LHCf consists in two imaging calorimeters made of tungsten plates, plastic scintillator and position sensitive sensors and they are installed at zero degree collision angle ±140m from ATLAS interaction point (IP1) inside the TAN.
The TANs (Target Neutral Absorber) are massive zero degree neutral absorbers located in the proximity of IP1 and IP5 in order to protect the outer superconducting beam separation dipoles D2 from neutral particle debris from the IP. Charged particles from the IP are swept aside by the inner beam separation dipole D1 before reaching the TAN. Inside TAN the beam vacuum chamber makes a Y shaped transition from a single common beam tube facing the IP to two separate beam tubes joining to the arcs of LHC. In the crotch of this "Y-chamber" there is an instrumentation slot of 96mmx607mmx1000mm.
The two detectors of LHCf are inserted in the TAN instrumentation slots on both sides of IP1. Each occupies a 300mm length in the most upstream position of the instrumentation slots followed by BRAN (a luminosity monitors) and the ATLAS ZDCs.
Both of the LHCf detectors consist of a pair of small sampling and imaging calorimeters made of plastic scintillators interleaved with tungsten converters and a position sensitive layers in order to provide incident shower positions.
The two detectors are very similar, but use different geometry for the calorimetric part and two different sistems for the tracking part.
Conventionally we call detector 1 (ARM 1) the detector installed in the IP8 side and detector 2 (ARM 2) the detector installed in the IP2 2 side.
Both detectors are supported by manipulators mounted to the top surface of the TAN in order to have the capability of remotely moving the detectors vertically by a 120mm stroke. In default setting, the center of the smaller calorimeters is placed on the horizontal midplane. Using the manipulators to move the detectors vertically from their default positions increases the range of transverse momentum (PT) that can be measured. The geometry of detector 2 is designed to maximize the PT coverage without scanning and/or employing a finite crossing angle.
In front of each detector, a Front Counter (FC) made of plastic scintillators is inserted. They provide useful trigger information by covering a larger aperture than the calorimeters.