Liquid Argon Sampling Calorimeters Research Articles

Performance of the ATLAS Liquid Argon Calorimeter after three years of LHC operation and plans for a future upgrade

The ATLAS experiment is designed to study the proton-proton collisions produced at the Large Hadron Collider (LHC) at CERN. Liquid Argon sampling calorimeters are used for all electromagnetic calorimetry covering the pseudorapidity region up to 3.2, as well as for hadronic calorimetry in the range 1.4-4.9. The electromagnetic calorimeters use lead as passive material and are characterized by an accordion geometry that allows a fast and uniform azimuthal response. Copper and tungsten were chosen as passive material for the hadronic calorimetry; whereas a parallel plate geometry was adopted at large polar angles, an innovative one based on cylindrical electrodes with thin argon gaps was designed for the coverage at low angles, where the particles flow is higher. All detectors are housed in three cryostats kept at 88.5 K. After installation in 2004-2006, the calorimeters were extensively commissioned over the three years period prior to first collisions in 2009, using cosmic rays and single LHC beams. Since then, around 27 fb−1 of data have been collected at a unprecedented center of mass energies between 7 TeV and 8 TeV. During all these stages, the calorimeter and its electronics have been operating with performances very close to the specification ones. After 2019, the instantaneous luminosity will reach 2-3 × 1034 cm−2s−1, well above the luminosity for which the calorimeter was designed. In order to preserve its triggering capabilities, the detector will be upgraded with a new fully digital trigger system with a refined granularity. In 2023, the instantaneous luminosity will ultimately reach 5-7 × 1034 cm−2s−1, requiring a complete replacement of the readout electronics. Moreover, with an increased particle flux, several phenomena (liquid argon boiling, space charge effects…) will affect the performance of the forward calorimeter (FCal). A replacement with a new FCal with smaller LAr gaps or a new calorimeter module are considered. The performance of these new calorimeters is being studied in highest intensity particle beams. This contribution covers all aspects of the first three years of operation. The excellent performance achieved is especially detailed in the context of the discovery of the Higgs boson announced in July 2012. The future plans to preserve this performance until the end of the LHC program are also presented.

Status of the ATLAS Liquid Argon Calorimeter; Performance after 2 years of LHC operation

The ATLAS experiment is designed to study the proton-proton collisions produced at the Large Hadron Collider(LHC) at CERN. Liquid argon sampling calorimeters are used for all electromagnetic calorimetry covering the pseudo-rapidity region up to 3.2, as well as for hadronic calorimetry in the range 1.4–4.9. The electromagnetic calorimeters use lead as passive material and are characterized by an accordion geometry that allows a fast and uniform azimuthal response without any gap. Copper and tungsten were chosen as passive material for the hadronic calorimetry; whereas a classic plate geometry was adopted at large polar angles, an innovative one based on cylindrical electrodes with thin argon gaps was designed for the coverage at low angles, where the particle flow is higher. All detectors are housed in three cryostats kept at about 88 K. After installation in 2004–2006, the calorimeters were extensively commissioned over the three years period prior to first collisions in 2009, using cosmic rays and single LHC beams. Since then, around 9 fb−1 (as of June, 2012) of data have been collected at a center of mass energy of 7 and 8 TeV. During all these stages, the calorimeter and its electronics have been operating almost optimally, with performances very close to the specifications.

Status of the ATLAS Liquid Argon calorimeter and its performance after two years of LHC operation

The ATLAS detector is designed to study proton–proton collisions produced at the Large Hadron Collider (LHC) at CERN. Its calorimeter system, for the electromagnetic and forward part and for a fraction of the hadronic part, is made of liquid argon sampling calorimeters. During the 2011 data taking period, ATLAS operated with an excellent efficiency and recorded more than 5fb−1 integrated luminosity of 7TeV center of mass energy proton–proton collisions. The ATLAS Liquid Argon calorimeters played a significant role in this achievement. Its main features are first described. Then, some of its performances over the past two years are given. Highlights are put on its operation, calibration, stability and data taking efficiency, as well as on the related physics performances.

Onset of space charge effects in liquid argon ionization chambers

Using a thin-gap liquid argon ionization chamber and Strontium-90 beta sources we have measured ionization currents over a wide range of gap potentials. These precision “HV plateau curves” advance the understanding of liquid argon sampling calorimeter signals, particularly at high ionization rates. The order of magnitude differences in the activities of the beta sources allow us to estimate where the ionization chamber is driven into the space-charge dominated regime.

Monte Carlo studies of response of a liquid argon calorimeter with lead absorber

Using the EGS4 Monte Carlo simulation program, we have examined the properties of a liquid argon sampling calorimeter that uses lead as main absorber. Response to electrons and to minimum-ionizing particles (muons) has been investigated as a function of energy cutoffs, and thickness of cladding-material applied to absorber plates.

Monte Carlo investigations of aspects of liquid argon calorimeters

Using the EGS3 Monte Carlo program we have examined the response of liquid argon sampling calorimeters for different absorber materials. In particular, the response to electrons (compared to muons) has been studied as a function of the width of the argon gap. Effects due to cladding of absorber plates have also been investigated.

Theoretical Estimation of Energy Resolution in Liquid Argon Sampling Calorimeter

A new method for estimating the energy resolution of the multi-lead-plate liquid argon calorimeter (Rm≈tc) and a new formula for the energy resolution of the multi-iron-plate liquid argon calorimeter (Rm≫tc) are presented, where Rm is the average range of secondary electrons or positrons and tc is the thickness of the unit cell. In the former case, the estimation is made by considering that the energy resolution is given by the sampling fluctuation of deposited energy in the liquid argon gap, assuming that the ionization losses in liquid argon gaps due to secondary particles have no correlation with each other. In the latter case, we cannot accept such an assumption, so a new formula for the energy resolution is derived from an analogy with Fano's formalism for the straggling of the total ionization produced by ionizing radiation with a fixed energy. A comparison with the results of Monte Carlo calculation and the experimental data shows that these can be used for estimating the energy resolution of the multi-plate liquid argon calorimeter with a wide liquid argon gap.