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Analogies between nuclear clusters and fullerene cages
Nuclear Physics A738 (2004) 459462
ELSFVIER
www.elsevier.comllocatelnpe
Analogies between nuclear clusters and fullerene cages L. Qu, Y. L. Zhao*, Z. L. Chen, X. F. Gao, Z. F. Chai, J. Zhang, G. M Xing Institute of High Energy Physics, The Chinese Academy of Sciences. P. 0.Box 918, Beijing 100039, China
Similarities in behaviors of atomic nuclei and C60 fullerene cages are investigated to explore evidcnces for a new concept that if a superheavy nucleus can bc a cage structurc similar to aC6" fullercne. It is found that neutron-induced fusion reactions of C60 and/or C70 cages yield stable heavy molecular clusters similar to nuclear molccular clusters surviving in fission process. The experimental results indicate analogies in geometric shapes and spatial alignment of the nuclear molecular clustcr and atomic molecular cluster.
1. INTRODUCTION The atomic cluster C60 [l] consisting of 60 carbon atoms (with 30 double bindings) possesses a spherical structure with an innermost hollow space. Recently. Greincr et al. brought forward a new idea: if replacing each carbon atom of C60 (Fig. l(a)) by one alpha particle, we can obtain a Z=120 superheavy nucleus (Fig. l(b) [2]) consisting of 60 helium nuclci with additional neutrons being as bindings in between helium nuclei. Here, the 30 double bindings in the C60 cage (atomic cluster) are replaced by neutron bindings in thc Z=120 nuclcus (nuclear cluster). Theoretical calculations with the framework of Skyrme-Hatree-Fock and relativistic mean-field models showed a strong depression in thc central density [3], implying an innermost hollow structure of charge distribution of the Z=120, N=184 nucleus. This arises an important question: if the conventional rcgularitics about the nuclear density do no longer work in the superheavy nuclci. In a framework of the sophisticated structure of a nucleus, a superheavy nucleus is at the limit of Coulomb instability and hence unstable against spontaneous fission. The prescnt knowledge making a superheavy nucleus be stable is the large shcll-correction energy. which creates an additional binding and a fission bamcr of up to 8 MeV. Experimentally, Hamilton and Mutterer et al. have discovered an existence of nuclear molecules in the spontaneous fission of 252Cf[4-61. Meanwhile, nuclear clusters with the closed shell have been proved to play key roles in fusion processes to form a superheavy nucleus [7-91. Thus, in addition to thc stability arising from shell-correction energy, thc new stability may arise from such geometric structure as the proposed fullerene-like innermost hollow sphere, which is propitious to decrcase Coulomb instability and hence enhance the stability of superheavy nuclei of a largc * Corresponding
0375-9474/$
~
author, Email addrcss zhi)OTiiiliaun’iiihcn ac cn
see front matter 0 2004 Published by Elsevier B.V
doi: 10.1016/j.nuclphysa.2004.04.086
460
L. Qu et al. /Nuclear. Physics A738 (2004) 459-462
number of protons.
Figure 1. (a) Thc structure of C60 fullerene, (b) a schematic illustration of the spcculatcd hollow structure for Z=120 and N=184 nucleus (see the text for details). To reach the definite conclusion about the validity of above speculation, exploring experimental evidences is of most important. Experimentally, similarity in behavior of fullerene cages and atomic nuclei may be one of observables. In this paper we discuss the similarity between atomic nuclei and atomic clusters based on the new experimental findings: neutron-induced fusion reactions of c60 and/or C70 with carbon atoms yielded new products of hcavier clusters C l ~ and l C131. They wcre identified by laser ionization time-of-flight mass spectral techniques, and the geometric shapc of the new products was characterized by in situ scanning tunncling microscopy (STM) method, and calculated with ab initio Hartree-Fock method. 2. EXPERIMENTAL TECHNIQUE The c 6 0 and c 7 0 were produced by Kratschmer-Huffman method [ I O J and purified using the method similar to the previous works [ 1 I]. They were prepared to be targets for reactions with neutron beams. Two targets, 30 mg c60 and 30 mg [c60+c701 mixture (wt/wt=l: 1) were separately irradiated for 2 hours by neutron beams provided from Heavy Water Research Reactor (HWRR), China Institute of Atomic Energy (CIAE). The beam composition was: fast neutrons of - 5 . 4 ~ 1 0 cm-s~ and thermal neutrons of - 5 . 4 ~ 1 0 cm-s-l, ~ with a fast-to-thermal neutron flux-ratio of about 10 %. After reactions, the samples were measured for 3000 SCC using a high-purity germanium detector (HPGe) to check the radioactivity. Then, the separation and isolation of the reaction products were performed using a two-step high performance liquid chromatography coupling with a preparation Buckyprep column. The products were identified by laser ionization time-of-flight mass spectrometer. The shape and structure of the products were measured by the in situ high-resolution STM method. 3. RESULTS AND DISCUSSION After separation and purification, the products from neutron-irradiated C60 and [c6O+c7()] were mcasured by the time-of-flight mass spectral method. New mass peaks appearing at m/z=1453.1 (Fig. 2(a)) from neutron-irradiated C ~ and O at m/z=1573.4 (Fig. 2(b)) from neutron-irradiated [C60+C70] mixture were observed (the mass resolution, FWHM/m, was less than 1110000). Theoretical calculations indicate that the masses of m/z=1453.3 and 1573.4 are ncw carbon clusters of Cl2l and C131, respectively. Also the observed isomeric distributions of CiZ1and C131 were completely consistent with the thcorctical predictions. The geometric shapes of the new products, obtained from high-resolution STM images. are shown in figure 3(a-I) for Cl21 and 3-(b-I) for C131 molecules. Two slightly distorted C60 cages or C60 and C70 cages are connected with a neck to form a dumbbell-like shape.
L. Qu et al./Nuclear Physics A738 (2004) 459-462
1400
1 6 0 0 mlz
1400
46 1
1600 mlz
Fig. 2, The measured mass spectra for the Clzl (a) and C131 (b) formed from fusion reactlons of [c(j()+C+c60]and [c60+c+c70]respectively. , The inset shows the geometric structure of the most stable isomer of corresponding clusters. They were obtained from theoretical calculations with the PM3 Hamiltonian and ab initio Hartree-Fock calculations. One knows that the dumbbell-like shape of a nucleus survives both in the transition from a single nuclear system to two fragmental nuclei in fission and in the evolution from a binary system to a monosystem in fbsion. The theoretical calculations by Moller et al. revealed dumbbcll-like nuclear shapes from the ground state through saddlc points to the final configurations of separated fission fragments [12]. The nuclear shape in 3(a-II) is the mass-symmetric configuration leading to the symmetric fission-fragment with higher kinetic energies, while that in 3(b-II) represents the mass-asymmetric configuration leading to the asymmetric fission-fragment with lower kinetic energies[ 13,141. The similarity in geometrical shapes of atomic clusters and atomic nuclei is seen. The illustrations in 3(a-111) and 3(b-III) exhibit the outline of binding electrons in carbon clusters of the dumbbell-like fullerenes.
Fig. 3, (a-I) and (b-I): the STM images from an angle showing the shape of molecular cluster C ~ O = C = Cand ~ O the C7O=C=C60 fullerenes; (a-11) and (b-11): the shapes of a heavy nucleus in nuclear fission process [12]; (a-111) and (b-111): the outline of the binding electrons in the corresponding carbon molecular clusters shown in (a-I) and (b-1). Recently, a high-efficient measurement of angular correlations between neutrons, light
462
L. Qu et al. /Miclear Physics A738 (2004) 459-462
Fharged particles. and main fission fragments revealed intermediate light charged particles, ’He, ’He and *Li p.61 in the 252Cf fission. A triple nuclear molecule [146Ba-’oBc-’6Sr] consisting of a small cluster "Be in the middle with IaBa and %Sr in both sides of "Be [4,15] \vas observed by the y-y-y coincidence measurement. In the rearrangement process of fullcrcne cages, the molecular cluster [c6O-C1-c701 consisting of atomic clusters c6o and c70 with a carbon atom locating in between them was observed. The similarity is seen in the spatial alignment of the nuclear molecular cluster and atomic molecular cluster. Note that the experimental techniques [4-6,15,16] might be applicable mcans to search evidences for nuclear molecular clusters in the region of heavy and superhcavy nuclei. In summary, similarities between the nuclear cluster and the atomic cluster C6o fullerene cages were studied. Neutron-induced fusion reactions of fullerene cages, forming the stable heavy molecular clusters, were observed. Experimental results exhibited the existence of similarities in geomctric shapes, fusion process, and spatial structure of molecular clusters of nuclei and fullerenes. In addition to the stability arising from shell-correction energy, the new stability arising from geometric structure (for instance, a structure of fullerene-like innermost hollow sphere) is probably important for reducing the force of Coulomb repulsion and increasing the stability of nuclei in the superheavy region. ACKNOWLEDGEMENTS. Y.L.Z would like to thank Prof. H.Q. Zhang of China Institute of Atomic Energy for intensive discussions; Prof. Itaya and Dr Yoshimoto of Tohoku Univ., Japan for the STM measurement; and acknowledge the grand support of National Natural Science Foundation (10275071 and a major project) and Ministry of Science & Technology (KS-02). REFERENCES 1. H. W. Kroto, et al, Nature, 318 (1985) 162. 2. W. Greiner, Fission and Properties of Neutron-Rich Nuclei, p l , eds. by 1. H. Hamilton, et al., World Scientific, 1999. 3. K. Rutz ct al., Phys. Rev. C 56 (1997) 238. 4. J. H. Hamilton, et al. Fission and Properties ofNeutron-Rich Nuclei, p126, cds. by J. H. Hamilton, et al., World Scientific, 1999. 5 . Yu. N. Kopach, P. Singer, M. Mutterer, M. Klemcns, A. Hotzcl, D. Schwalm, P. Thirolf. M. Hesse, F. Gonnenwein, Phys. Rev. Lett. 82 (1999) 303. 6. Yu. N. Kopach, M. Muttcrer, D. Schwalm, P. Thirolf and F. Gonnenwein, Phys. Rev C 65 (2002) 044614. 7. Yu. Ts. Oganessian et al., Nature. 400 (1999) 242. 8 S . Hoffman, Rep. Progr. Phys. 61 (1998) 639. 9. K. Morita, et al., Eur. Phys. J. A; and J. Phys Soc. Jp, in press, (2004). 10. W. Kratschmer. L. D.Lamb, K. Fostiropoulos, D. R. Huffman, Nature 347 (1990) 354. 11. A. Akiyama, Y. L. Zhao, et al., J. Am. Chem. Soc. 123 (2001) 181. 12. P. Moller, D. G. Madland. A. J. Sierk,A. Iwamoto, Nature 409 (2001) 785. 13. Y. L. Zhao, et al., Phys. Rev. Lett 82 (19%) 3408; 14. Y. L. Zhao, et al., Phys. Rev. C 62 (2000) 014612. 15. A. V. Rmayya, ct al.. Phys. Rev. Lett. 8 1 (1998) 947. 16. J . H. Hamilton ct al., J. Phys. G 20 (1994) L85.
ELSFVIER
www.elsevier.comllocatelnpe
Analogies between nuclear clusters and fullerene cages L. Qu, Y. L. Zhao*, Z. L. Chen, X. F. Gao, Z. F. Chai, J. Zhang, G. M Xing Institute of High Energy Physics, The Chinese Academy of Sciences. P. 0.Box 918, Beijing 100039, China
Similarities in behaviors of atomic nuclei and C60 fullerene cages are investigated to explore evidcnces for a new concept that if a superheavy nucleus can bc a cage structurc similar to aC6" fullercne. It is found that neutron-induced fusion reactions of C60 and/or C70 cages yield stable heavy molecular clusters similar to nuclear molccular clusters surviving in fission process. The experimental results indicate analogies in geometric shapes and spatial alignment of the nuclear molecular clustcr and atomic molecular cluster.
1. INTRODUCTION The atomic cluster C60 [l] consisting of 60 carbon atoms (with 30 double bindings) possesses a spherical structure with an innermost hollow space. Recently. Greincr et al. brought forward a new idea: if replacing each carbon atom of C60 (Fig. l(a)) by one alpha particle, we can obtain a Z=120 superheavy nucleus (Fig. l(b) [2]) consisting of 60 helium nuclci with additional neutrons being as bindings in between helium nuclei. Here, the 30 double bindings in the C60 cage (atomic cluster) are replaced by neutron bindings in thc Z=120 nuclcus (nuclear cluster). Theoretical calculations with the framework of Skyrme-Hatree-Fock and relativistic mean-field models showed a strong depression in thc central density [3], implying an innermost hollow structure of charge distribution of the Z=120, N=184 nucleus. This arises an important question: if the conventional rcgularitics about the nuclear density do no longer work in the superheavy nuclci. In a framework of the sophisticated structure of a nucleus, a superheavy nucleus is at the limit of Coulomb instability and hence unstable against spontaneous fission. The prescnt knowledge making a superheavy nucleus be stable is the large shcll-correction energy. which creates an additional binding and a fission bamcr of up to 8 MeV. Experimentally, Hamilton and Mutterer et al. have discovered an existence of nuclear molecules in the spontaneous fission of 252Cf[4-61. Meanwhile, nuclear clusters with the closed shell have been proved to play key roles in fusion processes to form a superheavy nucleus [7-91. Thus, in addition to thc stability arising from shell-correction energy, thc new stability may arise from such geometric structure as the proposed fullerene-like innermost hollow sphere, which is propitious to decrcase Coulomb instability and hence enhance the stability of superheavy nuclei of a largc * Corresponding
0375-9474/$
~
author, Email addrcss zhi)OTiiiliaun’iiihcn ac cn
see front matter 0 2004 Published by Elsevier B.V
doi: 10.1016/j.nuclphysa.2004.04.086
460
L. Qu et al. /Nuclear. Physics A738 (2004) 459-462
number of protons.
Figure 1. (a) Thc structure of C60 fullerene, (b) a schematic illustration of the spcculatcd hollow structure for Z=120 and N=184 nucleus (see the text for details). To reach the definite conclusion about the validity of above speculation, exploring experimental evidences is of most important. Experimentally, similarity in behavior of fullerene cages and atomic nuclei may be one of observables. In this paper we discuss the similarity between atomic nuclei and atomic clusters based on the new experimental findings: neutron-induced fusion reactions of c60 and/or C70 with carbon atoms yielded new products of hcavier clusters C l ~ and l C131. They wcre identified by laser ionization time-of-flight mass spectral techniques, and the geometric shapc of the new products was characterized by in situ scanning tunncling microscopy (STM) method, and calculated with ab initio Hartree-Fock method. 2. EXPERIMENTAL TECHNIQUE The c 6 0 and c 7 0 were produced by Kratschmer-Huffman method [ I O J and purified using the method similar to the previous works [ 1 I]. They were prepared to be targets for reactions with neutron beams. Two targets, 30 mg c60 and 30 mg [c60+c701 mixture (wt/wt=l: 1) were separately irradiated for 2 hours by neutron beams provided from Heavy Water Research Reactor (HWRR), China Institute of Atomic Energy (CIAE). The beam composition was: fast neutrons of - 5 . 4 ~ 1 0 cm-s~ and thermal neutrons of - 5 . 4 ~ 1 0 cm-s-l, ~ with a fast-to-thermal neutron flux-ratio of about 10 %. After reactions, the samples were measured for 3000 SCC using a high-purity germanium detector (HPGe) to check the radioactivity. Then, the separation and isolation of the reaction products were performed using a two-step high performance liquid chromatography coupling with a preparation Buckyprep column. The products were identified by laser ionization time-of-flight mass spectrometer. The shape and structure of the products were measured by the in situ high-resolution STM method. 3. RESULTS AND DISCUSSION After separation and purification, the products from neutron-irradiated C60 and [c6O+c7()] were mcasured by the time-of-flight mass spectral method. New mass peaks appearing at m/z=1453.1 (Fig. 2(a)) from neutron-irradiated C ~ and O at m/z=1573.4 (Fig. 2(b)) from neutron-irradiated [C60+C70] mixture were observed (the mass resolution, FWHM/m, was less than 1110000). Theoretical calculations indicate that the masses of m/z=1453.3 and 1573.4 are ncw carbon clusters of Cl2l and C131, respectively. Also the observed isomeric distributions of CiZ1and C131 were completely consistent with the thcorctical predictions. The geometric shapes of the new products, obtained from high-resolution STM images. are shown in figure 3(a-I) for Cl21 and 3-(b-I) for C131 molecules. Two slightly distorted C60 cages or C60 and C70 cages are connected with a neck to form a dumbbell-like shape.
L. Qu et al./Nuclear Physics A738 (2004) 459-462
1400
1 6 0 0 mlz
1400
46 1
1600 mlz
Fig. 2, The measured mass spectra for the Clzl (a) and C131 (b) formed from fusion reactlons of [c(j()+C+c60]and [c60+c+c70]respectively. , The inset shows the geometric structure of the most stable isomer of corresponding clusters. They were obtained from theoretical calculations with the PM3 Hamiltonian and ab initio Hartree-Fock calculations. One knows that the dumbbell-like shape of a nucleus survives both in the transition from a single nuclear system to two fragmental nuclei in fission and in the evolution from a binary system to a monosystem in fbsion. The theoretical calculations by Moller et al. revealed dumbbcll-like nuclear shapes from the ground state through saddlc points to the final configurations of separated fission fragments [12]. The nuclear shape in 3(a-II) is the mass-symmetric configuration leading to the symmetric fission-fragment with higher kinetic energies, while that in 3(b-II) represents the mass-asymmetric configuration leading to the asymmetric fission-fragment with lower kinetic energies[ 13,141. The similarity in geometrical shapes of atomic clusters and atomic nuclei is seen. The illustrations in 3(a-111) and 3(b-III) exhibit the outline of binding electrons in carbon clusters of the dumbbell-like fullerenes.
Fig. 3, (a-I) and (b-I): the STM images from an angle showing the shape of molecular cluster C ~ O = C = Cand ~ O the C7O=C=C60 fullerenes; (a-11) and (b-11): the shapes of a heavy nucleus in nuclear fission process [12]; (a-111) and (b-111): the outline of the binding electrons in the corresponding carbon molecular clusters shown in (a-I) and (b-1). Recently, a high-efficient measurement of angular correlations between neutrons, light
462
L. Qu et al. /Miclear Physics A738 (2004) 459-462
Fharged particles. and main fission fragments revealed intermediate light charged particles, ’He, ’He and *Li p.61 in the 252Cf fission. A triple nuclear molecule [146Ba-’oBc-’6Sr] consisting of a small cluster "Be in the middle with IaBa and %Sr in both sides of "Be [4,15] \vas observed by the y-y-y coincidence measurement. In the rearrangement process of fullcrcne cages, the molecular cluster [c6O-C1-c701 consisting of atomic clusters c6o and c70 with a carbon atom locating in between them was observed. The similarity is seen in the spatial alignment of the nuclear molecular cluster and atomic molecular cluster. Note that the experimental techniques [4-6,15,16] might be applicable mcans to search evidences for nuclear molecular clusters in the region of heavy and superhcavy nuclei. In summary, similarities between the nuclear cluster and the atomic cluster C6o fullerene cages were studied. Neutron-induced fusion reactions of fullerene cages, forming the stable heavy molecular clusters, were observed. Experimental results exhibited the existence of similarities in geomctric shapes, fusion process, and spatial structure of molecular clusters of nuclei and fullerenes. In addition to the stability arising from shell-correction energy, the new stability arising from geometric structure (for instance, a structure of fullerene-like innermost hollow sphere) is probably important for reducing the force of Coulomb repulsion and increasing the stability of nuclei in the superheavy region. ACKNOWLEDGEMENTS. Y.L.Z would like to thank Prof. H.Q. Zhang of China Institute of Atomic Energy for intensive discussions; Prof. Itaya and Dr Yoshimoto of Tohoku Univ., Japan for the STM measurement; and acknowledge the grand support of National Natural Science Foundation (10275071 and a major project) and Ministry of Science & Technology (KS-02). REFERENCES 1. H. W. Kroto, et al, Nature, 318 (1985) 162. 2. W. Greiner, Fission and Properties of Neutron-Rich Nuclei, p l , eds. by 1. H. Hamilton, et al., World Scientific, 1999. 3. K. Rutz ct al., Phys. Rev. C 56 (1997) 238. 4. J. H. Hamilton, et al. Fission and Properties ofNeutron-Rich Nuclei, p126, cds. by J. H. Hamilton, et al., World Scientific, 1999. 5 . Yu. N. Kopach, P. Singer, M. Mutterer, M. Klemcns, A. Hotzcl, D. Schwalm, P. Thirolf. M. Hesse, F. Gonnenwein, Phys. Rev. Lett. 82 (1999) 303. 6. Yu. N. Kopach, M. Muttcrer, D. Schwalm, P. Thirolf and F. Gonnenwein, Phys. Rev C 65 (2002) 044614. 7. Yu. Ts. Oganessian et al., Nature. 400 (1999) 242. 8 S . Hoffman, Rep. Progr. Phys. 61 (1998) 639. 9. K. Morita, et al., Eur. Phys. J. A; and J. Phys Soc. Jp, in press, (2004). 10. W. Kratschmer. L. D.Lamb, K. Fostiropoulos, D. R. Huffman, Nature 347 (1990) 354. 11. A. Akiyama, Y. L. Zhao, et al., J. Am. Chem. Soc. 123 (2001) 181. 12. P. Moller, D. G. Madland. A. J. Sierk,A. Iwamoto, Nature 409 (2001) 785. 13. Y. L. Zhao, et al., Phys. Rev. Lett 82 (19%) 3408; 14. Y. L. Zhao, et al., Phys. Rev. C 62 (2000) 014612. 15. A. V. Rmayya, ct al.. Phys. Rev. Lett. 8 1 (1998) 947. 16. J . H. Hamilton ct al., J. Phys. G 20 (1994) L85.