Re: ion chamber vs GM

[tdc@ehssun.lbl.gov]

A thin window GM instrument (1.5 mg/cm^2), either end window or pancake,
would be quite sufficient to survey your equipment.  In many ways it is
better than an IC instrument in that for the same or better sensitivity in
a GM the detector area is smaller and thus can localize leaks better.  Also
- using the audio features the GM allows quicker response and therefore
quicker survey.

The problem with the GM is of course that it is energy dependant and
tends to over respond to low energy photons - unless too low to get through
the window.  So - for dose rate measurements the IC is essential - a thin
window IC!

For analytical x-ray machines here at LBNL all the facilities are issued
thin windowed GM instruments for routine user surveys.  For health physics
surveys whether done by an RCT or by an HP the procedure is to first survey
with the GM to find any leaks and then to use the IC to quantify then (if,
of course, they cannot be eliminated).

IF I were limited to only one instrument - I'd pick the pancake GM!  I'd
rather eliminate leaks than measure them - and I can find them better,
faster and with greater sensitivity with the GM.

BTW - an area monitor would also be a good idea if you are concerned with
wide area leakage - as you may be with 200 keV (which is fairly high energy
in the x-ray business).  At that energy MAYBE an external probe GM unit
would fill the bill - but it will be directional.  Health Physics
Instruments make an IC area monitor instrument that we use exclusively here
for the energies you are concerned with and considerably lower.  It
response is adequately sensitive and not very directional.

Ted de Castro
X-ray Safety Officer
tdc@ehssun.lbl.gov
University of California Lawrence Berkeley Laboratory
Bldg 75B Rm 112A
Berkeley, CA 94720
(510) 486-5256
(510) 486-5506 - FAX