[Aging of gas detector wire]{.underline}

After seeing the detector at Fermi National Laboratory, the relation
between glow discharge polymerization and the material deposition on the
anode wire in the gas detector has become somewhat clearer. The
knowledge gained in glow discharge polymerization has relevance to the
deposition of material on the anode of gas detector in the following
manner.

In DC glow discharge of organic compounds, which could be taken as the
most relevant case in consideration of the aging of gas detector, the
most significant material formation occurs near the cathode, simply
because electrons are ejected from the cathode surface. In contrast to
this situation, electrons in the gas detector are ejected along the path
of a particle, and the cathode and the anode are not related to the
ionization of gas phase. In the gas detector, what happen in the cathode
region of glow discharge polymerization could occur in the vicinity of
the anode wires in the gas detector.

When electrons are pulled towards small diameter wire, electron density
increases and could cause electron avalanche and the break down of the
gas phase. The role of organic gas, such as ethane, added to argon is
understood to prevent break down of the gas phase by absorbing a part of
the energy of electron impact. In this sense, a larger gas can absorb
more energy than a smaller molecule, and the energy input parameter used
in glow discharge polymerization seems to indicate this situation.

The energy input parameter for material formation in luminous gas phase
is given by W/FM, where W is wattage, F is molar or volume flow rate,
and M is the molecular weight of gas, which represents the energy input
per mass of gas given by J/kg. In a flow system, FM represents the mass
of gas in unit time. The larger the total mass that passes through the
luminous gas phase, the higher energy input is necessary for the
formation of material deposition. Thus, the use of higher molecular
weight gas and higher flow rate would reduce the material deposition on
the anode of the gas detector. On the other hand, a gas with high
molecular weight; e.g., C4H~10~ vs. CH~4~, is closer to the critical
molecular weight for deposition (leaving gas phase), and could lead to
more deposition depending on other factors such as flow rate of gas and
the concentration in the gas mixture.

The recent discovery of the molecular dissociation glow in DC discharge
of organic molecules has a significant influence in interpretation of
the mechanism of absorbing electron energy by organic gas molecules.
According to the principle of molecular dissociation of organic
molecules, it is not necessary to ionize gas, which causes the break
down of gas phase, in order to absorb energy on electron impact. On the
other hand, free radicals are formed in the gas phase that is not
electrically broken down.

The growth mechanism of gas molecules is recombination of free radicals.
In the gas detector that operates in atmospheric pressure and with large
amount of argon, e.g., 50/50 mole percent, the growth of molecular size
is very slow because the chance of repeating recombination of free
radicals to increase the molecular size to the critical value for
deposition within unit time is very low. However, this creates the
possibility that the larger size molecules that is close to the critical
size for the deposition remaining in the gas phase, particularly if the
gas flow rate is not high enough to remove such species from the system
before they reach the critical size for deposition.

Another factor that should be taken into account is the possible role of
negative ions that is formed by the attachment of electrons. The
formation of negative ions occurs only with elements with high electro
negativity. In glow discharge polymerization, there is not enough data
that indicate the role of negative ions, because neutrals out number
charged species in the environment of glow discharge polymerization.
However, in the case of gas detector, chemically reactive species are
created in the vicinity of the anode, and if negative ions were formed,
they would be pulled to the anode enhancing the deposition of material
on the anode. In this sense, the role of F or O (electro negative
element) containing compound in reducing the aging effect is somewhat
questionable.

Both F containing and O containing organic molecules are reluctant to
form polymeric deposition. From the viewpoint of the deposition rate of
plasma polymer, these compounds retard the formation of plasma polymers,
and hence the inclusion of these compounds might be helpful to reduce
the aging effect. However, the lower deposition rate for these compounds
could due to the tendency to form negative ions. If this was the case,
the deposition to the anode of gas detector might be enhanced because
the formation of polymerizable species takes place in the vicinity of
the anode under strong electrical field. Unfortunately, there is no data
to check this possibility. The electron temperature measurement with
perfluorocarbons suggested the presence of negative ions from the shape
of i-V curves.

In the power input parameter of glow discharge polymerization, W/FM, the
effective energy input W~e~ is the energy above the break down energy
W~b~, i.e., W~e~ = W - W~b~. Therefore, the molecules that have high
break down energy are expected to show slower polymerize rates, if the
deposition rates are compared at the same W/FM value. Unfortunately,
there is not enough data for the break down voltage as a function of
chemical structure and the presence of electro negative elements.

If we could collect data on 1) break down voltage and chemical structure
of molecules, 2) the influence of electro negative element on break down
voltage, and 3) the influence of negative ions on the material
deposition on the anode of DC glow discharge, one could construct the
guideline for what kinds of molecules should be used to increase the
gain of gas detector. Without such a guideline, the selection of gas
molecule must rely on the experience and guess work.