Re: Radiation Output Measurement of the Primary Beam of X-Ray

[rkathren@tricity.wsu.edu (Ron L. Kathren)]

On several occasions in the course of my checkered career in health physics,
I have performed such measurements and, as I recvall, have described the
techniques in incident reports that are usually confidential for obvious
reasons.  Probably the most single useful method is to use TLD, suitably
calibrated against a K-fluorescence source, at various distances from the
window, thus providing a dose rate vs. distance curve (which will NOT follow
inverse square at short distances from the window).  This curve can be
extrapolated to zero window distance.  Because of their thickness, TLD
measurements will require an attenuation correction.  However, at the
energies involved, scattering is not a problem as the photoelectric effect
accounts for virtually all of the interactions.  Most units seem to operate
with a Cu target, and hence the 'effective' energy is at about 8.9 keV; the
spectrum, and indeed most of the dose, is from the Cu  K characteristic.

I will be pleased to provide you with more details and the benefit of my
experiences in this regard; please feel free to telephone me at 509-375-5643.  

Ron Kathren


, >Dear Radsafers:
>
>
>	I wish to solicit the expert advice of any Radsafer knowledgable in the
>following regard.  Does any Radsafer know of any professional journal
>articles wherein the authors have actually performed radiation
>measurements of the <italic>primary</italic> beam on x-ray diffraction
>(XRD) units?  I am searching for articles that quote numerical values of
>radiation intensity of such units, and preferably professional articles
>that describe the radiation measurement technique, what they used, how
>they did it, etc.  Perhaps Melissa Woo, who has compiled internet
>radiation safety guidance for such units, might know of such articles.
>
>
>	My search so far indicates that few such articles exist.  This is not
>surprising when one considers that measuring the primary beam of XRDs is
>confounded by at least four complicating factors:
>
>
>		a.  Small  primary beam cross-sectional area, which prevents the full
>sensitive volume of many detectors from being fully exposed to the
>primary beam photons;
>
>
>		b.  Low photon energy, which on an XRD operated at even a 'high' x-ray
>tube kilovoltage of say 45 kVp, yields photons the majority of which are
>10 keV or less--a photon energy region where the energy response of TLDs
>and ion chambers begins to dramatically fall off.
>
>
>		c.  High beam intensity on the order of many thousand R/min, in which
>case care must be taken to avoid detector saturation problems.
>
>
>		d.  The likely inaccuracy of the inverse square law at such low photon
>energies due to absorption via the photoelectric effect of the photons in
>air which implies that it may not be at all accurate to measure the
>intensity at a long distance and inverse square your way back inward to
>the point at which you really desire to know the beam intensity.
>
>
>	The American National Standards Institute (ANSI) standard regarding
>XRDs, ANSI N43.2, Radiation Safety for X-Ray Diffraction and Fluorescence
>Analysis Equipment, on page 9 quotes an exposure rate figure of 400,000
>R/minute at the x-ray tube exit port; however, the literature citation
>from which this figure allegedly came is in "Radiological Health Data and
>Reports," Vol. 8, May 1967, pp. 245-249, by Howley and Robbins which is
>out of print and which I have not been able to locate a of the pertinent
>pages copy.
>
>
>	Another paper quotes a figure of 36,000 R/sec, but the authors due not
>specify a reference where this figure came from.  This paper is from the
><italic>International Tables for X-Ray Crystallography</italic>, Vol III,
>Physical and Chemical Tables, 1968, Chapter 6, Kynoch Press, Birmingham,
>England.  This 36 kR/sec figure is the largest figure I have ever seen
>regarding the radiation intensity of XRDs.  Weigensberg et al., in
><italic>Health Physics</italic> (HPJ) 39:237-241 (1980) estimated an XRD
>primary beam intensity in the range of 16,000 to 27,000 R/min using a
>250R Victoreen ionization chamber and the inverse square law.
>
>
>	Two papers by R. Jenkins and D.J. Haas published in <italic>X-Ray
>Spectrometry</italic> in 1973, Vol 2 and in 1975, Vol 4 talk about XRDs
>but with no mention of output measurement values or techniques.  The same
>can be said for a long article by R. Rudman, "Journal of Chemical
>Education," Vol 44, Jan 1967, and an article by on "Radiation Hazards
>Associated with X-Ray Diffraction Techniques," published in Acta
>Crystallographica, Vol 16, 1963, pp. 324--328, as well as an article by
>Lubenau, J.O., et al., in the HPJ Vol 16; 739-746, 1969.
>
>
>	The Howley article cited above seems to be frequently referenced as an
>authoritative source of XRD primary beam output measurement data.  If any
>Radsafer has a copy of it, could you please FAX it to me.
>
>
>	Presumably, the 'best' way to measure the radiation output of XRDs is
>via TLDs, but as mentioned above, care would have to be exercised to
>calibrate the TLDs to the low photon energies involved, given that the
>TLD energy response curve, for LiF, drops of below about 10 keV.
>
>
>	If any Radsafer can refer me to any professional journal articles that
>describe the measurement of XRD primary beam intensity, what detectors
>were used, how they did it, what values they measured, I would sincerely
>appreciate it.  I suspect that this topic may be of interest to many
>Radsafers; however, please feel free to reply to me individually at 
>lee_david_w@lanl.gov
>
>
>	Thank-you.  Best regards,
>
>
>David W. Lee, PO Box 1663, MS K483, Los Alamos, NM  87545; Ph (505)
>667-8085; FAX: (505) 667-9726
>
>
>
>
>David W. Lee
>
>Radiation Protection Policy
>
>& Programs Analysis Group (ESH-12)
>
>Los Alamos National Laboratory
>
>PO Box 1663, MS K483
>
>Los Alamos, NM  87545
>
>Ph:  (505) 667-8085
>
>FAX: (505) 667-9726
>
>
>
>