Joint ICTP-IAEA Workshop on Nuclear Data for Analytical Applications October 2013

Transcription

1 Joint ICTP-IAEA Workshop on Nuclear Data for Analytical Applications October 2013 Ion Beam Analysis Techniques for non-destructive Profiling Studies M. Kokkoris Department of Physics National Technical University of Athens Athens Greece

2 Ion Beam Analysis Techniques for non-destructive Profiling Studies M. Kokkoris Department of Physics, National Technical University of Athens, Zografou Campus , Athens, Greece 1

3 What is IBA? Ion Beam Analysis (IBA) is based on the interaction, at both the atomic and the nuclear level, between accelerated charged particles and the bombarded material. When a charged particle moving at high speed strikes a material, it interacts with the electrons and nuclei of the material atoms, slows down and possibly deviates from its initial trajectory. This can lead to the emission of particles or radiation whose energy is characteristic of the elements which constitute the sample material. 2

4 IBA METHODS: Method Αcronym Interaction Particle-Induced X-ray Emission Rutherford Backscattering Spectrometry PIXE RBS Characteristic X-ray emission following ionization by the primary beam Elastic scattering at backward angles Εlastic or Nuclear (non- Rutherford) Backscattering Spectrometry Elastic Recoil Detection Analysis EBS ERDA Elastic scattering at backward angles Elastic recoil at forward angles, not necessarily Rutherford Nuclear Reaction Analysis NRA Nuclear reaction between incident beam and nuclei in the target, producing a light charged particle Particle Induced Gamma ray Emission PIGE Prompt γ-ray emission during ion beam irradiation 3

5 Summary of interactions of accelerated ion with atomic nucleus: Elastic scattering (Coulomb and nuclear) Inelastic scattering (residual nucleus is excited) Nuclear reactions with emission of particles and γ-rays Positive Common Characteristics of IBA techniques: They are generally not destructive (least) and are thus suitable for use with delicate materials. They are to a certain extent multielementary and produce high-accuracy quantitative results. They require little or no preparation of the sample with the result that a specimen (like an artifact) could be directly analyzed. Only very small quantities (mg) of sample are needed. They permit the analysis of a very small portion of the sample by reducing the diameter of the ion beam to less than 0.5 mm. 4

6 Negative Common Characteristics: Some damage cannot be avoided (thermal, carbon buildup etc.)! A VdG type of accelerator is required. In most of the cases the experiments are carried out in vacuum chambers. Several experimental issues need to be addressed, thus a minimum knowledge of nuclear physics (experimental and theoretical) is mandatory. No direct information about the chemical environment can be produced. Issues like dating and authenticity testing can be addressed only indirectly. Unlike NAA, the IBA analysis concerns only a few microns below the surface of the samples. In most of the cases, a combination of techniques is required to solve a problem, and this implies time consuming experiments! THUS, DO WE REALLY NEED IBA? YES, because it can solve problems that cannot be addressed by all the other existing techniques! 5

7 IBA PROFILING TECHNIQUES: Rutherford Backscattering Spectroscopy / Nuclear Backscattering Spectroscopy (RBS / EBS) / Channeling Elastic Recoil Detection Analysis (ERDA) Nuclear Reaction Analysis (particle-particle and particle-gamma reactions) NON-PROFILING: Charged Particle Activation Analysis (CPAA), Particle Induced X-Ray Emission (PIXE) Neutron Activation Analysis (NAA), Secondary Ion Mass Spectroscopy (SIMS) SPECIFIC REQUIREMENTS FOR ALL IBA TECHNIQUES: Electrostatic accelerator (mainly VdG, single-ended or tandem) Scattering chamber (vacuum or in-air) Electronics Software for acquisition and spectral analysis 6

8 p+ 7 Li ---> 4 He + 4 He 7

9 Single ended VdG accelerator Tandem type VdG accelerator 8

10 ION SOURCES (ESPECIALLY DESIGNED TO PRODUCE NEGATIVE IONS): Duoplasmatron (for gaseous materials) Cs - sputter (for solid materials) 9

11 (a) (b) (c) Several types of vacuum chambers: (a) with open ends, (b) end station / compact geometry, (c) end station with full vacuum system attached to the accelerator line (rotary + turbo pumps) Typical external beam facility for milli or micro - beam 10

12 Typical circuit diagram: 11

13 Typical example of software analysis: SIMNRA 12

14 A short introduction in physics: The interaction of charged particles with matter Stopping power = Average energy loss per unit length inside the material Bragg peak (near the end of range region) 13

15 Ernest Rutherford ( ) A great talent from New Zealand. Maybe the greatest experimental physicist of his era after Faraday Discovery and study of α and β radiation. Distinction on the basis of penetrability and measurement of charge. Professor in McGill, Canada till He named γ-rays. He discovered the law of radioactivity and the existence of nuclear reactions. He measured the age of Earth along with Soddy and Hahn. Professor in Manchester till Famous gold foil experiment with Geiger and Μarsden. First nuclear model suggestion. First study of a nuclear reaction: 14 Ν +α--> 17 Ο + p Professor at Cavendish, Cambridge in the place of J.J. Thomson till his death. Collaboration with Ν. Bohr in the new atomic model. Theoretical prediction for the existence of neutron. Brilliant team of students and collaborators: Cockroft, Walton, Chadwick, Wilson, Appleton. Nobel prize in Chemistry (!), In his speech he said: It was the quickest transition in my life! 14

16 : 100 YEARS OF NUCLEAR PHYSICS Ra source Ernest Marsden, ( ) ZnS Hans Geiger ( ) E. RUTHERFORD: It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre, carrying a charge. 15

17 16

18 A short introduction in physics: The definition of the cross section σ Differential cross section: 17

19 a + X b + Y 18

20 A short introduction in physics: The Rutherford scattering d = distance of closest approach b = impact parameter (random position) The problem presents cylindrical symmetry (or φ invariance) 19

21 (nx = N t ) (f = fraction of the incident particles with impact parameters less than b) (df = fraction of the incident particles that pass through the annular ring) 20

22 21

23 And taking into account that: 22

24 (Differential cross section / Rutherford formula in the c.m. system) 23

25 Rutherford Backscattering Spectrometry (RBS ) Light substrate SSB Experimental Yield Atoms/cm 2 24

26 The relative yield differs between high and low-z nuclei by almost two orders of magnitude, thus RBS is ideal for heavy elements on light substrates. Actual cross-sections deviate from Rutherford at low energies for all projectiletarget pairs. This is caused by partial screening of the nuclear charges by the electron shells surrounding both nuclei. This screening is taken into account by a correction factor F:σ=Fσ R 25

27 The kinematic factor varies dramatically with the ion beam mass and approaches unity for protons impinging on heavy elements. It depends only on the mass ratio M 1 /M 2 and on the scattering angleθ. To obtain good mass resolution, the coefficient of Μ 2 has to be as large as possible. To accomplish this one can: Increase E 0 Increase M 1 Setθvery close to 180 o 26

28 Two basic phenomena affecting RBS measurements: Energy straggling of the incident ions and Limited resolution of the SSB detectors 27

29 Step by step analysis of two thin Ta x Si y silicides on a Si substrate, using 2.2 MeV α-particles RBS spectra are analyzed from high to low energies (right to left) 28

30 Multilayered structure Ceramic glass thick target 29

31 2,000 1,900 1,800 1, Au on SiO 2 Energy [kev] 2,000 1,900 1,800 1,700 Energy [kev] Au/SiO 2 multilayers 1,600 1,600 1,500 1,500 1,400 1,400 1,300 1,300 1,200 1,200 Counts 1,100 1, Counts 1,100 1, Channel Channel Energy [kev] Soda-lime glass Experimental Simulated O Na Mg Al Si Ca 2 MeV 4 He, θ=165 o 200 Counts RBS examples demonstrating the power of the technique Channel

32 Main RBS difficulties: 500 kev 4 He 2 MeV 4 He θ=165 ο 1.5 MeV 4 He 2.5 MeV 1 H Plural scattering Rough surfaces Crystalline targets Light elements on heavy substrates 31

33 Other important RBS difficulties: Measurement of the charge Charging of surface in the case of insulators Other problems (sputtering, heating, solid angle calibration, accelerator calibration, non-uniformity of targets, m, signal mixing etc.) 32

34 Incident Beam θ A = point of dechanneling B = point of backscattering A φ B The problem of crystalline targets: The channeling case Detector SSB Channeled fraction of the beam Crystal Surface θ = alignment angle with crystal axis φ = small dechanneling angle 33

35 Polar Scan Si <1 1 1> Angle Scan Si <1 1 1> Counts Angle SiO 2 (c-axis) Channeling E α = 3.05 MeV Random 8000 Counts Channel 34

36 The problem of light element detection: ERDA (Elastic Recoil Detection Analysis) 35

37 ERDA is complementary to RBS 36

38 New trends in ERDA measurements: TOF-ERDA Main problems: 1. Need for high energy heavy ions (big accelerators) 2. Limited analyzing depth (~1 µm) 37

39 Are there any quantum mechanical corrections in the Rutherford formula? Mott scattering: (for 0 + ground state nuclei) 38

40 Other solutions in the problem of light element detection: EBS, resonances in elastic scattering a+x C* a+x e.g. E p +Q=E R + Potential Scattering (Rutherford) = Always an Increase? NO, (dual nature) 39

41 Energy [kev] Energy [kev] EBS: Formation of a compound nucleus Interference between resonant and potential scattering Counts 1, MeV 4 He Counts MeV 4 He Ultra thin oxidized Fe on a pure Fe substrate Channel Channel MeV protons on carbon and silicon at 165 o 40

42 RBS/EBS study of the roof at the Municipality of Stockholm, using 1.5 MeV protons rough samples, complicated matrix! 41

43 Main difficulties in employing EBS: 1. Need for accurate/evaluated differential cross sections from literature 2. Overlapping resonances of different light elements 3. The (heavy) matrix may sometimes be important, impeding the analysis E p = 1.8 MeV (Si, C, total) E p = 1.8 MeV (Thin SiC layer on Au) 42

44 Nuclear Reaction Analysis (particle-particle) particle) NRA: Well established nowadays as one of the principal IBA techniques for accurate quantitative depth profiling of light elements in complex matrices. Based on the use of nuclear reactions. More frequently used: 1. (p,α) α): Low Q-value ( 6 Li, 9 Be, 10 B, 27 Al) and high Q-value ( 7 Li, Na, 31 P). No absorber foil can be applied. Highly selective. 11 B, 18 O, 19 F, 2. (α,p): Very few elements have positive Q-values ( 10,11 11 B, 19 F, 23 Na, 27 Al, 31 P, 35 Cl) thus the background is severely reduced. Cross sections are high enough only at high beam energies. 3. (d,p) and (d,α): Almost all light isotopes have high positive Q-values. They permit simultaneous analysis of many light elements in complex matrices (e.g. C, O, N, B, S etc.) at the expense of peak overlaps or background interference in some cases. Require very low beam energies. Radiation safety precautions are mandatory because the (d,n) reaction channel is almost always open. 4. Less frequently used: (p,d), (p, 3 He), 3 He-NRA 43

45 Similar experimental setup e.g. The carbon problem : RBS is weak, EBS can be applied only in certain cases (no other interfering light elements present, no high-z matrix, very case-specific measurements): Counts ,000 6,500 6,000 5,500 5,000 4,500 4,000 3,500 3,000 2,500 2,000 1,500 1, Energy [kev] Thin layer 70% Au + 30% C on Au backing E p =1.8 MeV (~resonance) Channel C Counts Energy [kev] E 130 d =1.2 MeV C(d,p 80 0 ) Channel 44

46 834 kev deuterons on a thick SiO 2 /Si sample, using an absorber foil in front of the detector Interesting points: Excitation of all possible kinematically permitted states Excitation of several isotopes Excitation of unwanted, background isotopes! 45

47 Special NRA Characteristics: POSITIVE High isotopic selectivity The matrix is not so important Clear isolated peaks with practically no background If the deuteron beam is adopted, one can achieve simultaneous analysis of most of the main light elements (C, O, N, F, B, Li) NEGATIVE Not many cross sections available in literature / theoretical evaluation pending Usually time-consuming studies Not all the elements present low enough MDLs Radiation safety is sometimes an issue Typical sensitivities: 1:10 4 in atomic proportion 46

48 Nuclear Reaction Analysis (particle-gamma) or PIGE Use of a γ-ray detector (preferably HPGe) Use of standards in identical conditions: C i,s = (C st S s Y i,s ) / (S st Y i,st ) Dealing with the problem of different stopping powers between standard and target Lack of generally accepted computer simulation code Lack of accurate/evaluated cross section data for γ- inducing nuclear reactions in literature 2.4 MeV protons 47

49 Demonstration of the mechanism: The depth resolution: Changes in the beam energy Use of narrow, strong, isolated resonances for superior depth profiling of light isotopes not always possible 48

50 Hydrogen profiling indirectly addresses the dating problem, therefore PIGE information is unique (pre-colombian quartz artifacts measured at AGLAE, Louvre)! 49

51 Extra NRA (particle-gamma) Characteristics: POSITIVE Excellent for hydrogen profiling, fluorine and aluminum (MDL~1-10 ppm) Quite satisfactory for carbon, nitrogen, oxygen, magnesium, silicon Relatively easy to implement, due to the standards used Excellent for machine calibration (accelerator tuning)! NEGATIVE HPGe detectors are very expensive, fragile, and need liquid nitrogen cooling! Usually very time-consuming studies Very element-specific technique Overlap of resonances might occur if the sample is relatively thick Efficiency calibration of the HPGe detector is a requirement! 50

52 Ion Beam Analysis Profiling Techniques: Summary, Present Situation, Future Perspectives SUMMARY 1. Rutherford backscattering (RBS) is ideal for depth-profiling of heavy elements on lighter substrates. 2. Elastic recoil detection analysis (ERDA) is excellent for depth-profiling of very light elements in thin films (for very small depths < 1µm). 3. Nuclear reaction analysis (NRA), is excellent for high resolution depthprofiling of specific isotopes. PRESENT SITUATION 1. A lot of work is being done in channeling, EBS and NRA (cross section measurements and evaluations). 2. Micro-beams and measurements in air (Louvre) have enhanced IBA capabilities. FUTURE PERSPECTIVES 1. New techniques are always evolving (e.g. HR-RBS, TOF-ERDA). 2. Channeling analytical algorithms? 3. CAN WE SOLVE ALL THE PROBLEMS??? NO (BUT MANY YES ) 51