利用斜向入射與藍寶石晶體提升X光共振腔輸出頻譜效率與能量解析度之研究

The improvement of hard X-ray resonators in efficiency and energy resolution using inclined incidence geometry and sapphire crystals

吳玉忻(Wu, Yu Hsin)

指導教授 ： 張石麟

國立清華大學/理學院/物理學系/博士(2015年)

https://doi.org/10.6843/NTHU.2015.00522

摘要

利用晶體繞射作為反射機制之X光共振腔於1967年提出，由於實驗有賴高能量解析度之光源以及樣品製程之困難，直至2005年才首次觀察到X光Fabry–Pérot共振腔之干涉條紋：利用高能量解析單光儀（high resolution monochromator，HRM）於能量14.4388 keV時產生能量解析度為0.36 meV之X光，量測Si（12 4 0）布拉格背向繞射在共振腔之輸出頻譜。但由於矽晶體之鑽石結構特性，使得多光繞射無法避免，輸出頻譜不盡理想。為提升X光Fabry–Pérot共振腔之實用性，本論文提出兩種方式改善晶體製成之共振腔所遇到之困難：
1. 使用藍寶石（Al2O3）取代矽（Si）作為共振腔之繞射材料：藍寶石之結構為hexagonal，在能量E=14.3148 keV時，背向繞射(0 0 0 30)僅為兩光繞射，少了多光繞射之干擾，預期可提升晶體反射率以及輸出頻譜之能量解析度。
2. 斜向入射X光Fabry–Pérot共振腔：由於晶體繞射之機制很容易遇到多光繞射之問題，我們利用光之可逆性，將入射光利用複繞射面直接反射回晶體，這種方式可減少晶體吸收，以提升反射率以及共振效率。
根據模擬以及實驗結果分析，此兩種方式對於改善共振腔之效率以及能量解析度有顯著之效果，而斜向入射共振腔之效果更為突出，僅透過入射路徑之改變，可使相同結構之共振腔在共振效率（peak efficiency）上提升大約30倍，大幅提升了晶體製程之X光Fabry–Pérot共振腔之實用性。我們可將斜向入射之機制應用於不同能量與不同材料上，期待此元件在未來能應用於高能量解析X光之相關領域研究。

關鍵字

X光 ； 共振腔 ； 斜向入射 ； 藍寶石

並列摘要

Hard X-ray Fabry–Pérot resonators using Bragg-back-reflection has been proposed and explored since 1967. The ideas were brought into effect until 2005 when Chang et al. directly observed cavity resonance fringes in a Si crystal with the size of 40~150 μm. The performance of cavities using Si (12 4 0) back diffraction at 14.4388 keV with energy resolution ΔE of 0.36 meV was barely satisfactory for the intrinsic limits of crystal –based resonators from crystal absorption and 24-beam diffractions.
In this thesis, we proposed two models of resonators to improve the practicability of hard X-ray resonators:
(1) Using Al2O3 ( 0 0 0 30) back diffraction for X-ray resonators at 14.3148 keV: For its less absorption and hexagonal structure, the resonator of sapphire crys-tals underwent a pure 2-beam diffraction which could enhance the resonance interference and improve finesse compared with the one of silicon crystals.
(2) Hard X-ray resonators with inclined-incidence geometry:
Utilizing one of the multiple diffractions as incident beam to generate back dif-fraction in a crystal cavity for resonance and demonstrate FP resonance with ul-trahigh efficiency, highly purified resolving power in the sub-meV range and low background.
Both the experimental results show clear resonance fringes for the enhance-ment in Finesse and peak efficiency, especially for the inclined incidence, only by changing the path of incidence, the visibility was enhanced nearly 30 times than normal incidence. The compact-sized resonator with these promising fea-tures can be widely implemented to different energies and materials and antici-pated to apply to ultrahigh-resolution X-ray optics for X-ray diffraction, spec-troscopy, and imaging applications.

並列關鍵字

X-ray ； Fabry-Perot ； cavity ； resonator ； inclined incidence ； sapphire

參考文獻

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[2] S.-L. Chang, Yu. P. Stetsko, M.-T. Tang, Y.-R. Lee, W.-H. Sun, M. Yabashi, and T. Ishikawa, “X-ray resonance in crystal cavities: realization of Fabry-Perot reso-nator for hard x rays,” Phys. Rev. Lett. 94, 174801 (2005).

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[5] International Tables of Crystallography, Vol.3 3rd edition (IUCr, 2004).