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PC4250 – ADVANCED ANALYTICAL TECHNIQUES Part I – Ion Beam Analysis using High Energy Beams

PC4250 – ADVANCED ANALYTICAL TECHNIQUES Part I – Ion Beam Analysis using High Energy Beams. REFERENCES. PIXE: A Novel Technique for Elemental Analysis Sven A. E. Johansson and John L. Campbell Publisher: John Wiley & Sons, 1988

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PC4250 – ADVANCED ANALYTICAL TECHNIQUES Part I – Ion Beam Analysis using High Energy Beams

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  1. PC4250 – ADVANCED ANALYTICAL TECHNIQUESPart I – Ion Beam Analysis using High Energy Beams

  2. REFERENCES • PIXE: A Novel Technique for Elemental AnalysisSven A. E. Johansson and John L. CampbellPublisher: John Wiley & Sons, 1988 • Materials Analysis using a Nuclear Microprobe M B H Breese, D N Jamieson and P J C KingPublisher: John Wiley & Sons, 1996 • Handbook of Modern Ion Beam Materials AnalysisEdited by Joseph R. Tesmer and Michael NastasiPublisher: Materials Research Society, Pittsburgh, Pa., 1995 • Handbook of X-Ray SpectrometryEdited by ReneE.Van Grieken and Andrzej A. Markowicz Publisher: Marcel Dekker, Inc., 2nd Edition, 2002

  3. Contents • Introduction to ion-solid interactions • Ion beam analysis techniques: a summary • Ion sources • Experimental setups • Nuclear microprobes • Si(Li) detectors (PIXE) • Surface barrier detectors (RBS, ERDA)

  4. 1. INTRODUCTION TO ION-SOLID INTERACTIONS

  5. In its passage through matter, an ion may interact with • THE ATOMIC ELECTRONS • and/or • THE ATOMIC NUCLEI

  6. Such interaction will result in: • IONIZATION – the electron is ejected from its atomic orbit • or • ATOMICEXCITATION – the electron is raised to an outer orbit The interaction of an ion with an atomic electron is purely Coulomb (i.e. interaction governed by the Coulomb’s law). An ionized/excited atom will eventually return to its ground state, accompanied by the emission of one or more x-rays/photons.

  7. SECONDARY ELECTRONS & BREMSSTRAHLUNG • An electron ejected from its atomic orbit is called a secondary electron. It may further ionize or excite another atom, resulting in the emission of more x-rays/photons. • An secondary electron may also be decelerated by the coulomb field of a nucleus, losing part or all its energy in form of bremsstrahlung (braking radiation).

  8. The interaction of an ion with an atomic nucleus can be • COULOMB ELASTIC SCATTERING • COULOMB INELASTIC COLLISION • COULOMB EXCITATION • NUCLEAR INELASTIC SCATTERING • NUCLEAR TRANSFORMATION

  9. EFFECTS OF ION-NUCLEUS INTERACTIONS:

  10. RADIATIONS EMITTED IN ION-NUCLEUS INTERACTIONS • An excited nucleus will eventually return to its ground state accompanied by the emission of one or more g rays. • For ions with incident energy of a few MeV, radiations emitted from nuclear reactions are usually p, n, a and/or g.

  11. Stopping and Ranges of Ions in Matter (SRIM) • This is a very useful program which is used in many research and technology areas, from ion energies of eV to GeV, in semiconductor manufacturing (why ?), ion beam analysis, nuclear physics, high-energy physics, etc • It can be used to find the range, electronic and nuclear energy losses of any ion in any material. • Also used to study recoil events when heavy ions are incident on a light material. • Download from SRIM website http://www.srim.org/ • Or from course website here

  12. 2. ION BEAM ANALYSIS TECHNIQUES: A SUMMARY

  13. ION BEAM ANALYSIS TECHNIQUES: Often two or more of these techniques are carried out simultaneously in order to obtained complementary information.

  14. TYPICAL ION BEAMS AND INCIDENT ENERGIES USED IN VARIOUSIBA TECHNQUES:

  15. Comparison between X-ray spectra using EPMA and PIXE (a) 10 keV electrons High brehmsstrahlung X-ray energy (b) 3 MeV protons X-ray energy

  16. Example : RBS spectrum of hard-disk Layer structure: Protective polymeric material (~200A) Co-Pt-Fe alloy (~200A) Cr (~10A) Co-Pt.Fe alloy (~200A) Cr (~1000A) Ni3(PO4)2 (~100,000A) Al substrate

  17. Elastic Recoil Detection Analysis (ERDA) • An important application of ERDA is the analysis of hydrogen using 3He or 4He. • The kinematics of elastic collisions allows the recoil to occur only in the forward hemisphere. • For ERDA, the mass of the incident particle must be greater than that of the target nucleus.

  18. Nuclear Reaction Analysis (NRA) • Nuclear reaction analysis is based on the detection of the prompt g-rays or prompt particles emitted as a result of the nuclear interaction between the incident particles and the target nuclei. • The cross sections of nuclear reactions vary rather irregularly. When using light ion beams of only a few MeV, nuclear reaction cross sections are high enough for analysis of only low- and medium-Z elements. • The most popular application of PIGE is the determination of F in biomedical sample through the reaction 19F(p,p'g)19F. • PIGE is often used in conjunction with PIXE for analyzing light elements such as Li, Na, Mg and Al in aerosol and geological samples. • Deuterons are more commonly used than protons when prompt particles are measured in NRA. Useful reactions for determinations of C and N include 12C(d,p)13C and 14N(d,p)15N. • The 16O(d,pg)17O* reaction has also been used in conjunction with DIXE (Deuteron-Induced X-ray Emission) for stoichiometric analysis of Y-Ba-Cu-O superconductors.

  19. OTHER POSSIBLE EFFECTS & CHANNELING • Ions incident upon a target may break chemical bonds and produce light, UV radiation or sputter atoms from the target surface. • For a crystalline target, the incident ions may even channel through the ordered rows of atoms.

  20. The channelling process Reprinted from “Channeling in Crystals” by W. Brandt,  Scientific American

  21. 3. ION SOURCES

  22. ION SOURCES There are a number of methods for ion generation, but the use of radio-frequency power to produce ions from neutral gas in a low-pressure discharge bottle is by far the most popular way.

  23. RF ION SOURCE – THEORY OF OPERATION: • Neutral gas is bled into the discharge tube from the pressurized gas bottle through the palladium leak. Palladium is porous to low-Z gases and the porosity is a function of temperature. Hence the pressure in the discharge tube can be controlled by adjusting the output of the heating coil power supply. • Free electrons in the discharge tube are excited into oscillation in the RF electric field and quickly acquire enough kinetic energy to cause ionizations, hence producing +ve ions. • The ions are pushed by the positively biased electrostatic probe to the tube exit at the opposite end and are drawn into the acceleration tube by the extraction electrode.

  24. RF oscillator Extraction electrode (V-) Electrostatic probe (V+) Discharge tube IONS Coupling clips Heating coil power supply Palladium leak & heating coil Compressed gas A RADIO-FREQUENCY ION SOURCE:

  25. 4. EXPERIMENTAL SETUPS

  26. Ge(Li) -ray detector (for PIGE) VACUUM CHAMBER COLLIMATORS Target Ion beam Faraday cup Annular particle Detector (for RBS) Annular particle detector (for ERDA) Si (Li) x-ray detector (for PIXE) CONVENTIONAL IBA EXPERIMENTAL SETUP:

  27. C O M P U T E R DETECTOR BIAS X-RAY DETECTOR ANALOG-DIGITAL CONVERTER MAIN AMPLIFIER PIXE PRE-AMPLIFIER DETECTOR BIAS PARTICLE DETECTOR ANALOG-DIGITAL CONVERTER RBS MAIN AMPLIFIER PRE-AMPLIFIER ERDA DETECTOR BIAS PARTICLE DETECTOR ANALOG-DIGITAL CONVERTER MAIN AMPLIFIER PRE-AMPLIFIER FARADAY CUP CHARGE DIGITIZER ELECTRONIC COMPONENTS FOR SIGNAL PROCESSING:

  28. 5. NUCLEAR MICROPROBES

  29. MICROPROBE IBA: • Using an ion beam with a micron or sub-micron spot size for elemental analysis adds a new dimension to IBA analytical power – i.e. elemental imaging (measuring the elemental distributions of the various elements in specimens). • Microprobe IBA has applications in a large variety of disciplines, including bio-medicine, earth sciences, metallurgy, solid state physics, electronics, archaeology and aerosol study, etc. • Although most of the applications of microprobes are analytical, it is now being used for many non-analytical works, such as micro-machining of polymers and semiconductors.

  30. Trajectories of 3MeV protons and 3MeV 4He in silicon 8 Very little radial beam spread for MeV ions Unlike keV electrons ! protons radial beam spread (m) Helium ions 0 0 100 Distance travelled in silicon (m) • Microprobes using MeV ion beams are difficult to focus because of the high ion mass • However, once the beam is focused, it is this same property which prevents beam “blow-up”, unlike focused keV electrons in a SEM • Microprobes are very good at analysing “thick” layers with high resolution

  31. Magnetic rigidity B This is a measure of how difficult charged particles are to bend. It depends on the particle mass M, energy E and charge Q B = (ME) Q Ions Bi= (MiEi)(singly charged) Electrons Be = (MeEe) Proton mass is 1836 greater than electron mass, So a 2 MeV proton requires a magnetic field strength of 430 times that needed to focus a 20 keV electron !  MeV ions have a high B compared with keV electrons  Very difficult to focus using magnetic solenoid lenses, so use a quadrupole lens focusing system.

  32. Quadrupole Lenses Lorentz Force:F = q vB i.e. quadrupole lens gives a focusing force because v and B are perpendicular

  33. COMPONENTS OF A NUCLEAR MICROPROBE FACILITY: Additional components needed for a nuclear microprobe facility includes: • A set of object slits to define the geometrical image component of the final spot size. • A beam focusing device such as magnetic quadrupole lenses. • A scanning system to raster the beam over the specimen.

  34. Nuclear Microprobe Layout Beam divergence sample • MeV ion beam from Van de Graaff accelerator is focused • onto the sample (target). • Focused beam spot size is 0.05 m to 1 m, depending on the amount of beam current used. • Focused beam is scanned over sample surface and the (x,y) position and relevant detector signal is measured.

  35. Schematic of microprobe chamber X-ray detector (PIXE) Focusing microscope Backscatter detector (RBS) Trans- mission detector Beam sample Viewing microscope

  36. Microprobe Chamber

  37. CIBA Nuclear Microprobe Switcher magnet Proton accelerator Object aperture Collimator aperture Focusing system quadrupole lenses Scan coils Scan Controller Sample chamber

  38. CIBA. View of 3 beamline facilities: Proton Beam Micromachining (10), Nuclear microprobe (30) and Ion Channeling facility (45)

  39. High-Resolution microprobe beamline World’s best spatial resolution of 35 nm

  40. P S Ca Fe Ti W Nuclear microprobe PIXE elemental maps from 400 mm x 400 mm scan over a section of a lung tissue taken from a patient suffered from hard metal lung disease:

  41. 6. Si(Li) DETECTORS

  42. The Si(Li) detector is basically a semiconductor diode fabricated using high-purity p-type silicon cylindrical wafer doped with lithium on one side. The electrode contacts of the diode are formed with thin metal (normally Au) films evaporated on opposing surfaces of the silicon wafer When the diode is reversed biased, a carrier-free charge depletion region is created and the only current that flows between the electrodes is due to thermally generated carriers. Detector bias (-) Au contact Si dead layer Depletion region (Active region) High-purity p-type Si Li-diffused region Au contact Si(Li) DETECTOR - STRUCTURE

  43. Si(Li) DETECTOR – PRINCIPLE OF X-RAY DETECTION • In traversing the charge depletion region, an X-ray may interact with a Si atom through photoelectric absorption, spending part of its energy in knocking out an electron from the inner shell of a Si atom and transferring the rest of its energy to the photoelectron (i.e. the electron ejected from the inner shell of a Si atom). It may also scattered by an electron, dissipating only part of its energy in the charge depletion region. • The dissipation of energy by an X-ray in the charge depletion region of the Si(Li) detector will result in the production of free electron-hole pairs which are swiftly collected by the electrodes as a current pulse. The number of electron-hole pairs produced is proportional to the energy dissipated by the incident X-ray in the charge depletion region, Hence, the amplitudes of a current pulse generated by the photoelectric absorption is proportional to the energy of the incident X-ray.

  44. Si(Li) DETECTOR – SIGNAL PROCESSING • The current pulse generated by the Si(Li) detector must be processed electronically to such an amplitude and a shape suitable for analog to digital conversion. The pulse processing is done in two stages using two types of amplifiers. The first stage is charge integration which is carried out by using a pre-amplifier. The second is a combination of voltage amplification and pulse shaping which is done with a spectroscopy amplifier. • It is necessary to operate the Si(Li) at liquid nitrogen temperature (77 K) so the diode is usually mounted on one end of a cryostat finger and is placed inside an aluminum vacuum enclosure. The other end of the cryostat finger is immersed in liquid nitrogen and the vacuum enclosure has a thin Be window for the X-rays to pass through. • Several components of the preamplifier, a field effect transistor (FET) and the feedback elements, are also mounted on the cryostat finger within the vacuum enclosure to reduce the thermal noise.

  45. Be window Si crystal Al vacuum enclosure FET & feed back elements Wire feed through Vacuum seal To detector bias Cold finger To pre-amplifier Si(Li) DETECTOR ASSEMBLY

  46. Pre-amplifier Main amplifier Si(Li) diode Pulse shaper FET Vacuum enclosure of cryostat Detector bias Si(Li) DETECTOR – SIGNAL PROCESSING ELECTRONICS

  47. Si(Li) DETECTOR – ENERGY RESOLUTION • X-rays of the same energy may not produce the same number of electron-hole pairs, and the electron-hole pair production is governed the Poisson statistics and hence its standard deviation is equal to the square root of the average number of electron-holes produced. • The shape of an X-ray line is near Gaussian and its full width at half maximum (FWHM) is a function of two independent factors: the electronic noise of the detection system and the statistical fluctuation of the electron-hole production: • (FWHM)2 = (Enoise)2 + (Epair )2 • and (Epair )2 = (2.35)2 EF • where = average energy for producing a electron-hole pair, • E = Energy of the incident X-ray, • F = the so-called Fano factor introduced to correct the • departure of the electron-hole production from • Poisson statistics due to other competitive processes, • 2.35 = the constant that converts the electron-hole production • standard deviation to FWHM.

  48. Si(Li) DETECTOR – ENERGY RESOLUTION (cont.) • Si(Li) detectors are operated at liquid nitrogen temperature (77 K). At this temperature, the average energy  for producing a electron-hole pair is 3.76 eV and the Fano factor F is ~0.12. Typical state-of-the-art Si(Li) detectors offers an energy resolution (FWHM) of ~175 eV at 5.9 keV. • In practice, the shape of the energy peak produced by a Si(Li) departs from Gaussian, and this is due partly to incomplete charge collection by the electrodes and partly to the dissipation of the X-ray energy in the active region of the diode through processes other than the photoelectric effect.

  49. Si(Li) DETECTOR – DETECTION EFFICIENCY • The detector efficiency of the Si(Li) is a function of the energy of • the incident X-ray and also of the following: • absorption of the Be window • absorption of the gold contact • absorption of the Si dead layer • thickness of the Be window • thickness of the gold contact • thickness of the Si dead layer • photoelectric mass absorption of Si • thickness of the active region of the Si diode

  50. Detection efficiency (%) Energy (keV) The low energy cutoff is determined by absorption of the Be window and the high-energy limit is established by the photoelectric cross section and the thickness of the Si crystal. Si(Li) DETECTOR – EFFECTS OF WINDOW & CRYSTAL THICKNESSES ON ITS EFFICIENCY

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