Jackson Choate

 High-energy electrons and photons lose energy primarily through Bremsstrahlung and pair production, respectively.  Bremsstrahlung and pair production produce more electrons and photons at lower energies.

We see that for denser media, photons are more likely to produce electron-positron pairs and electrons lose energy predominantly through Bremsstrahlung

The denser the media, the less energy more massive charged particles lose per unit length.

 To measure the energy of lighter charged particles and photons, a dense absorber needs to be used with an ionizable medium.  Solution? Lead sheets clad in stainless steel with a liquid argon medium.  Showers in the argon liberate electrons to be collected and recorded.  Argon has the property of being resistant to ionizing and neutron radiations.

“Accordion” structure allows for faster readings and reduced dead zones while providing a radiation depth of 24 X o.

EM Calorimeter End-Cap uses the same accordion structure as the barrel, but positioned orthogonally in a “Spanish Fan” configuration. However, due to the orientation of the end-caps, this will produce variations in the distance traveled by cascades and affect measurements.

To minimize the effects of the sampling fraction and argon gap variations, the high voltage (which defines drift velocity) is varied. This causes the average drift velocity to remain constant throughout the calorimeter.

Cryostat maintains the -185 o C temperature to keep the liquid argon from becoming a gas.

Simulated result of shower inside of the EM Calorimeter Accordion

1.Presampler – Single thin layer of liquid argon to correct for energy losses in Inner Detector 2.1 st Sampling – Provides excellent resolution for photon/neutral pion separation 3.2 nd Sampling – Clusters below 50 GeV are fully contained 4.3 rd Sampling – Only the highest energy electrons will reach this deep

Electrodes are kept at a potential of V, creating an electric field of 1 MV/m between the absorber and the electrode.

 Signals collected at the electrodes are transferred through vacuum- sealed tubes called “feedthroughs.”  These feedthroughs are designed to preserve the signal during the transition from the cold liquid argon to the warmer electronics area.

Schematic of an end-cap feedthrough

As seen, the EM Calorimeter has a very precise energy resolution. It’s spatial resolution is also very precise, capable of measuring pseudorapidity and perpendicular plane angles within radians.

 D. M. Gingrich et al. Performance of a large scale prototype of the ATLAS accordion electromagnetic calorimeter. Nucl. Instrum. Meth., A364: ,  Egede, Ulrik. "The liquid argon calorimeter." N.p., 08 Jan Web. 3 Nov  Froidevaux D, Sphicas P Annu. Rev. Nucl. Part. Sci. 56:375 – 440  Krieger, Peter. "The ATLAS Liquid Argon Calorimeter." University of Toronto, 26 OCT  P. Schwemling. The European Physical Journal C - Particles and Fields Volume 34, Supplement 1, s129-s137, DOI: /epjcd/s x