In the field of decommissioning nuclear facilities and radiation safety, knowledge of the presence of alpha emitters is important in deciding whether a material can be released or has to be treated as radioactive waste. Yet, the real-time measurement of materials with a low activity of alpha emitters is difficult because of the varying background of natural alpha emitters from Radon and its progenies. Therefore, samples of the material usually have to be taken and analyzed by radio analytics in a laboratory to decide whether a material can be released or has to be treated as radioactive waste. This procedure makes the release process very time-consuming.For this reason, it was investigated whether a more efficient release process without sampling would be possible by the use of a Timepix3 detector with a 5 mm thick CdTe sensor.The Timepix3 readout chip is currently being further developed by the "Medipix4 Collaboration" at CERN. It offers 256 x 256 pixels with a native pixel pitch of 55 x 55 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and can be bump bonded to different semiconductor sensor materials such as Si, GaAs, CdTe, and CdZnTe. The Timepix3 detector features energy and time-of-arrival information for each incident. Due to charge sharing, thicker sensors feature a lower spatial resolution than thinner sensors when photons are counted without a correction for charge sharing. However, if a correction for charge sharing is applied by cluster analysis, both a higher spatial resolution and better energy information can be achieved than when using thin sensors.To investigate whether a spectroscopic distinction between Radon and its progenies and other relevant alpha nuclides is possible, a cluster-based per-pixel calibration of the detector was developed. The analysis of the alpha spectra will be presented here as well as its use for the release process in nuclear decommissioning and radiation protection.