Fig. 1. Artist's impression of a module, with cutaway showing the read-out structure.

Fig. 2. Schematic of the arrangement of the read-out structure in the 8.5 mm inter-plate gap.

Fig. 3. Histogram of the module height acheived to date in the 20 HEC1 and 13 HEC2 modules produced.

Fig. 4. Schematic of the HEC beam test electronics, showing the longitudinal segments (LS), the preamplifying and summing boards (PSB), the front-end board (FEB), the FEB drivers (DR), the calibration board and the analog-to-digital converter (ADC) modules.

Fig. 5. The setup used for data taking in the beam tests. The trigger is defined by the scintillation counters B1, F1 and F2 and the scintillation counter walls VM, M1 and M2.

Fig. 6. View of the setup of the HEC1 and HEC2 partial wheels in the cryostat. Each partial wheel is built out of three modules, the final calorimeter will have 32 modules per wheel.

Fig. 7. Block diagram of the data acquisition system.

Fig. 8. Schematics of the HEC electronic chain, showing the calibration generator board, the preshaper (PR), the shaper and FEB driver (SH&DR) located in the warm and the calibration distribution board (CDB), the strip line (SL), the detector gap (DET) and the preamplifier and summing board (PSB) located in the cold LAr.

Fig. 9. The calibration signal (left) and the prediction for the ionization signal (right) together with the residuals with respect to the fit (lower figures).

Fig. 10. O₂ contamination estimated from the signal ratio of the ²⁴¹Am cell and ²⁰⁷Bi cell. An increase of 1.6 ppb/day has been observed during the run period.

Fig. 11. Time dependence of the liquid argon temperature as measured by five sensors positioned either in a tube at different height (S) or at the rear plates of individual modules (P).

Fig. 12. Distribution of the liquid argon temperature measurements as obtained from a single probe (S1) during a 12 h operation period.

Fig. 13. The beam impact points (open squares) on the front face of the calorimeter setup with three φ-wedges. Data taken at these points have been used for the energy scans. The open circles locate the tie rods. The dashed circle represents the area accessible through the cryostat window, while the dashed rectangle represents the area accessible through beam bending (vertical) and cryostat motion (horizontal).

Fig. 14. Reconstructed normalized signal waveform for electrons of different energies.

Fig. 15. The relative amount of the energy in a cluster with respect to the total visible energy in HEC modules for electrons as a function of the number of read-out channels used in the cluster.

Fig. 16. The relative amount of the leakage energy with respect to the pion beam energy as a function of the beam energy.

Fig. 17. The relative amount of the energy in a cluster with respect to the total visible energy in all HEC modules for charged pions as a function of the number of readout channels used in the cluster.

Fig. 18. Calibration signal: measured data (points) and the result from the fit (solid line) for the read-out channel related to the impact point J (left figure) and the residuals from the fit (right figure).

Fig. 19. Ionization signal from electrons. Shown are the data for two different HV settings for the read-out channel related to the impact point J (left figures). Also shown are the predicted signals (solid line) and the related residuals (right figures).

Fig. 20. The dependence of the electron drift velocity on the electric field. Shown are the data for the read-out channels related to two different impact point (dots and squares). The solid line represents recently published data (see text for reference).

Fig. 21. The initial ionization current (points) and the fit to the Thomas–Imel model (solid lines) for the two impact points.