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Production of Heavy Elements in Neutron Stars 354 Nature Vol. 263 September 23 1976 mismatch between substrate and over­ Production of heavy may well be terminated by neutron­ growth4. induced fission in the mass region The orientation relationship that we elements in neutron stars A ~ 280. It should, however, be pointed report results from a correlation of out that the exact point of this termin­ d-spacings from electron diffraction, and CHECHETKIN and Kowalski1 have pro­ ation is very uncertain because of an the more accurate d-spacings from X-ray posed that the production of heavy extrapolation of surface-asymmetry and diffraction; such a correlation cannot be elements in nature occurs by the ejection closed-shell effects from the region of obtained if the orientation relationship of matter from a neutron star surface. known nuclei and because of the use of proposed by Mackay1 is assumed. Nor is Their suggestion relies on the results of the Strutinsky procedure. The termination 2 3 there evidence to postulate a relaxation in Chechetkin and Bisnovatyi-Kogan • , who point may be somewhat higher (A ~ 350), the Laue condition, since scanning find such matter to be composed before consistent with the possible discovery of electron microscopy (see Fig. 3, ref. 3) ejection of 'jumbo' nuclei (Z ~ 160; superheavy elements in pleochroic halo indicates that the green rust crystals A ~ 640). These results were derived inclusions 8 and their production in a 9 are "" 0.1 j.lm in thickness or greater. The using a highly uncertain extrapolation of conventional r process • In spite of the elongation of diffraction spots is usually binding energies into the extreme neutron­ uncertainties of the calculations of fission 6 7 associated with microcrystals which have rich region and in the absence of fission. barriers • , we believe that the existence of critical dimensions of much less than 0.1 Their justification of the neglect of fission nuclei as large as A = 640, N = 480, j.lm. Electron diffraction suggests that is based on a necessarily very crude Z = 160 is definitely ruled out. We the green rust platelets are well-crystal­ estimate of fission lifetimes. should also mention that similarly large lised. An inspection of the rhombohedral The nuclear stability against spon­ nuclei had been predicted in equilibrium 10 11 unit cell shows that a flat crystal with a taneous fission is the result of a barrier conditions in neutron star matter • • (5 6 6) habit plane may have hexagonal in the potential energy curve of a nucleus This, however, has also been con­ geometry. as a function of deformation. In the clusively shown to be incorrect12 - 14• In Turning to the point raised by Brindley framework of the macroscopic-micro­ conclusion, the nuclear basis of Chechet­ 4 5 •5, and Bish : in our experiments green rust scopic method the potential energy kin and Kowalski's suggestion for the is grown in closed vessels by the corrosion can be divided into two parts: (1) a production of superheavy elements is of cast iron in solutions which are deoxy­ macroscopic contribution that takes into unfounded although it may well be that genated by purging with nitrogen. Thus account the smooth variation of the these elements can be produced in an the level of dissolved carbon dioxide is energy as a function of nucleon number r process occurring in the surface material very low in non-carbonate solutions. For and deformation, and (2) a microscopic of neutron stars. Studies along these lines example, green rust and magnetite were contribution that includes shell effects are under way. formed by the corrosion of cast iron at which are significant only near the This work was supported in part by the 50 oc in distilled, deionised water con­ magic numbers for neutrons or protons. NSF. We acknowledge the hospitality of taining 0.4 p.p.m. of oxygen. Chemical The macroscopic energy is calculated the Aspen Center for Physics. analysis of the water gave the following using the liquid-drop or droplet model results: before corrosion, pH= 7.4, alka­ of the nucleus, whereas the microscopic J.- ROBERT BUCHLER linity= 5.0 p.p.m.; after corrosion, pH= contribution is evaluated from the energy 6.6, alkalinity= 5.5 p.p.m.; alkalinity= spectrum associated with an appropriate Department of Physics and Astronomy, 2 potential well. [C03 -] + [HC0 3 -] + [C0 ] + [OH-]. University of Florida, 2 As the proton number and the neutron Since, for H 2 C0 3 at 50 oc, K 1 = 5.19 x Gainesville, Florida 32611 7 excess of the nucleus are increased, the I0- and K 2 = 6.73 X w-u (ref. 6) the bicarbonate ion is the predominant spe­ stability against fission resulting from the WILLIAM A. FOWLER cies in this pH range, and the carbonate macroscopic energy is reduced both by MICHAEL J. NEWMAN ion concentration is extremely low. the increased repulsion of the Coulomb In another experiment, green rust pro­ force with the addition of protons and by the decreased surface tension of the W. K. Kellogg Radiation Laboratory, duced in sodium carbonate solution California Institute of Technology, nucleus with the addition of neutrons. It [COa"-] = 200 p.p.m. was examined Pasadena, California 91125 using infrared spectroscopy. The presence is the rapid decrease of the macroscopic of the hydroxyl group was clearly shown energy as a function of deformation that MICHAEL HOWARD but the carbonate ion was not detected. In causes nuclei beyond the actinide region view of the comments of Brindley and (A ~ 260) to be unstable with respect to 5 Department ofPhysics, Bish , a more extensive infrared spectro­ spontaneous fission. Beyond the actinide region, only strong single-particle effects Northwestern University, scopic investigation is being undertaken, Evanston, lllinois 6020 I but at present, our experiments do not near neutron or proton closed shells (as support the view that the presence of the for the case of the superheavy island A ~ 298, N ~ 184, Z ~ 114) can offer 1 Chechetkin, V. M., and Kowalski. M., Nature, 249, carbonate ion is particularly significant in 643-644 (1976). the formation of green rusts by the corro­ stability against spontaneous fission. In 2 Chechetkin, V. M., Soviet A.<tr., 15, 4S-SO (1971). l Bisnovatyi-Kogan, G. S., and Chechctkin, V. M., sion of cast iron. the immediate region surrounding the A.<trophys. Spare Sri., 26, 2S-46 (I 974). superheavy island, the fission barriers of • Nix, J. R., A. Rev. nud. Sci., 22, 65-120 (1972). These and other points relating to pro­ 'MoHer, P., and Nix, J. R., Pror. Third TAEA Symp. nuclei are virtually non-existent". Physics ancl Chemistry of Fisdon, Roc:ht~.tt~r. cesses involved in corrosion of cast iron 1971, I, 103 (IAEA, Vienna, 1974). will be developed elsewhere. Furthermore, in a neutron capture the • Howard, W. M., and Nix, J. R., ibid., 145. neutron separation energy in the com­ 7 Bengtsson, R., Boleu, R., Larsson, S. E., and Randrup, J., Proc. Nobel Symposium on Super­ pound nucleus plus the kinetic energy of heavy Elements, Ron11ehy, Sweden,J971,l0A(l914). (edit. by Nilsson S. G., and Nilsson, N. R.) the incoming neutron easily lead to (Aiunquist and Wikse11, 1974). subsequent fission. Consequently, any • Gentry, R. V., eta/., Phy.<. Rev. Lett., 37, II (1976). 9 Schramm, D. N., and Fowler, W. A., Nature, 231, I Mackay, A. L., Nature, 263, 3S3 (1976). 2 process of successive neutron captures 103-106 (1971). Bernal, I. D., Schwelur Arch. angew. Wiss. Tech. IO Bethe, H. A., Borner, G., and Sato, K., Astr. 26 69 (1960). • will be terminated by neutron-induced A.•trophy.,., 7, 279-288 (1970). l McGill, I. R., McEnaney, B., and Smith, D. C., II Baym, G., Bethe, H. A., and Pethick, C. J., Nuc/. Nature, 259, 200-201 (1976). fission before significant spontaneous 6 7 Phy.,., A 175, 225-271 (1971). • Pashley, D. W ., Epitaxial growth, Part A (edit. by fission can occur. Recent calculations • of 12 Buchler, J.-R., and Barkat, Z. K., Phys. Rev. Lett., )~~~at thews, J. W .), 2 (Academic, London, 197S). 27, 48-S1 (1971). 5 Brindley, G. W., and Bish, D. L., Nature, 263, 3S3 fission barrier heights and neutron sepa­ 13 Barkat, Z. K., Buchler, J.-R., and Ingber, L., (1976). 6 ration energies in the r-process region A.•trophy.•. J., 176,723-738 (1972). Panons, R., Handbook of Electrochemical Constants, 14 Negele, J. W., and Vautherin, D., Nucl. Phys., 47 (Butterworths, London,19S9). indicate that the astrophysical r process A207. 298-320 (1973). © 1976 Nature Publishing Group.
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