10 - H1 - Desy

46 The H1 experiment at HERA
the electromagnetic section varies between 20-30 radiation length 7 X 0 depending on the
impact angle.
Since for most absorbers, the nuclear interaction length λ 8 is roughly one order of magnitude
larger than X 0 , hadronic shower penetrate much deeper into the absorber material.
The hadronic section therefore extends the electromagnetic part and consists of 19 mm
thick stainless steel absorber and liquid argon filled gaps of of twice 2.4 mm width. All
together, the whole LAr calorimeter, including the electromagnetic section yields 5 − 8
nuclear interaction lengths λ. Figure 3.7 shows the longitudinal cross section of the LAr
calorimeter drawn together with lines of constant radiation length 20X 0 and constant
nuclear interaction length λ.
Figure 3.7: Longitudinal cross section of the Liquid Argon calorimeter showing lines of
constant radiation length 20X 0 and constant nuclear interaction length λ
As illustrated in figure 3.6, the orientation of the absorber plates varies between wheels,
being horizontal in all three CB wheels and vertical elsewhere.
Both the electromagnetic and the hadronic section are highly segmented in the transverse
and longitudinal direction with about 44000 cells in total, which is illustrated in
figure 3.8. The cell granularity is finer in the forward direction on account of the higher
particle concentration and in the electromagnetic section in order to resolve the compact
electromagnetic showers induced by electrons and photons. The longitudinal segmentation
varies from three (central) to four (forward) layers of cells in the electromagnetic
section, where the first layer has a thickness of three to six radiation lengths, and from
four to six layers in the hadronic section. Transversally a basic granularity of the electromagnetic
readout cells is twice the Molière radius 9 2R M measured at the entrance of
the electromagnetic calorimeter. In the BBE, CB1 and CB2 the transverse resolution is
relatively poorer. For particles incident from the interaction point, the laterally projected
7 The radiation length is a material dependent constant, which measures the mean distance over which
a high-energy electron loses all but 1/e of its energy by bremsstrahlung. A good approximation of the
radiation length X 0 is [99] X 0 =
716.4g/cm2 A
Z(Z+1)ln(287/ √ Z)
, where A is the atomic weight and Z the atomic charge.
8 Nuclear interaction length λ is defined as an average free path between inelastic interaction.
9 The Molière radius R M is defined i.e. in [100,101] and is a measure of the transverse dispersion of
the electromagnetic shower. Roughly 95% of the shower is contained within the radius of 2R M .