estimated range of fission half life for 247Cm. The two major unknowns are the initial Pu to Cm ratio in the early solar system and the fission half life of 247Cm. Of these the latter is amenable to experimental determination, though this may be complicated by the lack of low molecular weight, easily seperable, volatile curium compounds. An experimental determination of the Xe fission spectrum would also be valuable, should the spon- taneous fission half life prove to be low enough for 247Cm to have been a major source of fission Xe in the early solar system.


s 18 r y

- 16

e f

i 14 l


l 12 a h

10 n o

i 8 s s

i 6 f

g 4 o

L 2 0 235 240 245 250 255 260 Atomic mass (odd numbers)

Figure 1 shows the relationship between the atomic mass of odd numbered isotopes and spontaneous fission half life. A simple linear interpolation has been performed. The plot is equivalent to a line passed though the peaks of each element's spontaneous fission half life half life curve. The expected position of 247Cm is shown by the grey rectangle. The upper and lower dotted lines rep- resent the upper and lower fission half life limits (420 Ma – 3500 Ga)