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

With a 256x256 pixel matrix on a 300μm 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 55μm 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 55μm 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 <1mrad and positions with respect to the detector coordinates often with sub-μm precision. Close tracks can be resolved down to 100μm 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 three-dimensional position-sensitive volume