The Photon Counting Webcam

Cai-Oliver Thier, Tobias Kaufmann

project report

A photon counting camera is the key to spectacular and convincing physics experiments relating to the wave-particle duality of photons. However, such cameras are too expensive for schools. Can such measurements be realized with a normal digital camera? No, a normal CCD sensor is not sensitive enough to detect single photons of light. But X-ray photons are much more energetic. Can a normal digital camera detect X-rays without a fluorescent screen? Our experiments with a webcam and an X-ray apparatus for students showed several exposed pixels per image during X-ray illumination. We tried to record interference patterns resulting from X-ray interference on crystals by adding up the images using the software Giotto. Initially, we did not succeed with the webcam as its CCD sensor is much smaller than the interference pattern. For safety reasons, we could not use a digital SLR with a larger CCD, as the X-ray apparatus would not have closed. The breakthrough was achieved by recording X-ray spectra exploiting Bragg diffraction and the turning crystal method. Bragg diffraction can only be explained by interference of waves, while the recorded pixels show that the detected radiation consists of particles. The familiar intensity distribution emerges when adding up the images using Giotto.

Our low cost method allows physics students to discover experimentally in a double period that electromagnetic waves are neither classical waves nor classical particles. They can discover that photons are quantum objects whose impact on the CCD or CMOS chip can only be described by a probability distribution. The interference pattern known form classical physics only emerges after a large number of photons.

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