5 . X-ray spectroscopy

1. 5. X-ray spectroscopy AIT

2. What are X-rays? • electromagnetic radiation • very short wavelengths, meaning high frequencies and high energies • wavelength range of the X‑ray region is 0.01 to 10 nm, compared to 400‑800 nm for the visible region • up to 80000 x more energetic than visible radiation • an older unit ‑ the Angstrom ‑ is often used in measures of wavelength • 1Å is equal to 0.1 nm • X-ray range is 0.1 to 100 Å • keV also used – measure of energy • 1 nm = 1.24 keV; range from 124 to 0.124 keV (inverse)

3. X‑rays are produced in three ways: • when high energy electrons collide with a surface • loss of energy produces a continuous band of X‑ray wavelengths (used in X-ray sources) • emissions from decaying radioactive nuclei • also used in X-ray sources • emission of X‑rays from matter which has been irradiated with a X‑ray beam • single wavelengths, and of different energies to the excitation beam (X-ray fluorescence)

4. Safety aspects • high energies of X‑rays mean that they are potentially harmful to living organisms • the energy will be absorbed by molecules in the organism • cause destruction or alteration to the molecules absorbing the energy • direct • the photon breaks bonds in an important molecule • indirect • the photon splits water, forming the very reactive OH radical • this reacts with something important

5. Safety aspects • if the species affected is genetic material (DNA) • chance exists for mutated cells to be produced • leading to the formation of cancers • effect is cumulative • a limited number of X‑rays that a person can have with in a set period of time • too many increases the number of damaged molecules • the chance of serious health consequences

6. Safety aspects • instruments using them are thoroughly sealed so that no radiation should escape • seals are checked on a regular basis • removable/opening panels have micro-switches • trip out the X‑ray source within milliseconds • operators of X‑ray instruments wear radiation badges • developed each month to check the levels of exposure • still appear to be film-based Why won’t we ask you to wear one? • one or two pracs are not enough potential exposure

7. X‑ray spectroscopic techniques X‑ray diffraction (XRD) • where X‑rays are diffracted through characteristic angles by the crystal structure of the matter • allows distinction between different forms of the same element, e.g. between the various crystal structures that occur in steel, X‑ray fluorescence (XRF) • emission of characteristic wavelength X‑rays from matter irradiated by an X‑ray beam is used to identify and quantify particular elements X‑ray absorption (XRA) • the familiar use of X‑rays in medicine and materials testing • the varying density of matter causes variation in the absorption of the X­-ray beam • only XRF part of this course

8. A bit of XRF history • 1895 - the discovery of X-rays is also the (not realised) discovery of XRF • 1913 – emitted X-rays are shown to be characteristic of different elements and related to atomic number • helps fix some errors in the periodic table • 1928 – first use of X-rays to generate XRF for analysis of real samples; limited by other instrumental problems • 1948 – first true working XRF spectrometer • 1950s – first commercial instruments • 1970 – use of semiconductor detection

9. X-ray fluorescence • emission technique • released X-rays of lower energy than those absorbed • X-rays are energetic enough to penetrate to the inner electron orbitals and eject an electron completely from the atom • this creates a “hole” which the atom will fill by electrons from higher orbitals dropping down • in doing so they release energy • some of this energy is in the form of X-rays

10. M shell M & L electrons drop down L shell X-rays emitted X-ray K electron ejected K shell X-ray fluorescent transitions

11. X-ray transitions: • L K • MK • ML • NL • K, L, M and N shells are the old name for the 1st, 2nd, 3rd and 4th orbitals

12. Transition labels • transitions in XRF are given labels which identify them when a spectrum is recorded • labels indicate: • the shell from which the electron was ejected • K, L (change to notes: M rare, N no) • the shell from which the filling electron came • Greek letters , , and  • the filling electron came from 1, 2 or 3 shells respectively, from the hole

13. M shell M & L electrons drop down L shell X-rays emitted X-ray K electron ejected K shell Exercise 5.2 (a) What are the two transitions shown in Figure 5.1? L K

14. N M L K Exercise 5.2 (b) Draw transitions for K and L. L K

15. Why are these labels important? • important trends in wavelength and intensity that can be followed for these transitions • help identify the presence of species in a sample • shorthand way of referring to certain characteristic emissions

16. N M L K Exercise 5.3 • Explain the order of energies for the four transitions • the bigger the jump, the greater the energy L La Ka K

17. Figure 5.2 Why use anything other than K? Not all X-rays can be excited efficiently or detected

18. Instrumentation • two basic designs of XRF instruments: • wavelength‑dispersive • the older type • uses a conventional monochromator-based design • energy‑dispersive • uses a special detector which doesn't require a monochromator

19. sample holder emitted radiation single  detector path many s excitation X-rays detector collimator dispersing crystal X-ray source Wavelength dispersive some have moving crystal & fixed detector

20. Wavelength dispersive • in high resolution WD-XRFs both the detector & the diffracting crystal move around a circular path • this is known as the goniometer • this is the same as for the ICP • a large radius circular path gives better resolution than a rotating crystal through an exit slit

21. Energy dispersive X-ray source no monochromator! excitation X-rays detector emitted radiation sample collimator

22. Energy dispersive • no moving parts => able to be made portable • no scanning => instant spectrum • lower resolution => lesser ability to distinguish close wavelengths • lower sensitivity

23. Portability

24. window X-rays target electrons cathode Instrumental components – radiation source • an X‑ray tube consists of a heated wire cathode, which emits electrons • accelerated towards to the anode ‑ a block of metal known as the target • collision releases energy in the form of a continuous spectrum of X‑rays

25. Output variables for X-ray tube • applied voltage between the electrodes • current • the material used on the surface of the target

26. Exercise 5.4

27. Other aspects • targets are water-cooled copper block with a surface coating of the target element • ours has a rhodium (Rh) target • the target element adds a number of X-ray lines to the continuous spectrum • alternative sources to the high current (30-50 kV) standard are: • radioactive isotopes (no power required) • low current X-ray tubes (portable or small desktop) Exercise 5.5 What would be the main disadvantage of these low current X-tubes? • lower current => lower intensity => lower fluorescence

28. The importance of tube voltage • voltage determines the energy of the exciting photons • => which elements will be excited. • the keV energy unit for X-rays is related to the voltage needed to excite a particular element line • to excite the fluorescence to generate that X-ray typically requires a tube voltage twice the keV value Example 5.1 The K line of calcium has an energy of 3.69 keV. What tube voltage needed to excite this line? • 2 x 3.69 = 7.38 kV

29. Exercise 5.6 The X-ray tube in our instrument has a maximum voltage of 30kV. What is the maximum atomic number for an element that can have its K line excited? • 30 ÷ 2 = 15 keV

30. Detectors • three main types: • semiconductor • gas-filled/flow/proportional • scintillation • a is used in energy-dispersive • b & c are used together in wavelength-dispersive • neither covers the entire range

31. Detectors Semiconductors • produce an output (a current pulse) that is proportional to the energy of the incoming photon • can do this for polychromatic radiation equally well • the number of pulses of each amount of current (e.g. 1, 1.1, 1.2 uA) gives the spectrum • correlation between energy and current must be made by some internal setting in the detector • eg 1.2 uA = 2.5 keV

32. Detectors - semiconductors • silicon-drift • do not need a monochromator • all the multi-channel-type advantages • limitations • resolution • sensitivity

33. Detectors Proportional/flow counters • X-rays ionise argon atoms • electrons generated can ionise other atoms, producing a chain reaction • this generates a current related to the number of photons • proportional relates to region where applied voltage is proportional to intensity • flow because of the throughput of gas • only measures lighter elements (higher wavelengths)

34. Detectors Scintillation counters • the generation of multiple photons of visible light from matter, which has absorbed a higher energy photon, e.g. an X‑ray • some crystalline materials do this • one X-ray can generate thousands of visible photons • detected by a photomultiplier tube • best for heavier elements (shorter wavelengths) • works in conjunction with flow counter to give full spectrum

35. Qualitative analysis XRF analysis: • is non-destructive • works better with solids • is of medium sensitivity (10 mg/kg upwards) • a surface technique • does not distinguish between different forms of an element • case studies • Victoria crosses • archaeological artefacts • medieval painting

36. Quantitative analysis • can quantitatively analyse solids • substantial matrix interferences • peak position is the same, but intensity varies greatly • sources of interference include: • preferential absorption of the excitation radiation • absorption of the analyte fluorescence • emission by matrix components • particle size and homogeneity variations

37. Means of countering matrix errors • matrix‑matched standards • standard addition • internal standard • standardless

38. Sample prepration • physical state affects intensity • equivalent treatment for standards and samples • solids need cleaning • powders need grinding & pressing or borax fusion

39. Other matters Use of helium • used for elements lighter than calcium for best sensitivity • low energy photons generated by these elements absorbed by air  Filters • placed between tube and sample • absorb a particular range of excitation X-rays • tube lines can be removed by this method • loss of sensitivity Current control • a current that is too high will generate a large broad background “bump” in the spectrum • particularly if the sample has high concentrations of light elements