~6LLi(p,γ)~7Be低能反应截面的实验研究
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摘要
6Li(p, γ)7Be反应及其逆反应与宇宙大爆炸核合成过程密切相关，其低能反应截面的研究并不透彻，值得我们进一步进行实验研究。根据低能核天体物理实验特点，课题团队依托兰州320kV高压平台建立了一个新的低能核天体物理实验装置。平台可以提供强流的质子，α粒子以及其他重离子束流。束流能量利用已知的11B(p, γ)12C共振反应和12C(p, γ)13N非共振反应进行了刻度，能量精度好于0.5keV。实验终端的探测器包含有高纯锗Clover探测器、Si探测器和塑料闪烁体探测器。Clover探测器的绝对效率由60Co和152Eu标准源以及14N(p,γ)15O共振反应给出。塑料闪烁体探测器能对宇宙射线本底起到有效的抑制，抑制系数也按能区分别给出。
6Li(p, γ)7Be的低能反应截面测量实验在新建成的低能核天体物理实验装置上完成。实验同时对γ反应道和α反应道的产额进行了测量，并利用现有的α反应道的截面数据，相对计算出6Li(p, γ)7Be在50keV～250keV低能区的天体物理S因子。实验结果显示，在200keV以下能区，6Li(p, γ)7Be反应的天体物理因子出现反常的降低，这与之前理论学家的预期不符。R矩阵拟合结果预示在7Be核中可能存在一个从未被发现的ER≈195keV的共振态。该共振态对应能级的自旋宇称可能为3/2+。这个能级的指认不但能很好的拟合我们得到的S因子谱，同时也能解释6Li(p, α)3He反应角分布系数a1不为零的问题。我们的实验结果并不能改变现有的大爆炸标准模型或基于大爆炸理论的SUSY模型所预言的6Li和7Be的丰度，但是新的精确反应截面数据使得进一步的理论研究有了更可靠的实验基础。
6Li(p, γ)7Be reaction and its reverse reaction are closely related to the big bangnucleosynthesis (BBN). It is worthwhile to make a experimental study of thelow-energy cross-section of6Li(p, γ)7Be, which has not been studied well. A newexperimental setup for nuclear astrophysics study has been established in Lanzhouby our group. The setup is driven by a320kV high voltage platform, which canprovide intense currents of proton, alpha and many heavy ion beams. The energy of aproton beam has been calibrated against the nominal platform high voltage by usinga well-known resonant reaction of11B(p, γ)12C and a non-resonant reaction of12C(p,γ)13N. The accuracy was achieved to be better than0.5keV. The detection systemconsists of a Clover-type high-purity germanium detector, a silicon detector and aplastic scintillator. The absolute efficiency of the Clover detector was given by thestandard γ-ray sources of60Co and152Eu with known activites combined with aresonant reaction of14N(p, γ)15O. The plastic scintillator can suppress thebackground of cosmic-rays efficiently, and background suppression factors wereobtained for different energy regions.
The low-energy astrophysical S-factors of the6Li(p, γ)7Be reaction have beeninvestigated on the new experimental setup. The yields of γ-rays from γ-channel andcharge particles from α-channel were measured simultaneously. The astrophysicalS-factors of6Li(p, γ)7Be were determined relatively to the well-known S-factors of6Li(p, α)3He reaction at50～250keV energy region. The experimental S-factor ofthis reaction shows an interesting sizable drop contrary to any existing theoreticalexpectations at energies below200keV. Our R-matrix calculation shows that theremay be an interesting new positive-parity3/2+resonance at ER≈195keV in7Be. Such a resonance state of7Be can not only fit the structure of the obtained S-factorsbut also interpret the nonzero coefficient of a1in6Li(p, α)3He reaction. Although, ourresults discussed in a SUSY assisted BBN model hardly change the abundance of6Liand7Be, the new cross section based on a solid experimental ground is a good startfor the realistic calculations.
6Li(p, γ)7Be的低能反应截面测量实验在新建成的低能核天体物理实验装置上完成。实验同时对γ反应道和α反应道的产额进行了测量，并利用现有的α反应道的截面数据，相对计算出6Li(p, γ)7Be在50keV～250keV低能区的天体物理S因子。实验结果显示，在200keV以下能区，6Li(p, γ)7Be反应的天体物理因子出现反常的降低，这与之前理论学家的预期不符。R矩阵拟合结果预示在7Be核中可能存在一个从未被发现的ER≈195keV的共振态。该共振态对应能级的自旋宇称可能为3/2+。这个能级的指认不但能很好的拟合我们得到的S因子谱，同时也能解释6Li(p, α)3He反应角分布系数a1不为零的问题。我们的实验结果并不能改变现有的大爆炸标准模型或基于大爆炸理论的SUSY模型所预言的6Li和7Be的丰度，但是新的精确反应截面数据使得进一步的理论研究有了更可靠的实验基础。
6Li(p, γ)7Be reaction and its reverse reaction are closely related to the big bangnucleosynthesis (BBN). It is worthwhile to make a experimental study of thelow-energy cross-section of6Li(p, γ)7Be, which has not been studied well. A newexperimental setup for nuclear astrophysics study has been established in Lanzhouby our group. The setup is driven by a320kV high voltage platform, which canprovide intense currents of proton, alpha and many heavy ion beams. The energy of aproton beam has been calibrated against the nominal platform high voltage by usinga well-known resonant reaction of11B(p, γ)12C and a non-resonant reaction of12C(p,γ)13N. The accuracy was achieved to be better than0.5keV. The detection systemconsists of a Clover-type high-purity germanium detector, a silicon detector and aplastic scintillator. The absolute efficiency of the Clover detector was given by thestandard γ-ray sources of60Co and152Eu with known activites combined with aresonant reaction of14N(p, γ)15O. The plastic scintillator can suppress thebackground of cosmic-rays efficiently, and background suppression factors wereobtained for different energy regions.
The low-energy astrophysical S-factors of the6Li(p, γ)7Be reaction have beeninvestigated on the new experimental setup. The yields of γ-rays from γ-channel andcharge particles from α-channel were measured simultaneously. The astrophysicalS-factors of6Li(p, γ)7Be were determined relatively to the well-known S-factors of6Li(p, α)3He reaction at50～250keV energy region. The experimental S-factor ofthis reaction shows an interesting sizable drop contrary to any existing theoreticalexpectations at energies below200keV. Our R-matrix calculation shows that theremay be an interesting new positive-parity3/2+resonance at ER≈195keV in7Be. Such a resonance state of7Be can not only fit the structure of the obtained S-factorsbut also interpret the nonzero coefficient of a1in6Li(p, α)3He reaction. Although, ourresults discussed in a SUSY assisted BBN model hardly change the abundance of6Liand7Be, the new cross section based on a solid experimental ground is a good startfor the realistic calculations.
引文
[1] R. A. Alpher, H. Bethe, G. Gamow. The Origin of chemical elements. Phys. Rev.,1948, Vol.73:803～804
[2]向守平.天体物理概论.合肥：中国科学技术大学出版社，2008
[3] K. M. Burbidge, G. R. Burbidge, W. A. Fowler. Synthesis of elements in stars.Rev. of Mod. Phys,1957, Vol.29：547～650
[4] A. G. W. Cameron. Stellar evolution, Nuclear Astrophysics and nucleogenesis.Atomic Energy of Canada, A series of lectures given at Purdue University,1957
[5] J. N. Bahcall, S. H. Lubow, W. F. Huebner. New Solar-Neutrino Flux Calculati-ons and Implications Regarding Neutrino Oscillations, Phys. Rev. Lett,1980, Vol.45:945～948
[6] S. E. Woosley，R. D. Hoffman. The Alpha-Process and the R-Process. Astrophys.J.,1992, Vol.395202～239
[7]苏俊.6Li(n，γ)7Li天体物理反应率的间接测量.中国原子能科学研究院博士论文，2010
[8]卢希庭，江栋兴，叶沿林.原子核物理.北京：原子能出版社，2000
[9] Z. E. Switkowski, J. C. P. Heggie, D. L. Kennedy. Nucl. Phys. A,1979, Vol.33150～60
[10] R. Prior, M. Spraker, A. Amthor. Energy dependence of the astrophysical Sfactor for the6Li(p, γ)7Be reaction. Phys. Rev. C,2004, Vol.70:055801.
[11] J. T. Huang, C. Bertulani, V. Guimar es. Radiative capture of nucleons atastrophysical energies with single-particle states.At. Data Nucl. Data Tables,2010,Vol.96:824～847.
[12] F. C. Barker, Netron and proton capture by6Li. Aust. J. Phys.,1980, Vol.33:159～176
[13] K. Arai, D. Baye, P. Descouvemont. Microscopic study of the6Li(p, γ)7Be and6Li(p, α)3He reactions. Nucl. Phys. A,2002, Vol.699:963～975
[14] F. E. Cecil, D. Ferg, H. Liu. Radiative capture of protons by light nuclei at lowenergies. Nucl. Phys. A,1992. Vol.539:75～96
[15] G. Audi, A. Wapstra, C. Thibault. The Ame2003atomic mass evaluation. Nucl.Phys. A,2003, Vol.729:129～336
[16] C. Rolfs. Spectroscopic factors from radiative capture reactions. Nucl. Phys. A,1973, Vol.217:29～70
[17]吴治华，齐卉荃，沈能学.原子核物理实验方法.北京：原子能出版社，1997
[18]张宁涛.139Pm高自旋能级结构的研究与反康谱仪的研制.中国科学院研究生院博士论文，2011
[19] C. Rolfs, W. Rodney. Cauldrons in the Cosmos. The University of ChicagoPress,1988
[20]申庆彪.低能和中能核反应理论.北京：科学出版社，2005
[21]侯素青.大爆炸动力学方程的研究与计算.北京化工大学硕士论文，2010
[22] S. Sengupta, P. B. Pal. Constraints on cosmic electric charge asymmetry andneutrino charge from the microwave background. Phys. Lett. B, Vol.365:450
[23]侯素青.大爆炸动力学方程的研究与计算.北京化工大学硕士论文，2010
[24] L. Sun, H. Zhao, J. Li. A high charge state all-permanent magnet ECR ionsourcefor the IMP320kV HV platform. Nucl. Ins.&Meth. B,2007, Vol.263:503～512
[25]谢一冈，陈昌，王曼.粒子探测器与数据获取.北京：科学出版社，2003
[26] http://www.eljentechnology.com/index.php/products
[27] Roush, Wilson, Hornyak. Pulse Shape Discrimination. Nucl. Ins.&Meth.,31(1964)112.
[28] T. Huus, R. Day. The Gamma Radiation from11B Bombarded by Protons. Phys.Rev.,1953, Vol.91:599～605.
[29] C. Rolfs, W. S. Rodney, S. Durrance. A new method for measurement of protonbeam energies. Nucl. Phys. A,1975, Vol.240:221～234
[30] T. Freye, H. Lorenz-Wirzba, B.Cleff. Voltage Calibration of Low-EnergyAccelerators. Z. Phys. A,1977, Vol.281:211～218
[31] M. Wang, G. Audi, A. H. Wapstra. The Ame2012atomic mass evaluation.Chinese Phys. C,2012, Vol.36:1603～2014
[32] R. Longland, C. Iliadis, A. E. Champagne. Nuclear astrophysics studies at theLENA facility: The g-ray detection system. Nucl. Instr. and Meth. A,2006, Vol.566:452～464
[33] C. Angulo, M. Arnould, M. Rayet. A compilation of charged-particle inducedthermonuclear reaction rates. Nucl. Phys. A,1999, Vol.656:3～183
[34] F. Ajzenberg-selove. Energy levels of light nuclei13-15. Nucl. Phys. A,1991,Vol.523:1～196
[35] S. Agostinelliae, J. Allisonas, K. Amako. Geant4—a simulation toolkit. Nucl.Instr. and Meth. A,2003, Vol.506:250～303
[36] R. Brun, F. Rademakers. ROOT-An object oriented data analysis framework.Nucl. Instr. and Meth. A,1997, Vol.389:81～86
[37] Lan Morison. Introduction to Astronomy and Cosmology. John Wiley&Sons,2008.
[38] http://radware.phy.ornl.gov/gf3/gf3.html
[39] S. Engstler, G. Raimann, C. Angulo. Isotopic dependence of electron screeningin fusion reactions. Z. Phys. A,1992, Vol.342:471～482
[40] C. Iliadis. Nuclear Physics of Stars. Wiley-VCH,2007
[41] J. Cruz, H. Luis, M. Fonseca. Experimental study of proton-induced nuclearreactions in6,7Li. J.Phys.G:Nucl.Part.Phys,2008, Vol.35:014044
[42]朱永生.实验物理中的概率和统计(第二版).北京：科学出版社，2006
[43]胡济民.原子核理论(第二版).北京：原子能出版社，1996
[44] A. M. Lane, R. G. Thomas. R-Matrix Theory of Nuclear Reactions. Rev. ofMod. Phys,1958, Vol.30：257～353
[45] R. E. Azuma, E. Uberseder, E. C. Simpson. AZURE: An R-matrix code fornuclear astrophysics. Phys. Rev. C,2010, Vol.81:045805
[46] F. C. Barker. Electron screening in reactions between light. Nucl. Phys. A,2002,Vol.707:277～300
[47] M. Kusakabe, T. Kajino, G. J. Mathews. Li6production by the radiative decayof long-lived particles. Phys. Rev. D,2006, Vol.74:023526
[2]向守平.天体物理概论.合肥：中国科学技术大学出版社，2008
[3] K. M. Burbidge, G. R. Burbidge, W. A. Fowler. Synthesis of elements in stars.Rev. of Mod. Phys,1957, Vol.29：547～650
[4] A. G. W. Cameron. Stellar evolution, Nuclear Astrophysics and nucleogenesis.Atomic Energy of Canada, A series of lectures given at Purdue University,1957
[5] J. N. Bahcall, S. H. Lubow, W. F. Huebner. New Solar-Neutrino Flux Calculati-ons and Implications Regarding Neutrino Oscillations, Phys. Rev. Lett,1980, Vol.45:945～948
[6] S. E. Woosley，R. D. Hoffman. The Alpha-Process and the R-Process. Astrophys.J.,1992, Vol.395202～239
[7]苏俊.6Li(n，γ)7Li天体物理反应率的间接测量.中国原子能科学研究院博士论文，2010
[8]卢希庭，江栋兴，叶沿林.原子核物理.北京：原子能出版社，2000
[9] Z. E. Switkowski, J. C. P. Heggie, D. L. Kennedy. Nucl. Phys. A,1979, Vol.33150～60
[10] R. Prior, M. Spraker, A. Amthor. Energy dependence of the astrophysical Sfactor for the6Li(p, γ)7Be reaction. Phys. Rev. C,2004, Vol.70:055801.
[11] J. T. Huang, C. Bertulani, V. Guimar es. Radiative capture of nucleons atastrophysical energies with single-particle states.At. Data Nucl. Data Tables,2010,Vol.96:824～847.
[12] F. C. Barker, Netron and proton capture by6Li. Aust. J. Phys.,1980, Vol.33:159～176
[13] K. Arai, D. Baye, P. Descouvemont. Microscopic study of the6Li(p, γ)7Be and6Li(p, α)3He reactions. Nucl. Phys. A,2002, Vol.699:963～975
[14] F. E. Cecil, D. Ferg, H. Liu. Radiative capture of protons by light nuclei at lowenergies. Nucl. Phys. A,1992. Vol.539:75～96
[15] G. Audi, A. Wapstra, C. Thibault. The Ame2003atomic mass evaluation. Nucl.Phys. A,2003, Vol.729:129～336
[16] C. Rolfs. Spectroscopic factors from radiative capture reactions. Nucl. Phys. A,1973, Vol.217:29～70
[17]吴治华，齐卉荃，沈能学.原子核物理实验方法.北京：原子能出版社，1997
[18]张宁涛.139Pm高自旋能级结构的研究与反康谱仪的研制.中国科学院研究生院博士论文，2011
[19] C. Rolfs, W. Rodney. Cauldrons in the Cosmos. The University of ChicagoPress,1988
[20]申庆彪.低能和中能核反应理论.北京：科学出版社，2005
[21]侯素青.大爆炸动力学方程的研究与计算.北京化工大学硕士论文，2010
[22] S. Sengupta, P. B. Pal. Constraints on cosmic electric charge asymmetry andneutrino charge from the microwave background. Phys. Lett. B, Vol.365:450
[23]侯素青.大爆炸动力学方程的研究与计算.北京化工大学硕士论文，2010
[24] L. Sun, H. Zhao, J. Li. A high charge state all-permanent magnet ECR ionsourcefor the IMP320kV HV platform. Nucl. Ins.&Meth. B,2007, Vol.263:503～512
[25]谢一冈，陈昌，王曼.粒子探测器与数据获取.北京：科学出版社，2003
[26] http://www.eljentechnology.com/index.php/products
[27] Roush, Wilson, Hornyak. Pulse Shape Discrimination. Nucl. Ins.&Meth.,31(1964)112.
[28] T. Huus, R. Day. The Gamma Radiation from11B Bombarded by Protons. Phys.Rev.,1953, Vol.91:599～605.
[29] C. Rolfs, W. S. Rodney, S. Durrance. A new method for measurement of protonbeam energies. Nucl. Phys. A,1975, Vol.240:221～234
[30] T. Freye, H. Lorenz-Wirzba, B.Cleff. Voltage Calibration of Low-EnergyAccelerators. Z. Phys. A,1977, Vol.281:211～218
[31] M. Wang, G. Audi, A. H. Wapstra. The Ame2012atomic mass evaluation.Chinese Phys. C,2012, Vol.36:1603～2014
[32] R. Longland, C. Iliadis, A. E. Champagne. Nuclear astrophysics studies at theLENA facility: The g-ray detection system. Nucl. Instr. and Meth. A,2006, Vol.566:452～464
[33] C. Angulo, M. Arnould, M. Rayet. A compilation of charged-particle inducedthermonuclear reaction rates. Nucl. Phys. A,1999, Vol.656:3～183
[34] F. Ajzenberg-selove. Energy levels of light nuclei13-15. Nucl. Phys. A,1991,Vol.523:1～196
[35] S. Agostinelliae, J. Allisonas, K. Amako. Geant4—a simulation toolkit. Nucl.Instr. and Meth. A,2003, Vol.506:250～303
[36] R. Brun, F. Rademakers. ROOT-An object oriented data analysis framework.Nucl. Instr. and Meth. A,1997, Vol.389:81～86
[37] Lan Morison. Introduction to Astronomy and Cosmology. John Wiley&Sons,2008.
[38] http://radware.phy.ornl.gov/gf3/gf3.html
[39] S. Engstler, G. Raimann, C. Angulo. Isotopic dependence of electron screeningin fusion reactions. Z. Phys. A,1992, Vol.342:471～482
[40] C. Iliadis. Nuclear Physics of Stars. Wiley-VCH,2007
[41] J. Cruz, H. Luis, M. Fonseca. Experimental study of proton-induced nuclearreactions in6,7Li. J.Phys.G:Nucl.Part.Phys,2008, Vol.35:014044
[42]朱永生.实验物理中的概率和统计(第二版).北京：科学出版社，2006
[43]胡济民.原子核理论(第二版).北京：原子能出版社，1996
[44] A. M. Lane, R. G. Thomas. R-Matrix Theory of Nuclear Reactions. Rev. ofMod. Phys,1958, Vol.30：257～353
[45] R. E. Azuma, E. Uberseder, E. C. Simpson. AZURE: An R-matrix code fornuclear astrophysics. Phys. Rev. C,2010, Vol.81:045805
[46] F. C. Barker. Electron screening in reactions between light. Nucl. Phys. A,2002,Vol.707:277～300
[47] M. Kusakabe, T. Kajino, G. J. Mathews. Li6production by the radiative decayof long-lived particles. Phys. Rev. D,2006, Vol.74:023526