Re[3]: 137Cs check sources -Reply

[TDC@ehssun.lbl.gov]

>I thought, and have taught several thousand students, that the problem was
>because of the long resolving time of the GM tube itself.  The tube takes
>several hundred microseconds to recover from the electron cascade and is
>unable to detect a second event during that time.  If this is not the
>case, I'll change my tune.

No - that just causes it to miss the next pulse - or a few.

A GM tube on what is called a "non-paralyzible" circuit will simply top
out.  The idea - roughly as I understand it - is to not allow the GM to
draw enough current to totally shut down - I've also heard it referred to
as running is a "current starved" mode.

I even put this to the test once is a very extreme manner in a situation
where the purpose of the GM was to alert at fairly low levels - but since
the source was pulsed there could be very high instantaneous levels.

This particular tube and circuit would top out at about 100 mr/hr but to
test its high field response I ran it to about 1 megaR/hr (a cm or so from
a 2500 Ci Co-60 source.  It still read 100 mr/hr.

Any manufacturer who knows anything about GMs and instrumentation will use
non paralyzing circuits.  For a while - and long after this problem was
identified and solved - they would even advertise this as a spec.  It is so
commonplace now that it is hardly even mentioned.

BUT many who remember how it used to be still bring it up as a problem.

Ted de Castro
tdc@ehssun.lbl.gov
University of California Lawrence Berkeley Laboratory
Bldg 75B Rm 112A
Berkeley, CA 94720
(510) 486-5256
(510) 486-7207 - FAX

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