Jackson Choate ATLAS Calorimeters The Science Behind It Highenergy electrons and photons lose energy primarily through Bremsstrahlung and pair production respectively Bremsstrahlung and pair production produce more electrons and photons at lower energies ID: 159688
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Slide1
ATLAS EM Calorimeter
Jackson ChoateSlide2
ATLAS CalorimetersSlide3
The Science Behind It…
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
.Slide4
Science (cont.)
We see that for denser media, photons are more likely to produce electron-positron pairs and electrons lose energy predominantly through BremsstrahlungSlide5
Science (cont.)
The denser the media, the less energy more massive charged particles lose per unit length.Slide6
EM Calorimeter Design
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. Slide7
EM Calorimeter BarrelSlide8
EM Calorimeter Accordion
“Accordion” structure allows for faster readings and reduced dead zones while providing a radiation depth of 24 X
o.Slide9
EM Calorimeter Accordion (cont.)Slide10
EM Calorimeter
End-Cap
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.Slide11
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.Slide12
Barrel & End-Cap SetupSlide13
EM Cryostat
Cryostat maintains the -185
o
C temperature to keep the liquid argon from becoming a gas. Slide14
EM Calorimeter
Data Collection
Simulated result of shower inside of the EM Calorimeter AccordionSlide15
Presampler
– Single thin layer of liquid argon to correct for energy losses in Inner Detector
1
st
Sampling – Provides excellent resolution for photon/neutral
pion
separation
2
nd
Sampling – Clusters below 50
GeV
are fully contained
3
rd
Sampling – Only the highest energy electrons will reach this deepSlide16
EM Calorimeter Data Collection
Electrodes are kept at a potential of +2000 V, creating an electric field of 1 MV/m between the absorber and the
electrode
. Slide17
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.Slide18
Schematic of an end-cap feedthroughSlide19
Energy Resolution of EM Calorimeter
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 0.025 radians. Slide20Slide21
Works Cited
D
. M. Gingrich et al.
Performance
of a large scale prototype
of
the ATLAS accordion electromagnetic calorimeter.
Nucl
.
Instrum
. Meth.
, A364:290-306, 1995.
Egede
,
Ulrik
. "The liquid argon calorimeter."
N.p
., 08
Jan 1998
.
Web
. 3 Nov 2010.
<
http://
www.hep.lu.se/atlas/thesis/egede/thesis- node43.html
>. Froidevaux D, Sphicas P. 2006. Annu. Rev. Nucl. Part. Sci. 56:375 – 440Krieger, Peter. "The ATLAS Liquid Argon Calorimeter." University of Toronto, 26 OCT
<http://www.physics.utoronto.ca/~krieger /talks/Krieger_NSS05_Talk.pdf>P. Schwemling. The European Physical Journal C
- Particles and Fields Volume 34, Supplement 1, s129-s137, DOI: 10.1140/
epjcd
/s2004-04-014-x