The Analog Hadron Calorimeter Physics Prototype

The CALICE analog hadron calorimeter prototype consists of a sandwich structure of 38 active layers and 38 absorber plates. While the first version of the AHCAL was equipped with steel as absorber material, we refer here only to the second version in which a tungsten alloy was chosen as absorber material. The active layers were the same in both versions. The tungsten absorber plates have a lateral dimension of

Wavelength Shifting Fiber SiPM

Figure 3.13: Picture of a scintillator cell of the CALICE AHCAL with embedded wavelength shifting fiber and SiPM (left) and of a complete sensitive HCAL layer equipped with cell of this kind pointing out its high granularity (right).

1×1m2 _{and a thickness of 1cm. Each active layer has a finely segmented pattern of}

individual calorimeter cells (see Figure 3.13, right). The heart of the AHCAL prototype are scintillator tiles with a lateral size of 3×3cm2 and a thickness of 5mm (see Figure 3.13, left). A wavelength shifting fiber is embedded into each tile. A reflective mirror is coupled to one end of the fiber and a silicon photomultiplier to the other. The used SiPMs were produced at MEPhI and provide 1156 photon sensitive pixels (see Figure 3.7). 216 scintillator cells were assembled to the particle sensitive area of an active calorimeter layer (see Figure 3.13, right). The cell size increases towards the outside to 6×6cm2 and12×12cm2. This represents a balance between shower sampling and cost [60]. In the current design plans of the International Large Detector (see Chapter 2) - the large scale ILC or CLIC detector with a scintillator based HCAL - all cells will have a size of 3×3cm2. Detailed studies showed that for an analog scintillator tile calorimeter the Particle Flow performance and therefore the jet energy resolution is not significantly improved by a further reduction of the cell size [34]. The cells are covered with reflective mirror foil and the whole scintillator plane is placed inside a cassette structure consisting of two steel plates with a thickness of2mm each. The prototype has a total of 7608 readout channels.

In the AHCAL prototype, 38 boards for calibration and monitoring and for the readout electronics are located on the outside of the calorimeter, but the space restrictions of a full collider detector require a highly integrated and more compact design of the active layers. In the past years, extensive R&D work has been done and a second generation AHCAL design for the active layers has been developed. Several prototype modules have been constructed and their performance under test beam conditions is about to be evaluated within the CALICE test beam campaign 2012. In this new design, the active layer has a thickness of only5.3mm (see Figure 3.14, top) [61]. The electronics is accommodated on one side of a thin PCB, while the scintillator cells are assembled on the other side (see Figure 3.14, bottom). The thickness of the cells is reduced to3mm to meet the space constraints, but the light yield of the SiPM-tile entity is approximately the same as for the 1st _{generation prototype because of the usage of a new type of}

Figure 3.14: New integrated design of the active layers for the second generation CALICE AHCAL prototype [61]. See text for details.

SiPMs. Furthermore, the cell architecture was subject to optimizations. In the current version, the cells do still comprise a wavelength shifting fiber (see Figure 3.14, bottom), but it is under discussion to equip the next batch of produced active layers with cells with a fiberless scintillator tile design similar to the one used within the T3B experiment (explained in Section 4.2). This promises an easier and more time and cost efficient

machining and production of the active cells.

The design of the 2nd _{generation AHCAL prototype modules allows for an automated}

assembly and should be capable of fulfilling the requirements in terms of stability and rigidness, modularity and operational reliability in the future. It is therefore well suited for mass production and scalable to a calorimetric system used within a full collider detector. Furthermore - in contrast to the 1st _{generation prototype - the capability to}

timestamp energy depositions is an inherent part of the design of the 2nd generation AHCAL prototype [62] [63]. This is further elaborated in Section 3.4.3.

With a scintillator cell size of 3×3cm2, an analog quantification of energy depositions is feasible in terms of costs, data processing and storage considering the number of readout channels of a full collider detector (O(107_{)channels in the HCAL of the ILD).}

The analog information can be exploited to improve the energy resolution for neutral hadrons which is a key performance parameter of a HCAL system optimized for Particle Flow.

The precision of the energy reconstruction of neutral hadrons dominates the Particle Flow performance for jet energies up to approximately 100GeV [34] (see Section

2.4.2). The application of an offline software compensation algorithm aims to equalize the calorimetric response to electromagnetic sub-showers and purely hadronic shower depositions based on their different spacial characteristics. This reduces the sensitivity to event-to-event fluctuations in the electromagnetic shower fraction and consequently improves the overall energy resolution. Note that the e/h ratio cannot be measured directly. Instead, one quantifies the ratio of the calorimetric response to electrons relative to the response to pions of the same energy - thee/π-ratio. Despite the intrinsically high hydrogen content of the scintillator material which increases the calorimetric response to the hadronic shower fraction, studies showed that the AHCAL prototype - in this study located within a steel absorber structure with a thickness of on average17.4mm per layer - features an e/π-ratio of around 1.2 (in an energy range between 10GeV and 80GeV) and is therefore non-compensating [64]. This undercompensation can be corrected on an event-by-event basis by an intelligent weighting of the detected energy depositions based on their height. While a high electromagnetic shower fraction and therefore a high energy density is expected within the core of a hadron shower, the surrounding halo and the tails are dominated by the hadronic shower component and exhibit a lower particle density. Thus, the software compensation algorithm reduces the

e/h-ratio by weighting high energy depositions down and low energy depositions up (details in [64]). Utilizing the analog nature of the recorded cell signals of the AHCAL prototype (see Figure 3.12, right), the hadronic energy resolution could be improved by up to20 % [64].