Re: Low energy x-ray (8 kev) survey meter

[Mike Grissom <mikeg@SLAC.Stanford.EDU>]

Radsafers,

As an addendum to this discussion on looking for
leakage from XRDs and other low-energy radiation
generating devices, an old colleague, now long retired,
used to conduct surveys using a series of "mirrors" of
many different sizes that had been coated with a
scintillator (I believe he had had them custom made using
zinc sulfide). These ranged from dental type coated mirrors
to much larger area ones. He was able to access areas that
no pancake GM or thin window NaI detector could reach
around radiation generating devices such as XRDs and
electron microscopes. He also used film if he found a
suspicious area or faint fluorescence using his "mirror"
detectors! By leaving the film taped to the questionable
area (paper technique radiography film which passes enough
photons even at very low energies), he was able to "image"
the general shape of the leak and in many cases facilitate
the repair.

This solution indicates that his concern was finding
qualitatively ANY radiation leakage, which also indicated
that the machine was loosing vacuum or otherwise not
functioning properly, and not any grand effort to precisely
measure the quantity.

He did, of course, need a VERY dark room!

Ciao,

MikeG.

At 09:11 AM 6/10/98 -0500, you wrote:
>
>	Ted,
>
>	At 8 kev, you have "LESS" than 0.01% efficiency with a G.M. (pancake,
even less with a end-window), but are exceeding 10% efficiency with a low
energy scintillation probe!  With an end-window G.M., you may as well be
using florescent paper in a darkened room!  You are missing a LOT of
photons there my friend!  I wonder if you have ever done a survey and
compared the two - I think not... Sure I use a pancake G.M. too initially,
to look for the big leaks, but when I am through, I always go back and do
another survey and look more carefully with a scintillation probe - that
extra 5 - 10 minutes is WELL worth it as far as detecting scattering angles
(leaks) no-one would have imagined before hand...  As you, Ron and I all
mentioned in our posts, the idea is not to measure the "dose" (rate), but
to find and eliminate unwanted leaks (ALARA).  Why limit yourself to a G.M.
or a ion chamber whose response and efficiency are orders of magnitude less
than a scintillation probe????  
>
>	Another 2 cents,
>
>	Joel Baumbaugh
>	baumbaug@nosc.mil
>...