Testing for lead in paint and other products using radiation.

There is a lot of squawking going on concerning China imports containing lead in  paint, especially on children's toys, and rightfully so.

Decades ago, we in the U.S. used lead in many products, including gasoline and paint, now banned. As a resident of Missouri, a big lead mining and processing state I am particularly keen on the subject of detecting lead where it should not be. It should probably be mentioned that some of my former clients were St. Joe Lead, National Lead ( NL Industries), and Ethyl Corp.

All atoms are made up of nucleons and electrons. When an electron is removed from an inner shell, it must be replaced by another electron to maintain stability in the atom. In atoms heavier than hydrogen, this action releases energy, in the form of X-Rays. There are several sources from which the missing electron may be replaced, and in heavy atoms, there are other shells from which the electron may go missing.

We will concentrate on the innermost electron shell, called the K shell. Extra energy introduced into the atom by using an X-Ray source, Gamma source, a Beta or other charged particle source, can cause the K electron to gain energy, and leave the shell, heading off to parts unknown at great speeds. As mentioned this space must be filled, if from the L shell, this reaction is called  K-alpha. If it gets replaced by the M shell, we call that K-beta and this reaction has a different energy.

Most elements fill the K shell hole from the L shell, giving rise to a distinct X-Ray, unique to that element, called the "K-alpha CHARACTERISTIC X-RAY ENERGY"

For lead (Pb) this K-a energy is 74.96 keV.

The procedure is simple, excite the material under test with an X-Ray tube or isotope  whose energy level is substantially higher that 74.96 keV. Then at the same time, scan the DUT (Device Under Test) for the tell-tale 74.96 keV emission. Apparatus are available to do all the needed functions such as:

http://www.niton.com/Lead-Paint-Testing/Applications/beyond-lead-paint.aspx

http://www.rmd-lpa1.com/

Many apparatus variants exist, as mentioned, using protons, beta particles, Cd-109 and Fe-55, Co-57,  as well as Am-241 as the excitation source. Virtually any element can be examined using this X-Ray Fluorescence method, except hydrogen. The acronym PIXE (Particle Induced X-Ray Emission) is applied if the excitation source is a charged particle. I have read of Alpha Particle excitation but have never been able to verify that personally.

Making the element X-Ray Fluorescence is very easy, I use a simple Kr-85 beta source and test jig. Detecting the characteristic X-Ray is a bit more problematic in lighter atoms, due to the low energies involved.  HPGE probe, CdTe, even Mercuric Iodide probes are preferred, but many NaI(Tl) give great results with the heavier elements. Virtually ANY detector will pick up the 74.96 keV from Pb.

Of course a Gamma Spectrum Analyzer (MCA) is required to read the exact energy, but the simplest SCA's will work too. System calibration is easy by using Am-241's 59.5 keV and 88 keV from Cd-109 as standards.

Lead's 74.96 X-Rays can be exited by Cosmic Rays too. For this reason and others, when we shield a probe in the lab, we use a lot of lead, but then shield the lead with tin or cadmium, then shield that with copper. Such a "graded shield", if built with pre WW2  or ancient lead, will make an exceptionally effective probe shield. The shield for my 3" X 3" probe is 300+ pounds, the one for my Marinelli Beaker probe is well over 1000 lbs. A BIG one would run nearer to 3000 lbs.

XRF can obviously scan for the other restricted ROHS elements, Hg, Cd, Cr, Br,

Compact, portable instruments are costly, can run 20,000 USD easily. There is no excuse for Corporate HQ at Wal-Mart, Mattel, et.al. not having ONE unit and someone who knows how to use it.

Lab setups can be significantly more expensive, but at what cost compared to company brands like Mattel losing all?

Below is an amateur lead (Pb) XRF detector system, all homemade from commercial components.

Pb XRF on Gamma Spectrum Analyzer

Setup: 2" NaI well scintillator with thin side sections. Known to be OK with
low energy Gammas, but not optimized for this service- calibrated with
Cs-137. Home made base and dynode string, single coax feed. Adapted to 2
coax instrumentation via a home made splitter box.
Spectrum Analyzer is SpecTech UCS-20, USB connected to an HP laptop.
Source = 4 uCi Kr-85, Betas blocked from direct interaction with probe by a
stack of 3x5 cards. Distance from source to detector= 4".
Thin lead foil used in second step, in transmission mode to stimulate
X-Rays.
Foil approx 3 mils thick. Can't be sure because it has a thin plastic
coating which I can't remove. Laminate total thickness is 4 mils. Experiment
set up so plastic is away from the Beta source, with the Betas striking bare
lead foil. X-Rays must penetrate the plastic on way to probe, so some
attenuation anticipated.

Charts
1: Spectrum chart showing Kr-85 only. Note nice peak at 514 keV which is the
characteristic Gamma for Krypton85.

UPDATE- This article was first printed in winter of 2004. Since then my setup has improved drastically. I still use the same old exempt quantity Krypton 85 source though, it wound up being the best of all. Most of the improvements in this system were to the probe shield and the probe itself was upgraded a notch to a 3" X 3" Bicron NaI(Tl), pretty much the standard lab probe I think.


Have Fun

Geo