Microscopic imaging of muons and 120 GeV/c pion interactions in a single, low-noise 256x256 Si pixel detector

With a 256x256 pixel matrix on a 300\mum thick silicon sensor, placed parallel to the particles in the CERN H6 beam, it proves possible to record electronically in real time the microscopic details of particle trajectories and interactions. The Medipix2 readout chip, matched to this matrix of 55\mum square pixels, contains a tuneable discriminator and a pulse counter in each pixel. The noise of the signal processing chain in each pixel is on average 135e^- (equivalent electrons) r.m.s. A threshold not much higher than 800e^- allows discrimination of full signals ~3800e^- as well as partial signals from a minimum ionizing particle in adjacent 55\mum thick pixels. With binary pixel information, exploiting charge diffusion and redundancy in the large matrix, the vectors of trajectories can be reconstructed with angular accuracy <1mradian and positions with respect to the detector coordinates often with sub-\mum precison. Close tracks can be resolved down to 100\mum distance. The width of the trail in the matrix sometimes can provide information on the energy deposition as well. A variety of applications can be imagined, the more so if several such detectors could be stacked to create a true 3-dimensional position-sensitive volume.