Material Analysis using X-Ray Fluorescence (XRFA)

Transcription

1 Material Analysis using X-Ray Fluorescence (XRFA)

2 Material Analysis (XRFA) The excitation of characteristic X-rays in a sample of material is the preferred method of material analysis. In this experiment a radioactive g-source ( 241 Am) or a X-ray tube is used for the excitation depending on the energy range or Z- region of interest. A high resolution Si-X-ray detector and a multichannel analyzer enables us to distinguish neighbor elements which differ by one unit in the proton number (Z). The X-ray fluorescence is an efficient and quick method. It doesn t harm the investigated sample. Several examples are given below. The XRFA-method is complimented by the method of proton induced X- ray measurement (PIXE) which requires higher expenditures, shown in the example of the mask of Queen Sat-Djehuti.

3 X-Ray Fluorescence Process Example (Ti) : An electron in the K shell is ejected from the atom by an external primary excitation x-ray, creating a vacancy.

4 The L Lines When a vacancy is created in the L shell by either the primary excitation x-ray or by the previous event, an electron from the M or N shell "jumps in" to occupy the vacancy. In this process, it emits a characteristic x-ray unique to this element and in turn, produces a vacancy in the M or N shell. The K Lines An electron from the L or M shell "jumps in" to fill the vacancy. In the process, it emits a characteristic x-ray unique to this element and in turn, produces a vacancy in the L or M shell.

5 "Auger" Electron The excitation energy from the inner atom is transferred to one of the outer electrons causing it to be ejected from the atom.

6 ThinkCentre IBM Computer Canberra Si(Li) X-ray Detector Canberra Multiport II X-ray Tube PreAmp HV Power Supply Canberra 3122D Spectroscopy Amplifier Canberra 2020

7 Material Analysis using X-Ray Fluorescence (XRFA) : Required Knowledge Gam Tun Comp Gei Comp Int Bet TRI Principles of Principles of What? What? Tech Comp Principles of

8 Material Analysis using X-Ray Fluorescence (XRFA) : Tasks and Goals Set-up Produce Set-up Determine Determine Determine energy Determine the Measure energy Determine Compare energy WARNINGS Be careful. Shut down Never touch Remove source after measurement

9

10

11

12 EXAMPLES

13 The Restoration of Michelangelo's David Typical spectrum before and after cleaning treatment Taking an x-ray fluorescence spectrum of David Mapping sulphates on Michelangelo's David using portable EDXRF (from "Exploring David: diagnostic tests and state of conservation," edited by S. Bracci et al., GIUNTI, Florence, 2004) Giovanni Buccolieri, Alfredo Castellano, Marina Donativi, Stefano Quarta Universita di Lecce, Dipartimento di Scienza dei Materiali

14 The Restoration of Cellini's Perseus AmpTek in collaboration with the University of Sassari, the University of Roma and Lecce and with the Central Institute for Restoration of Roma

15 Example of the influence of a green patina on the detected spectrum. It is easy to see that the photons generated in tin atoms (corresponding to the peak at 25.2 kev) penetrate easily through the patina while the copper and lead ones are strongly absorbed. In this way, you can check the status of the patina and verify its composition. Example of a spectrum of the Perseus right knee showing that the bronze alloy is composed by copper, tin (~3.6%), lead (~6%), antimony (~1%), iron (<1%) and silver (<1%). The composition of the alloy slightly changes in other points of the statue. The x ray tube is equipped with a W-anode and the HV is 35 KV.

16 The Gold Ibex from Akrotiri, Santorini, Greece "Unexpected among the many and wealthy finds of the excavations made for the foundations of the pillars of the new shelter, is the gold figurine found on December Unique of its kind, it was discovered in mint condition, inside a wooden box (only the imprint has survived) inside a clay chest (larnax), next to a large pile of horns, mainly of goats. Excavation of the find-spot is still in progress and it is therefore too early to draw conclusions about the figurine's significance (I would say that it was offered as a gift from someone coming from the East or that it is a sacrificial object related to worship or other rituals). The figure is hollow and was cast by the "lost wax" method. The legs, neck, and tail of the animal were soldered on after the removal of the inside core. In the finishing process the figure was hammered, as deduced from the tool-marks."

17 Below is a comparison of two spectra taken from the Gold Ibex. The Goat_1 spectrum was taken from just above the front left leg. This position is free of visual contaminants and considered to be the base spectrum. The Goat_4 spectrum is from the braze/weld of the tail. It shows an increase in the amount of Copper (Cu).

18

19 PIXE investigation of funeral mask of Queen Sat-Djehuti, ~1650 b.c. Proton beam