Portable X-Ray Fluorescence to Determine the Composition of

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

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

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 Xrays with energy equal to the difference between the two energy levels.

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.

Setup • • • Amptek Mini-X X-ray tube. Generates X-rays by using accelerated electrons to excite electrons in a silver target, which then emits Xrays at its characteristic energy 22. 16 ke. V. Continuous Bremsstrahlung radiation also produced.

Setup • • Amptek XR-100 SDD - silicon drift detector Amptek PX 4 - 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. PX 4 measures pulse height and sorts into channels based on X-ray energy, producing a spectrum.

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.

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

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.

Spectrum Analysis • • The collected spectrum is displayed in Amptek’s ADMCA program. The program is also used to control the PX 4, to start and stop data acquisition.

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

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

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

Quantitative Analysis

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.

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.

Results This is a children’s bracelet that I found in my house, probably from the late 1990 s. 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.

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.

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

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

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

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.