论文标题
研究裂变中角动量效应
Study of angular momentum effects in fission
论文作者
论文摘要
背景:长期以来讨论了角动量在裂变中的作用,但可观察到的效果很难量化。目的:我们讨论与裂变和当前定量插图中角动量相关的各种影响。方法:我们采用裂变模拟模型$ \ mathtt {freya} $,它非常适合此目的,因为它遵守了过程的每个步骤,包括线性和角度动量保护。我们首先讨论在$ \ mathtt {freya} $中的角动量的实现,然后评估特定的可观察物,包括各种相关的可观察结果。我们还研究了中子诱导的$^{235} $ u相对于基态的低洼异构体裂变的潜在影响。结果:裂变过程中固有的波动确保初始化合物核的自旋对片段旋转的影响很小,因此几乎不相关。裂变片段的旋转幅度与光子多重性之间存在明显的相关性。我们还考虑了由蒸发片段的旋转引起的动力学各向异性,尤其是研究预测的中子中性 - 中子开口角的分布,表明虽然它因蒸发后坐后的影响而支配,但可以通过傅立叶分解来提取动态各向异性的信号。最后,我们注意到,使用异构体目标,$^{235 {\ rm m}} $ u($ n _ {\ rm th} $,f)可以增强对称屈服,因此可能导致低碎片动能的较高中子多重性。
Background: The role of angular momentum in fission has long been discussed but the observable effects are difficult to quantify. Purpose: We discuss a variety of effects associated with angular momentum in fission and present quantitative illustrations. Methods: We employ the fission simulation model $\mathtt{FREYA}$ which is well suited for this purpose because it obeys all conservation laws, including linear and angular momentum conservation at each step of the process. We first discuss the implementation of angular momentum in $\mathtt{FREYA}$ and then assess particular observables, including various correlated observables. We also study potential effects of neutron-induced fission of the low-lying isomeric state of $^{235}$U relative to the ground state. Results: The fluctuations inherent in the fission process ensure that the spin of the initial compound nucleus has only a small influence on the fragment spins which are therefore nearly uncorrelated. There is a marked correlation between the spin magnitude of the fission fragments and the photon multiplicity. We also consider the dynamical anisotropy caused by the rotation of an evaporating fragment and study especially the distribution of the projected neutron-neutron opening angles, showing that while it is dominated by the effect of the evaporation recoils, it is possible to extract the signal of the dynamical anisotropy by means of a Fourier decomposition. Finally, we note that the use of an isomeric target, $^{235 {\rm m}}$U($n_{\rm th}$,f), may enhance the symmetric yields and can thus result in higher neutron multiplicities for low total fragment kinetic energy.