Noiseless direct detection of electrons in Medipix2 for electron microscopy

We describe the application of a hybrid pixel detector (HPD), Medipix2, containing 256×256 pixels to electron microscopy. Our previous tests on HPDs were made with Medipix1, which was an earlier version of the present detector. Apart from an increase in the size and number of pixels, the main advantage of Medipix2 for our applications is in the ability to set a fairly high threshold voltage. We show that Medipix2 has a high detection efficiency for 120 keV electrons, with no readout noise. The spatial resolution is also excellent as measured by comparing the power spectrum obtained by imaging a standard electron microscopy grid: at Nyquist frequency the power is 76% compared to film. The special properties of Medipix2 allow electron detection in single pixels. No radiation damage was detectable up to a dose of ∼6 Mrads—equivalent to many years normal usage in the electron microscope.