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cpm conversion - low E xrays



Most of the technical responses related to
the higher energy xrays of the xray generator,
which of course is the normal source of
concern in xray diffraction units.

But if one is interested in the diffracted
xrays you are dealing with energies in the
6 to 20 or 30 keV range.  Then the 'typical'
response factor is not relevent since you
normally have to measure these thru an open 
window.  The GM detector then becomes simply
a photon counter and I assure you that the
conversion factor to mrem (or Sv) is not a
constant.

Even for the source xray, you might be pushing
the underlying assumption on constancy (which
assumes most of the spectrum is above the low 
energy cut off of a compensated GM).  The vast
majority of diffraction units at NIST are less
than 45 kVp.  I do not know what is typical
elsewhere, but at this low an energy (and typical
operating usage at even lower energies) I do
not trust a GM response and prefer ion chambers.
And even with those (having a lowered response
below 30 keV) one needs to take care.

A little risk adds spice to life.
slaback@MICF.NIST.gov