Hadronic Calorimeter (HCAL)

The hadronic calorimeter [23,24,25], like the electromagnetic calorimeter, has a barrel region and two endcap regions covering roughly the same regions as the ECAL, however, HCAL also has a hadron poletip calorimeter which covers regions were the momentum resolution of the central detectors is poor. HCAL uses the iron return yoke of the OPAL magnet as passive absorbing material. Layers of the iron are sandwiched by planes of detectors.

Due to the large amount of material between HCAL and the interaction point, most hadronic showers are likely to have initiated long before reaching HCAL, this means that the hadronic energy measurement is made by adding the energy deposited in HCAL with that deposited in ECAL. The energy resolution for all parts of HCAL are similar, although there is more variation with energy for the poletip calorimeter. The resolutions, depending on the energy measured are;

$$\frac{\sigma_{E}}{E} \simeq \frac{100\%}{\sqrt{E}} {\rm for} E < 15 {\rm GeV}$$ (2.2)

$$\frac{\sigma_{E}}{E} \simeq \frac{140\%}{\sqrt{E}} {\rm for} E < 50 {\rm GeV}$$ (2.3)