Technical Design Report on the project to provide higher-granularity, higher-resolution and longitudinal shower information from the Liquid Argon calorimeter to the Level-1 trigger processors.

ATLAS Liquid Argon Calorimeter Phase-I Upgrade Technical Design Report

Upgrade Plans for ATLAS Forward Calorimetry for the HL-LHC

We have uniformly irradiated liquid argon ionization chambers with betas from high-activity 90 Sr sources. The radiation environment is similar to that in the liquid argon calorimeters which are part of the ATLAS detector installed at CERN׳s Large Hadron Collider (LHC). We measured the resulting ionization current over a wide range of applied potential for two different source activities and for three different chamber gaps. These studies provide operating experience at exceptionally high ionization rates. In particular they indicate a stability at the 0.1% level for these calorimeters over years of operation at the full LHC luminosity when operated in the normal mode at an electric field E = 1.0 kV / mm . We can operate these chambers in the normal mode or in the space-charge limited regime and thereby determine the transition point between the two. This transition point is parameterized by a positive argon ion mobility of μ + = 0.08 ± 0.02 mm 2 / V s at a temperature of 88.0±0.5 K and at a pressure of 1.02±0.02 bar. In the space-charge limited regime the ionization currents are degraded and show signs of instability. At the highest electric fields in our study (6.7 kV/mm) the ionization current is still slowly rising with increasing electric field.

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Space-charge effects in liquid argon ionization chambers

In this paper it is shown that a measurement of the relative luminosity changes at the LHC may be obtained by analysing the currents drawn from the high voltage power supplies of the electromagnetic section of the forward calorimeter of the ATLAS detector. The method was verified with a reproduction of a small section of the ATLAS forward calorimeter using proton beams of known beam energies and variable intensities at the U-70 accelerator at IHEP in Protvino, Russia. The experimental setup and the data taking during a test beam run in April 2008 are described in detail. A comparison of the measured high voltage currents with reference measurements from beam intensity monitors shows a linear dependence on the beam intensity. The non-linearities are measured to be less than 0.5% combining statistical and systematic uncertainties.

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Relative luminosity measurement of the LHC with the ATLAS forward calorimeter

Works on the steering of particle beams using channeling in aligned bent crystals have been carried out at IHEP over a number of years. Comprehensive theoretical and experimental studies have led to the creation of actually operating systems on the U-70 accelerator. In particular, a slow proton beam extraction with unprecedentedly high parameters, an extraction efficiency of about 85% at a beam intensity of 10[Formula: see text] particles per cycle, has been realized on U-70 using particle channeling in short bent crystals. In recent years, successful experiments on the extraction of carbon ions were performed. At present, crystal elements are used in regular U-70 runs and provide half of the particle beams for physical experiments. Here, we summarize the results of this unique experience in the world practice of accelerators and outline its prospects.

Positive features of crystal extraction of protons and carbon ions from the U-70 on basis of long-term experience

We project the performance of the ATLAS liquid argon endcap and forward calorimeters at the planned high luminosity LHC option HL-LHC by exposing small calorimeter modules of the electromagnetic, hadronic, and forward calorimeters to high intensity beams at IHEP/Protvino. The beam intensity extends well beyond the maximum expected for these calorimeters at HL-LHC. The signal reconstruction and calorimeter performance have been studied in full detail.

http://inspirehep.net/record/1194814/files/ATL-LARG-PROC-2011-012.pdf

A. Grekhov

Papers

Liquid argon calorimeter performance at high rates