More details on the EMCal

ALICE EMCal is Electromagnetic Calorimeter with large acceptance. It covers almost the full length of the ALICE Time Projection Chamber and central detector, and a third of its azimuth placed back-to-back with the ALICE Photon Spectrometer.

The EMCAl enables triggering and full reconstruction of high energy jets in ALICE, and augments existing ALICE capabilities to measure high momentum photons and electrons. The EMCal enables a comprehensive study of jet interactions in the medium produced in heavy ion collisions at the LHC. The interaction and energy loss of high energy partons in matter provides a sensitive tomographic probe of the medium generated in high energy nuclear collisions.

The EMCal is located inside the solenoidal magnet of ALICE. It occupies a cylindrical integration volume approximately 110 cm deep in the radial direction, with front face 450 cm from the beam line. This volume is sandwiched between the ALICE space-fame which supports the entire central detector and the magnet coils. The EMCal azimuthal coverage is 107 degrees while in the longitudinal direction the EMCal has a length of 700 cm, covering η < 0.7.

The structural units of the calorimeter are the EMCal Super Modules that form a continuous arch, spanning 20o in azimuth. The towers are grouped into two types of Super Modules: 10 full size and two one-third size Super Modules in the full detector acceptance. Each full-sized Super Module is assembled from 12 x 24 = 288 modules arranged in 24 strips modules of 12 x 1 modules each while the one-third size Super Module is assembled from 4 x 24 = 96 modules.

A module is a single self-contained detector unit comprising four independent detection channels. In total the 3000 modules of the EMCal correspond to more than 12000 channels of the EMCal. The module is a sandwich of 77 plastic scintillator layers and 76 lead foils. A particle passing through the module and interacting with the lead produces an electromagnetic shower, which produces light (photons) in the plastic scintillators.

These photons are collected and guided by the fibers going through the module in vertical direction until they reach the Avalance Photodiode. The latter converts the light into an electrical signal, with an amplitude proportional to the energy of the original incoming particle.

Read here the EMCal Technical Design Report. 

After months of commissioning and running with cosmic rays, at the end of 2009 it was possible to see the response of EMCAL with the first proton-proton collisions at the center-of-mass energy of 900 GeV and in 2010 at 7 TeV. Since the early commissioning phase of the LHC, it was possible to have the first events analyzed and displayed in the counting room by the offline reconstruction software and the online reconstruction software implemented in the high level trigger analyzing the event in real time, indicating the start of the physics exploitation of the ALICE experiment. The four EMCal Super Modules were continuously present in the data taking. Invariant mass spectra of π0  are obtained in different energy bins, an example shown in the figure below. 

π0  mass reconstruction in the transverse momentum bin 5.0 < p< 5.5 GeV 

 

The whole calorimeter has been build by universities and laboratories in France, Finland, Italy and USA.

DCal, the second component of the ALICE Di-Jet calorimeter, is situated in the ALICE central barrel, 180 degrees in azimuth from the EMCal, and completes the calorimetric coverage of the open regions of ALICE. This configuration enables back-to-back coincidence measurements of jets with various triggers, including jets, photons, and high momentum hadrons and electrons. The DCal thereby expands the ALICE jet programme significantly.

 6 DCAL super-modules (in gray) with the PHOS super-modules (in orange) in between.

The DCal structure, which is identical to that of the ALICE EMCal, is based on supermodules, each of which consists of 1100 individual calorimeter towers and weighs about 8 tons. The DCal is made up of six full-sized supermodules and two 1/3-sized supermodules.

Together, the DCAL and EMCAL form a two-arm electromagnetic calorimeter. The DCal was not initially part of the EMCal project, but followed the successful completion of the EMCal project in 2011. The DCal effort required redesign of the ALICE support structure, fabrication and assembly of the DCal components, and finally, integration and installation of the full detector in ALICE. The installation was completed in autumn 2014.