The intense beams and large cross sections at the LHC give rise to high flux of background throughout the ATLAS detector cavern. This background, composed mainly of low energy neutrons and photons, can cause radiation damage to detector elements and front-end electronics. The induced hits increase detector occupancy. It is important to properly simulate this background so we can understand the sources and attempt to shield them. Cavern background is also crucial for sLHC upgrade.
ATLAS carried out such a calculation several years ago. It used the detector and shielding geometries known at that time. This calculation needs to be updated now to reflect the geometries as expected for the start-up of beams in late 2009. SLAC is spearheading this effort.
Tool
It is generally accepted that the physics modeling in the FLUKApackage gives the best agreement with data for the low-energy neutrons and photons that are important here. On the other hand, ATLAS has little expertise with FLUKA but has much experience with Geant4, which is regarded as having a more flexible geometry definition. We therefore decided to use FLUGG, which couples Geant4 geometry with FLUKA physics.
Validation
The earlier studies made by the ATLAS Radiation Task Force have been reported in ATL-GEN-2005-001. We will use the same geometry, materials, etc as in this report, and compare our FLUGG results with the FLUKA results from the Task Force.
Start-Up for 2009-2010
There is already a very detailed Geant4 description of the ATLAS detector used in physics event simulation; however, it is far too complicated for this application. We will simplify it so that the outlines are correct -- open space between detectors will channel backgrounds and must be simulated properly. We will simplify internal geometries, which are important for detector response but not for this application. It is important to maintain the proper isotopic composition of materials as neutron cross sections are sensitive.
We will calculate the fluences expected for the start-up in late 2009. Predictions will be compared with radiation monitors, detector occupancies and so on. Differences have to be traced back to possible inaccuracies in the modeling.
sLHC Studies
ATLAS will be replacing the Inner Detector tracker for Super LHC. It is expected to be an all-Silicon detector with pixels and strips. We believe additional shielding is needed for the FCAL. It is also expected to replace part of the muon spectrometer detectors. The scope of the muon upgrade has great uncertainty at this time. It can be as little as replacing the endcap detectors at small radii, or it can be replacing all muon detectors, depending on the background level. One of the goals of the cavern background calculation is to provide a more reliable prediction.
Status
As of May 2009, the validation geometry and materials used by the Radiation Trask Force have been implemented. First resultsfrom FLUGG are available. The next step is to define the outputs necessary for comparison with the Task Force results.
Work has started on the 2009 start-up geometry.
Meetings and Material
The cavern background group meets at 17:30 (CERN) biweekly on Thursday. Agendas can be found at http://indico.cern.ch/categoryDisplay.py?categId=225.