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...
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 1030 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 13 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 1020 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 , 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.
А также другие работы, которые могут Вас заинтересовать
|11224.||Talk about the problems a newly-independent state is confronted with||23.5 KB|
|Talk about the problems a newlyindependent state is confronted with. Chinese people say that the worst thing is to live in the time of changes. With the rich choice of possible way of further development comes a bunch of problems as well. Its especially true for newlyindependent states. Id like to illustrate this on the example of Samoa. For centuries time stood still in Samoa. The people worked at banana plantations and respected the customs that the family chiefs presented abso...|
|11225.||Talk on how parents feel about their children travelling on their own for the first time||23.5 KB|
|Talk on how parents feel about their children travelling on their own for the first time. It goes without saying that parents is always get worried when their children go abroad to visit new places and get to know other ways of living. Without any doubt traveling is necessary for all of us as it broadens our minds and horizons its kind of relaxation and a wonderful opportunity to get away from it all. When children go on a journey they relax their body refresh their spirits and renew ...|
|11226.||Talk about stereotyped ideas that people may have about the way of life in oyher countries||24.5 KB|
|Talk about stereotyped ideas that people may have about the way of life in oyher countries. Nobody wants to be average. So generalizations about nations arent usually welcomed though sometimes they are quiet accurate. The dangers may go even deeper when someone tries to generalize from his own limited experience. The Americans for instance are the nation about which numerous stereotyped ideas exist. They are created as a rule by the people who have never been to the USA but n...|
|11227.||Talk about madame tussaud’s as one of london’s famous museums||24.5 KB|
|Talk about madame tussauds as one of londons famous museums. It often comes as a shock for Londoners that Madame Tussauds museum is one of the capital top tourist attractions. Many find it gruesome and frightening. Others maintain that its collection of wax statues has no artistic merit. Yes despite this criticism Madam Tussauds has become a world famous. Millions of tourists from overseas and from other parts of Britain wouldnt consider their trip to the capital worthwhile with...|
|11228.||TAIK ABOUT HOLYWOOD AND THE EPICS PRODUCED THERE IN THE 1930S AND 1940S||23.5 KB|
|TAIK ABOUT HOLYWOOD AND THE EPICS PRODUCED THERE IN THE 1930S AND 1940S. Nowadays for million if people theres nothing better than watching a good film and the vast majority of films are produced in Hollywood. But what we really know about this really know about this greatstar factory To tell the truth everything we read about the great days of Hollywood in the 30s and 40s seems like exaggeration. The studious always said their film were superb and colossal because vast s...|
|11229.||TALK ABOUT THE LANGUAGE OF SHAKESPEAR AND THE DIFFICULTIES OF PRESENTING SHAKESPEAR’S CHARACTERS TO A MODERN AUDIENCE||23.5 KB|
|TALK ABOUT THE LANGUAGE OF SHAKESPEAR AND THE DIFFICULTIES OF PRESENTING SHAKESPEARS CHARACTERS TO A MODERN AUDIENCE. For any Englishman there are never can be any discussion on topic who is the worlds greatest poet and dramatist. Only one name possibly suggest itself to him: that of W Shakespeare. All of us use words phrases and quotation from Shs writings that have become part of the common property of Englishspeaking people. Shakespeare more perhaps than any other writer ...|
|11230.||DESCRIBE AN ENGLISH JOURNALIST’S IMPRESSIONS OF TRAVELLING THROUGH THEVAST SPASES OF RUSSIA, CHINA AND MONGOLIA||23.5 KB|
|DESCRIBE AN ENGLISH JOURNALISTS IMPRESSIONS OF TRAVELLING THROUGH THEVAST SPASES OF RUSSIA CHINA AND MONGOLIA. Its no wonder that trains are a literary genre all on their own. Writers from Graham Green to Agatha Christie realized that there is nothing quite like curtain sleeping compartments quicken the narrative pulse. To board the train that crosses countries and continents is to feel that anything might happened. From the first days of a journey on board a TransSiberian Expr...|
|11231.||THE ETHICAL AND CULTURAL HERITAGE OF THE BIBLE: CHRISTIAN VIRTUES SA DESCRIBED IN THE BIBLE AND THEIR ETERNAL VALUE||24 KB|
|THE ETHICAL AND CULTURAL HERITAGE OF THE BIBLE: CHRISTIAN VIRTUES SA DESCRIBED IN THE BIBLE AND THEIR ETERNAL VALUE. To become a real Christian you must obtain certain qualities of mind and heart which are called Christian virtues. The main virtues are: love hope faith humility patience meekness selfcontrol. Love is surely the most important of all Christian qualities. Firstly a man must love God with all his heart soul mind and strength. Secondly a man should love his neig...|
|11232.||THE ETHICAL AND CULTURAL HERITAGE OF THE BIBLE: SINS AND STRUGGLE WITH THEM AS DESCRIBED IN THE BIBLE||24 KB|
|THE ETHICAL AND CULTURAL HERITAGE OF THE BIBLE: SINS AND STRUGGLE WITH THEM AS DESCRIBED IN THE BIBLE. To start with I should say that there are some sins called deadly sins because they are especially ruinous for ones soul. They are pride covetousness lust envy gluttony anger sloth other sins are idle talk filthy language slander bribery sponging murder magic and witchcraft adultery fornication violation incest and cruelty. All of them are originated from the 7 ...|