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X-ray energy and getting the terms right

While reading the article entitled “the art detectives” in the RSC’s Chemistry World magazine I saw the statement that high energy X-rays are used for XRF of elements such as zinc. I strongly suspect that a misunderstanding has occurred, for example the zinc k lines will come at 8.6 keV which is hardly high when compared with the X-ray photons commonly for the industrial radiography of steel objects. To excite an atom in a X-ray fluorescence (XRF) experiment only moderate energy photons are needed (tube voltage of 40 kV is acceptable) while for industrial X-ray radiography it is common to use much higher accelerating voltages (100 kV and higher). For very thick metal objects photons in the MeV range are used.

What I think the article should have stated is that the object in XRF was illuminated with a high intensity of x-ray photons, to my mind intensity (photons cm-2 s-1) is very different to photon energy. But why would anyone use an expensive intense x-ray source rather than a weaker and cheaper one ?

If we assume that the increase measured above background is directly proportional to the concentration of an element and the intensity of the incoming exciting x-ray beam, then if the background is 10 cps, then with a weak x-ray source then we could get a reading of 20 cps on a spot on a painting. As for a random events the standard deviation on the count number is the square root of the count number after 1 second then the sum of the two SDs is 7.634 which is close to the difference between the two count numbers. If we were to use a source ten times brighter then the sum of the standard deviations (10.49 + 3.16 = 13.65) is small compared to the difference in counts after 1 second.

The great problem is that people writing about science sometimes tend to throw words about, almost randomly, without thinking about the fact that the word already has a meaning. To write clearly about science we must first avoid confusion.


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