论文标题:用特洛伊木马方法间接研究天体核反应～9Be（p,α）～6Li
Study of the Reaction ～9Be(p,α)～6Li via the Trojan Horse Method
论文作者
论文导师 柳继锋；周书华,论文学位 硕士,论文专业 理论物理
论文单位 广西师范大学,点击次数 96,论文页数 71页File Size18256K
论文网 http://www.lw23.com/lunwen_865757347/
THM method; Quasi-free condition; Particle identification;S(E)factor
Li、Be、B丰度及其演化对核天体物理的诸多领域都具有重要的意义。~9Be的原初丰度是检验大爆炸原初核合成非均匀模型准确性的一个关键性参数;年轻恒星中~9Be丰度的研究,和其它轻元素Li、B一起,可以作为恒星内部运作机制的探针,用于研究恒星的结构和演化。 无论是在大爆炸原初核合成环境还是恒星演化天体环境中,Li、Be、B的毁灭过程都主要是由质子引发的(p,α)反应道发生的,其相应的天体环境能量从~10keV(恒星核合成)到~100 keV(原初核合成)。这些能量远低于库仑位垒(通常在MeV量级),核反应只能通过隧道穿透效应发生,反应截面随能量降低呈近似指数下降。天体能区的反应截面通常利用随能量变化相对平缓的天体因子S(E)从较高能区的测量数据走势向低能端外推得到。然而,由于可能存在的阈下共振及低能核反应中电子屏蔽效应的影响,这种外推过程会引入较大不确定性。 为避免外推,近些年来,发展了许多间接方法,特洛伊木马方法就是其中之一,特别适于间接测量带电粒子两体反应在低能区的裸核截面。该方法建立在准自由反应机制基础上,在适当的运动学条件下,选取一个与两体反应密切相关的三体反应,其反应在库仑位垒之上发生,从而避免了库仑位垒对截面的抑制,电子屏蔽效应也可以忽略不计。借助于核反应理论,两体反应截面可以从三体反应数据中提取出来。 本论文工作中,特洛伊木马方法被用于间接测量~9Be(p,α)~6Li核反应——它是~9Be燃烧过程的主要反应道之一。该反应借助于一个与之相匹配的三体反应d(~9Be,α~6Li)n来研究。其中,氘核作为“木马核”,它可以看作是由质子和中子两个结合得相对比较松散的体系构成的:d=(p+n)。在三体反应过程中,质子作为核反应的实际参与者,而中子作为两体反应p+~9Beα+~6Li的旁观者。如果~9Be束流能量高于库仑位垒,三体反应可以在库仑位垒之上顺利进行,~9Be(p,α)~6Li反应借助于三体反应在其核相互作用区域内发生,库仑位垒影响和电子屏蔽效应可以被忽略,裸核反应截面的间接测量成为可能。 本次d(~9Be,α~6Li)n实验是在北京串列加速器核物理国家实验室完成的。串列加速器提供的22.44MeV的~9Be束流轰击CD2靶,流强1-5nA,使用1.5mm左右宽线靶,靶厚约257μg/cm2。没有直接的粒子鉴别,探测器放置于被称为“准自由角度对”的位置——准自由过程在这个区域占主要地位。两探测器的符合输出作为触发信号。为了获得较高的角度分
In order to understand many astrophysical processes a complete knowledge of microphysics is required. Nuclear Astrophysics plays a key role in the description of astrophysical phenomena: it studies the nuclear processes that have been taking place in the universe since its beginning. Beryllium primordial abundances can provide a powerful test to discriminate between homogeneous and inhomogeneous primordial nucleosynthesis. Moreover, the study of beryllium abundances in young stars, together with lithium and boron, can provide a strict test for understanding the stellar structure and discriminating between possible non-standard mixing processes in stellar interiors. In both stellar and primordial environments, however, Li,Be and B are mainly destroyed by proton-capture reactions via the (p,α) channel with a Gammow energy EG ranging from ~10keV (for stellar nucleosynthesis) to ~100 keV (for primordial nucleosynthesis). These energies are much lower than the Coulomb barrier EC, which is usually of the order of MeV"s. Thus the reactions take place via tunnel effect with an exponential decrease of the cross section. Due to the exponential suppression, the behavior of the cross sections at astrophysical energies are usually extrapolated from that at higher energies by using the definition of the astrophysical factor S(E), which varies smoothly with energies. Nevertheless this extrapolation procedure can introduce some uncertainties due to the presence of unexpected sub-threshold resonances or electron-screening effects. In recent years, many indirect methods have been developed in order to extract the S(E)-factor without extrapolations. In particular, the Trojan-Horse Method (THM) is a powerful tool, which selects, under appropriate kinematical conditions, the quasi-free (QF) contribution of a suitable three-body reaction performed at energies well above the Coulomb barrier to extract a charged particle two-body cross section at astrophysical energies, free of Coulomb suppression and electron-screening effect. The THM is applied to derive the bare nucleus cross section of the ~9Be(p,α)~6Li reaction, which plays a key role in beryllium burning processes, from the cross section measurement of the suitable three-body process d(~9Be,α~6Li)n. In this case, the deuteron is used as“Trojan Horse
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