One applies the HV to the wire and grounds the aluminized mylar
sheets. There's some gas mixture that makes nice ion multiplier
effects. The wire is almost invisible, just a few mils in diameter, so
the electric field gradient close to the wire in insane. There's a
huge ion multiplier gain, close to geiger mode but not quite.
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From: John Larkin<
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Newsgroups: sci.electronics.design
Subject: Re: gamma ray detection
Date: Mon, 30 Aug 2010 17:20:35 -0700
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One applies the HV to the wire and grounds the aluminized mylar
sheets. There's some gas mixture that makes nice ion multiplier
effects. The wire is almost invisible, just a few mils in diameter, so
the electric field gradient close to the wire in insane. There's a
huge ion multiplier gain, close to geiger mode but not quite.
As I recall, we terminated each end of the wire, amplified the pulses
a bit, and triggered an ADC on the peak. Some simple math mapped the
two pulse amplitudes into position along the wire, with some
calibrations maybe. The product was a Safeway shopping cart sort of
thing with a big, like 1m square, detector on the bottom. The idea was
to sweep a floor looking for hot particles. Lots of facilities need to
do this. This sort of thing is a low-rate detector that will get
confused by multiple hits.
I also did a classic wire chamber thing for UCLA/CERN. That had a
zillion parallel wires per plane, with an amplifier, discriminator,
and time-digital converter per wire. Looking at the timing data, one
can interpolate the hit position to a fine fraction of the wire pitch.
Multiple planes gathered X-Y data and particle path curvature in the
magnetic fields. These were Gev particles that made a lot of ions in a
lot of chambers without slowing down much. These arrays generate
mountains of data on thousands of channels, and the problem is to
process huge rates of junk hits down to a set that's possible to
archive and analyze. We used a bunch of FPGAs in a data flow
peristaltic sort of thing. Messy.
John