6 Calorimetry
251
implying a 7.8% correction at 100 GeV. Such non-linearities affect in particular
high energy jets in which several showers may be superimposed, thus affecting the
result in a way difficult to correct.
While this technique was still adequate at LEP, gas calorimeters were not
considered for the LHC. With a very small cell size allowing a binary readout, they
may find some application in hadronic calorimetry for the ILC (see for example
[100]). An exception at the LHC concerns the very forward region in which, due
to the high density of energy deposits, gas ionization chambers (ie without any
amplification) are being used for specific purposes, including beam loss monitoring
and luminosity measurements [101].
6.3.5 High Rate Effects and Radiation Damage
High particle rates and associated backgrounds impact both on the performance and
the useful operating time of calorimeters. Radiation damage needs to be considered
for the active readout material and signal processing electronics. Particle rates drive
the choice of the calorimeter technology and construction.
Calorimeters with gaseous readout are particularly vulnerable to the high radiation environment due to the ageing effects associated with internal gas amplification,
as discussed in Chap. 4.
Such radiation damage is essentially absent in noble liquids making this technology one of the most intrinsically radiation-hard techniques used to date. However,
care has to be taken to select adequately radiation resistant components, including
electronics, to limit deterioration of the performance (e.g. due to out-gassing).
Particularly vulnerable are plastic insulators used in multilayer electrodes or in
signal cables. Among the insulators highly resistant to radiation and suitable for
calorimeter construction are polyimide (like Kapton) and PEEK. A fundamental
limitation of noble liquid calorimeters are space charge effects due to the low drift
speed of the positive ions (typically in the range of few cm/s at a nominal electric
field around 1 kV/mm). At high incident rates these ions form locally a charged
domain which effectively shields the electrons in the gaps from the externally
applied field, reducing the drift velocity and thus the signal. These space charge
effects are inversely proportional to the square of the detector gaps [102]. For this
reason the forward calorimeters [103] of the ATLAS experiment feature gaps down
to 250 μm.
Scintillators suffer from the formation of colour centres, which absorb part
of the emitted light. The qualification of PbWO 4 as a candidate for the CMS
crystal calorimeter required a world-wide R&D programme to study the radiation
damage effects and to develop methods of crystal growth improving the radiation
hardness. Several impurities were identified, which affect transparency in the useful
wavelength range (above 350 nm). The best radiation resistance was obtained for
crystals grown in Pb/W stoechiometric conditions, with the addition of a small
quantity (~100 ppm) of Nb and Y [104]. These crystals showed a light loss of
251
implying a 7.8% correction at 100 GeV. Such non-linearities affect in particular
high energy jets in which several showers may be superimposed, thus affecting the
result in a way difficult to correct.
While this technique was still adequate at LEP, gas calorimeters were not
considered for the LHC. With a very small cell size allowing a binary readout, they
may find some application in hadronic calorimetry for the ILC (see for example
[100]). An exception at the LHC concerns the very forward region in which, due
to the high density of energy deposits, gas ionization chambers (ie without any
amplification) are being used for specific purposes, including beam loss monitoring
and luminosity measurements [101].
6.3.5 High Rate Effects and Radiation Damage
High particle rates and associated backgrounds impact both on the performance and
the useful operating time of calorimeters. Radiation damage needs to be considered
for the active readout material and signal processing electronics. Particle rates drive
the choice of the calorimeter technology and construction.
Calorimeters with gaseous readout are particularly vulnerable to the high radiation environment due to the ageing effects associated with internal gas amplification,
as discussed in Chap. 4.
Such radiation damage is essentially absent in noble liquids making this technology one of the most intrinsically radiation-hard techniques used to date. However,
care has to be taken to select adequately radiation resistant components, including
electronics, to limit deterioration of the performance (e.g. due to out-gassing).
Particularly vulnerable are plastic insulators used in multilayer electrodes or in
signal cables. Among the insulators highly resistant to radiation and suitable for
calorimeter construction are polyimide (like Kapton) and PEEK. A fundamental
limitation of noble liquid calorimeters are space charge effects due to the low drift
speed of the positive ions (typically in the range of few cm/s at a nominal electric
field around 1 kV/mm). At high incident rates these ions form locally a charged
domain which effectively shields the electrons in the gaps from the externally
applied field, reducing the drift velocity and thus the signal. These space charge
effects are inversely proportional to the square of the detector gaps [102]. For this
reason the forward calorimeters [103] of the ATLAS experiment feature gaps down
to 250 μm.
Scintillators suffer from the formation of colour centres, which absorb part
of the emitted light. The qualification of PbWO 4 as a candidate for the CMS
crystal calorimeter required a world-wide R&D programme to study the radiation
damage effects and to develop methods of crystal growth improving the radiation
hardness. Several impurities were identified, which affect transparency in the useful
wavelength range (above 350 nm). The best radiation resistance was obtained for
crystals grown in Pb/W stoechiometric conditions, with the addition of a small
quantity (~100 ppm) of Nb and Y [104]. These crystals showed a light loss of