2.8 X-ray fluorescence and Moseley's Law with MCA The emission of ...

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2.8 X-ray fluorescence and Moseley's Law with MCA The emission of characteristic X-rays caused by irradiation with photons of high energy is called X-ray fluorescence. The binding energy of the innermost electrons increases with atomic number and so does the energy of the characteristic X-rays which is measured in this experiment. The energy resolution of a scintillation counter is sufficient for such examinations. The dependence of X-ray energy on atomic number was examined by Moseley. In this experiment Rydberg's constant will be determined using the Mosely's law. Related Topics Equipment Binding energy, photoelectric eftect, structure of electron Americium-241 source, 370 kBq 09090.11 1 shells, characteristic X-rays, ry-spectrometry, X-ray spectro- Source Cs-137, 37 kBq 09096.01 1 metry. Absorption material, lead 09029.01 1 Support rod -PASS-, square, l = 250 mm 02025.55 3 Principle Right angle clamp -PASS- 02040.55 3 Photons originating from nuclear transitions are called Support base -PASS- 02005.55 1 r--quanta and photons originating from electron transitions of Universal clamp 37715.00 1 high energy are called X-rays. If matter is irradiated with pho- Source holder, swivel-type 18461.88 1 tons of high energy, part of them are absorbed by electrons Barium sulfate, 500 g 30035.50 1 transferring the energy and momentum of the photon to the Tin rods, 100 g 31991.25 1 electron which is called photoelectric effect. If the energy is Silver plate, 25 g 31839.04 1 sufficient, also the innermost i.e. strongest bound electrons of Cer (IV) sulfate, 25 g 31194.04 1 an atom can be removed from the atom by this process. The lodine resublimed, 25 g 30093.04 1 states of the missing electrons are filled with other electrons Crocodile clips, pack of 10 07274.03 1 under emission of characteristic X-rays or Auger-electrons. Plastic bags, DIN A5, pack of 100 46444.01 1 The emission of characteristic X-rays caused by irradiation Screened cable, BNC, l = 750 mm 07542.11 1 with photons of high energy is called X-ray fluorescence. The Gamma detector 09101.00 1 binding energy of the innermost electrons increases with Operating unit f. gamma detector 09101.93 1 atomic number and so does the energy of the characteristic High-voltage connecting cable 09101.10 1 X-rays which is measured in this experiment. The energy res- Multi Channel Analyzer (MCA) 13726.99 1 olution of a scintillation counter is sufficient for such examina- MCA Software 14524.61 1 tions. The dependence of X-ray energy on atomic number was RS232 data cable 14602.00 1 examined by Moseley. PC, Windows 895 or higher Fig. 1: Experimental set-up PHYWE - Related Topics Equipment Binding energy, photoelectric eftect, structure of electron Americium-241 source, 370 kBq 09090.11 1 shells, characteristic X-rays, ry-spectrometry, X-ray spectro- Source Cs-137, 37 kBq 09096.01 1 metry. Absorption material, lead 09029.01 1 Support rod -PASS-, square, l = 250 mm 02025.55 3 Principle Right angle clamp -PASS- 02040.55 3 Photons originating from nuclear transitions are called Support base -PASS- 02005.55 1 r--quanta and photons originating from electron transitions of Universal clamp 37715.00 1 high energy are called X-rays. If matter is irradiated with pho- Source holder, swivel-type 18461.88 1 tons of high energy, part of them are absorbed by electrons Barium sulfate, 500 g 30035.50 1 transferring the energy and momentum of the photon to the Tin rods, 100 g 31991.25 1 electron which is called photoelectric effect. If the energy is Silver plate, 25 g 31839.04 1 sufficient, also the innermost i.e. strongest bound electrons of Cer (IV) sulfate, 25 g 31194.04 1 an atom can be removed from the atom by this process. The lodine resublimed, 25 g 30093.04 1 states of the missing electrons are filled with other electrons Crocodile clips, pack of 10 07274.03 1 under emission of characteristic X-rays or Auger-electrons. Plastic bags, DIN A5, pack of 100 46444.01 1 The emission of characteristic X-rays caused by irradiation Screened cable, BNC, l = 750 mm 07542.11 1 with photons of high energy is called X-ray fluorescence. The Gamma detector 09101.00 1 binding energy of the innermost electrons increases with Operating unit f. gamma detector 09101.93 1 atomic number and so does the energy of the characteristic High-voltage connecting cable 09101.10 1 X-rays which is measured in this experiment. The energy res- Multi Channel Analyzer (MCA) 13726.99 1 olution of a scintillation counter is sufficient for such examina- MCA Software 14524.61 1 tions. The dependence of X-ray energy on atomic number was RS232 data cable 14602.00 1 examined by Moseley. PC, Windows 895 or higher Fig. 1: Experimental set-up PHYWE - Tasks Set-up and Procedure 1. Perform an energy calibration of the setup using the Set up the equipment as seen in Fig. 1. Before turning on the 59.5 keV line of 241 Am and the 32.2 keV line of 137Cs. operating unit for the scintillation counter, connect the high 2. Record spectra of the fluorescence radiation exited with voltage cable correctly to operating unit and photomultiplier the radiation of the 241 Am source for different specimen. and read the instructions in the manual of the gamma-detec- 3. Plot the energy of the fluorescence peak vs. the function tor. Set the multiturn potentiometer of the operating unit to (Z - 1)2 of the specimen's nuclear mass number Z and cal- 2.00. Connect the MCA to the computer's USB port or a culate the Rydberg constant from the slope of the RS232 serial interface and start the "measure" program. obtained graph. Select the Gauge "Multi Channel Analyzer" (MCA). Multi Channel Analyser 1. First adjust the detector: Start with the 241 Am source. Select "Spectra recording", use the "Continue" button and in the spectra recording window (see Fig. 3) set the "Gain" to "Level 4" and the "Offset" to 1 % and choose "Channel number" as x-Data. Place the source in a distance to the PHTWE detector such that the counting rate is slightly below 1000 cts/s. Adjust the multiturn potentiometer on the detector's operating unit so that the 59.5 keV peak moves to the right end of the spectrum (around channel 3500). Leave this setting unchanged throughout the measure- ment - for low drift turn on the detector some time before measurement. "Cancel" the measurement. Please select the measurement mode Spectra recording Detector calibration Single channel analyser Detector Calibration: 2-point calibration 137Cs 241Am Integration measurement Comment none Mode: Settings and Calibration 2-point calibration Gain: Level 4. Offset 0% Equation: 0,1-<channel> keV 5.2 keV Description Allows to calibrate the detector connected to the MCA The Calibrate Bave Delete calibrations can be saved for later use. Device information Port COM3 Device version: 1.02-2 Perform hardware adjustment Continue Cancel Fig. 2: Start window of the MCA Fig. 4: Calibration window Impulses Control Impulses Calibration # Start Stop # Mode 1739 3488 2-point calibration Reset 80- Unit keV MCA Settings Gain Channel Energy Level 4 70- 1739 32,2 keV 300 3488 59,5 keV Offset[%] 1 60 Clear diagram Histogram X-Data 50- MCA Settings 200- Channel number Gain Interval width [channels] Level 40- Offset[%] 30- 100 20 Channel 10 0- 500 1000 1500 2000 2500 3000 3500 4000 hs Q + + Accept data Channel Apply 500 1000 1500 2000 2500 3000 3500 4000 Total impulses 264373 Impulse rate: 800,3 #/s Cancel Detector: calibration 137Cs, 241Am hs a + I+ ++ Cancel Fig. 3: Window for spectrum recording - here the spectrum of Fig. 5: Performing the calibration - here with 241 Am 241Am with gain level 4. </channel>Now calibrate the MCA so that the corresponding energy Impulse of each channel is known: Start the MCA gauge of "mea- sure" again and select "Settings and Calibration". The win- dow as seen in Fig.4 appears - use the "Calibrate" button. 30 BASO4 Set the "Gain" to "Level 4", the "Offset" to 1 % and select "2-point calibration" Move one bar to the 59.5 keV peak, then remove the 241 Am and bring the 137 Cs in direct vicin- 20 ity to the detector. Use the "Clear diagram" button and move the other bar to the then appearing 32.2 keV peak and type the energy values in the appropriate fields and finally use the "Apply" button and then the "Save" button 10 of the window seen on Fig. 4 and enter a name for your calibration. 2. Now choose the program part "Spectra recording" again with "Gain" "Level 4" and 1 % "Offset". Put 3 cm of lead 10 20 30 40 50 60 70 keV shielding between the 340 kBq 241 Am source and the detector with the source close to the detector. Check the Fig. 7: Fluorescence peaks spectrum that gets recorded now and the counting rate - the detector should "see" as few of the source's radiation as possible - the presence of the source shouldn't Theory and evaluation increase the background rate a lot. Then put the fluores- Fig. 6 shows the decay scemes of the used nuclids. The pro- centing specimen in the vicinity of the detector so that it is portions of the energy scale are not displayed correctly and exposed to the source's radiation and so that the fluores- the term sceme of No is strongly simplified - in the experi- cence radiation can reach the detector. Move it until the ment only the 59.5 keV of No is of importance. It is to be counting rate gets maximal. The counting rate should be kept in mind, that the exited states of the daugther nuclids can distinctly higher with specimen than without. Reset the also disintegrate by inner conversion in the case of 137 Ba lead- spectrum and in the now recorded spectrum the fluores- ing to a strong 32 keV X-ray line. cence peak should be clearly visible and much lower than In the recorded spectra use "Display options" to change the the 60 keV peak. Save the recorded spectra with the displayed area to 10 to 60 keV and on the "Channels" chart "Accept data" button. If the specimen is powder and is change the "Interpolation" from "Bars" to "Straight lines". packed in a plastic bottle, simply the whole package can Then use the "Smooth" tool to generate a new diagram where be used. Glass packages absorb too strongly and show a the position of the fluorescence peak is more clearly visible. broad fluorescence spectrum themselves - specimen in With the "Survey" function read out the corresponding energy glass bottles have to be removed from the bottle and may of the peak. Fig. 7 shows results combined into one graph be put into e.g.plastic bags. Metal pieces need no pack- with "Measurement" "Assume channel". aging. 5/2- 432,2 c 5637,8 keV 241 a 100% 7/2+ Am. 95 146 30,07 a 1175,6 keV ß- 100% 137CS82 11/2- 94,4% 661,7 keV 2,551 min 7/2- 13,0% 103 keV 9/2+ 1/2+ 0,000058% 283,5 keV 0,02% 75,9 keV 5/2- 84,5% 59,5 keV 3/2+ W 0,22% 7/2+ 5,6% 0 keV stable 33,2 keV 137 56 Ba 81 5/2+ 0,34% 0 keV stable 237 Np 93 144 Fig. 6: Decay scemes of 137Cs and 241 Am E Create new measurement X keV General settings Title: Manually created measurement 30 Peakenergy Number of channels: 1 Number of values: 5 x-Data 20 Title Symbol Unit Digits Atomic number Z 10 Channels Title Symbol Unit Digits Peak energy E keV 1 (2-1) 500 1000 1500 2000 2500 3000 # Fig. 10: Plot of Moseley's law Continue Cancel The fluorescence peak that dominates here is from Ko radia- Fig. 8: Window for creating measurements tion. The corresponding energy is the energy difference between the state of the innermost electron in the atom, the lowest energy state, and the energy of an electron state one Channel modification x shell above that. Kn radiation is thus the radiation emitted Source channel when an innermost electron is lost by photo effect and it's 1: Z Atomic number Calculate state is filled up with an electron of the next shell and the ener- 2 [off] Cancel gy difference is emitted as photon. Moseley's law now states that this energy is connected to the ionization energy of atom- Help ar hydrogen, the Rydberg constant Roo, in a simple manner: Operation f (Z-1)22 differentiate integrate with atomic number Z and Planck's quantum h. progressive average value Fig. 10 shows that this approximation is astonishingly well valid - though one would expect strong deviations because of Destination channel the complicated shell structure of heavy atoms - e.g. Cer is a add new y-channel lanthanoid and has an electron in the f-shell and Barium overwrite Title: belongs to the second main column of the table of elements Atomic number (Z-1)² and has none. into new measurement Symbol: (Z-1)² The slope in Fig. 10 reads 0.0107 keV and with h = as x-channel 6.626 10-34 Js = 4.136 . 10-18 keVs is then Unit: as y-channel 1 Fig. 9: Window for channel modification = S Create a measurement with "Measurement" > "Enter data Literature values for R.: manually II as seen in Fig. 8. Then change the x-channel with Roc/co = 1.097 - 10 1/m "Analysis" > "Channel modification. II to (Z-1)² as seen in R. = 3.290 - 1015 1/s Fig. 9. The number of digits beyond the point can be changed = 13.6 eV with the "Information" button. The slope of the graph can be h.R. = 2.178 - 10-18 J evaluated with the "Regression" tool.

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