论文标题:利用（d，p）反应确定不稳定核的天体物理（p，γ）反应率
论文作者
论文导师 柳卫平；李志宏,论文学位 博士,论文专业 粒子物理与核物理
论文单位 中国原子能科学研究院,点击次数 630,论文页数 144页File Size6555K
2007-05-01论文网 http://www.lw23.com/lunwen_280527/
Hydrogen burning;; Transfer reaction;; Asymptotic normalization coefficient;; Radiative capture reaction;; Astrophysical S factor;; Reaction rate
在高温高密度的天体环境中，丰质子不稳定核的质子俘获反应率可能接近或超过其β~+衰变的速率，在爆发性氢燃烧过程中起主导作用。如何确定这些天体物理重要反应的截面对于核物理和核天体物理都是一个巨大的挑战。直接测量是最可靠的方法，然而由于天体物理感兴趣能区(p，γ)反应截面极小(只有nb-pb量级)，不稳定核束流强度较低，所以直接测量非常困难。因此，通过间接测量方法研究不稳定核的(p，γ)反应截面十分重要。 近年来，单质子转移反应的测量结合渐进归一化系数(ANC)方法已经广泛应用于质子辐射俘获反应的研究。该方法可以通过测量截面较大(mb量级)的单质子转移反应的角分布以及扭曲波玻恩近似(DWBA)分析，导出剩余核虚衰变的质子ANC，进而得出质子辐射俘获反应的直接俘获截面以及天体物理S因子和反应率。 本论文工作采用的是结合镜像核电荷对称性的ANC方法，该方法与常规的ANC方法不完全相同。镜像核是质量数相等的两个核，交换其中一个核的质子数与中子数就得到另外一个核。由于核力与电荷近似无关，所以一对镜像核的两条对应能级的结构具有相似性。假设B核与D核互为镜像核，那么我们就可以通过研究更容易测量的B核来间接地得到目标核D的信息。以研究C(p，γ)D反应为例来说，常规的ANC方法中需要测量质子转移反应，比如C(d，n)D反应，然后通过DWBA分析导出D核虚衰变的质子ANC，进而可以得出C(p，γ)D反应直接俘获的天体物理S因子和反应率。结合镜像核电荷对称性的ANC方法中测量的是镜像核中子转移反应，比如A(d，p)B反应，通过DWBA分析导出B核虚衰变的中子ANC，然后利用镜像核电荷对称性导出D核虚衰变的质子ANC以及质子宽度，进而可以得出C(p，γ)D反应的天体物NS因子和反应率。 值得一提的是，本方法既可以得到(p，γ)反应的直接俘获贡献，也可以得到共振俘获贡献的部分信息，即质子宽度。 本方法有两个特点：(1)能够更充分利用HI-13串列加速器次级束流线GIRAFFE上已有次级束流，拓宽其实验研究范围；(2)能够间接得出仍无实验数据的质子辐射俘获反应截面或减小其不确定性。以我们的两个目标反应~8B(p，γ)~9C和~(26)Si(p，γ)~(27)P为例。研究~8B(p，γ)~9C反应需要~8B束流，目前GIRAFFE还没有产生~8B束流，但是已经产生了纯度较高、品质较好的~8Li束流。~8B和~8Li是镜像核，可以通过测量中子转移反应~8Li(d，p)~9Li的角分布来间接地研究~8B(p，γ)~9C反应。因此，在现有束流的基础上，利用该方法可以拓宽GIRAFFE的实验研究范围。同样也可以利用该方法通过分析已有的~(26)Mg(d，p)~(27)Mg反应的角分布来间接研究~(26)Si(p，γ)~(27)P反应。由于稳定束的强度要远远大于不稳定束的强度，因此可以大大减小结果的统计误差。此外，本论文利用此方法第一次实验上得出了该反应中的直接俘获贡献。 ANC方法作为一种间接的实验方法，它的可靠性非常值得关注，也就是说ANC方法给出的结果在多大程度上可以信赖。本论文中我们利用~(16)O(p，γ)~(17)F反应天体物理S因子的直接测量数据对结合镜像核电荷对称性的ANC方法作了验证。我们分析了~(16)O(d，p)~(17)O反应布居~(17)O基态和第一激发态角分布的数据，利用本方法导出了~(16)O(p，γ)~(17)F反应布居~(17)F基态和第一激发态的天体物理S因子，并与直接测量的天体物理S因子作了比较。结果表明，ANC方法给出的结果无论幅值还是激发函数形状与直接测量结果在误差范围内都符合很好。因此，当直接测量数据难以获得的情况下，ANC方法能够为辐射俘获反应的截面以及反应率提供一个实验的约束。这正是ANC方法作为一个间接测量方法的价值所在。此外，ANC方法的结果也可以与直接测量结果进行交叉检验，因为直接测量实验不可能在极其低能区进行，所以直接测量必然涉及低能外推的问题。 本论文工作采用结合镜像核电荷对称性的ANC方法研究了恒星演化进程中氢燃烧阶段的四个重要反应，分别是~8B(p，γ)~9C、~(11)C(p，γ)~(12)N、~(13)N(p，γ)~(14)O和~(26)Si(p，γ)~(27)P四个反应。此外，该方法也可以用于其它一系列天体物理重要反应的研究，例如~7Be(p，γ)~8B，~(17)F(p，γ)~(18)Ne，和~(22)Mg(p，γ)~(23)Al等反应。 此外，论文中阐述了攻读博士期间参加的其它两个工作。第一，我们与加拿大TRIUMF实验室合作研究了利用低能质子束产生高强度放射性束流的方法，目前利用该方法已经成功地产生了满足实验要求的高强度~(11)C束流。第二，在中国原子能科学研究院的GIRAFFE次级束流线上测量了~(13)N(d，n)~(14)O反应的角分布，间接研究了~(13)N(p，γ)~(14)O反应。 论文布局如下：第一章概述了核天体物理和恒星演化进程中的氢燃烧；第二章阐述了恒星热核反应率相关知识及其实验研究方法；第三章是本论文工作的主体，在这一章中，我们阐释并验证了镜像核电荷对称性与ANC方法，然后将该方法应用于四个氢燃烧中重要反应的研究；第四章简述了攻读博士期间参加的其它两个工作；第五章是论文工作的总结和讨论。
The proton capture reactions on proton-rich nuclei play an important rolein explosive hydrogen burning in the peculiar astrophysical sites where the tem-perature and density are so high that the capture reactions become faster thanthe competingβdecays. Determination of the cross sections of these astrophys-ical reactions is a major challenge for nuclear physics and nuclear astrophysics.Direct measurement of cross sections is the most exact, however, many of thesereactions are difficult to measure directly with currently available experimentaltechniques because the cross sections at low energies are very small (nb-pb) andthe available intensities of the radioactive ion beams are very low. It is thereforeimportant to explore indirect alternative methods for determining the (p,γ) re-action cross sections on unstable nuclei. Recently, the proton transfer reactions and ANC approach have been exten-sively applied to the study of radiative capture reactions. In this method, theANC of virtual decay can be derived from proton transfer reactions with largecross sections (mb), and then used to calculate the astrophysical S factors andrates of direct captures in radiative capture reactions. In this thesis, I used the ANC method combined with charge symmetry ofmirror nuclei which is a little bit different from traditional ANC approach. Mir-ror nuclei include a pair of nuclei that have the same mass number. If the proton-and neutron numbers are interchanged in the one of mirror pair, this nucleus willbe transformed into the other. The corresponding levels of mirror pair are ana-logical due to charge symmetry of strong interaction. Assume B and D are mirrornuclei, then some of the information of D nucleus can be found by studying Bnucleus. In order to study the C(p,γ)D reaction, the proton transfer reactionsuch as C(d, n)D needs to be measured in traditional ANC approach. The pro-ton ANC of D virtual decay can be then extracted and employed to computethe astrophysical S factors and rates of direct captures in the C(p,γ)D reaction.While the neutron transfer reaction such as A(d,p)B will be measured in the ANC method combined with charge symmetry of mirror nuclei. The neutronANCs of B virtual decays can be derived through DWBA analysis. The protonANC of D virtual decay can be then extracted according to charge symmetryand utilized to calculate the astrophysical S factors and rates of the C(p,γ)Dreaction. It should be mentioned that both the direct capture contribution and protonwidths of resonant captures can be obtained by using this method. This method has two advantages. (1) The preexisting radioactive beamsproduced by GIRAFFE can be used to study new radiative capture reactionsbesides those studied by traditional ANC approach. (2) The experimentally un-known cross sections of radiative capture reaction may be indirectly determined,or the statistical uncertainty of cross sections can be significantly reduced throughthis method. The ~8B(p,γ)~9C and ~(26)Si(p,γ)~(27)P reactions are just the examples ofthese two advantages. In order to study ~8B(p,γ)~9C, the ~8B beam is needed intraditional ANC approach, which has not been produced yet by GIRAFFE. How-ever, the ~8Li beam is available at GIRAFFE. Thus, one can measure the angulardistribution of ~8Li(d, p)~9Li, and then indirectly study the ~8B(p,γ)~9C reaction bythe ANC method combined with charge symmetry. In addition, the angulardistributions of ~(26)Mg(d,p)~(27)Mg can be applied to the study of the ~(26)Si(p,γ)~(27)Preaction by this method. Because the intensity of stable deuteron beam is sig-nificantly larger than that of unstable ~(26)Si beam, the statistics will be largelyimproved. The contribution from direct capture in the ~(26)Si(p,γ)~(27)P reaction hasbeen determined experimentally in the present work, for the first time. The ANC method is an indirect experimental approach, therefore, its re-liability is rather concerned. In this thesis, I have derived the astrophysical Sfactors of the ~(16)O(p,γ)~(17)F reaction leading to the ground and first excited statesof ~(17)F from the angular distributions of the ~(16)O(d,p)~(17)O reaction leading to theground and first excited states of ~(17)O using the ANC method combined withcharge symmetry. The present S factors have been compared with those fromthe direct measurement experiment. It shows that the results by ANC methodare in good agreement with those from the direct measurement at astrophysicalenergies within the uncertainties. Therefore, the results from ANC method can provide an experimental constraint to the cross sections and rates of radiativecapture reactions in the case that the data from direct measurement are notavailable. In addition, the data at very low energies from the extrapolation ofdirect results at higher energies can be checked by those from ANC method. In this dissertation, the ANC method combined with charge symmetry isutilized to study four reactions of importance in hydrogen burning, namely~8B(p,γ)~9C, ~(11)C(p,γ)~(12)N, ~(13)N(p,γ)~(14)O and ~(26)Si(p,γ)~(27)P. In addition, this methodcan be applied to the study of many other reactions of astrophysical importance,for example, ~7Be(p,γ)~8B, ~(17)F(p,γ)~(18)Ne, ~(22)Mg(p,γ)~(23)Al and so on. In addition, two more experiments are presented in this thesis. The firstone is the production of intense radioactive beams using low energy protons atTRIUMF lab; the second one is the measurement of the ~(13)N(d, n)~(14)O angular dis-tribution at GIRAFFE facility of CIAE and the indirect study of the ~(13)N(p,γ)~(14)Oreaction. The outline of this thesis is as follows. Chapter 1 will briefly introduce nu-clear astrophysics and hydrogen burning in evolution of stars. Chapter 2 willpresent the thermonuclear reaction formalism and experimental methods. Chap-ter 3 is the principal part of this thesis, where the ANC method combined withcharge symmetry of mirror nuclei will be introduced and tested for its reliability,and then used to study four important reactions in hydrogen burning. Chapter 4will present two other works during my PHD course. Chapter 5 has a summaryof this dissertation and a discussion on ANC method.
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