RE: GM response to pulsed fields

[Wes Van Pelt <vanpelt@ios.com>]

Hi Keith and RadSafers,

The reason GM meters do not work with pulsed radiation is the time characteristics, not the energy characteristics.  That is, the radiation may be "flattened out" in energy spectrum, but it is not flattened out in time.

At the speed of light, photons will travel 300 m in one microsecond, which is on the order of the time response of a GM.  The 15 feet distance will not significantly flatten out the time sequence of the pulses.

Cheers,

Wes Van Pelt
vanpelt@ios.com 

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From: 	welch[SMTP:welch@CEBAF.GOV]
Sent: 	Friday, November 03, 1995 5:35 PM
To: 	Multiple recipients of list
Subject: 	Re: GM response to pulsed fields

I'm assuming the post on GM response was due to my post the other day
indicating that we had seen some surprisingly "good" results out of a GM
Electronic Dosimeter.  I'm also skeptical, and I wouldn't think of placing
one of these things on someone when they were going into an area with a
pulsed field.  I mentioned that the reason that I think we  lucked out in
the particular cases we tested was that the dosimeters were placed in a
geometry where there was very little direct, unscattered radiation from the
pulsed beam.  In fact they were located "upstream" from the target and saw
radiation that was scattered almost 180 degrees 'backward'.  I'm still
skeptical though, and would like to do further testing.  At our facility we
also have some penetrations to our beam tunnel where some dose rates are
encountered.  In testing these penetrations we have found that at some depth
into the penetration, a GM will begin to saturate and count beam pulses, but
at the "top" of the pentration, outside the enclosure, the instruments begin
to track with ion chambers.  Again it appears that through multiple
scattering up the pentration (about 15 feet), and reduction of dose rate
from distance, the pulse effects are "flattened" out.

Keith Welch
CEBAF 
welch@cebaf.gov
KW