There is not a large group discussing ionization chambers. However,
I'll send this message to the whole list anyway. If you are not
interested in liquid argon ionization chambers, there's no need to read
further.
I worked on liquid argon (LAr) ionization chambers from 1976 to 1978.
The work was done in the Electronics Division of Los Alamos National
Laboratory. I published two papers from that work. References to
those papers are at the end of this message. I'll answer a few
questions in this e-mail. Those with more interest can consult the
papers. I would also be happy to answer questions by e-mail... but
remember this is 20-year-old work and it gets fuzzier every year.
Liquid argon and liquid xenon were predicted to have great potential
for use in ionization chambers operated in proportional mode. They
would have reasonably high density, and should yield energy resolution
better than sodium iodide scintillation detectors, but not as good as
germanium ionization detectors. The cost should be less than sodium
iodide, and much less than germanium. Several different groups worked
on this possibility in the late 1970's.
A question was asked about the signal obtained. Ultimately one is
trying to measure charge, because the number of ions created in the LAr
is proportional to the energy of the radiation being detected.
Therefore we used "charge sensitive preamplifiers." This means we
were basically using an operational amplifier with capacitive feedback.
The input stage was a FET used in a "cascode circuit."
In this type of charge measurement it is a common misconception that
the charge measurement takes place when the charges hit the chamber
plates (or wires) and go into the preamplifier. In reality the charge
measurement takes place while the charges are drifting through the
ionization chamber medium. As the electrons drift toward the positive
chamber plate, an "image current" flows toward it from the capacitor in
the charge-sensitive pre-amp. In essence, the pre-amp is integrating
this current signal (hence charge) and produces a voltage output
proportional to the integrated current, i.e. voltage proportional to
the moving charge.
In LAr the positive ions are immobile. That essentially cuts the
signal in half as only the electrons give rise to a current that can be
integrated.
The electron drift-velocity in LAr is fairly slow compared to that in
germanium or silicon. Hence, the integration is done over a fairly
long time (several microseconds) even with chamber voltages of 10,000
V/cm.
Also, a question was asked about recombination. This is indeed a
problem, as is trapping of electrons by electronegative impurities in
the LAr such as oxygen. That is another reason to have the
ionization-chamber voltage very high. The immobility of positive ions
also means that the chamber must have a grid separating the ionization
portion from the drift portion. Hence, this work involved high
voltages, gridded chambers, purification techniques to reduce oxygen to
ppb levels, and extremely low noise electronics.
As stated in an earlier message, the work was discontinued because it
did not produce the desired results and we did not know where to go
next. At least two other groups were working on this and I believe
they quit about the same time I did. We all had similar results,
meaning that we were all getting energy resolution many times worse
than the theorists had predicted.
Here are the references to the papers I published.
"Energy Resolution Considerations in Liquid Ionization Chambers," M.D.
Edmiston and C.R. Gruhn, IEEE Transactions on Nuclear Science, February
1978, Vol NS-25, No.1, pg 352.
"Geminate Recombination of Alpha-Particle-Excited Carriers in Liquid
Argon," C.R. Gruhn and M.D. Edmiston, Physical Review Letters, Vol. 40,
No. 6, February 1978, pg 407.
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