4 Gaseous Detectors
109
Here c e is the electron velocity, c 2 the relative thermal velocity between O 2 and
X In an Ar/CH 4 (80/20%) mixture at 8.5 atm with an O 2 contamination of 1 ppm,
an absorption of 3%/m was measured at a drift speed of 6 cm/μs.
4.2.3 Avalanche Amplification
4.2.3.1 Operation Modes
Gas detectors generally use gas amplification in the homogeneous field of a parallel
plate geometry or, more frequently, in the inhomogeneous field around a thin wire.
We shall start with the discussion of the second case.
Near a wire with a charge q s per cm, the electric field at a distance r from its
centre is
E = q s / (2πε 0 r) .
(4.42)
When raising the field beyond the ionization chamber regime, in which all
primary charges are collected without any amplification, at some distance from the
wire a field is reached, in which an electron can gain enough energy to ionize the gas
and to start an avalanche. The avalanche will grow until all electrons have arrived
on the anode wire. For a gas amplification A of 1000 ~ 2 10 , some 10 ionization
generations are required. As the mean free path between collisions is of the order
of microns, the field to start an avalanche has to be several times 10 4 V/cm. This is
usually achieved by applying a voltage of a few kV to a thin wire, with a diameter
in the 20 − 50 μm range.
Besides ionization, excitation will always occur and with it photon emission. A
fraction of these photons may be energetic enough to produce further ionization in
the gas or on the cathode. Only those photons which ionize outside the radius r av of
the moving electron avalanche may be harmful, as their avalanches will arrive later.
If γ called the second Townsend coefficient, is the probability per ion pair in the first
avalanche to produce one new electron, and if A denotes the amplification of the
first avalanche, breakdown will occur for
Aγ > 1.
(4.43)
In this case the first avalanche will be followed by a bigger one, this by an even
bigger and so on, until the current is limited by external means. If Aγ < < 1, Aγ
gives the probability for producing an after-discharge. If a photoionization takes
place inside r av , the effect will be an increase of A.
The resulting need to suppress far-traveling photons produced in the rare gases
is the reason for the use of ‘quench gases’ like Methane, Ethane, CO 2 , etc., which
have large absorption coefficients for UV photons.
109
Here c e is the electron velocity, c 2 the relative thermal velocity between O 2 and
X In an Ar/CH 4 (80/20%) mixture at 8.5 atm with an O 2 contamination of 1 ppm,
an absorption of 3%/m was measured at a drift speed of 6 cm/μs.
4.2.3 Avalanche Amplification
4.2.3.1 Operation Modes
Gas detectors generally use gas amplification in the homogeneous field of a parallel
plate geometry or, more frequently, in the inhomogeneous field around a thin wire.
We shall start with the discussion of the second case.
Near a wire with a charge q s per cm, the electric field at a distance r from its
centre is
E = q s / (2πε 0 r) .
(4.42)
When raising the field beyond the ionization chamber regime, in which all
primary charges are collected without any amplification, at some distance from the
wire a field is reached, in which an electron can gain enough energy to ionize the gas
and to start an avalanche. The avalanche will grow until all electrons have arrived
on the anode wire. For a gas amplification A of 1000 ~ 2 10 , some 10 ionization
generations are required. As the mean free path between collisions is of the order
of microns, the field to start an avalanche has to be several times 10 4 V/cm. This is
usually achieved by applying a voltage of a few kV to a thin wire, with a diameter
in the 20 − 50 μm range.
Besides ionization, excitation will always occur and with it photon emission. A
fraction of these photons may be energetic enough to produce further ionization in
the gas or on the cathode. Only those photons which ionize outside the radius r av of
the moving electron avalanche may be harmful, as their avalanches will arrive later.
If γ called the second Townsend coefficient, is the probability per ion pair in the first
avalanche to produce one new electron, and if A denotes the amplification of the
first avalanche, breakdown will occur for
Aγ > 1.
(4.43)
In this case the first avalanche will be followed by a bigger one, this by an even
bigger and so on, until the current is limited by external means. If Aγ < < 1, Aγ
gives the probability for producing an after-discharge. If a photoionization takes
place inside r av , the effect will be an increase of A.
The resulting need to suppress far-traveling photons produced in the rare gases
is the reason for the use of ‘quench gases’ like Methane, Ethane, CO 2 , etc., which
have large absorption coefficients for UV photons.