Threshold characterisation of the Medipix1 chip

The Medipix1 chip is a hybrid pixel detector working in photon counting mode and has the ability to put a discriminating threshold on each photon signal. To increase the image quality the threshold voltage can be electronically fine tuned with a 3 bit threshold adjust for each pixel. This electronic adjustment can only equalise the inhomogeneities of the electronics, not those of the physical frontend (conversion material and bump bonds). The remaining noise of the threshold has an obvious impact on the image quality. To optimise the uniformity over the whole chip we did threshold scans with a cadmium X-ray source analysing every pixel. With those scans we were able to create a bit mask including the physical frontend. Comparing the results of the different methods we can judge the quality of the electronically generated mask and draw conclusions about the properties of the physical frontend. Furthermore, simulations have been carried out for comparison with the measured data.