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Re: IONIZATION CHAMBER, was ...

Good question... and let me admit right off that I am not an expert
here... I was following advice from electrical engineers. In my case I
was trying to achieve high energy resolution using an ionization medium
that had a fairly high ionization energy. In other words, I had a very
small signal, and I was using high amplification. Electronic noise was
a severe problem. Several types of electronic noise (such as Johnson
noise) are resistance dependent. So I was trying to keep resistance as
low as possible. I also had to worry about thermocouple effects if I
made junctions of dissimilar metals, and the amplifiers, of course,
used copper conductors.

In a conventional ionization chamber in which high energy resolution is
not the goal, lead resistance might not be a problem. Indeed, it may
not have been a problem in my case either; I am not sure. What I do
know is that I eventually abandoned this project because I could not
get sufficient signal-to-noise ratio to achieve the type of energy
resolution the theorists had predicted would be possible with liquid
argon. Some other groups also tried liquid argon and I believe they
gave up also. We tried building our own amplifiers and cooling our FET
input stages, etc. but the electronic noise was always killing us
(which is also synonymous to saying our signal was too small).

Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817



-----Original Message-----
From: Ludwik Kowalski [SMTP:KowalskiL@MAIL.MONTCLAIR.EDU]
Sent: Monday, June 14, 1999 11:21 AM
To: PHYS-L@LISTS.NAU.EDU
Subject: IONIZATION CHAMBER, was ...

In the "stainless conductivities" thread Michael Edmiston wrote:


... I have done a lot of instrument design and construction involving
the need for electrical conduction/insulation as well as thermal
conduction/insulation. For example, I have built nuclear detectors
using liquid argon as the ionization-chamber medium. Even though
I would have liked to have used stainless steel electrical
connections
(because of its low thermal conductivity) I never considered doing
so because of its low electrical conductivity. ....

Why would several ohms (or tens of ohms) would be objectionable in
the case of an ionization chamber? To my understanding, the current
depends predominantly on what happens in the ionized medium, at
least in a common air chamber. Is this not also true for a liquid?

Ludwik Kowalski