A Basic Description of the ATLAS Shielding

The high levels of radiation in the ATLAS environment impose stringent requirements on shielding, which is vital for the operation of the muon spectrometer. Special measures have also to be taken to reduce the radiation damage to the silicon of the inner detector.

The dominant source of the radiation in ATLAS is proton-proton collisions at the interaction point. Other sources such as beam halo and beam-gas interactions are very small in comparison. Most of the collision products are absorbed in the calorimeters or in the copper absorber protecting the first machine quadrupoles. But shower leakage produces backgrounds in the muon spectrometer, and particles from backsplash will affect the inner detector. Gaps between calorimeters can be the dominant effect in certain regions. Careful consideration must be given to background rates, detector ageing and activation of material, and steps taken to minimize them.

The interaction point is not directly visible from the muon spectrometer, but particles produced in the proton-proton collisions can strike material such as the beam pipe, vacuum pumps and supports as well as the calorimeters, initiating secondary and tertiary particles which could emerge into the ATLAS detector. The major secondary sources are the inner edge of the forward liquid-argon calorimeter and the copper absorber in front of the machine quadrupoles. Fortunately there is sufficient space in the latter region to provide a thick shield.

High-energy particles from the interaction point or from secondary and tertiary interactions initiate showers when they enter material, and if the material is thick enough, most charged particles will be absorbed. There is a residue of neutrons and associated photons, and in certain regions, punch-through muons. Neutrons can cause radiation damage to the silicon detectors, and need to be reduced to below 100 KeV in energy using a moderator. Capture of thermal neutrons frequently gives rise to gamma rays, which in turn produce electrons and positrons, causing stray hits in the detectors.

The basic principle for shielding the muon spectrometer, is to construct as thick a shield as is affordable and as space allows from a material of fairly short interaction length such as iron or copper, but one which does not produce too many secondary neutrons. Charged particles in the shower are largely absorbed, but there is a residue of neutrons and gamma rays. The remaining neutrons are then moderated in a hydrogen-rich material such as polyethylene, and then stopped. We choose to dope the polyethylene with a lithium compound since lithium, unlike boron, absorbs neutrons of low energy without the emission of capture gammas. There still remains a flux of photons which is removed where practicable by a few centimeters of lead, which has a very small cross-section for producing gamma rays from neutron capture.