Manual X-ray fluorescence Manual remote experiment Project e-xperimenteren+

Transcription

1 Manual X-ray fluorescence Manual remote experiment Project e-xperimenteren+ J.M.Mulder, J. Snellenburg

2 Colofon Manual X-ray fluorescence Manual remote experiment Project e-xperimenteren+ Stichting Digitale Universiteit Oudenoord 340, 3513 EX Utrecht Postbus 182, 3500 AD Utrecht Phone Fax Author(s) J.M.Mulder, J. Snellenburg Copyright Stichting Digitale Universiteit De Creative Commons Naamsvermelding-GeenAfgeleideWerken-NietCommercieel-licentie is van toepassing op dit werk. Ga naar om deze licentie te bekijken. Date page 2 of 15

3 Table of contents 1 Introduction 4 2 Theory 4 3 Setup Schematic view Pictures 6 4 Remote interface Settings Measure Calibrate Analyse Journal 13 5 Constants and parameters 14 6 References 14 page 3 of 15

4 1 Introduction This manual explains the working of a remotely controlled X-ray fluorescence experiment. X-ray fluorescence is a technique that can be used to determine the surface composition of a sample non-destructively. Related to this technique are various processes such as attenuation of x- rays in materials, absorption, fluorescence yields, coherent and incoherent scattering. In this experiment some of these processes can be studied using a 241 Am source. The experimental setup is located at the Vrije Universiteit Amsterdam and is maintained by the Physics student laboratory. The web-experiment can be accessed from practically any location in the world, provided the user s system meets the minimum system requirements as listed in Appendix A. Before starting the experiment it is recommended to formulate a research plan to work as productively as possible. To this end, besides reading this manual, it may help to study the background theory available on the website to find out what the possibilities and limitations are of online experimenting. 2 Theory X-ray fluorescence occurs when an electron from the inner electron shell of an atom (molecule) is ejected by a high energy X-ray photon. The absorption of x-ray photons can be described by the law of Lambert-Beer. The created hole is filled up by an electron from one of the outer shells (an electron from the L, M, state). The energy difference from this transition is emitted as a fluorescence photon. Fig. 2.1: A Bohr-atom with filled K and L shells and one electron in the M shell. These energies are characteristic for each element. A measurement of emitted fluorescence photons coming from a target results in fluorescence spectrum. The composition of the exposed target can be determent by comparing the energies in this spectrum with the known transition energies of all elements. See the table of emission energies. page 4 of 15

5 An electron from the L to the K shell is a Kα transition and from the M to the K shell is a Kβ transition. For an introduction to the theory of fluorescence see refs. 1, 2 and 3. Elements you should be familiar with are (notations of) electron states, characteristic lines, selection rules, fluorescence and intensity yield, absorption and scattering of x-rays. 3 Setup 3.1 Schematic view The most important parts of the experimental setup are the x-ray source, the target materials and the x-ray detector. (Fig. 3.1) X-ray source Fig. 3.1: schematic overview of experimental setup X-ray source The x-ray source is a 241 Am (10 mcu). The x-ray spectrum of this source together with the identification of some peaks is shown in fig The target materials In the target wheel different materials can be selected. For the list of materials that can be chosen see 4.1: Remote interface settings The x-ray detector Specifications of the detector (type XR-100CR) can be found in ref. 4. If an x-ray photon interacts with the silicon plate in the x-ray detector, it produces a number of electron/hole pairs depending on the energy of this photon. This results in a short signal pulse. After amplification these pulses are selected on height, counted and stored by the MCA. The resulting x-ray spectrum is then read by the computer. The detector can be placed at different angles. The angle between the x-ray source and the x-ray detector can be varied from 40 to 180 degrees. page 5 of 15

6 3.2 Pictures Below some pictures of the setup. Legend: A B C D E F G X-ray detector X-ray source Target wheel Power supply x-ray detector and preamplifier Multi channel analyzer Stepper motor control unit web-cams D E A X-ray detector with power supply Multi channel analyzer (MCA) page 6 of 15

7 G A B C D F Overview of the experimental setup A B C Close-up of x-ray detector, target wheel and x-ray source. page 7 of 15

8 4 Remote interface Below is a description of the software interface of the experiment. The panel is organized into several tabs: 1. Settings 2. Measure 3. Calibrate 4. Analyse 5. Journal Each tab is described in the next paragraphs. 4.1 Settings Various experimental parameters can be adjusted: The angle between the detector and X-ray source Normally set at an angle of 40 degrees for fluorescence measurements and 180 degrees for absorption measurements. A change of this angle has to be confirmed with the confirmation button which appears below this control. A type of target One can choose targets with known materials such as copper, iron, zirconium for calibration purposes, or targets with a priori unknown compositions like the sand from the shores of the Dutch island of Texel to analyse their content. Also some compounds can be chosen such as brass or a euro coin for this purpose. A pinhole A pinhole is only used with absorption measurements. During fluorescence measurements it should always be set to none. Webcam views Two types of web cam views can be chosen: an upper view of the setup and a side view of the chosen target in front of the source. page 8 of 15

9 Figure 4.1: screenshot of the SETTINGS tab in the remote panel overview. Set angle detector X-ray source: a new angle can be set by changing the value or by dragging the arrow in the picture control to a new position. The actual change to this new position is done by clicking the confirm button, which appears when an change is first initiated Confirm: clicking this button will result in moving the x-ray detector to the new position. Choice target: with this popup button you can select one of the following materials: 0. empty mm Silver mm Copper mm Nickel mm Copper mm Iron mm Copper mm Aluminum mm Copper 12. Euro coin (50 cts) mm Lead 13. Sand (from the sea bottom of Texel) mm Tantalum mm Zirconium mm Tin 15. Brass Choice pinhole: You can choose between the following diameters: 2, 4, 8, and 16 mm. None should always be selected for fluorescence measurements. Large image: when selected the color will change to blue and a large image is displayed. Small image: when selected the color will change to blue and a small image is displayed. Camera upper view: shows an upper view of the experimental setup Camera targets and carrousel: shows a part of the target wheel and the x-ray detector page 9 of 15

10 4.2 Measure The measured energy spectrum of the X-ray detector is displayed. This energy spectrum is measured with a Multi Channel Analyzer (MCA). Controls of this instrument are: start or stop the measurement, clear memory (and graph), number of channels to set the resolution of the energy spectrum and preset time to set the measuring time. When set to zero the measurement can only be stopped manually. The measured time corrected for dead time is shown in the lower indicator. The indicator Measurement settings and variables shows all the settings of the displayed energy spectrum. The data can be saved and previously measured spectra can be read. Read MCA buffer control displays the last measured energy spectrum that is still in the buffer. Figure 4.2: screenshot of the MEASURE tab in the remote panel overview. Energy spectrum: the current energy spectrum is displayed here. Number of photons against channel number (or against energy in kev if the MCA is calibrated). Preset time [sec]: the preset live time to measure the energy spectrum. If zero the measuring time is infinite. Measured time [sec]: the measured live time that has past. Measurement settings and variables: Information about the displayed energy spectrum. Start measurement: Starts the measurement until the preset time is reached or the measurement is stopped manually. Stop measurement: stops the measurement. Empty MCA buffer and graph: clears the buffer of the multi channel analyzer and the graph. Number of channels: determines the resolution of the measurement. Possible values are 256, 512, 1024, 2048, 4096 and 8192 channels. Read MCA buffer: reads the buffer of the MCA. page 10 of 15

11 Write data to file: Saves the displayed energy spectrum in a data file and the information shown in measurement settings and variables indicator in an info file. Read data from file: reads previously saved data- and info-file from disk and displays it in the energy spectrum graph and measurement settings and variables indicator. 4.3 Calibrate This tab gives the possibility to calibrate the x-axis of the measured energy spectrum. The X-ray emission energies of all selected known targets, of which energy spectra are measured, are shown in the upper indicator/control. On the right side of this tab you give input for at least two channel numbers with each a selected X-ray emission energy. After pressing in sequence the buttons Determine straight line fit and calibrate x-axis the x-axis will be calibrated and changed into energy in kev. This is also done for the spectra under the other tabs. The extra controls cursor active will activate an extra cursor. Snap to channel means that the cursor is locked to each point of the energy spectrum. Figure 4.3: screenshot of the CALIBRATE tab in the remote panel overview. X-ray emission energies in kev: here all the emission energies are shown of all the known target materials of which energy spectra are measured. spectrum: displays all the line spectra of the known materials in the upper popup control. page 11 of 15

12 Energy spectrum: the current energy spectrum is displayed here. Number of photons against channel number (or against energy in kev if the MCA is calibrated). Slope: a parameter of the linear fit. Intercept y-as: a parameter of the linear fit. Mean squared deviation: goodness of the linear fit. Cursor active: if selected a cursor is shown in the graph and a active value in the linear fit. Channel number: the position of the cursor in the graph. Energy value kev: here the channel number is set to the emission energy values in kev s. Snap to channel: If selected the cursor is locked to a channel number. Determine straight line fit: A fit is made with the channel numbers and the energy values corresponding to the active cursors. Calibrate x-axis: After a fit is calculated all energy spectra will be shown in kev. 4.4 Analyse On this tab the energy spectrum and a panel with all elements and the corresponding X-ray emission energies is displayed. By selecting elements from this panel it is possible to investigate which elements contribute to the energy spectrum. When an element is selected (clicked on) a line spectrum with the tabulated X-ray energies from this element is shown in the graph. The RESET ALL button will remove all selected elements from the graph. Figure 4.4: screenshot of the ANALYSE tab in the remote panel overview. Energy spectrum: the current energy spectrum is displayed here. Number of photons against channel number (or against energy in kev if the MCA is calibrated). page 12 of 15

13 Elements: here an element can be selected, which results in colored lines in the upper energy spectrum at the specific energy values (kev s) given in this table. All selected elements are also set in the legend on the right hand side of the graph Reset all: clears all colored lines of the chosen elements in the upper graph 4.5 Journal In this tab you can enter items to your lab journal. This is done by selecting a Journal Item and Journal Status from the lists at the top of this tab and entering a text in the field Journal Text. Optionally you can add a reference to an earlier journal entry or log entry. This is done by clicking the entry in the list box at the bottom of the tab or entering its number in the reference field. By clicking the button Submit, the journal entry is added to the log. Note that some actions are automatically logged. Figure 4.5: screenshot of the JOURNAL tab in the remote panel overview. Journal Item: here it is possible to choose a category for your journal entry. Journal Status: here you can choose a qualification of your journal entry. Journal Text: here the actual text of the journal entry can be typed. Reference: You can refer to an earlier journal entry or experiment log entry by clicking in the table of entries or choosing a number in the Reference field. Submit: by clicking the button Submit the journal entry is added to the experiment log. page 13 of 15

14 5 Constants and parameters Name Description Value / range, error, unit X-ray source 241 Am 10 mcu Source/detector The angle between the x-ray source and the 40 to 180 degrees angle x-ray detector. Efficientie Ref. 3 detector Emission energies of all download/tabel.pdf elements Size pinholes Diameter pinhole (lead) 2,4,8 and 16 mm 6 References 1. R. Tertian and F. Claisse, Principles of Quantitative X-ray Fluorescence Analysis 2. B. Dziunikowkski, Energy dispersive X-ray Fluorescence Analysis Website of Amptek ( page 14 of 15

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