Portable X-Ray Fluorescence to Determine the Composition of

Household ObjectsKatherine 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 X-rays 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 X-rays at its characteristic energy 22.16 keV.

• Continuous Bremsstrahlung radiation also produced.

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.

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 demonstrations.

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 PX4, 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 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.

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 1970s.

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.

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.