118
H. J. Hilke and W. Riegler
contributes a constant error. Near the anode wire, the effects of the clustering of
the primary charges adds a significant error. At large distances from the anode, the
contribution from diffusion grows as square root of distance. Resolutions achieved
are typically 50 − 200 μm.
A detailed discussion of limits to space resolution is presented in [19] and for the
particular case of proportional tubes in [15].
4.2.6 Ageing of Wire Chambers
Deterioration of performance with time has been observed since the early days of
gas detectors but has gained importance with the ever increasing radiation loads
due to the demand for higher detection rates over long periods. Typical effects
of ageing are: pulse height decrease, a broadening of the energy resolution and
increase in dark current, in the extreme also electrical breakdown or broken wires.
An enormous number of studies has been carried out. They are well documented in
the proceedings of workshops [54] and several reviews [55].
Upon opening of damaged chambers, deposits have been observed on anode
wires and/or on cathodes. On the wire they can take any form from smooth layers
to long thin whiskers [56], see Fig. 4.15. On the cathodes, deposits usually consist
in spots of thin insulator. Defects of this latter kind can often be correlated with a
discharge pattern, which may be interpreted as Malter effect [57]: under irradiation,
charges build up on the insulator until the electric field is strong enough to extract
electrons from the cathode through the layer into the gas where they initiate new
avalanches. The facts that the buildup time decreases with higher ionization rate
and that the discharges take some time to decay after irradiation is timed off, give
support to this explanation, as does the observation that addition of water vapour is
reducing the discharges, probably introducing some conductivity.
Analysis of the layers and whiskers on the anode wires often indicate carbon
compounds, more surprisingly also often silicon, sometimes other elements: Cl,
O, S.
The aging results are often characterized by a drop in pulse height PH as
function of integrated charge deposition in Coulomb per cm wire, although it was
found in some cases that the rate of the charge deposition has an influence. Typical
values with classical gas mixtures containing hydrocarbons are
ΔP H /P H ∼ 0.01 − 0.1%/0/mC/cm for small detectors,
ΔP H /P H ≈ 0.1 − 1%/mC/cm
for large detectors.
(4.63)
It is obvious that the control of ageing is one of the major challenges for the LHC
experiments, possibly even the major one.
Unfortunately, however, it has not been possible to establish a common fundamental theory, which could predict lifetimes of a new system. On the other hand, the
H. J. Hilke and W. Riegler
contributes a constant error. Near the anode wire, the effects of the clustering of
the primary charges adds a significant error. At large distances from the anode, the
contribution from diffusion grows as square root of distance. Resolutions achieved
are typically 50 − 200 μm.
A detailed discussion of limits to space resolution is presented in [19] and for the
particular case of proportional tubes in [15].
4.2.6 Ageing of Wire Chambers
Deterioration of performance with time has been observed since the early days of
gas detectors but has gained importance with the ever increasing radiation loads
due to the demand for higher detection rates over long periods. Typical effects
of ageing are: pulse height decrease, a broadening of the energy resolution and
increase in dark current, in the extreme also electrical breakdown or broken wires.
An enormous number of studies has been carried out. They are well documented in
the proceedings of workshops [54] and several reviews [55].
Upon opening of damaged chambers, deposits have been observed on anode
wires and/or on cathodes. On the wire they can take any form from smooth layers
to long thin whiskers [56], see Fig. 4.15. On the cathodes, deposits usually consist
in spots of thin insulator. Defects of this latter kind can often be correlated with a
discharge pattern, which may be interpreted as Malter effect [57]: under irradiation,
charges build up on the insulator until the electric field is strong enough to extract
electrons from the cathode through the layer into the gas where they initiate new
avalanches. The facts that the buildup time decreases with higher ionization rate
and that the discharges take some time to decay after irradiation is timed off, give
support to this explanation, as does the observation that addition of water vapour is
reducing the discharges, probably introducing some conductivity.
Analysis of the layers and whiskers on the anode wires often indicate carbon
compounds, more surprisingly also often silicon, sometimes other elements: Cl,
O, S.
The aging results are often characterized by a drop in pulse height PH as
function of integrated charge deposition in Coulomb per cm wire, although it was
found in some cases that the rate of the charge deposition has an influence. Typical
values with classical gas mixtures containing hydrocarbons are
ΔP H /P H ∼ 0.01 − 0.1%/0/mC/cm for small detectors,
ΔP H /P H ≈ 0.1 − 1%/mC/cm
for large detectors.
(4.63)
It is obvious that the control of ageing is one of the major challenges for the LHC
experiments, possibly even the major one.
Unfortunately, however, it has not been possible to establish a common fundamental theory, which could predict lifetimes of a new system. On the other hand, the