Abstract - Neutron transmission radiography can serve as a complementary diagnostic method to X-ray radiography. It can produce contrast images of materials, which are indistinguishable in X-ray images (typically materials containing Hydrogen). Good performance of a neutron detecting device is essential for the acquisition of high quality neutron images. The properties of the neutron pixel detector based on the single X-ray photon pixel detector device Medipix-1 (64 x 64 square pixels with pitch of 170 μm) were already demonstrated and published. Slow neutrons are captured and converted in a surface layer containing Li-6 to tritons and alpha particles which are subsequently detected by a silicon pixel detector. A Medipix-2 detector (256 x 256 square pixels with pitch of 55 μm) was adapted in a similar way. The detector performance was tested at the NEUTRA station of the Paul Scherrer Institute using a beam of slow neutrons from the spallation source SINQ with an intensity of 3⋅10 6 neutrons/cm 2 s. The spatial resolution, detection efficiency and other detector properties have been determined and are compared with several types of contemporary neutron imaging systems (Medipix-1, CCD camera, imaging plates). The results demonstrate the superiority of the Medipix-2 based neutron imager in terms of spatial resolution, linearity and dynamic range. The system is very promising in applications for micro-neutron tomography, especially when large detector areas will become available. I. INTRODUCTION HE hybrid silicon pixel device of Medipix type developed at CERN [1] was originally designed for position sensitive single X-ray photon detection. The Medipix device consists of a semiconductor detector chip bonded to a readout chip. The silicon detector chip is equipped with a single common backside electrode and a rectangular front side matrix of electrodes. Each element of the matrix (pixel) is connected to its respective preamplifier, discriminator and digital counter integrated on a readout chip. Each individual pixel counts ionizing particles of adequate energies crossing its Manuscript received October 26, 2004. Work carried out within the CERN Medipix Collaboration. Work was supported in part by the EU project “3D- RID”. J. Jakubek, T. Holy, S. Pospisil, J. Uher and D. Vavrik are with the Institute of Experimental and Applied Physics, Czech Technical University in Prague, Horska 3a/22, CZ-12800 Prague 2, Czech Republic telephone: + 420 224 359 181, fax: + 420 224 359 392, e-mail: jan.jakubek@utef.cvut.cz). S. B. Author, Jr., was with Rice University, Houston, TX 77005 USA. He is now with the Department of Physics, Colorado State University, Ft. Collins, CO 80523 USA (telephone: 970-491-6206, e-mail: author@lamar. colostate.edu). E. Lehmann is with the Paul Scherrer Institute, CH-5232 Villigen, Switzerland (e-mail: eberhard.lehmann@psi.ch). J. Vacik is with the Nuclear Physics Institute, Academy of Sciences of the Czech Republic, Rez near Prague, CZ-25068, Czech Republic (e-mail: vacik@ujf.cas.cz). area. Basic properties of Medipix devices are summarized in Tab. 1. Tab. 1. Medipix 1 and 2 devices. Thermal neutrons can be hardly detected by the silicon detector itself. Therefore we deposited a layer of neutron converter on the Medipix surface [2] as illustrated in Fig. 1. The following nuclear reactions were considered for converter design: 6 Li + n → α (2.05 MeV) + 3 H (2.72 MeV) 10 B + n → α (1.47 MeV) + 7 Li (0.84 MeV) + γ (0.48MeV) (93.7%) 10 B + n → α (1.78 MeV) + 7 Li (1.01 MeV) (6.3%) 113 Cd + n → 114 Cd + γ (0.56MeV) + conv. electrons 155 Gd + n → 156 Gd + γ (0.09, 0.20, 0.30 MeV) + conv. electrons 157 Gd + n → 158 Gd + γ (0.08, 0.18, 0.28 MeV) + conv. electrons Thermal neutrons are converted in the layer into secondary radiation (heavy charged particles, electrons) which can be subsequently detected by the pixel detector. The signal created by secondary charged particles is high enough to set the discriminator threshold well above noise and possible gamma background. Count of events in each pixel obeys a Poisson distribution with standard deviation determined only by the number of neutrons reacting in the converter. Therefore, the signal to noise ratio can be improved to any arbitrary level by extending the exposition time. Fig. 1. Medipix device with 300μm thick Silicon sensor chip covered by a 6 LiF converter with illustration of a neutron conversion in the converter layer. Properties of Neutron Pixel Detector based on Medipix-2 Device Jan Jakubek, Tomas Holy, Eberhard Lehmann, Stanislav Pospisil, Josef Uher, Jiri Vacik, Daniel Vavrik T Medipix-1 Medipix-2 Pixels 64 x 64 256 x 256 Pixel size 170 x 170 μm 2 55 x 55 μm 2 Counter 15 bit 13 bit Read out 384 μs parallel (at 10 MHz) 266 μs parallel or 9 ms serial (at 100 MHz) Technology 1 mm SACMOS 6-metal 0.25mm CMOS 0-7803-8701-5/04/$20.00 (C) 2004 IEEE