Определение элементного состава образца методом PIXE (Proton Induced X-ray Emission)



Лекция 31 Определение элементного состава образца методом PIXE Proton Induced Xray Emission. Метод PIXE русский аналог РФА – рентгеновский флуоресцентный анализ является малораспространенным как следует из его названия основан на возбуждении ускоренными протонами линий характе...



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Лекция 31

Определение элементного состава образца методом PIXE (Proton Induced X-ray Emission).

Метод PIXE (русский аналог РФА – рентгеновский флуоресцентный анализ является малораспространенным), как следует из его названия, основан на возбуждении ускоренными протонами линий характеристического рентгеновского излучения с последующим определением элементов, испустивших эти линии.

Для возбуждения линий характеристического рентгеновского излучения (ХРИ) энергия, переданная протоном электрону, должна больше энергии связи электрона Есв. Так как максимальная переданная энергия 4E0 mp me/(mp + me)2  4E0 me /mp  E0 /500, то при Есв ~ 1 кэВ необходим протонный пучок с энергией больше 0,5 МэВ. Если использовать более тяжелые ионы, то потребуются еще большие энергии. Обычно в данном методе используются либо протонные пучки с энергиями 2-3 МэВ или ионы гелия с энергиями 6-8 МэВ.

Важнейшим преимуществом метода PIXE является возможность проводить анализ на воздухе без помещения образца в вакуумную камеру. Вывод ионного пучка из вакуумной камеры производится через сверхтонкие Si4N3 фольги. Использование магнитных линз позволяет сжать ионный пучок до диаметра 10–30 мкм при токе пучка ~ 100 пA. Подобные пучки обычно называют микропучками.

Увеличение скорости счета рентгеновских квантов обеспечивается использованием широкоаппертурных детекторов. В частности, при использовании кольцевых кремниевых дрейфовых детекторов с общей площадью 60 мм2 телесный угол составляет 1,09 ср. При охлаждении до азотных температур энергетическое разрешение подобных детекторов удается довести до 170 эВ.

A compact expression for the number of detected X-rays from the element i, present in the target with the weight fraction xi, is given by

where DX is the detector solid angle and Np, the number of projectiles that hit the target. The quantities labeled by i depend on the energy of emitted X-rays that are characteristic for specific element: the detector efficiency i, the transmission of absorbers between target and detector i, and the thick target factors Ti that involves the X-ray production cross-section rxi and the attenuation coefficient i:

Here S(E) is the projectile stopping force in the target and (E) is the path-length that the induced X-rays had to pass through the target. We assume that in thick target, the projectile energy reduces from its impact value E1 to zero. Since xi are weight fractions, Eq. (1) also contains atomic mass Mi and Avogadro_s number NA. Calculation of Ti requires the data base of X-ray production cross-sections, stopping forces and attenuation coefficients. For the K-shell, the values for protons are fixed since 1989. The uncertainty of reference cross-sections is a few percent and reduces below 1% for most favorable cases (as for 1–3 MeV protons in Cu).

Technical development on the exit parts of the in-air measuring stations include: focusing of the proton beam to the size of several 10 мкм, improvements in positioning of the target, and measurements of the impact particle number. The beam size, produced in combination of magnetic optics, very thin exit windows (Si4N3) and a very short air-path (up to 3 mm) flushed with helium results in space resolution of about 10–20 мкм, which is convenient enough for the studies of the objects of cultural heritage. A similar space resolution is achieved by a competing technique, confocal Xray fluorescence induced by synchrotron radiation.

Target positioning at elaborated measuring stations can be done in a remote way. Some set-ups provide possibility of pre-selecting a series of measuring points, which is a certain advantage for analyzing large flat targets (papers and paints). Rather accurate positioning of the paper documents (12.5 мкм) was achieved by an automatic pattern recognition method using a laser beam and a camera mounted at an angle sensitive for parallax.

The normalization of measurements to the preselected number of projectiles is done in several ways: the number of impact particles is measured by (1) X-ray or RBS signal from the exit window, (2) X-ray or current signal from a chopper periodically intercepting the beam, (3) X-ray or visible light fluorescence from the air gap between the exit window and target. The advantage of collecting Ar K-lines in the measuring spectrum is twofold: for most of applications, the lines do not coincide with other L and M lines; the ratio of line intensities does not depend on pile-up effects. On the other side, however, argon normalization cannot be used in combination with strong absorbers.

The methods for determination of concentration profiles split in two classes. One assumes that the shape of the concentration profile is approximately known and given with an analytical function, and several measurements are used to determine the parameters of this function. The second, more general methods make no assumption on the profile shape, but work with a limited space resolution since they slice the target into layers.

The number of layers is equal or smaller than the number of measurements. A series of measurements, aimed to probe an unknown profile, is selected in a way that projectiles reach different parts of the concentration profile. This is achieved by varying the projectile energy (Fig. 3), by tilting the sample, or by a combination of both. The tilting techniques requires samples with a uniform flat surface. The position of the rotational axis has to be determined with certain precision, since it lies in the surface plane of the sample.

For the analysis of the objects of art, usually exhibiting irregular, structured surface, the technique of varying the projectile energy at perpendicular impact appears more applicable. This technique is also less sensitive to the surface roughness of the sample, provided the beam size largely exceeds the size of surface irregularities. A simple method of rapidly selecting the projectile energy by varying the length of projectile air-path was proposed in [29], but the disadvantage of this method is radial broadening of the beam, which renders measurements in a selected point impossible. Varying the accelerator energy then remains the only reasonable solution. Fast variation of the projectile impact energy can also be achieved by a series of stopping foils.

Using either the variation of the impact energy or tilting the sample, the X-ray production is strongest in the target region just below the surface. Increasing the projectile energy or the impact angle increases the X-ray production from the regions deeper in the target, but this contribution is generally smaller than that from the near-surface region. The de-convolution of the measurements into a concentration profile is then a hard numerical problem, rather sensitive to accidental variation of the X-ray yields. Solution of the deconvolution algorithm is the matrix xij, which denotes the concentration of the element i in the layer j:

For the end, it is certainly tempting to predict further development of the PIXE method. Magnetic optics tends to be more compact, and shorter focusing lengths would allow smaller dimensions of the whole set-up. Beam size may drop to a few 10 nm for routine measurements. Complex target manipulators, already seen at newer setups, will inspire development of software for more automated measurements. Similar trends could be observed at external beams. It is hard to predict the smallest beam size attainable, but it will certainly be progress in automatic execution of measurements. Of particular components, new types of detectors, avoiding cooling with liquid nitrogen may come into wider use, supporting spread of portable systems.


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