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<p>[QUOTE=&quot;-jeffB, post: 25245826, member: 27832&quot;]XRF machines work by sending high-energy (short-wavelength) X-rays into a sample, then measuring lower-energy (longer-wavelength) X-rays that are re-emitted from the various kinds of atoms in the sample.</p><p><br /></p><p>X-rays can&#039;t penetrate dense metals very well. So the high-energy X-rays get absorbed before they can penetrate very deeply, and the lower-energy fluorescence X-rays get absorbed even more easily on their way back <i>out</i> of the sample.</p><p><br /></p><p>I found <a href="https://www.researchgate.net/post/Is-there-an-easy-way-to-calculate-what-the-penetration-depth-is-for-X-ray-florescence-in-gold" target="_blank" class="externalLink ProxyLink" data-proxy-href="https://www.researchgate.net/post/Is-there-an-easy-way-to-calculate-what-the-penetration-depth-is-for-X-ray-florescence-in-gold" rel="nofollow">one discussion</a> that says 50keV x-rays (the kind that might be generated by an XRF gun) can penetrate gold to a characteristic depth of about 70 microns, but the generated X-rays (that the gun detects to do its analysis) can only penetrate 4-6 microns.</p><p><br /></p><p>So if you&#039;ve got a gold-<i>plated</i> object, where the gold is typically a few microns or less thick, the XRF analyzer will detect a signal from the underlying base metal. But if you&#039;ve got a fake bar that&#039;s a millimeter or two of gold wrapped around a tungsten core, XRF can&#039;t see through the outer layer. An EM checker <b>might</b> be able to.</p><p><br /></p><p>(When I posted earlier, I was confident that an EM checker <b>would</b> be able to see through a thick layer of gold. But I&#039;ve done some fact-checking on myself, and it depends on the specific frequencies the tester uses. Sigma claims 1.5mm, which would be deep enough to detect tungsten-cored <i>coins</i>, but possibly not 10-oz or larger bars.)[/QUOTE]</p><p><br /></p>

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