• Blog Stats

    • 77,570 hits
  • Archives

  • Enter your email address to subscribe to this blog and receive notifications of new posts by email.

    Join 157 other followers

  • Copyright notice

    This blog entry and all other text on this blog is copyrighted, you are free to read it, discuss it with friends, co-workers and anyone else who will pay attention.

    If you want to cite this blog article or quote from it in a not for profit website or blog then please feel free to do so as long as you provide a link back to this blog article.

    If as a school teacher or university teacher you wish to use content from my blog for the education of students then you may do so as long as the teaching materials produced from my blogged writings are not distributed for profit to others. Also at University level I ask that you provide a link to my blog to the students.

    If you want to quote from this blog in an academic paper published in an academic journal then please contact me before you submit your paper to enable us to discuss the matter.

    If you wish to reuse my text in a way where you will be making a profit (however small) please contact me before you do so, and we can discuss the licensing of the content.

    If you want to contact me then please do so by e-mailing me at Chalmers University of Technology, I am quite easy to find there as I am the only person with the surname “foreman” working at Chalmers. An alternative method of contacting me is to leave a comment on a blog article. If you do not know which one to comment on then just pick one at random, please include your email in the comment so I can contact you.

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.

Advertisements

Go on, Have your say !

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: