POWDER DIFFRACTION FOR CULTURAL HERITAGE ON INES AT ISIS

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Isabella Wilkinson
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1 POWDER DIFFRACTION FOR CULTURAL HERITAGE ON INES AT ISIS SoNS School of Neutron Scattering seminar 1

2 Summary 1. What is neutron diffraction? 2. What are the properties that neutron diffraction can measure? 3. An example of application: cultural heritage (CH) 4. CH experiment on INES 2

3 Fundamentals of Neutron Diffraction 3

4 Bragg s law A Bragg peak is obtained if the reflected radiations off successive layers of atoms interfere constructively. This condition is satisfied by the Bragg s law: nλ= 2dsenϑ K=2π/λ DIFFRACTION PATTERN 2ϑ De Broglie relationship: λ= h/mv Thermal neutrons (λ=1,8 A) have λcomparable with the interatomic spaces 4 Giorgia Albani - Università Milano-Bicocca 27/05/14

5 Powder diffraction A randomly oriented polycrystalline sample (e.g. a powder) contains a very large number of crystallites Debye-Scherrer cones A beam impinging on the sample will find a representative number of crystallites in the right orientation for diffraction Diffraction occurs only at specific angles, those where Bragg s law is satisfied 5

6 Time of Flight Neutron Diffraction d-spacing d( A) = h t(µs) 2m n L(m)sinϑ = 1 t(µs) L(m)sinϑ Time of flight Distance Scattering angle 6

7 Neutron diffraction for CH Analyzing the composition of an archeological sample in a non-destructive way could give indicators about: The period: WHEN? The culture: WHO? The region: WHERE? The technology: HOW? An example: metallurgic evolution: Native copper Arsenicated copper Bronze (Cu-Sn) Brass (Cu-Zn) 9500 BC 3200 BC 2000 BC Romans (753 BC) 7

8 ToF - Neutron Diffraction measurements for CH Determine the wt% of crystal phases present in the sample Alloys compositions (elements wt%) Texture (preferred cristallite orientations) Cristallite size Residual stress 8

9 Italian Neutron Experimental Station: 9

10 TS1 at ISIS 10

11 Spallation at ISIS An 800 MeV proton accelerator produces intense pulses of protons with a 50 Hz frequency The tungsten target is bombarded with these pulses of high energy protons. This gives an extremely intense neutron pulse (FWHM=300ns performance 15-20n/p) The neutrons are slowed to speeds useful for condensed matter research by an array of hydrogenous moderators around the target They are then directed to a suite of neutron instruments, each optimised to explore different properties of materials 11

12 INES: Italian Neutron Experimental Station Scattered neutrons 3 He tubes Incident n beam Transmitted n beam sample position 12

13 Incident n beam INES: a powder Scattered neutrons diffractometer 3 He tubes Transmitted n beam sample position 316 K water moderator λrange: 0,17A -3,24A (E~keV) Large sample holder tank (1m 3 ) 144 diffraction detectors ( 3 He) 9 banks d-spacings: A Angle range: 11,6-170,6 High resolution: 0.10% High signal to noise ratio Beam size: 40x40 mm 2 Jaws to shape the beam: min 2x2mm 2 Laser point to align Sample changer 13

14 A special case: analysis of bronze artefacts Neutron Diffraction (ND) and Neutron Resonance Transmission (NRT) on INES applicated on metallurgy for Cultural Heritage (CH) 14

15 The aim of the experiment Bronze fibulas were found in the archeological site of Guardamonte (AL). The analysis of the composition and construction technology can help archeologists to understand the role of this human settlement in the 5 th century BC. ToF-ND measurements allows for the identification of mineral phases and quantification of alloys components. The latter result is achieved through quantitative Rietveld analysis through the measurement of lattice parameters of the alloy, and the application of the Vegard s law. Limitation of Vegard s law: only applicable to binary alloys or ternary alloys with segregates NRT measurements for the identification of a sample content 15

16 Rietveld refinement The Rietveld refinement minimizes the difference of the measured and theoretical diffractogram The theoretical model of the diffractogram has many parameters (crystal structures, d-spacings, molecular angles, etc) that are used to do a multiparameter fit Intensities of Bragg peaks at specific ϑ are related with the relative composition of phases present in the sample and presence of texture Position of the Bragg reflection is related with lattice parameter and diffractometer characteristics Broadening of Bragg reflection is related with presence of microstrain and grains size. Bragg reflections are usually described in terms of Gaussian and Lorentzian component 16

17 Vegard s law Measured lattice parameter of the alloy a = a A 0 (1 X B )+ a B 0 (X B ) Concentration of minority element B Approximate and empirical Concentration of element A We can measure lattice parameter a of a binary alloy and derive the elemental composition, if we have a calibration curve It allows segregation and corrosion problems to be overcome It s a non-destructive method to derive the chemical composition of a binary alloy 17

18 Elemental composition of a single phase metal alloy Bronze (Cu, Sn) Lattice parameter determination through Rietveld refinement Calibration curve It s possible to derive the Sn content in bronzes and leaded bronzes measuring the lattice parameter of Cu phase in objects 18

19 Elemental composition of a single phase metal alloy Brass (Cu, Zn) Cu, As alloy Cu, As alloy is the only case of non linear relationship. A quadratic term is needed. 19

20 A fibula sample Body Spring Visual inspection: 1. Body: lightly corroded grayish colour 2. Spring: heavily corroded greenish colour Question: Do fragments have the same composition and conservation status? 20

21 Phase composition Body diffractogram bronze Rietveld refinement Determination of the weight fraction of the phases composing the sample Cu 2 O lead The fibula body is made of leaded bronze and corrosion products in small quantity 21

22 Neutron diffraction results Bronze peaks Body Spring Spring Body Bronze 47% 72% Cuprite 31% < 1% Malakite 12% < 1% Nantokite 5% < 1% Lead 5% 28% Lattice parameter Average composition 3,68 A 3,64 A Beam size: 40x40 mm 2 Measuring time: 6hours Typical d-spacing lengthens: 0,05-0,1 A 22

23 Neutron resonance transmission Fibula s body NRT spectra associated with ND measurements allows for distinguishing between binary tin bronze, arsenic copper and zinc bronze (brass) thus overcoming one of the limitations encountered by ND Fibula body: Cu-As bronze Fibula spring: Cu-Sn bronze Fibula s spring Hypotesys: the body and the spring of the object were cast separately and eventually joint together 23

24 Conclusions A set of bronze samples from the site of Guardamonte have been investigated with ND and NRT techniques at ISIS. Both the techniques are completely non-invasive ND has higher penetration and can give info on: Chemical composition Way of production of the samples (provided calibration curves for Vegard s law) NRT it s crucial to distinguish, different Cu alloys The two techniques are complementary 24

25 Thank you! 25

Introduction to X-Ray Powder Diffraction Data Analysis

$Introduction to X-Ray Powder Diffraction Data Analysis$ Introduction to X-Ray Powder Diffraction Data Analysis Center for Materials Science and Engineering at MIT http://prism.mit.edu/xray An X-ray diffraction pattern is a plot of the intensity of X-rays scattered