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Re: [Phys-l] Ionization type smoke detectors.

A couple follow-up comments...

* * * CO Monitors * * *

Don Polvani wondered if a regular smoke detector would be just as good
at detecting carbon monoxide (from problems with a gas furnace or gas
water heater) as specific CO monitors one can purchase.

Certainly an ionization smoke detector could detect a malfunctioning
natural-gas furnace if a reasonable amount of products of combustion
would be getting into the household air near the furnace. However, the
detectors specific to CO work on a different principle and are more
sensitive. I would recommend that near your furnace you have both
detectors. They don't cost that much. I suspect you could experience
health effects from low levels of CO that a CO detector would detect,
but a smoke detector would not detect. I would think you would need a
fairly major furnace problem to trigger the smoke detector.

You can find out about how CO monitors work at an American Chemical
Society page:

* * * Ion velocity in ionization chambers * * *

Ludwik commented that ionization chamber detected current is
proportional to n*q*v.

Quite correct. I remember when this was a surprise to me, and I suspect
others might likewise be surprised. In a former life, I designed,
built, tested nuclear detectors. Many were ionization chambers.

When I first started working with ionization chambers I assumed the ions
travel to the electrodes, hit the electrodes, and that was when they
were detected. This is wrong. Once the ions hit the electrode, the
detection is finished (not started). The detection of ions in an
ionization chamber results from "seeing" the charges move through the
space between the electrodes. In order to get high signal-to-noise
ratio, you not only prefer large n and large q, you also want large
drift velocity. One way to help get large drift velocity is high
voltage across the chamber, and that is one big reason for operating
ionization detectors at high voltage.

Another velocity determining factor is the ionization medium. I did a
lot of work with liquid argon as the ionization medium, and I measured a
lot of drift velocities of electrons in liquid argon as a function of
temperature, pressure, purity, etc. A problem with using liquid argon
as an ionization-chamber medium is only half the ions are mobile. When
the ionizing radiation produces Ar+ and electrons, the electrons are
quite mobile, but the Ar+ is not. That means the signal from Ar+ is
practically nil. By contrast, a silicon or germanium radiation detector
is also an ionization chamber with solid Si or Ge as the medium, and
here the charge carriers are electrons and holes. Both are very mobile,
so the signal is at least twice the size from a Si or Ge chamber as from
a liquid-Ar chamber.

Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton University
Bluffton, OH 45817