Energy sensitive Timepix silicon detector for electron imaging

We present the first measurements with the energy sensitive Timepix pixel detector for electron imaging. The hybrid pixel detector consists of a silicon detector, 300 µm thick, bump-bonded to the Timepix readout chip developed by the Medipix2 collaboration (256×256 pixels, 55 µm pitch, 14.08×14.08 mm2 sensitive area). Each Timepix pixel can be independently operated in one of three modes: 1. counting of the detected particles; 2. measurement of the particle energy; 3. measurement of the time of particle interaction. The energy measurement in the second mode is performed via the determination of the ???Time-Over-Threshold??? (TOT). The ionization charge generated by the particle along its track is often registered by several adjacent pixels forming a cluster. The shape of the cluster is affected also by lateral charge spread. It is often possible to determine a particle type, its energy, entrance point and direction by on-line or off-line analysis of shapes of recorded clusters. This way an influence of background or noise can be significantly reduced in measured data. The energy spectrum for 90Sr/90Y electrons was measured by Timepix detector and compared with the ??- decay spectrum and the Monte Carlo simulated spectrum. In order to improve spatial resolution, we analyze the tracks of all electrons and substitute each cluster with the position of its centroid. We measured the spatial resolution with a 90Sr/90Y source irradiating, at 10 cm distance, a 100 µm thick steel edge slightly tilted with respect to the detector lines. The oversampled Line Spread Function shows a FWHM of 27.5±1.1 ??m. The Timepix Si detector will be used for digital autoradiography with ??- and ??+ tracers, and it could be used for electron microscopy. First tests were performed with a 14C autoradiography sample.