微束斑X射线源的理论与实验研究
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摘要
微束斑X射线源是指能够产生束斑直径为1.0～100μm的高亮度X射线源，它在生物医学、生化反应动力学、工业无损探伤、X射线显微成像、X-CT等科学研究与技术应用中，发挥着举足轻重的作用。
目前，产生微束斑X射线源的主要方法是通过传统的电子束打靶X射线管、同步辐射X射线源、激光等离子体X射线源等与波带片、多层膜、X射线透镜等X射线光学元件相配合组成的。这些射线源要么束斑偏大、亮度不够，要么造价昂贵、结构庞大、使用不便，严重限制着微束斑X射线光学的研究与发展。
高速运动的高密度小束斑电子束直接轰击金属靶面可以辐射出小束斑的X射线，依据此原理，本论文研制的微束斑X射线源主要由三部分组成，即具有优良电子发射能力的LaB_6阴极电子枪发射系统、等径双圆筒静电聚焦系统以及金属靶。LaB_6电子枪发射的电子束经过静电聚焦系统，被会聚为微米级的电子束斑，该微束斑电子束与固体金属靶相互作用，产生出微束斑的X射线，从而形成微束斑X射线源。
电子束打靶产生X射线已经是很成熟的理论，因此，本论文的理论研究重点是选择合适结构的电子源系统与合适结构的聚焦系统并将电子束会聚为微米级的细小束斑。在理论分析中，先后采用边界元方法、差分方法、有限元方法等科学数值计算方法，编制程序，对X射线源的电子枪发射系统、聚焦系统以及发射系统与聚焦系统的组合系统等的电场分布进行了严格的计算，在准确求得各系统电场内各个剖分点的电场场强、偏导数等参量的基础上，采用蒙特-卡罗模拟方法和不等距龙格-库塔方法相结合，追踪由LaB_6单晶阴极表面发射出的大量电子束在电场内的运动轨迹，求出点扩展函数，并根据点扩展函数的优劣，反复调节、大量计算对比，挑选出X射线源最佳的电极结构及其组合。在最佳条件下的X射线源，当LaB_6阴极加热温度为1900～2000K、饱和发射电流为58.3～141.4μA时，计算所得轰击金属靶面的电子束焦斑的半值宽度仅有1.0μm左右。
依据理论设计组装的微束斑X射线源样机，既可连续发射也可脉冲辐射X射线；其各项性能，在满足其正常工作条件的综合测试仪上通过了实际运行测试。当阴极采取连续发射电子的工作模式(发射电流40μA)时，记录测量到的最小电子束焦斑直径不大于22μm；而在相同条件下，以脉冲方式发射电子束时，其焦斑直径不大于15μm，原因是短时间的电子轰击减小了靶面的热堆积效应等。
本论文所研制的微束斑X射线源，可连续发射也可脉冲辐射，不仅可以发射具有足够亮度的微米级束斑X射线，而且仪器重量轻，体积小，可灵活移动，价廉经济，完全可以为一般大专院校的普通实验室及小研究团体接受。相信该微束斑X射线源的成功研制，对于微束斑X射线光学的研究与发展，促进生物学、医学、生命科学以及材料科学等的发展将具有重要的现实意义。
The x-ray source with micro-beam is an x-ray source with 1.0~100um x-ray spot's diameter and with high enough brightness. The source is very important in such scientific researches and applications as biomedicine, biochemical dynamics, industrial nondestructive examination and detection, x-ray microscopy and micro-CT.
Several different types of x-ray source with micro-beam are currently being used, such as conventional x-ray tubes and other electron-beam devices, synchrotron radiation source, laser plasmas bombing target, being cooperated with one or two x-ray optical elements as Bragg-Fresnel zone plates, multilayer mirrors into the x-ray regime, or x-ray lens. Most of these x-ray sources are large, complex, and expensive.
The x-ray source with micro-beam that we developed avoids several fundamental limitations in comparison with traditional ones. The novel x-ray source is based on the fact that when the high energy electron-beams with micro-focus bombard metal target, it would radiate x-rays with micro-beam. The source has two fundamental parts, electron-emitting system and focusing system. The former consists of LaBe crystal cathode, Wehnelt grid and an accelerating anode, and the latter is comprised of two equal radius concentrating cylinder electrodes.
It's well known that the x-ray radiation emitted by electron-beams acting on metal target surface has well developed for about 100 years. So, our main work is to obtain a satisfactory electron beam with diameter about micrometers by means of choosing a suitable LaBe cathode electron-gun system and a suitable focusing system.
In theory, firstly, the numerical simulation methods, such as the boundary element method (BEM), the finite difference method (FDM) and the finite element method(FEM), were utilized to calculate the distribution of electric field intensity and potential of the electron-gun system, the focusing system and the whole equipment, respectively. Then, the trajectories of the electron-beam were traced by using of the Runge-Kutta method and the Monte-Carlo method associatively. The point spread function (PSF) was also calculated. Finally, It is according to the size of the whole instrument and the PSF that we chose the optimal electrodes' parameters, e.g. configuration and position and potential. The results show that when the temperature of LaB6 cathode is about 1900-2000K and it emits current saturation 58.3-141.4uA, the full width of half maximum (FWHM) of electron-beam spot focused onto the surface of metal target is only l.Oum or so.
In the subsequent experiments, when the LaBe cathode emitted continuously and the current is 40uA, the minimum diameter of electron-beam spot recorded was 22 um. While the LaB6 emitted pulse, the minimum diameter of the focus decreased and wasn't more than 15um. The reason is that the heat accumulated on the foil-film, which enlarged the focus dimension, reduced remarkably.
The x-ray source with micro-beam we developed, not only can be located in
individual research laboratories and used by researchers because it's a convenient and simple source which is so small and so low-cost, but also can be facilely and safely moved and manipulated. Furthermore, the source can be used to ameliorate and enhance the spacial resolution of existing medical transmission imaging and x-ray micro-CT images. It is well-founded believed that our x-ray source with micro-beam will help the advance in the studies of biology, medicine, life sciences, and material science, etc.
目前，产生微束斑X射线源的主要方法是通过传统的电子束打靶X射线管、同步辐射X射线源、激光等离子体X射线源等与波带片、多层膜、X射线透镜等X射线光学元件相配合组成的。这些射线源要么束斑偏大、亮度不够，要么造价昂贵、结构庞大、使用不便，严重限制着微束斑X射线光学的研究与发展。
高速运动的高密度小束斑电子束直接轰击金属靶面可以辐射出小束斑的X射线，依据此原理，本论文研制的微束斑X射线源主要由三部分组成，即具有优良电子发射能力的LaB_6阴极电子枪发射系统、等径双圆筒静电聚焦系统以及金属靶。LaB_6电子枪发射的电子束经过静电聚焦系统，被会聚为微米级的电子束斑，该微束斑电子束与固体金属靶相互作用，产生出微束斑的X射线，从而形成微束斑X射线源。
电子束打靶产生X射线已经是很成熟的理论，因此，本论文的理论研究重点是选择合适结构的电子源系统与合适结构的聚焦系统并将电子束会聚为微米级的细小束斑。在理论分析中，先后采用边界元方法、差分方法、有限元方法等科学数值计算方法，编制程序，对X射线源的电子枪发射系统、聚焦系统以及发射系统与聚焦系统的组合系统等的电场分布进行了严格的计算，在准确求得各系统电场内各个剖分点的电场场强、偏导数等参量的基础上，采用蒙特-卡罗模拟方法和不等距龙格-库塔方法相结合，追踪由LaB_6单晶阴极表面发射出的大量电子束在电场内的运动轨迹，求出点扩展函数，并根据点扩展函数的优劣，反复调节、大量计算对比，挑选出X射线源最佳的电极结构及其组合。在最佳条件下的X射线源，当LaB_6阴极加热温度为1900～2000K、饱和发射电流为58.3～141.4μA时，计算所得轰击金属靶面的电子束焦斑的半值宽度仅有1.0μm左右。
依据理论设计组装的微束斑X射线源样机，既可连续发射也可脉冲辐射X射线；其各项性能，在满足其正常工作条件的综合测试仪上通过了实际运行测试。当阴极采取连续发射电子的工作模式(发射电流40μA)时，记录测量到的最小电子束焦斑直径不大于22μm；而在相同条件下，以脉冲方式发射电子束时，其焦斑直径不大于15μm，原因是短时间的电子轰击减小了靶面的热堆积效应等。
本论文所研制的微束斑X射线源，可连续发射也可脉冲辐射，不仅可以发射具有足够亮度的微米级束斑X射线，而且仪器重量轻，体积小，可灵活移动，价廉经济，完全可以为一般大专院校的普通实验室及小研究团体接受。相信该微束斑X射线源的成功研制，对于微束斑X射线光学的研究与发展，促进生物学、医学、生命科学以及材料科学等的发展将具有重要的现实意义。
The x-ray source with micro-beam is an x-ray source with 1.0~100um x-ray spot's diameter and with high enough brightness. The source is very important in such scientific researches and applications as biomedicine, biochemical dynamics, industrial nondestructive examination and detection, x-ray microscopy and micro-CT.
Several different types of x-ray source with micro-beam are currently being used, such as conventional x-ray tubes and other electron-beam devices, synchrotron radiation source, laser plasmas bombing target, being cooperated with one or two x-ray optical elements as Bragg-Fresnel zone plates, multilayer mirrors into the x-ray regime, or x-ray lens. Most of these x-ray sources are large, complex, and expensive.
The x-ray source with micro-beam that we developed avoids several fundamental limitations in comparison with traditional ones. The novel x-ray source is based on the fact that when the high energy electron-beams with micro-focus bombard metal target, it would radiate x-rays with micro-beam. The source has two fundamental parts, electron-emitting system and focusing system. The former consists of LaBe crystal cathode, Wehnelt grid and an accelerating anode, and the latter is comprised of two equal radius concentrating cylinder electrodes.
It's well known that the x-ray radiation emitted by electron-beams acting on metal target surface has well developed for about 100 years. So, our main work is to obtain a satisfactory electron beam with diameter about micrometers by means of choosing a suitable LaBe cathode electron-gun system and a suitable focusing system.
In theory, firstly, the numerical simulation methods, such as the boundary element method (BEM), the finite difference method (FDM) and the finite element method(FEM), were utilized to calculate the distribution of electric field intensity and potential of the electron-gun system, the focusing system and the whole equipment, respectively. Then, the trajectories of the electron-beam were traced by using of the Runge-Kutta method and the Monte-Carlo method associatively. The point spread function (PSF) was also calculated. Finally, It is according to the size of the whole instrument and the PSF that we chose the optimal electrodes' parameters, e.g. configuration and position and potential. The results show that when the temperature of LaB6 cathode is about 1900-2000K and it emits current saturation 58.3-141.4uA, the full width of half maximum (FWHM) of electron-beam spot focused onto the surface of metal target is only l.Oum or so.
In the subsequent experiments, when the LaBe cathode emitted continuously and the current is 40uA, the minimum diameter of electron-beam spot recorded was 22 um. While the LaB6 emitted pulse, the minimum diameter of the focus decreased and wasn't more than 15um. The reason is that the heat accumulated on the foil-film, which enlarged the focus dimension, reduced remarkably.
The x-ray source with micro-beam we developed, not only can be located in
individual research laboratories and used by researchers because it's a convenient and simple source which is so small and so low-cost, but also can be facilely and safely moved and manipulated. Furthermore, the source can be used to ameliorate and enhance the spacial resolution of existing medical transmission imaging and x-ray micro-CT images. It is well-founded believed that our x-ray source with micro-beam will help the advance in the studies of biology, medicine, life sciences, and material science, etc.
引文
1. P.C.Cheng, R.Feder, D.M.Shinozaki, et al., Soft x-ray contact microscopy, Nucl. Instr. and Meth. A(246), 1986:668～674.
2. G.Schmahl, X-ray microscopy, Nucl. Instr. and Meth. 208, 1983:361～365.
3．郑世才，“射线实时成象检验技术”，《无损检测》，22(7)，2000：328～333．
4. P. Dhez, P. Chevallier, T. B.Lucatorto, et al., Instrumental aspects of x-ray microbeams in the range above 1keV, Rev.Sci. Instrum., 70(4), 1999:1907～1920.
5. T. Tomie, H.Shimize, T. Majima, M.Yanada, et al., Flash contact X-ray microscopy of biological specimen in water. Proc. SPIE, 1992,1741:118～128.
6. Robin Medenwaldt and Erik UggerhΦj. Description of an x-ray microscopy with 30nm resolution. Rev. Sci. Instrum., 69(8),1998:902～906.
7. C.Jacobsen, S.Williams, et.al, Diffraction-limited imaging in a Scanning transmission X-ray microscopy. Rev. Sci. Instrum., 86(3/4), 1991:351～364.
8. A.Krol, A.Ikhlef, J.C.Kieffer, et al, Laser-based microfocused x-ray source for mammography: Feasibility study, Med. Phys., 24(5), 1997:725～732.
9. R.E.Marrs, D.H.Schneider, J.W. McDonald, Projection X-ray microscope
powered by highly charged ions. Rev. Sci. Instum., 69(1),1998:204～209.
10. J.E.Bateman, G.R.Moss, P. Rockett, S.Webb, Digital x-ray microscopy with a microfocal x-ray generator and an MWPC area detector, Nucl.Instru. and Meth. A(273),1988:767～772.
11. S.Rondot, J.Cazaux, Quantitative mapping of species moving in solution by x-ray projection microscopy, Rev. Sci. Instrum., 68(4), 1997:1787～1791.
12．ъ．я．皮涅斯(前苏联)著，何犖译，《细聚焦X射线管及其在结构分析中的应用》，科学出版社，1963．6．
13. W. Ehrenberg, W. Spear, An Electrostatic Focusing System and Its Application to a Fine Focus X-Ray Generator, Proc.phys. soc. B 64(5), 1951:374-378.
14. E.J.Morton, S.Webb, J.E.Bateman, et al., Three-dimensional x-ray micro-tomography for medical and biological applications, Phys. Med. Biol., 35(1990):805-820.
15. K.Machin, S.Webb, Cone-beam X-ray microtomography of small specimens, Phys. Med. Biol., 39(1994):1639-1657.
16. J.Kirz, H.Rarback, Soft x-ray microscopes, Rev. Sci. Instrum., 56(1), 1985:1～13.
17. J.Lehr, J.B.Sibarita, J.M.Chassery, Image restoration in x-ray microscopy: PSF determination and biological applications, IEEE Trans. on Image. Proce., 7(2), 1998: 258～263.
18. Kouichi Tsuji, Tasaku Sato, et al., Preliminary experiment of total reflection x-ray fluorescence using two glancing x-ray beams excitation, Rev. Sci. Instrum., 70(3), 1999:1621～1623.
19. A.V. Bronnikov, Virtual alignment of x-ray cone-beam tomography system using two calibration aperture measurements, Opt. Eng., 38(2), 1999: 381～386.
20. G.T. Gullberg, B.M.W. Tsui, C.R.Crawford, et al., Estimation of geometrical parameters and collimator evaluation for cone beam tomography, Med.Phys., 17(2), 1990:264～272.
21. F.H.Seguin, P. Burstein, P.J.Bjorkholm, et al., X-ray computed tomography with 50-μm resolution, Appl. Opt., 24(23), 1985:4117～4123.
22．金春水，王占山，曹健林，“软X射线投影光刻技术”，《强激光与粒子束》，12(5)，2000：559～564．
23. T. Rayment, S.M. Schroeder, G.D.Moggridge, et al., Electron-yield x-ray absorption spectroscopy with gas microstrip detectors, Rev. Sci. Instrum.,
71(10), 2000:3640～3645.
24. A.Pogany, D.Gao, S.W. Silkins, Contrast and resolution in imaging with a microfocus x-ray source, Rev. Sci. Instrum., 68(7), 1997:2774～2782.
25. G.D.Stasio, B.Gilbert, T. Nelson, et al., Feasibility tests of transmission x-ray photoelectron emission microscopy of wet samples, Rev. Sci. Instrum., 71(1), 2000:11～14.
26. Yamamoto N., Hosokawa Y., Theoretical study on the high-energy electron energy loss spectroscopy compared with x-ray absorption spectroscopy, Ⅱ effects of single elastic scattering, Jpn.J. Appl. Phys., 28(9), 1989:1683～1694.
2. G.Schmahl, X-ray microscopy, Nucl. Instr. and Meth. 208, 1983:361～365.
3．郑世才，“射线实时成象检验技术”，《无损检测》，22(7)，2000：328～333．
4. P. Dhez, P. Chevallier, T. B.Lucatorto, et al., Instrumental aspects of x-ray microbeams in the range above 1keV, Rev.Sci. Instrum., 70(4), 1999:1907～1920.
5. T. Tomie, H.Shimize, T. Majima, M.Yanada, et al., Flash contact X-ray microscopy of biological specimen in water. Proc. SPIE, 1992,1741:118～128.
6. Robin Medenwaldt and Erik UggerhΦj. Description of an x-ray microscopy with 30nm resolution. Rev. Sci. Instrum., 69(8),1998:902～906.
7. C.Jacobsen, S.Williams, et.al, Diffraction-limited imaging in a Scanning transmission X-ray microscopy. Rev. Sci. Instrum., 86(3/4), 1991:351～364.
8. A.Krol, A.Ikhlef, J.C.Kieffer, et al, Laser-based microfocused x-ray source for mammography: Feasibility study, Med. Phys., 24(5), 1997:725～732.
9. R.E.Marrs, D.H.Schneider, J.W. McDonald, Projection X-ray microscope
powered by highly charged ions. Rev. Sci. Instum., 69(1),1998:204～209.
10. J.E.Bateman, G.R.Moss, P. Rockett, S.Webb, Digital x-ray microscopy with a microfocal x-ray generator and an MWPC area detector, Nucl.Instru. and Meth. A(273),1988:767～772.
11. S.Rondot, J.Cazaux, Quantitative mapping of species moving in solution by x-ray projection microscopy, Rev. Sci. Instrum., 68(4), 1997:1787～1791.
12．ъ．я．皮涅斯(前苏联)著，何犖译，《细聚焦X射线管及其在结构分析中的应用》，科学出版社，1963．6．
13. W. Ehrenberg, W. Spear, An Electrostatic Focusing System and Its Application to a Fine Focus X-Ray Generator, Proc.phys. soc. B 64(5), 1951:374-378.
14. E.J.Morton, S.Webb, J.E.Bateman, et al., Three-dimensional x-ray micro-tomography for medical and biological applications, Phys. Med. Biol., 35(1990):805-820.
15. K.Machin, S.Webb, Cone-beam X-ray microtomography of small specimens, Phys. Med. Biol., 39(1994):1639-1657.
16. J.Kirz, H.Rarback, Soft x-ray microscopes, Rev. Sci. Instrum., 56(1), 1985:1～13.
17. J.Lehr, J.B.Sibarita, J.M.Chassery, Image restoration in x-ray microscopy: PSF determination and biological applications, IEEE Trans. on Image. Proce., 7(2), 1998: 258～263.
18. Kouichi Tsuji, Tasaku Sato, et al., Preliminary experiment of total reflection x-ray fluorescence using two glancing x-ray beams excitation, Rev. Sci. Instrum., 70(3), 1999:1621～1623.
19. A.V. Bronnikov, Virtual alignment of x-ray cone-beam tomography system using two calibration aperture measurements, Opt. Eng., 38(2), 1999: 381～386.
20. G.T. Gullberg, B.M.W. Tsui, C.R.Crawford, et al., Estimation of geometrical parameters and collimator evaluation for cone beam tomography, Med.Phys., 17(2), 1990:264～272.
21. F.H.Seguin, P. Burstein, P.J.Bjorkholm, et al., X-ray computed tomography with 50-μm resolution, Appl. Opt., 24(23), 1985:4117～4123.
22．金春水，王占山，曹健林，“软X射线投影光刻技术”，《强激光与粒子束》，12(5)，2000：559～564．
23. T. Rayment, S.M. Schroeder, G.D.Moggridge, et al., Electron-yield x-ray absorption spectroscopy with gas microstrip detectors, Rev. Sci. Instrum.,
71(10), 2000:3640～3645.
24. A.Pogany, D.Gao, S.W. Silkins, Contrast and resolution in imaging with a microfocus x-ray source, Rev. Sci. Instrum., 68(7), 1997:2774～2782.
25. G.D.Stasio, B.Gilbert, T. Nelson, et al., Feasibility tests of transmission x-ray photoelectron emission microscopy of wet samples, Rev. Sci. Instrum., 71(1), 2000:11～14.
26. Yamamoto N., Hosokawa Y., Theoretical study on the high-energy electron energy loss spectroscopy compared with x-ray absorption spectroscopy, Ⅱ effects of single elastic scattering, Jpn.J. Appl. Phys., 28(9), 1989:1683～1694.