32ND I NTERNATIONAL COSMIC RAY CONFERENCE,BEIJING 2011 Position sensitive detector at the upgraded LHCf detector T. SUZUKI 1 , O. ADRIANI 2,3 , L. BONECHI 2 , M. BONGI 2 , G. CASTELLINI 2,3 , R. D. ALESSANDRO 2,3 , K. FUKATSU 4 , M. HAGUENAUER 5 ,Y.I TOW 4,6 , K. KASAHARA 1 , K. KAWADE 4 , T. MASE 4 , K. MASUDA 4 , H. MENJO 2,6 , G. MITSUKA 4 ,Y.MURAKI 4 , K. NODA 7 , P. PAPINI 2 , A.-L. PERROT 8 ,S. RICCIARINI 2,9 , T. SAKO 4,6 ,Y.SHIMIZU 1 , K. SUZUKI 4 , K. TAKI 4 , T. TAMURA 10 , S. TORII 1 , A. TRICOMI 7,11 , W. C. TURNER 12 1 Waseda Research Institue for Science and Engineering, Waseda University, Japan 2 INFN Section of Florence, Italy 3 University of Florence, Italy 4 Solar-Terrestrial Environment Laboratory, Nagoya University, Nagoya, Japan 5 Ecole-Polytechnique, Palaiseau, France 6 Kobayashi-Maskawa Institute for the Origin of Particles and the Universe, Nagoya University, Nagoya, Japan 7 INFN Section of Catania, Italy 8 CERN, Switzerland 9 Centro Siciliano di Fisica Nucleare e Struttura della Materia, Catania, Italy 10 Kanagawa University, Japan 11 University of Catania, Italy 12 LBNL, Berkeley, California, USA taku-1984@akane.waseda.jp Abstract: The LHC forward (LHCf) experiment is an experiment to measure the neutral particles emitted in the very forward region of the collision point of LHC. Its goal is to provide data to discriminate the various hadronic interaction models used in cosmic-ray physics. The first phase of the experiment has been achieved on July 2010 with center-of-mass energy √ s =7 TeV, and the first result has been submitted. The data analysis of data is still ongoing. The next phase is planned on 2014 at √ s = 14 TeV. An upgrade of the detector is necessary to cope with the radiaiton damage of our plastic scintillator and the position sensitive scintillating fiber (SciFi) due to the higher beam intensity with the higher energy than the 7 TeV case. For this purpose, GSO scintillator, which has a high radiation resistivity, has been chosen. Small scaled GSO scintillators (GSO bars) have been manufactured to construct a position sensitive detector. The detector, GSO bundle, consists of GSO bars in a single hodoscope plane. In this paper, we describe the expected performance of a GSO bar system in order to replace SciFi. For this purpose, we made a single layer hodoscope consisting of several GSO bars and performed tests using low-energy heavy ion (Carbon) beam as well as cosmic ray muons. We discuss mainly the position resolution and the simulated performance when the system is applied to cascade showers. Keywords: high-energy cosmic-ray, LHC, GSO scintillator, position sensitive detector, hadronic interaction 1 Introduction The LHCf experiment [1] is one of the six experiments of LHC (Large Hadron Collider), and is dedicated for cosmic ray physics. The aim of LHCf is to provide a crucial cali- bration on the existing interaction models used in high en- ergy cosmic ray simulations. To achieve that, LHCf mea- sures neutral particles emitted in the very forward region from the LHC interaction point. LHCf has two indepen- dent detectors named Arm1 and Arm2. Each detector is in- stalled on each side of the interaction point at a distance of 140m and is packed in an aluminium box of 90 mm w ×620 mm h ×290 mm l that fits to the massive zero degree neutral absorbers (Target Neutral Absorber, TAN) instrumentation slot. Each detector is compact, and has two shower sam- pling calorimeters with four position sensitive layers (each calorimeter is called “tower”). The cross section size of the towers are 20 mm×20 mm and 40 mm×40 mm for Arm1 and 25 mm×25 mm and 32 mm×32 mm for Arm2. Tow- ers are composed of 22 tungsten plates and 16 plastic scin- tillators. The total thickness is 44 radiation lengths. For the position sensitive layers, Arm1 uses SciFis and Arm2 uses silicon micro-strips sensors. The structure of the cur- rent Arm1 calorimeter is shown in Fig.1 and a picture of a SciFi belt is shown in Fig.2. This two calorimeters struc- ture allows us to reconstruct π 0 decaying into a γ pair. By measuring the energy and position of these γ ’s, the energy and momentum of the initial π 0 produced by proton-proton collisions at the interaction point can be derived. The first