LHCf is an experiment dedicated to the measurement of neutral particles emitted in the very forward region of LHC collisions. The physics goal is to provide data for calibrating the hadron interaction models that are used in the study of Extremely High-Energy Cosmic-Rays. This is possible since the laboratory equivalent collision energy of LHC is 10¹⁷ eV. Two LHCf detectors, consisting of imaging calorimeters made of tungsten plates, plastic scintillator and position sensitive sensors, are installed at zero degree collision angle +-140m from an interaction point (IP). Although the lateral dimensions of these calorimeters are very compact, ranging from 20mm x 20mm to 40mm x 40 mm, the energy resolution is expected to be better than 6% and the position resolution better than 0.2mm for gamma-rays with energy from 100 GeV to 7 TeV. This has been confirmed by test beam results at the CERN SPS. These calorimeters can measure particles emitted in the pseudo rapidity range >8.4. Detectors, data acquisition and electronics are optimized to operate during the early phase of the LHC commissioning with luminosity below 10³⁰ cm^-2s^-1. LHCf is expected to obtain data to compare with the major hadron interaction models within a week or so of operation at luminosity > 10²⁹ cm^-2s^-1.

Both LHCf detectors employ two imaging shower calorimeters. While the structure of the calorimeters is similar (except for their sizes and orientation), the position sensitive sensors are quite different. Detector 1 uses scintillating fibers (SciFi) and Multi-anode PMTs (MAPMTs), while detector 2 uses silicon strip sensors. Four X-Y pairs of the position sensors are installed at 6, 10, 30 and 42 X0 for detector 1 and 6, 12, 30 and 42X0 for detector 2. The first two pairs are optimized to detect the shower maximum of gamma-ray induced showers and the other two are for hadronic showers developed deep in the calorimeters. These position sensitive sensors are necessary not only to obtain the transverse momentum of the incident primary but also to correct for the effect of leakage from the edges of the calorimeters. Because the fraction of shower leakage with respect to the total energy is only a function of the position and independent of the energy, we can correct for this effect by measuring the position of the shower.
Calorimeter components (scintillator, tungsten and PMT), position sensors and their frontend electronics are packed in a 92mm^w x 620mm^h x 280mm^l aluminum box for each detector. The size is designed to fit the narrow instrumentation slot in the TAN. One wall of the box for detector 2 is made of copper to better dissipate the heat generated by the front-end circuitry for the silicon strips. The detectors in their boxes are attached to and supported by the manipulators. In front of each detector (facing the IP), a counter composed of thin plastic scintillators called the Front Counter (FC) is installed.