Portable X-Ray Fluorescence to Determine the Composition of Household Objects

1. Portable X-Ray Fluorescence to Determine the Composition of Household Objects • Katherine Spoth, mentor Peter Revesz • August 13, 2010

2. The Project • Learn the physics of XRF • Master the use of the XRF setup and related software • Collect samples • Run XRF measurements/collect spectra from each sample • Identify features in the spectrum • Determine which element produced each peak • Use software to carry out rough quantitative analysis

3. Review of XRF • Incoming X-ray radiation excites an inner electron and causes it to be ejected from an atom. • Outer electrons fall to fill this vacancy, releasing X-rays with energy equal to the difference between the two energy levels.

4. Review of XRF • Each element when excited emits a unique X-ray spectrum. • This is used to determine which elements are present in a spectrum from an unknown sample. The characteristic spectrum of copper.

5. Setup • Amptek Mini-X X-ray tube. • Generates X-rays by using accelerated electrons to excite electrons in a silver target, which then emits X-rays at its characteristic energy 22.16 keV. • Continuous Bremsstrahlung radiation also produced.

6. Setup • Amptek XR-100SDD - silicon drift detector • Amptek PX4 - digital pulse processor and multichannel analyzer • X-rays create electron-hole pairs in silicon material in the detector, produces electrical pulse with height proportional to X-ray energy. • PX4 measures pulse height and sorts into channels based on X-ray energy, producing a spectrum.

7. Setup • The sample is placed on a joystick-controlled 2-axis table. • Allows user to change locations or look at multiple samples without turning off the X-ray source and opening the enclosure.

8. Setup • Setup also includes a camera that allows the user to see where the sample is located. • Can use a fluorescent screen to determine the area that the X-ray beam impacts. • This can be marked onto the camera and used to determine which part of the sample is receiving X-rays. • Allows view of a sample to be shown on a projector for demonstrations.

9. Setup • X-ray source, sample, and detector all are contained in a lead acrylic enclosure, ensuring no radiation can escape. • Interlock system prevents door from being opened while X-ray source is powered on.

10. Spectrum Analysis • The collected spectrum is displayed in Amptek’s ADMCA program. • The program is also used to control the PX4, to start and stop data acquisition.

11. Spectrum Analysis • Peaks can be selected and their centroid energy will be displayed • Determine which element produces a peak at that energy, look for other peaks from the same element to be certain of its presence

12. Quantitative Analysis • After it is determined which elements are present in a sample, XRS-FP can be used to give approximate concentrations • This software processes the spectrum: removes background, smoothes the spectrum, removes artifacts from the source, subtracts a background spectrum, and deconvolves the spectrum to compute the intensity of each element peak • Uses fundamental parameters to compute concentrations from these intensities • This approach requires the user to input details about the geometry of the XRF setup and a list of the elements present

13. Quantitative Analysis A spectrum before and after processing in XRS-FP.

14. Quantitative Analysis

15. Quantitative Analysis • Method has several limitations: • X-rays from light elements attenuate in air too much to be detected by this setup • The mass fraction that is produced does not account for the possible presence of light elements • Approximations in user inputs also decrease the accuracy of the concentrations given by the program • These ideas need to be kept in mind when interpreting results from XRS-FP.

16. Results • Around 100 different samples were tested - very few were found to contain anything harmful • Samples that did contain elements like lead and cadmium tended to be older items • None of the samples we purchased for the project contained dangerous materials (face paints, cosmetics, crayons) • Some interesting samples are shown on the following slides.

17. Results This is a children’s bracelet that I found in my house, probably from the late 1990s. The beads are colored made of colored plastic so the lead found here is found in the beads, not just in a paint or surface coating.

18. Results The spectrum created by one sample of the face paint that was purchased in June. The spectra shown have all been processed in XRS-FP.

19. Results This is the spectrum from the green paint on this Pyrex mug made in the 1970s.

20. Results A soil sample collected near the lower Wilson ab parking lot.

21. Outreach • The portable XRF cart’s main purpose is outreach and teaching • Presented the setup to several groups of visiting teachers • Gave brief introduction to XRF • Described the apparatus and setup • Demonstrated the process using a sample provided from the audience • Allowed visitors to use the chart to match a peak’s energy to the element

22. Outreach • The XRF setup is a valuable teaching tool for several reasons: • Its portability allows it to be used in any location. • Spectra can be displayed on the monitor on the cart or the computer can be attached to a projector. • XRF can be easily explained to groups with almost any amount of science background. • The use of audience-contributed samples increases interest and participation.

23. Outreach • The XRF setup is a valuable teaching tool for several reasons: • Its portability allows it to be used in any location. • Spectra can be displayed on the monitor on the cart or the computer can be attached to a projector. • XRF can be easily explained to groups with almost any amount of science background. • The use of audience-contributed samples increases interest and participation.