Re: cpm conversion - low E xrays

["Michael P. Grissom" <mikeg@SLAC.Stanford.EDU>]

To all,

This is the critical issue:  What is the energy cut-off
for the detector being used.  Even many "thin-window"
instruments will not reliably detect photons at 6-30 keV.

This is one of the reasons some surveyors use film,
even conventional "paper" film used in radiotherapy
for beam localization can be useful, taped to the devices 
while in use for a period of time to enable visual 
inspection of critical points (seals, openings, fastener 
points, sample chamber doors, etc.).

Such integrating devices (TLDs, radiosensitive film, etc.)
with very thin-windows (paper or thin plastic wrappers) will 
more likely produce results than conventional survey 
instruments.  This is in part because of the low probability 
of significant leakage (that is, photon poor measurement
conditions) from most modern XRD systems.

Clearly an opinion about surveys for a category of
instruments that very rarely have a problem (as long
as the vacuum necessary for doing x-ray crystallographic or 
TEM/SEM work is properly maintained!).  Earlier postings on 
RADSAFE noted that real problems occurred due to home made XDR 
systems that had sample chamber doors rigged to defeat the 
interlocks!  In those cases, the environment becomes photon
rich (but still a very limited angle beam).

Somehow, all this reminds me of an ABHP Part II question
a few years ago... :-)

MikeG.

At 07:43 AM 8/29/96 -0500, you wrote:
>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.
>...
>slaback@MICF.NIST.gov

-----------------------
Michael P. Grissom
Special Assistant, SLAC
mikeg@slac.stanford.edu
Phone:  (415) 926-2346
Fax:    (415) 926-3030