Fresh-lands Environmental Actions, Caversham, UK.
Sci Prog. 2012;95(Pt 2):101-74. doi: 10.3184/003685012X13336424471773.
Muonium (Mu), may be regarded as a radioactive hydrogen atom with a positive muon as its nucleus, and is formed in a range of media which are irradiated with positive muons. This exotic atom can be considered as a second radioisotope of hydrogen, along with tritium. Addition of this light atom (with a mass 1/9th that of a normal hydrogen, protium, atom) to unsaturated organic molecules forms free radicals, in which the muon serves as a radioactive and magnetic probe of their kinetic and structural properties. Suitable examples are chosen to illustrate the very large functionality of organic radicals which have been measured using muons and various methods of muSR, where mu stands for muon, S for spin and R may refer to rotation, resonance or relaxation. The principal techniques illustrated are transverse-field muon spin rotation (TF-muSR), avoided level crossing muon spin resonance (ALC-muSR) and longitudinal-field muon spin relaxation (LF-muSRx). Structural studies of radicals, the determination of mechanisms for radical formation, the measurement of radical stabilisation energies, the determination of the kinetics of reactions of free muonium atoms and of free radicals have all been accomplished using TF-muSR methods. It is further shown that TF-muSR is most useful in measuring radical reaction rates in non-aqueous media, to provide information of relevance to cell membrane damage and repair Muonium may further be used as a mechanistic probe since it determines a true pattern of H-atom reactivity in molecules, against which results from similar radiolysed materials may be compared. [In many solid materials that are exposed to ionising radiation, apparent H-atom adduct radicals are detected but which originate from charge-neutralisation of positive holes (radical cations) and ejected electrons, without free H-atoms being formed. DNA is the superlative example of this. Free H-atoms normally feature in the province of radiolysed aqueous media]. The applications of ALC-muSR and LF-muSRx in studying the reorientation of reactive radicals on reactive surfaces forms the substantive proportion of the review: considered specifically are radicals sorbed in zeolites, in clays and in porous silica, in porous carbons and on ice-surfaces, in connection with their role as intermediates in catalytic systems, particularly hydrocarbon cracking and oxidation processes, and in atmospheric aerosol chemistry. The formation of muonium and other muon species in cation-exchanged zeolite-X samples are also considered, according to the evidence of longitudinal field repolarisation measurements. Finally, mention is given of the use of muSR techniques for studying radicals in the gas-phase.
正电子素（Mu）可以被视为带正电μ子的放射性氢原子，它在一系列被正μ子辐照的介质中形成。这种奇特的原子可以被认为是氢的第二种放射性同位素，与氚一起。将这种轻原子（质量为普通氢原子，质子的 1/9）添加到不饱和有机分子中会形成自由基，其中μ子可用作它们动力学和结构性质的放射性和磁性探针。选择合适的例子来说明已使用μ子和各种μSR 方法（μ 代表μ子，S 代表自旋，R 可能指旋转、共振或弛豫）测量的有机自由基的巨大功能。所说明的主要技术是横向磁场μ子旋转（TF-μSR）、避免能级交叉μ子自旋共振（ALC-μSR）和纵向磁场μ子自旋弛豫（LF-μSRx）。使用 TF-μSR 方法已经完成了自由基的结构研究、自由基形成机制的确定、自由基稳定能的测量、自由正电子素原子和自由基反应动力学的测量。进一步表明，TF-μSR 最适用于测量非水介质中自由基的反应速率，以提供与细胞膜损伤和修复相关的信息。正电子素还可以用作机制探针，因为它确定了分子中 H 原子反应性的真实模式，与类似放射性裂解材料的结果进行比较。[在许多暴露于电离辐射的固体材料中，检测到表观 H-原子加合物自由基，但它们源自正空穴（自由基阳离子）和射出电子的电荷中和，而没有形成自由 H-原子。DNA 就是这种情况的绝佳例子。在放射性裂解水介质中，自由 H-原子通常是特征]。在研究反应性自由基在反应性表面上的重定向方面，ALC-μSR 和 LF-μSRx 的应用构成了综述的主要部分：具体考虑的是沸石、粘土和多孔二氧化硅、多孔碳和冰表面上吸附的自由基，以及它们在催化系统中的作用作为中间体，特别是烃类裂化和氧化过程，以及大气气溶胶化学。根据纵向场再极化测量的证据，还考虑了在阳离子交换沸石-X 样品中形成正电子素和其他μ子物种的情况。最后，提到了使用μSR 技术研究气相中的自由基。
